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Walther Meissner

SUPERCONDUCTOR:

An element, inter-metallic alloy, or compound that will conduct electricity without

resistance below a certain temperature. Resistance is undesirable because it produces losses in the energy flowing through the material. Once set is motion, electrical current will flow forever in a closed loop of superconducting material – making it the closest thing to perpetual motion in nature. Scientist refer to superconductivity as a “microscopic quantum phenomenon”. THE HISTORY OF SUPERCONDUCTORS:

Superconductors, materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery. Not only have the limits of superconductivity not yet been reached, but the theories that explain superconductor behavior seem to be constantly under review. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University (shown). When he cooled it to the temperature of liquid helium, 4 °Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an "absolute" scale of temperature. Thus, it was necessary to come within 4

degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area.

Next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walther Meissner (above) and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field (below graphic). A magnet moving by a conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the "Meissner effect" (an eponym). The Meissner effect is so strong that a magnet can actually be

levitated over a superconductive material.

In subsequent decades other superconducting metals, alloys and compounds were discovered. In 1941 niobium-nitride was found to superconduct at 16 °K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 °K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi). High-energy, particle-accelerator electromagnets made of copper-clad niobium-titanium were then developed in the 1960s at the Rutherford-Appleton Laboratory in the UK, and were first employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.

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The first widely-accepted theoretical understanding of superconductivity was advanced in 1957 by American physicists John Bardeen, Leon Cooper, and John Schrieffer (above). Their Theories of Superconductivity became know as the BCS theory - derived from the first letter of each man's last name - and won them a Nobel prize in 1972. The mathematically-complex BCS theory

explained superconductivity at temperatures close to absolute zero for elements and simple alloys. However, at higher temperatures and with different superconductor systems, the BCS theory has subsequently become inadequate to fully explain how superconductivity is occurring.

Another significant theoretical advancement came in 1962 when Brian D. Josephson (above), a graduate student at Cambridge University, predicted that electrical current would flow between 2 superconducting materials - even when they are separated by a non-superconductor or insulator. His prediction was later confirmed and won him a share of the 1973 Nobel Prize in Physics. This tunneling phenomenon is today known as the "Josephson effect" and has been applied to electronic devices such as the SQUID, an instrument capabable of detecting even the weakest magnetic fields. (Below SQUID graphic courtesy Quantum Design.)

The 1980's were a decade of unrivaled discovery in the field of superconductivity. In 1964 Bill Little of Stanford University had suggested the possibility of organic (carbon-based) superconductors. The first of these theoretical superconductors was successfully synthesized in 1980 by Danish researcher Klaus Bechgaard of the University of Copenhagen and 3 French team members. (TMTSF)2PF6 had to be cooled to an incredibly cold 1.2K transition temperature (known as Tc) and subjected to high pressure to superconduct. But, its mere existence proved the possibility of "designer" molecules - molecules fashioned to perform in a predictable way.

Then, in 1986, a truly breakthrough discovery was made in the field of superconductivity. Alex Müller and Georg Bednorz (Photo), researchers at the IBM Research Laboratory in Rüschlikon, Switzerland, created a brittle ceramic compound that superconducted at the highest temperature then known: 30 °K. What made this discovery so remarkable was that ceramics are normally insulators. They don't conduct electricity well at all. So, researchers had not considered them as possible high-temperature superconductor candidates. The Lanthanum, Barium,

Copper and Oxygen compound that Müller and Bednorz synthesized, behaved in a not-as-yet-understood way. The discovery of this first of the superconducting copper-oxides (cuprates) won the 2 men a Nobel Prize the following year. It was later found that tiny amounts of this material were actually superconducting at 58 K, due to a small amount of lead having been added as a calibration standard - making the discovery even more noteworthy.

Müller and Bednorz' discovery triggered a flurry of activity in the field of superconductivity. Researchers around the world began "cooking" up ceramics of every imaginable combination in a

Brian Josephson

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quest for higher and higher Tc's. In January of 1987 a research team at the University of Alabama-Huntsville substituted Yttrium for Lanthanum in the Müller and Bednorz molecule and achieved an incredible 92 K Tc. For the first time a material (today referred to as YBCO) had been found that would superconduct at temperatures warmer than liquid nitrogen - a commonly available coolant. Additional milestones have since been achieved using exotic - and often toxic - elements in the base perovskite ceramic. The current class (or "system") of ceramic superconductors with the highest transition temperatures are the mercuric-cuprates. The first synthesis of one of these compounds was achieved in 1993 at the University of Colorado and by the team of A. Schilling, M. Cantoni, J. D. Guo, and H. R. Ott of Zurich, Switzerland. The world record Tc of 138 °°°°K is now held by a thallium-doped, mercuric-cuprate comprised of the elements Mercury, Thallium, Barium, Calcium, Copper and Oxygen. The Tc of this ceramic superconductor was confirmed by Dr. Ron Goldfarb at the National Institute of Standards and Technology-Colorado in February of 1994. Under extreme pressure its Tc can be coaxed up even higher - approximately 25 to 30 degrees more at 300,000 atmospheres. The world record Tc of 138 °K

The first company to capitalize on high-temperature superconductors was Illinois Superconductor (today known as ISCO International), formed in 1989. This amalgam of government, private-industry and academic interests introduced a depth sensor for medical equipment that was able to operate at

liquid nitrogen temperatures (~ 77K).

In recent years, many discoveries regarding the novel nature of superconductivity have been made. In 1997 researchers found that at a temperature very near absolute zero an alloy of gold and indium was both a superconductor and a natural magnet. Conventional wisdom held that a material with such properties could not exist! Since then, over a half-dozen such compounds have been found. Recent years have also seen the discovery of the first high-temperature superconductor that does NOT contain any copper (2000), and the first all-metal perovskite superconductor (2001).

Also in 2001 a material that had been sitting on laboratory shelves for decades was found to be an extraordinary new superconductor. Japanese researchers measured the transition temperature of magnesium diboride at 39 °Kelvin - far above the highest Tc of any of the elemental or binary alloy superconductors. While 39 K is still well below the Tc's of the "warm" ceramic superconductors, subsequent refinements in the way MgB2 is fabricated have paved the way for its use in industrial applications. Laboratory testing has found MgB2 will outperform NbTi and Nb3Sn wires in high magnetic field applications like MRI.

Though a theory to explain high-temperature superconductivity still eludes modern science, clues occasionally appear that contribute to our understanding of the exotic nature of this phenomenon. In 2005, for example, Superconductors.ORG discovered that increasing the weight ratios of alternating planes within the layered perovskites can often increase Tc significantly. This has led to the discovery of more than 40 new high-temperature superconductors, including a candidate for a new world record.

The most recent "family" of superconductors to be discovered is the "pnictides". These iron-based superconductors were first observed by a group of Japanese researchers in 2006. Like the high-Tc copper-oxides, the exact mechanism that facilitates superconductivity in them is a mystery. However, with Tc's over 50K, a great deal of excitement has resulted from their discovery.

Researchers do agree on one thing: discovery in the field of superconductivity is as much serendipity as it is science.

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SUPERCONDUCTOR TERMINOLOGY AND THE NAMING SCHEME

Anneal: To heat and then slowly cool a material to reduce brittleness. Annealing of ceramic superconductors usually follows sintering and is done in an oxygen-rich atmosphere to restore oxygen lost during calcination. The oxygen content of a ceramic superconductor is critical. For example, YBCO with 6.4 atoms of oxygen will not superconduct. But YBCO with 6.5 atoms will.

Anti-ferromagnetism: A state of matter where adjacent ions in a material are aligned in opposite or "anti-parallel" arrays. Such materials display almost no response to an external magnetic field at low temperatures and only a weak attaction at higher temperatures. There is evidence that anti-ferromagnetism in the copper oxides plays a role in the formation of Cooper pairs and, thus, in facilitating a superconductive state in some compounds.

BCS Theory: The first widely-accepted theory to explain superconductivity put forth in 1957 by John Bardeen, Leon Cooper, and John Schreiffer. The theory asserts that, as electrons pass through a crystal lattice, the lattice deforms inward towards the electrons generating sound packets known as "phonons". These phonons produce a trough of positive charge in the area of deformation that assists subsequent electrons in passing through the same region in a process known as phonon-mediated coupling. This is analogous to rolling a bowling ball up the middle of a bed. 2 people, one lying on each side of the bed, will tend to roll toward the center of the bed, once the ball has created a depression in the mattress. And, a 2nd bowling ball, placed at the foot of the bed, will now, quite easily, roll toward the middle.

Borocarbides: Superconducting borocarbides are compounds containing both boron and carbon in combination with rare-earth and transition elements; some of which exhibit the unusual ability to return to a normal, non-superconductive state at temperatures below Tc. For more on this, click here.

BSCCO: An acronym for a ceramic superconductor system containing the elements Bismuth, Strontium, Calcium, Copper and Oxygen. Typically, a small amount of lead is also included in these compounds to promote the highest possible Tc. BSCCO has probably found the widest acceptance among high-Tc superconductor applications due to its unique properties. BSCCO compounds exhibit both an intrinsic Josephson effect and anisotropic (directional) behavior. You can view a list of BSCCO compounds on the "Type 2" page.

Ceramics: Ceramic superconductors are inorganic compounds formed by reacting a metal with oxygen, nitrogen, carbon or silicon. The best-known of these are the copper-perovskites. Ceramics are typically hard, brittle, heat-resistant materials formed by a process known as solid-state reaction.

Charge Reservoirs: In superconductors, charge reservoirs are the layers that may control the oxidation state of adjacent superconducting planes (even though they themselves are not superconducting). In the layered cuprates, these consist of copper-oxide chains.

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Chevrel (phases): A class of molybdenum chalcogenides (compounds containing Group VI elements S, Se or Te along with molybdenum and a positively charged metal ion) - named for Roger Chevrel of the University of Rennes, whose research brought them to the attention of the scientific community in the early 1970's. Recently, the Superconductivity Group at the University of Durham (UK) reported a novel fabrication technique that increases Bc2 (upper critical field) in the chevrel PbMo6S8 from 50 T in bulk materials up to > 100 T. Click HERE to read a [technical] writeup on this (as a PDF file). Or click HERE to see a short list of some of these compounds alongside their Tc's.

Coherence Length: The size of a cooper pair - representing the shortest distance over which superconductivity can be established in a material. This is typically on the order of 1000Å; although it can be as small as 30Å in the copper oxides.

Cooper Pair: Two electrons that appear to "team up" in accordance with theory - BCS or other - despite the fact that they both have a negative charge and normally repel each other. (Named for Leon Cooper.) Below the superconducting transition temperature, paired electrons form a condensate - a macroscopically occupied single quantum state - which flows without resistance. However, since only a small fraction of the electrons are paired, the bulk does not qualify as being a "bose-einstein condensate". Click here to see an animation of a cooper pair.

DAC: An acronym for "diamond anvil cell". Often the Tc of a superconductor can be coaxed upward with the application of high pressure. The DAC is used to accomplish this in the laboratory. A DAC is composed of 2 specially-cut diamonds and a stainless steel gasket. The gasket goes between the diamonds and seals a small chamber in which a fluid is placed. Since neither the diamonds nor the liquid will compress, hydrostatic forces in excess of a million atmospheres can be brought to bear on a sample suspended within the fluid. Click here to see a graphic of a DAC.

Diamagnetism: The ability of a material to repel a magnetic field. Many naturally-occurring substances (like water, wood and paraffin, and many of the elements) exhibit weak diamagnetism. Superconductors exhibit strong diamagnetism below Tc. In a few rare compounds, a material may become superconductive at a higher temperature than the point at which diamagnetism appears. But, as a rule, the onset of strong diamagnetism is one of the most reliable ways to ascertain when a material has become superconductive

D-Wave: A form of electron pairing in which the electrons travel together in orbits resembling a four-leaf clover. Wave functions help theoreticians describe (and predict) electron behavior. The d-wave models have gained substantial support recently over s-wave pairing as the mechanism by which high-temperature superconductivity might be explained

Energy Gap: This is the energy required to break up a pair of electrons. According to BCS theory, the formula for determining the energy gap (in meV) is Eg=7/2 KTc. Where K = Boltzmann's constant (8.62e-5 eV/K). And where Tc is the critical transition temperature in Kelvin. Since electron-pairing is universally agreed to be the method by which superconductivity occurs, this is the amount of energy required to disrupt the superconducting state.

ESR: An acronym for "Electron Spin Resonance" (also EPR: Electron Paramagnetic Resonance). This is another mechanism by which superconductivity might be explained in some materials. Simply put, ESR is the response of electrons to electromagnetic radiation or magnetic fields at discrete frequencies. Electrons, as they move, create tiny magnetic moments. Nearby electrons are influenced either beneficially or adversely. When the moments are complementary, the electrons become paired and can help each other move through a crystal lattice.

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Ferrite: Ferrites are ceramics with magnetic properties. They are included on this page because many of the same elements used in ferrites (e.g. Ba, Sr, Tm, O) are also key constituents in ceramic superconductors. This may be an important clue in understanding high-temperature superconductivity.

Ferromagnetism: A state wherein a material exhibits magnetization through the alignment of internal ions (neighboring magnetic moments). This contrasts with paramagnetism, which is temporary, much weaker and results from unpaired electrons.

Flux-Lattice: A configuration created when flux lines from a strong magnetic field try to penetrate the surface of a Type 2 superconductor. The tiny magnetic moments within each resulting vortex repel each other and a periodic lattice results as they array themselves in an orderly fashion.

Fluxon: The smallest magnetic flux (flux quantum) that exists in nature. Just as electrons are quantized charge, fluxons are a quantized flux. The term is used in association with vortices, which result from magnetic fields penetrating Type 2 superconductors in single fluxon quanta. Click here to view a hollographic depiction of waves of fluxons on the surface of superconducting Niobium.

lux-Pinning: The phenomenon where a magnet's lines of force (called flux) become trapped or "pinned" inside a superconducting material. This pinning binds the superconductor to the magnet at a fixed distance. Flux-pinning is only possible when there are defects in the crystalline structure of the superconductor (usually resulting from grain boundaries or impurities). Flux-pinning is desirable in high-temperature ceramic superconductors in order to prevent "flux-creep", which can create a pseudo-resistance and depress Jc and Hc. Click here to see a superconductor suspended in air by flux-pinning.

Four-point Probe: The most common method of determining the Tc of a superconductor. Wires are attached to a material at four points with a conductive adhesive. Through two of these points a voltage is applied and, if the material is conductive, a current will flow. Then, if any resistance exists in the material, a voltage will appear across the other two points in accordance with Ohm's law (voltage equals current times resistance). When the material enters a superconductive state, its resistance drops to zero and no voltage appears across the second set of points. By using the four-point method, instead of just two points, resistance in the adhesive and wires can be ignored; as the second set of points do not themselves conduct any current and can, therefore, only reflect what voltage exists across the body of the material.

Hall Effect: When a magnetic field is applied perpendicularly to a thin metal film or semiconductor film that is conducting an electric current, a small voltage will appear perpendicular to the axis of both the film and the magnetic field. This voltage is proportional to the strength of the applied field. However, the output is typically not linear. The Hall resistance (the ratio of the Hall voltage to the current) changes in steps, pursuant to the laws of quantum mechanics. This is known as the Integral Quantum Hall Effect or just Quantum Hall Effect. Discovered in 1879, the Hall effect was named for its discoverer Edwin H. Hall, a graduate student at Johns Hopkins University.

Hc: The scientific notation representing the "critical field" or maximum magnetic field that a superconductor can endure before it is "quenched" and returns to a non-superconducting state. Usually a higher Tc also brings a higher Hc.

Heavy Fermions: Compounds containing the elements cerium, ytterbium or uranium; whose (inner shell) conduction electrons often have effective masses (called quasiparticle masses) several hundred times as great as that of a "free" (normal) electron mass. This gives them what's

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known as a low "Fermi energy" and makes them unlikely - and unusual - superconductors. Research suggests cooper-pairing in heavy fermion systems arises from the magnetic interactions of the electron spins.

Hole: A positively-charged vacancy within a crystal lattice resulting from the shortage of an electron in that region. Holes are typically induced by doping a material with an impurity. However, they can also be synthesized electronically with devices like the field-effect transistor (FET). Modern electronic devices rely heavily on holes (as p-type semiconductors) to function. There is evidence that the holes of hypocharged oxygen in charge-reservoirs are, in fact, what makes possible high-temperature superconductivity in the layered cuprates.

HTS: An acronym for "High-Temperature Superconductor" (or Superconductivity). There is no widely-accepted temperature that separates HTS from LTS (Low-Temperature Superconductors). However, all the superconductors known before the 1986 discovery of the superconducting oxocuprates would be classified LTS. The barium-lanthanum-cuprate fabricated by Müller and Bednorz, with a Tc of 30K, is generally considered to be the first HTS material. Certainly any compound that will superconduct above the boiling point of liquid nitrogen (77K) would be HTS.

Hysteresis (loop): Hysteresis, as it applies to a superconductor, relates to the dynamic response of a superconductor to a strong magnetic field impinged upon it. As the strength of a nearby magnetic field (H) increases, the critical transition temperature (Tc) of a superconductor will decrease. And, at some point superconductivity will completely disappear, as it becomes "quenched". However, as the magnetic field is gradually withdrawn, the superconductor may NOT immediately return to a superconductive state. Herein lies the hysteresis. The graph of H-vs-Tc is different retreating than it is advancing (creating a "loop" shape). This fact must be weighed carefully in high-current applications where the superconductor Hc may, even briefly, be exceeded; as significant power losses can result.

Infinite layer: Infinite layer compounds have no clear separation between molecules. Rather than electrostatic bonding between discrete molecules to form a bulk crystalline aggregate, all the atoms are bound together by covalent or co-ionic bonding to form the equivalent of one huge molecule. (Ba,Sr)CuO2 and Na2Ba6Si46 are examples of "infinite layer" or "infinite network" superconductor compounds.

Isotope Effect: The influence atomic mass contributes to the critical transition temperature of a superconductor. For example, 203.4Hg has a Tc of 4.126K. While 198Hg has a Tc of 4.177K. Since both forms of mercury have the same lattice structure, this difference in Tc can be attributed solely to the difference in mass. To learn more, click here.

Jc: The scientific notation representing the "critical current density" or maximum current that a superconductor can carry. Also note that, as the current flowing through a superconductor increases, the Tc will usually decrease.

Josephson Effect (also DC Josephson Effect): A phenomenon named for Cambridge graduate student Brian Josephson, who predicted that electrons would "tunnel" through a narrow (<10 angstroms) non-superconducting region, even in the absence of an external voltage. In a normal conductor, electrical current only flows when there's a voltage differential and contiguous electrical connection. It has been theorized that the Josephson Effect arises from the incoherent phase relationships between superconducting electrons in the two (separated) superconductors. The AC Josephson Effect is where the current flow oscillates as an external magnetic field impinged upon it increases beyond a critical value. [at a frequency of 2eV/h, where e is the electron charge, V

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is the voltage that appears, and h is Planck's constant] Sidebar: This oscillation frequency has, in fact, resulted in an upward revision of Planck's constant from 6.62559e-34 to 6.626196e-34.

Josephson Junction: A thin layer of insulating material sandwiched between 2 superconducting layers. Electrons "tunnel" through this non-superconducting region in what is known as the "Josephson effect" (see above). Sidebar: The standard volt is now defined as the voltage required to produce a frequency of 483,597.9 GHz in a Josephson Junction.

Kelvin: A scale of temperature measurement that starts at "absolute zero", the coldest theoretical temperature attainable. (Named for Lord William Thomson Kelvin.)

Meissner Effect: Exhibiting diamagnetic properties to the total exclusion of all magnetic fields. (Named for Walther Meissner.) This is a classic hallmark of superconductivity and can actually be used to levitate a strong rare-earth magnet. To see a movie of a magnet being levitated by a superconductor, click here.

Mott Transition: The Mott transition is the shift from an insulating to a metallic state in a material. The high- temperature copper oxides are composed of CuO2 planes that are separated from each other by ionic "blocking layers". Although it has one conduction electron (or hole) per Cu site, each CuO2 plane is originally insulating because of the large electron correlation. That behavior is typical of the Mott insulator state, in which all the conduction electrons are tied to the atomic sites. The superconducting state emerges when holes from the blocking layers dope the CuO2 layers in a way that alters the number of conduction electrons and triggers the Mott transition. Researchers believe that the strong antiferromagnetic correlation, which originates in the Mott-insulating CuO2 sheets and persists into the metallic state, could be a possible mechanism of high-temperature superconductivity. (courtesy Science Week)

Organics: Organic superconductors are a sub-class of organic conductors that include molecular salts, polymers and pure carbon systems (including carbon nanotubes and C60 compounds). They may also be referred to as "molecular" superconductors. They are typically large, carbon-based molecules of 20 or more atoms, consisting of a planar organic molecule and a non-organic anion. For a non-technical write-up on organics, click here. Or, for a more technical paper on this subject, click here.

Penetration Depth (also London Penetration Depth): This term relates to how deeply a magnetic field will penetrate the surface of a superconductor. An external magnetic field impinged upon a Type 2 superconductor will decay exponentially into the surface based on the paired electron density within the superconductor (only a small fraction of the electrons are in a superconductive state). The "London" name comes from brothers F. London and H. London, who in 1935 created a theoretical model of superconductivity. For a more technical explanation and the actual formula to calculate penetration depth, click here.

Perovskites: A large family of crystalline ceramics that derive their name from a mineral known as a perovskite. They are the most abundant minerals on earth and have a metal-to-oxygen ratio of approximately 2-to-3. Copper-oxide superconductors are layered perovskites. The perovskite name comes from Russian mineralogist Count Lev Aleksevich von Perovski.

Phase-Slip (also Quantum Phase-Slip): A point where a material in a superconductive state spontaneously changes from one state to another, generating a topological "defect". This defect causes paired electrons to become "out of step" with each other, producing a voltage and, ergo, non-zero electrical resistance. This phenomenon has been observed in ultra-thin wires less than a few tens-of-nanometers in diameter. Though bulk superconductivity may persist (T<Tc), one

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consequence of phase slip is a lower current-carrying state. A similar phenomenon occurs in Josephson Junctions.

Planar Weight Disparity (PWD): A term referring to the method by which Tc can often be increased by adjusting the relative weights of alternating layers in copper-oxide superconductors. The greatest improvements usually occur when making the insulating layers heavy/light OR the Cu-O2 planes heavy/light - but not both. Click HERE to read more about this discovery.

Proximity Effect: The phenomenon where a thin film of non-superconductive material in close proximity with a superconductor takes on superconductive properties. The Josephson junction is a device that takes advantage of this phenomenon. The Inverse Proximity Effect is where just the opposite occurs. A non-superconductive metal can enhance the Tc of an adjacent superconductor. This inverse effect has been observed with silver and lead.

P-Wave: A rare form of electron pairing in which two electrons travel together in spherical orbits; with both having the same direction of rotation. (See "D-Wave" explanation above.)

Quasiparticle: A bare particle that is "dressed" or "clothed" by a cloud of other surrounding particles. Quasiparticles behave similarly to bare (normal) particles, but usually have a larger effective mass due to this cloud moderating interactions with other particles.

Quench: The phenomenon where superconductivity in a material is suppressed; usually by exceeding the maximum current the material can conduct (Jc) or the maximum magnetic field it can withstand (Hc).

Re-entrant (behavior): A condition where a material retreats from its superconductive state and then re-enters it. This can be caused by a strong external magnetic field that dynamically exceeds the Hc of the material and/or is mis-aligned (in the case of some organic superconductors), a discordant temperature below Tc (in the case of some borocarbides), or by Jc hysteresis (momentarily exceeding the critical current density, causing the Tc to shift downward).

Resistance: The opposition of a material to the flow of electrical current through it. Energy lost due to resistance is a result of vibrations at the molecular level and manifests itself as heat in proportion to the square of the current flow. In a superconductor all resistance disappears below a certain temperature. However, this applies only to direct current (DC) electricity. Other types of losses result when transporting alternating current (AC). Examples of this include hysteresis, reactive-coupling and radiational losses. In the new high-temperature ceramic superconductors, the power loss in applications like transmission lines is inversely proportional to the critical current density for low magnetic field applications. This limitation can be compensated for to some degree by increasing the ratio of voltage to current. In Type 2 superconductors carrying high-frequency alternating current, "skin effect" losses also result as the energy tends to migrate to the surface where the conductive medium is incontiguous, producing a pseudo-resistance. In some materials the amount of resistance may also depend on the direction of current flow (anisotropic resistivity) and/or presence of an external magnetic field (hall effect).

Room-temperature Superconductor: There are NO confirmed room-temperature superconductors (as was once reported for lithium-beryllium-hydride and for lead-silver-carbonate). However, it has been theorized that a metallic form of hydrogen might be a room-temperature superconductor. In 1996 physicists at Lawrence Livermore Laboratory were able to briefly create metallic hydrogen. But, its existence was fleeting and no measurement of the Meissner effect was possible. Zero resistance has been observed at room temperatures in ballistic quantum wire.

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However, having one-dimensional geometry, this wire does not exhibit the Meissner effect, except when configured as a closed loop.

Sinter: The process of heating a material to just below its melting point. An extended period of sintering is the method by which the constituent components of a ceramic superconductor are combined in a solid-state reaction. Since ceramic superconductors are inherently brittle, sintering helps promote intergranular bonding and hardness.

SQUID: A superconducting loop interrupted in 2 places by Josephson junctions. When sufficient electrical current is conducted across the squid body, a voltage is generated proportional to the strength of any nearby magnetic field. The SQUID, an acronym for Superconducting QUantum Interference Device, is the most sensitive detector known to science

Stripes: Stripes are microscopic rivers of charge that flow across the surface of a Type 2 superconductor. It is theorized that stripes encourage "holes" to pair up and, as such, may play a role in facilitating charge transfer. Recently, at the Stripes 2000 conference in Rome, Italy, it was shown that there exists a critical value of micro-strain that must be exerted upon the CuO2 planes for stripes to form. Click here to learn more.

Superconductor: An element, inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature. However, this applies only to direct current (DC) electricity and to finite amounts of current. All known superconductors are solids. None are gases or liquids. And all require extreme cold to enter a superconductive state. Once set in motion, current will flow forever in a closed loop of superconducting material - making it the closest thing to perpetual motion in nature. Scientists refer to superconductivity as a "macroscopic quantum phenomenon". In addition to being classified Type 1 and Type 2, superconductors can be categorized further by their dimensionality. Most are 3-D. But some compounds, like surface-doped NaWO3 and some organic superconductors are 2-D. Li2CuO2 and single-walled carbon nano-tubes have shown rare 1-D superconductivity. In addition to repelling magnetic fields, enhanced thermal conductivity, higher optical reflectivity and reduced surface friction are also properties of superconductors. The term "superconductor" is also used in some instances to refer to materials that have near infinite thermal conductivity - such as carbon nanotubes. However, on this website it is used in the context of electrical conductivity only.

Susceptibility: A measure of the relative amount of induced magnetism in a material. Magnetic susceptibility is often used in lieu of resistance measurements to determine the transition temperature of a superconductor. Although, on occasion, the two techniques produce very different Tc's. In a typical superconductor, the (arbitrary) value of susceptibility will change from zero to a negative number as the temperature drops through Tc. However, in some materials it changes from positive to negative, as paramagnetism yields to diamagnetism.

S-Wave: A form of electron pairing in which the electrons travel together in spherical orbits, but in opposite directions. (See "D-Wave" explanation above.)

Tc: The scientific notation representing the critical transition temperature below which a material begins to superconduct. The sudden loss of resistance in a superconductive medium may occur across a range as small as 20 millionths of a degree or, in the case of some stoichiometrically imperfect compounds, tens of degrees. Click here to see a graphic example. ("Tc" is not to be confused with the atomic symbol for Technetium.)

Thin Film (Deposition): A method of fabricating ceramic superconductors to more precisely control the growth of the crystalline structure to eliminate grain boundaries and achieve a

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desired Tc. This can involve Pulsed-Laser Deposition (PLD) or Pulsed-Electron Deposition (PED) of the material. A variation of this technique can be used to increase the Tc of a superconductor by growing it on a supporting material with a smaller interatomic spacing. The supporting material acts as a molecular "girdle" to compress the atomic lattice of the superconductor, thereby raising its transition temperature. Superconductive tape is made using thin film deposition technology.

Translational Symmetry: As it applies to superconductivity, translational symmetry is where the process of charge transfer is repeated exactly as the charge carriers (paired electrons) traverse the solid. In a normal conductor, latent heat continuously vibrates the atomic lattice, deflecting mobile free electrons and preventing "perfect" translational symmetry. In a superconductor this scattering tendency is overcome.

Tungsten-bronze: A nebulous term used to describe alkali metal tungstenates, vanadates, molybdates, titanates and niobates. The term was originally coined to describe NaxWO3 compounds; the crystals of which look much like the copper-tin alloy known as bronze. There have been reports of superconductivity as high as 91K for a surface-doped sodium tungsten-bronze. This material was the first high-temperature superconductor discovered that does not contain any copper.

Ultraconductor: Materials known as ultraconductors™ display room-temperature resistance many orders of magnitude lower than the best metallic conductors. Examples of these materials include oxidized atactic polypropylene (OAPP) and other polymers. Since ultraconductor™ is a colloquial term, these materials might better be described as "hyperconductors". The Meissner effect cannot be confirmed in them, but strong (giant) diamagnetism is in evidence. Some of them may actually find acceptance in high-current applications ahead of superconductors as a result of their low losses at ambient temperatures and pressures

Undressing: The process by which a quasiparticle becomes more like a bare (normal) particle. It is theorized this may be a driving force behind superconductivity, as undressed electrons are significantly lighter and can, thus, conduct current more readily. To learn more, click here.

Unit Cell: A unit cell is the smallest assemblage of atoms, ions, or molecules in a solid, beyond which the structure repeats to form the 3-dimensional crystal lattice.

Vortices (plural of vortex): Swirling tubes of electrical current induced by an external magnetic field into the surface of a superconducting material that represent a topological singularity in the wavefunction. These are particularly evident in Type 2 superconductors during "mixed-state" behavior when the surface is just partially superconducting. Superconductivity is completely suppressed within these volcano-shaped structures. Recent research suggests that flux vortices may NOT possess quantum values (equal to multiples of Planck's constant divided by 2 times electron charge). But may instead have but a tiny fraction of the basic unit of magnetism. The movement of vortices can produce a pseudo-resistance and, as such, is undesirable. While superconductivity is a "macroscopic" phenomenon, vortices are a "mesoscopic" phenomenon. (See the graphic at the top of this page.)

YBCO: An acronym for a well-known ceramic superconductor composed of Yttrium, Barium, Copper and Oxygen. This was the first truly "high temperature" ceramic superconductor discovered; having a transition temperature well above the boiling point of liquid nitrogen - a commonly available coolant. Its actual molecular formula is YBa2Cu3O7, making it a "1-2-3" superconductor. YBCO compounds exhibit d-wave electron pairing. The patent for YBCO is held by Lucent Technologies. (You can view a list of the best-performing 1-2-3 compounds on the "Type 2" page.)

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With the discovery of the ceramic superconductors, came a need for a classification system to describe structure types. The cuprate superconductors all have blocks of conducting CuO2 planes, alternating with insulating, spacing and separating layers. This makes possible a systematic Naming Scheme that allows for identification and comparison. The scheme chosen uses four numbers. The first denotes the number of insulating layers between adjacent conducting blocks. The second represents the number of spacing layers between identical CuO2 blocks. The third gives the number of layers that separate adjacent CuO2 planes within the conducting block. And, the fourth is the number of CuO2planes

within a conducting block.

Using the TlBa2Ca2Cu3O9 molecule depicted at left as an example, there is 1 insulating TlO layer, 2 spacing BaO layers, 2 separating Ca layers, and 3 conducting CuO2 planes - making it a "1223" type.

There are also occasionally letters of the alphabet in the 5th and 6th positions. Suffixes like "T*", "C", and "F" describe to researchers structure detail in the oxygen positions.

To further complicate matters, several of the more popular structures have names that do NOT follow the 4-number scheme. For example, the compound Y1Ba2Cu3O7 is often referred to by the 3-number name "123". This delineates the number of metal atoms (the stoichiometry) - without regard to atom location - within the structure. Using the 4-number scheme, it would be classified a "1212".

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A TYPICAL SUPERCONDUCTORS AND THE FUTURE

As if ceramic superconductors were not strange enough, even more mysterious superconducting systems have been discovered. One is based on compounds centered around the "Fullerene". The fullerene name comes from the late designer-author Buckminster Fuller. Fuller was the inventor of the geodesic dome, a structure with a soccer ball shape. The fullerene - also called a buckminsterfullerene or "buckyball" - exists on a molecular level when 60 carbon atoms join in a closed sphere. When doped with one or more alkali metals the fullerene becomes a "fulleride" and has produced Tc's ranging from 8 K for Na2Rb0.5Cs0.5C60 up to 40 K for Cs3C60. In 1993

researchers at the State University of New York at Buffalo reported Tc's between 60 K and 70 K for C-60 doped with the interhalogen compound ICl.

Fullerenes, like ceramic superconductors, are a fairly recent discovery. In 1985, professors Robert F. Curl, Jr. and Richard E. Smalley of Rice University in Houston and Professor Sir Harold W. Kroto of the University of Sussex in Brighton, England, accidentally stumbled upon them. The discovery of superconducting alkali metal fullerides came in 1991 when Robert Haddon and Bell Labs announced that K3C60 had been found to superconduct at 18 K.

Larger, non-spherical pure carbon fullerenes that will superconduct have only recently been discovered. In April of 2001, Chinese researchers at Hong Kong University found 1-dimensional superconductivity in single-walled carbon nanotubes at around 15 Kelvin. And in February 2006, Physicists in Japan showed non-aligned, multi-walled carbon nanotubes were superconductive at temperatures as high as 12 K. Silicon-based fullerides like Na2Ba6Si46 will also superconduct. However, they are structured as infinite networks, rather than discrete molecules. Fullerenes are technically part of a larger family of organic conductors which are described below.

"Organic" superconductors are part of the organic conductor family which includes: molecular salts, polymers and pure carbon systems (including carbon nanotubes and C60 compounds). The molecular salts within this family are large organic molecules that exhibit superconductive properties at very low temperatures. For this reason they are often referred to as "molecular" superconductors. Their existence was theorized in 1964 by Bill Little of Stanford University. But the first organic superconductor (TMTSF)2PF6 was not actually synthesized until 1980 by Danish researcher Klaus Bechgaard of the University of

Copenhagen and French team members D. Jerome, A. Mazaud, and M. Ribault. About 50 organic superconductors have since been found with Tc's extending from 0.4 K to near 12 K (at ambient pressure). Since these Tc's are in the range of Type 1 superconductors, engineers have yet to find a practical application for them. However, their rather unusual properties have made them the focus of intense research. These properties include giant magnetoresistance, rapid oscillations, quantum hall effect, and more (similar to the behavior of InAs and InSb). In early 1997, it was, in fact (TMTSF)2PF6 that a research team at SUNY discovered could resist "quenching" up to a magnetic field strength of 6 tesla. Ordinarily, magnetic fields a fraction as strong will completely kill superconductivity in a material.

Organic superconductors are composed of an electron donor (the planar organic molecule) and an electron acceptor (a non-organic anion). Below are a few more examples of organic superconductors.

(TMTSF)2PF6 The first organic superconductor discovered.

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(TMTSF)2ClO4 [tetramethyltetraselenafulvalene + acceptor] (BETS)2GaCl4 [bis(ethylenedithio)tetraselenafulvalene + acceptor] (BEDO-TTF)2ReO4H2O [bis(ethylenedioxy)tetrathiafulvalene + acceptor]

Discovered in 1993 by Bob Cava (currently at Princeton University) and Bell Labs, "Borocarbides" are one of the least-understood superconductor systems of all. It has always been assumed that superconductors cannot be formed from ferromagnetic transition metals - like iron, cobalt or nickel. It's the equivalent of trying to mix oil and water. However, in some borocarbides there is a "soap" that acts to bring these adversaries together. The crystallographic sites for the magnetic ions are thought to be isolated from the conduction path. This allows the cooper pairs to detour around the magnetic ions. Further, when combined with an

element that has unusual magnetic properties - like holmium - "reentrant" behavior can also be in evidence in some borocarbides. Below Tc, where it should remain superconductive, there is a discordant temperature at which the material retreats to a "normal", non-superconductive state (see above graphic).

To date only one superconductor has been found that has zero resistance at a single temperature - the opposite of reentrant superconductivity. Click HERE to read more about this "resonant" superconductor.

Not only the borocarbides recede from a superconductive state at extreme low temperatures. In the compounds HoMo6S8 (Chevrel) and ErRh4B4 superconductivity suddenly disappears at around 1K. Click here to

read more about this phenomenon. As can be seen from some of the below examples, the first metal site in the molecule is always occupied by a rare earth atom.

YPd2B2C 23.0 K LuNi2B2C 16.6 K YNi2B2C 15.5 K TmNi2B2C 11.0 K (resistance increases below Tc) ErNi2B2C 10.5 K (ferromagnetic) HoNi2B2C 07.5 K (see above graphic) NOTE: Other elements that exhibit ferromagnetism have been - by one means or another -

integrated into superconducting compounds. See the below section on Heavy Fermions.

The "Heavy Fermions" sound like a family of overweight circus performers. But, they are yet another example of atypical superconductors. Heavy fermions are compounds containing rare-earth elements such as Ce or Yb, or actinide elements such as U. Their (inner shell) conduction electrons often have effective masses (known as

Flux lattices in UPt3 Courtesy Bell Labs

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quasiparticle masses) several hundred times as great as that of "normal" electrons, resulting in what's known as low "Fermi energy" (Ef). This makes them reluctant superconductors. Yet, at cryogenic temperatures, many of these materials are magnetically ordered, others show strong paramagnetic behavior, and some display superconductivity through a mechanism that quickly runs afoul of BCS theory. Research suggests cooper-pairing in the heavy fermion systems arises from the magnetic interactions of the electron spins (D-wave, P-wave, S-wave), rather than by lattice vibrations.

The first observation of superconductivity in a heavy fermion system was made by E. Bucher, et al, in 1973 in the compound UBe13; but, at the time was attributed to precipitated uranium filaments. Superconductivity was not actually recognized, per se, in a heavy fermion compound until 1979 when Dr. Frank Steglich of the Max Planck Institute for Chemical Physics in Solids (Dresden, Germany) realized it was a bulk property in CeCu2Si2. Heavy fermion superconductivity has since been observed in over 20 Ce compounds.

In April 2003 a heavy-fermion compound unambiguously exhibited the so-called "FFLO" state, where magnetism and superconductivity have a beneficial coexistence. The compound CeCoIn5 confirmed a theoretical model first put forth in 1964 by Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO). Below are some heavy fermion compounds that will superconduct, along with their Tc's. As can be seen, their transition temperatures are in the range of Type 1 superconductors, which severely limits their use in practical applications.

CeCoIn5 2.30 K (first confirmed FFLO compound) UPd2Al3 2.00 K Pd2SnYb 1.79 K URu2Si2 1.20 K UNi2Al3 1.00 K Al3Yb 0.94 K UBe13 0.87 K CeCo2 0.84 K CePt3Si 0.75 K (first heavy fermion superconductor without a center of symmetry) UPt3 0.48 K (see above graphic) CeCu2Si2 0.1-0.7 K

Note: UGe2 and URhGe2 exhibit simultaneous ferromagnetism and superconductivity. Read more about this phenomenon in the below section on Ruthenates.

In the mid 1990's, it was discovered that copper-oxygen planes are not the only superconducting facilitators within the layered perovskites. In 1994 physicists at IBM Zurich and at Hiroshima University collaborated to study the atomic planes of ruthenium-oxygen due to their similarity to copper-oxygen planes. Yoshiteru Maeno and colleagues found that the compound Sr2RuO4 exhibited superconductivity at 1.5 K. While this is an

extremely cold Tc for a superconducting perovskite, it revealed a new area of potential among what are known as "Ruthenates". Shortly after that SrRuO and SrYRuO6 were also found to superconduct at similarly low temperatures.

When the crystalline structure of some of these materials is broken apart, its surface becomes increasingly ferromagnetic at low temperatures. This phenomenon flies in the face of condensed matter theory. So much so, that researchers have characterized them as an analog to superfluid Helium-3. And, when copper is added to the mix, even stranger things happen. In June

Magnetic topography of Sr2RuO4

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1999 New Zealand researcher Dr. Jefferey Tallon and his German colleague Dr. Christian Bernard discovered a ruthenium-cuprate** whose bulk is both a superconductor and a magnet. Although it was not the first compound discovered that exhibits coexisting ferromagnetism and superconductivity, it's remarkably high Tc of 58 K makes it truly distinct in the world of superconductors. Unlike "normal" superconductors, this compound becomes diamagnetic at about one-half Tc.

[**RuSr2(Gd,Eu,Sm)Cu2O8 or any parenthetical element partially substituted by Y.]

There are many phase transitions that matter goes through on its way to another state (e.g. ice changing to water requires a sudden increase in heat energy). Among the superconductors this is also the case at Tc, Hc and Jc. However, a superconductor has been discovered that exhibits no measurable change in its specific heat (the amount of energy required to increase its temperature by one degree) while going through up to 3 different "critical" magnetic fields.

Ba0.6K0.4BiO3 seems to be the first material discovered that enters a "fourth order" phase transition

(according to Ehrenfest's phase transition classification scheme) - something that has never before been observed in nature. Roy Goodrich of Louisiana State University and Donovan Hall of the National High Magnetic Field Laboratory announced this discovery in May, 1999. Not only is this an amazing anomaly in the field of superconductivity. It suggests that even higher order phases may exist in nature.

While no one can predict what future discoveries will be made in the field of superconductivity, some recent developments in the tungsten-bronze system suggest a new vista may be emerging. In July of 1999 researchers Y. Tsabba and S. Reich of the Weizmann Institute in Israel reported possible superconductivity near 91 °K in the sodium-doped tungsten-bronze Na0.05WO3. This would be the first known HTS that is not a cuprate. Most tungsten-bronze compounds that are known to superconduct have Tc's below 4K - making this a truly tantalizing find.

Other categories of materials that theory suggests may produce superconductors are the higher silver fluorides and complex fluorides - known as fluoroargentates. Fluoroargentates bear a strong similarity to oxocuprates, compounds that currently have the highest transition temperatures of all known superconductors. In October 2003 researchers Wojciech Grochala, Adrian Porch and Peter P. Edwards reported sudden drops in magnetic susceptibility within a large number of samples of Be-Ag-F. They attribute this to possible spherical regions of superconductivity – with a Tc up to 64 °K - couched inside a ferromagnetic host.

With few exceptions (e.g. polysulphur-nitrides), most polymers resist being coaxed into a superconductive state. However, some organic polymers exhibit electrical resistance many orders of magnitude lower than the best metallic conductors. And, they do this at room temperature! These ultraconductors™, materials such as oxidized atactic polypropylene (OAPP), do not have zero

resistance. But, their enhanced conductivity at ambient temperatures and pressures may actually allow them to compete with superconductors in certain fields. Polypropylene, for example, is normally an insulator. In 1985, however, researchers at the Russian Academy of Sciences discovered that as an oxidized thin-film, polypropylene can have a conductivity 105 to 106 higher than the best refined metals. The Meissner effect - the classic criterion for superconductivity - cannot be observed, as the critical transition temperature appears to be above the point at which the polymer breaks down (>700K). However, strong (giant) diamagnetism has been confirmed

Ba0.6K0.4BiO3 Courtesy University of Texas

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TYPE 1 SUPERCONDUCTORS: AND A PERIODIC CHART COMPARISON

The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in "Cooper pairs" in order to help each other overcome molecular obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon-

mediated coupling because of the sound packets generated by the flexing of the crystal lattice.

Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state (see above graph) and "perfect" diamagnetism - the ability to repel a magnetic field completely. Below is a list of known Type 1 superconductors along with the critical transition temperature (known as Tc) below which each superconducts. The 3rd column gives the lattice structure of the solid that produced the noted Tc. Surprisingly, copper, silver and

gold, three of the best metallic conductors, do not rank among the superconductive elements.

Lead (Pb) 7.196 K FCC Lanthanum (La) 4.88 K HEX Tantalum (Ta) 4.47 K BCC Mercury (Hg) 4.15 K RHL Tin (Sn) 3.72 K TET Indium (In) 3.41 K TET Palladium (Pd)* 3.3 K (see note 1) Chromium (Cr)* 3 K (see note 1) Thallium (Tl) 2.38 K HEX Rhenium (Re) 1.697 K HEX Protactinium (Pa) 1.40 K TET Thorium (Th) 1.38 K FCC Aluminum (Al) 1.175 K FCC Gallium (Ga) 1.083 K ORC Molybdenum (Mo) 0.915 K BCC Zinc (Zn) 0.85 K HEX Osmium (Os) 0.66 K HEX Zirconium (Zr) 0.61 K HEX Americium (Am) 0.60 K HEX Cadmium (Cd) 0.517 K HEX Ruthenium (Ru) 0.49 K HEX Titanium (Ti) 0.40 K HEX Uranium (U) 0.20 K ORC

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Hafnium (Hf) 0.128 K HEX Iridium (Ir) 0.1125 K FCC Beryllium (Be) 0.023 K (SRM 768) HEX Tungsten (W) 0.0154 K BCC Platinum (Pt)* 0.0019 K (see note 1) Lithium (Li) 0.0004 K BCC Rhodium (Rh) 0.000325 K FCC

*Note 1: Tc's given are for bulk (alpha form), except for Palladium, which has been irradiated with.

He+ ions, Chromium as a thin film, and Platinum as a compacted powder.

Many additional elements can be coaxed into a superconductive state with the application of high pressure. For example, phosphorus appears to be the Type 1 element with the highest Tc. But, it requires compression pressures of 2.5 Mbar to reach a Tc of 14-22 K. The above list is for elements at normal (ambient) atmospheric pressure. See the periodic table below for all known elemental superconductors (including Niobium, Technetium and Vanadium which are technically Type 2).

**Note 2: Normally bulk carbon (amorphous, diamond, graphite, white) will not superconduct at

any temperature. However, a Tc of 15K has been reported for elemental carbon when the atoms are

configured as highly-aligned, single-walled nanotubes. And non-aligned, multi-walled nanotubes

have shown superconductivity near 12K. Since the penetration depth is much larger than the

coherence length, nanotubes would be characterized as "Type 2" superconductors.

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TYPE 2 SUPERCONDUCTORS:

Except for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys. The recently-discovered superconducting "perovskites" (metal-oxide ceramics that normally have a ratio of 2 metal atoms to every 3 oxygen atoms) belong to this Type 2 group. They achieve higher Tc's than Type 1 superconductors by a mechanism that is still not completely understood. Conventional wisdom holds that it relates to the planar layering within the crystalline structure (see above graphic). Although, other recent research suggests the holes of hypocharged oxygen in the charge reservoirs are responsible. (Holes are positively-charged vacancies

within the lattice.) The superconducting cuprates (copper-oxides) have achieved astonishingly high Tc's when you consider that by 1985 known Tc's had only reached 23 Kelvin. To date, the highest Tc attained at ambient pressure for a material that will form stoichiometrically (by formula) has been 138 K. And the highest Tc overall is 254 °°°°K for a material which does not form stoichiometrically (see below list). One theory predicts an upper limit for the layered cuprates (Vladimir Kresin, Phys. Reports 288, 347 - 1997). Others assert there is no limit. Either way, it is almost certain that other, more-synergistic compounds still await discovery among the high-temperature superconductors.

The first superconducting Type 2 compound, an alloy of lead and bismuth, was fabricated in 1930 by W. de Haas and J. Voogd. But, was not recognized as such until later, after the Meissner effect had been discovered. This new category of superconductors was identified by L.V. Shubnikov at the Kharkov Institute of Science and Technology in the Ukraine in 1936(1) when he found two distinct critical magnetic fields (known as Hc1 and Hc2) in PbTl2. The first of the oxide superconductors was created in 1973 by DuPont researcher Art Sleight when Ba(Pb,Bi)O3 was found to have a Tc of 13K. The superconducting oxocuprates followed in 1986.

Type 2 superconductors - also known as the "hard" superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of "mixed state" behavior. Since a Type 2 will allow some penetration by an external magnetic field into its surface, this creates some rather novel mesoscopic phenomena like superconducting "stripes" and "flux-lattice vortices". While there are far too many to list in totality, some of the more interesting Type 2 superconductors are listed below by similarity and with descending Tc's. Where available, the lattice structure of the system is also noted.

(Tl4Ba)Ba2Ca2Cu7O13+ (As a 9223 structure)

~254 K

(Tl4Ba)Ba4Ca2Cu10Oy (As a 9212/2212C intergrowth.) ~242 K

Tl5Ba4Ca2Cu10Oy (As a 9212/2212C intergrowth.)

~233 K

(Sn5In)Ba4Ca2Cu11Oy (As a B212/2212C intergrowth.)

~218 K

(Sn5In)Ba4Ca2Cu10Oy (As a B212/1212C intergrowth.)

~212 K

A Type 2 Layered Cuprate

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Sn6Ba4Ca2Cu10Oy (As a B212/1212C intergrowth.)

~200 K

(Sn1.0Pb0.5In0.5)Ba4Tm6Cu8O22+ (As a 1256/1212 intergrowth.)

~195 K

(Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20+ (As a 1245/1212 intergrowth.)

~185 K

(Sn1.0Pb0.5In0.5)Ba4Tm4Cu6O18+ (As a 1234/1212 intergrowth) ~163 K

Sn3Ba4Ca2Cu7Oy (As a 5212/1212C intergrowth.)

~160 K

(Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 138 K*

HgBa2Ca2Cu3O8 133-135 K

HgBa2Ca3Cu4O10+ 125-126 K

HgBa2(Ca1-xSrx)Cu2O6+ 123-125 K

HgBa2CuO4+ 94-98 K

Lattice: TET * Note: As a result of a topological "defect", Hg will also go into the Cu atomic sites. Thus, the

volume fraction of the intended structure type is considerably less than 100%.

Tl2Ba2Ca2Cu3O10 127-128 K

(Tl1.6Hg0.4)Ba2Ca2Cu3O10+ 126 K

TlBa2Ca2Cu3O9+ 123 K

(TlSn)Ba4TmCaCu4Ox ~121 K

(Tl0.5Pb0.5)Sr2Ca2Cu3O9 118-120 K

Tl2Ba2CaCu2O6 118 K

TlBa2Ca3Cu4O11 112 K

TlBa2CaCu2O7+ 103 K

Tl2Ba2CuO6 95 K

TlSnBa4Y2Cu4Ox 86 K

Lattice: TET

Sn4Ba4(Tm2Ca)Cu7Ox ~127 K (TmTm-Ca structure only)

Sn2Ba2(Tm0.5Ca0.5)Cu3O8+ ~115 K (Superconductors.ORG - 2005)

SnInBa4Tm3Cu5Ox ~113 K (Superconductors.ORG - 2005)

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Bi1.6Pb0.6Sr2Ca2Sb0.1Cu3Oy 115 K (thick film on MgO substrate)

Bi2Sr2Ca2Cu3O10*** 110 K

Bi2Sr2CaCu2O9*** 110 K

Bi2Sr2(Ca0.8Y0.2)Cu2O8 95-96K

Bi2Sr2CaCu2O8 91-92K

Lattice: ORTH

*** Though not always listed as a component, a small amount of Lead (x=.2-.26) is often used with

Bismuth compounds to help facilitate a higher-Tc crystalline phase.

(Ca1-xSrx)CuO2 110 K

YSrCa2Cu4O8+ 101 K (Superconductors.ORG - 2007)

(Ba,Sr)CuO2 90 K

BaSr2CaCu4O8+ 90 K (Superconductors.ORG - 2007)

(La,Sr)CuO2 42 K

*** The above 5 compounds are all "infinite layer".

Pb3Sr4Ca3Cu6Ox 106 K (Superconductors.ORG - 2007)

Pb3Sr4Ca2Cu5O15+ 101 K (Superconductors.ORG - 2005)

(Pb1.5Sn1.5)Sr4Ca2Cu5O15+ ~95 K (Superconductors.ORG - 2006)

Pb2Sr2(Ca, Y)Cu3O8 70 K (Cava, et al - 1989)

AuBa2Ca3Cu4O11 99 K (Kopnin, et al - 2001)

AuBa2(Y, Ca)Cu2O7 82 K

AuBa2Ca2Cu3O9 30 K

Lattice: ORTH

Sn3Ba4Tm3Cu6Ox 109 K (Superconductors.ORG - 2007)

Sn3Ba8Ca4Cu11Ox 109 K (One-of-a-Kind Resonant - 2006)

SnBa4Y2Cu5Ox 107 K (Superconductors.ORG - 2007)

Sn4Ba4Tm2YCu7Ox ~104 K (First Hi-Tc Reentrant - 2007)

Sn4Ba4TmCaCu4Ox ~100 K (Superconductors.ORG - 2007)

Sn4Ba4Tm3Cu7Ox ~98 K (Superconductors.ORG - 2006)

Sn2Ba2(Y0.5Tm0.5)Cu3O8+ ~96 K (Superconductors.ORG - 2007)

Sn3Ba4Y2Cu5Ox ~91 K (Superconductors.ORG - 2006)

SnInBa4Tm4Cu6Ox 87 K (Superconductors.ORG - 2005)

Sn2Ba2(Sr0.5Y0.5)Cu3O8 86 K(Aleksandrov, et al - 1989)

Sn4Ba4Y3Cu7Ox ~80 K (Superconductors.ORG - 2005)

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YBa3Cu4Ox (9223C structure) 177 K (Superconductors.ORG - 2009)

(Y0.5Lu0.5)Ba2Cu3O7 107 K (Superconductors.ORG - 2005)

(Y0.5Tm0.5)Ba2Cu3O7 105 K (Superconductors.ORG - 2005)

Y3Ba5Cu8Ox 105 K (Superconductors.ORG - 2008)

Y3CaBa4Cu8O18+ 99 K (Superconductors.ORG - 2010)

(Y0.5Gd0.5)Ba2Cu3O7 97 K (Superconductors.ORG - 2005)

Y2CaBa4Cu7O16 96 K (Superconductors.ORG - 2006)

Y3Ba4Cu7O16 96 K (Superconductors.ORG - 2005)

Y2Ba5Cu7Ox 96 K (Superconductors.ORG - 2008)

NdBa2Cu3O7 96 K

Y2Ba4Cu7O15 95 K

GdBa2Cu3O7 94 K

YBa2Cu3O7 92 K (See above graphic)

TmBa2Cu3O7 90 K

YbBa2Cu3O7 89 K

YSr2Cu3O7 62 K

Comment: "1-2-3" superconductors actually have the 1212C structure. Thus, the formula for YBCO

could be written CuBa2YCu2O7.

GaSr2(Ca0.5Tm0.5)Cu2O7 99 K (Superconductors.ORG - 2006)

Ga2Sr4Y2CaCu5Ox 85 K (Superconductors.ORG - 2006)

Ga2Sr4Tm2CaCu5Ox 81 K (Superconductors.ORG - 2006)

La2Ba2CaCu5O9+

79 K (Saurashtra Univ., Rajkot, India - 2002)

(Sr,Ca)5Cu4O10 70 K

GaSr2(Ca, Y)Cu2O7 70 K

(In0.3Pb0.7)Sr2(Ca0.8Y0.2)Cu2Ox 60 K

(La,Sr,Ca)3Cu2O6 58 K

La2CaCu2O6+ 45 K

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(Eu,Ce)2(Ba,Eu)2Cu3O10+ 43 K

(La1.85Sr0.15)CuO4 40 K

SrNdCuO**** 40 K

(La,Ba)2CuO4 35-38 K

(Nd,Sr,Ce)2CuO4 35 K

Pb2(Sr,La)2Cu2O6 32 K

(La1.85Ba.15)CuO4 30 K (First HTS ceramic SC discovered - 1986)

**** First ceramic superconductor discovered without a non-superconducting oxide layer.

Comment: All of the above are copper perovskites, even though their metal-to-oxygen ratios are not

exactly 2-to-3. The best performers are those compounds that contain one or more of the electron-

emitters BaO, SrO or CaO, along with a Period 6 heavy metal like Mercury, Thallium, Lead,

Bismuth, or Gold.

GdFeAsO1-x 53.5 K (Highest Tc iron-based compound)

(Ca,Sr,Ba)Fe2As2 38 K

LiFeAs 18 K

Comment: The above are members of the newly discovered iron pnictide family.

MgB2 39 K (Highest Tc Non-Fullerene Alloy)

Ba0.6K0.4BiO3 30 K (First 4th order phase compound)

Nb3Ge 23.2 K

Nb3Si 19 K

Nb3Sn 18.1 K

Nb3Al 18 K

V3Si 17.1 K

Ta3Pb 17 K

V3Ga 16.8 K

Nb3Ga 14.5 K

V3In 13.9 K

Lattice: A15

Comment: Among the binary alloys, these are some of the best performers; combining Group 5B

metals in a ratio of 3-to-1 with 4A or 3A elements.

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PuCoGa5 18.5 K (First SC transuranic compound)

NbN 16.1 K Comment: After NbTi (below) NbN is the most widely used low-temperature superconductor.

Nb0.6Ti0.4 9.8 K (First superconductive wire)

MgCNi3 7-8 K (First all-metal perovskite superconductor)

C 15 K (as highly-aligned, single-walled nanotubes)

Nb 9.25 K

Tc 7.80 K V 5.40 K

Lattice: C=Fullerene, Nb=BCC, Tc=HEX, V=BCC

Comment: These four are the only elemental Type 2 superconductors.

RuSr2(Gd,Eu,Sm)Cu2O8 Tc ~58 K (Ruthenium-oxocuprate)

ErNi2B2C Tc 10.5 K (Nickel-Borocarbide)

YbPd2Sn Tc ~2.5 K (Heusler compound)

UGe2 Tc ~1K (Heavy fermion)

URhGe2 Tc ~1K ( " )

AuIn3 Tc 50 uK

Comment: The above 6 compounds are all rare ferromagnetic superconductors.

Sr.08WO3 2-4 K (Tungsten-bronze)

Tl.30WO3 2.0-2.14 K (")

Rb.27-.29WO3 1.98 K (")

Lattice: TET

(1.) "History of Physics Research in Ukraine", by Oleksandr Bakai and Yurij Raniuk, Kharkov

Institute of Science and Technology, 1993.

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APPLICATION OF SUPERCONDUCTORS:

Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than

superconducting magnets. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally-funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line which opened on April 3, 1997. In December 2003, the MLX01 test vehicle (shown above) attained an incredible speed of 361 mph (581 kph).

Although the technology has now been proven, the wider use of MAGLEV vehicles has been constrained by political and environmental concerns (strong magnetic fields can create a bio-hazard). The world's first MAGLEV train to be adopted into commercial service, a shuttle in Birmingham, England, shut down in 1997 after operating for 11 years. A Sino-German maglev is currently operating over a 30-km course at Pudong International Airport in Shanghai, China. The U.S. plans to put its first (non-superconducting) Maglev train into operation on a Virginia college campus. Click this link for a website that lists other uses for MAGLEV.

An area where superconductors can perform a life-saving function is in the field of biomagnetism. Doctors need a non-invasive means of determining what's going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body's water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer. Magnetic Resonance

Imaging (MRI) was actually discovered in the mid 1940's. But, the first MRI exam on a human being was not performed until July 3, 1977. And, it

took almost five hours to produce one image! Today's faster computers process the data in much less time.

The Korean Superconductivity Group within KRISS has carried biomagnetic technology a step further with the development of a double-relaxation oscillation SQUID (Superconducting QUantum Interference Device) for use in Magnetoencephalography. SQUID's are capable of sensing a change in a magnetic field over a billion times weaker than the force that moves the needle on a compass (compass: 5e-5T, SQUID: e-14T.). With this technology, the body can be probed to certain depths without the need for the strong magnetic fields associated with MRI's.

Probably the one event, more than any other, that has been responsible for putting "superconductors" into the American lexicon was the Superconducting Super-Collider project

planned for construction in Ellis county, Texas. Though Congress cancelled the multi-billion dollar effort in 1993, the concept of such a large, high-energy collider would never have been viable

The Yamanashi MLX01 Maglev train.

MRI of a Human Skull.

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without superconductors. High-energy particle research hinges on being able to accelerate sub-atomic particles to nearly the speed of light. Superconductor magnets make this possible. CERN, a consortium of several European nations, is doing something similar with its Large Hadron Collider (LHC) recently inaugurated along the Franco-Swiss border.

Electric generators made with superconducting wire are far more efficient than conventional generators wound with copper wire. In fact, their efficiency is above 99% and their size about half that of conventional generators. These facts make them very lucrative ventures for power utilities. General Electric has estimated the potential worldwide market for superconducting generators in the next decade at around $20-30 billion dollars. Late in 2002 GE Power Systems received $12.3 million in funding from the U.S. Department of Energy to move high-temperature superconducting generator technology toward full commercialization.

Other commercial power projects in the works that employ superconductor technology include energy storage to enhance power stability. American Superconductor Corp. received an order from Alliant Energy in late March 2000 to install a Distributed Superconducting Magnetic

Energy Storage System (D-SMES) in Wisconsin. Just one of these 6 D-SMES units has a power reserve of over 3 million watts, which can be retrieved whenever there is a need to stabilize line voltage during a disturbance in the power grid. AMSC has also installed more than 22 of its D-VAR systems to provide instantaneous reactive power support.

Recently, power utilities have also begun to use superconductor-based transformers and "fault limiters". The Swiss-Swedish company ABB was the first to connect a superconducting transformer to a utility power network in March of 1997. ABB also recently announced the development of a 6.4MVA (mega-volt-ampere) fault current limiter - the most powerful in the world. This new generation of HTS superconducting fault limiters is being called upon due to their ability to respond in just thousandths of a second to limit tens of thousands of amperes of current. Advanced Ceramics Limited is another of several companies that makes BSCCO type fault limiters. Intermagnetics General recently completed tests on its largest (15kv class) power-utility-size fault limiter at a Southern California Edison (SCE) substation near Norwalk, California. And, both the US and Japan have plans to replace underground copper power cables with superconducting BSCCO cable-in-conduit cooled with liquid nitrogen. (See photo below.) By doing this, more current can be routed through existing cable tunnels. In one instance 250 pounds of superconducting wire replaced 18,000 pounds of vintage copper wire, making it over 7000% more space-efficient.

An idealized application for superconductors is to employ them in the transmission of commercial power to cities. However, due to the high cost and impracticality of cooling miles of superconducting wire to cryogenic temperatures, this has only happened with short "test runs". In May of 2001 some 150,000 residents of Copenhagen, Denmark, began receiving their

The General Atomics/Intermagnetics General superconducting Fault Current Controller, employing HTS superconductors.

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electricity through HTS (high-temperature superconducting) material. That cable was only 30 meters long, but proved adequate for testing purposes. In the summer of 2001 Pirelli completed installation of three 400-foot HTS cables for Detroit Edison at the Frisbie Substation capable of delivering 100 million watts of power. This marked the first time commercial power has been delivered to customers of a US power utility through superconducting wire. Intermagnetics General has announced that its IGC-SuperPower subsidiary has joined with BOC and Sumitomo Electric in a $26 million project to install an underground, HTS power cable in Albany, New York, in Niagara Mohawk Power Corporation's power grid. Sumitomo Electric's DI-BSCCO cable was employed in the first in-grid power cable demonstration project sponsored by the U.S. Department of Energy and New York Energy Research & Development Authority. After connecting to the grid successfully on July 2006, the DI-BSCCO cable has been supplying the power to approximately 70,000 households without any problems. The long-term test will be completed in the 2007-2008 timeframe.

The National Science Foundation, along with NASA and DARPA and various universities are currently researching "petaflop" computers. A petaflop is a thousand-trillion floating point operations per second. Today's fastest computers have reached "petaflop" speeds - quadrillions of operations per second. Currently the fastest is a U.S. Military Supercomputer call the "Road Runner" operating at 1.026 petaflops per second (with multiple CPU's). The fastest single processor is a Lenslet optical DSP running at 8 teraflops. It has been conjectured that devices on the order of 50 nanometers in size along with unconventional switching mechanisms, such as the Josephson junctions associated with superconductors, will be necessary to achieve the next level of processing speeds. TRW researchers (now Northrop Grumman) have quantified this further by predicting that 100 billion Josephson junctions on 4000 microprocessors will be necessary to reach 32 petabits per second. These Josephson junctions are incorporated into field-effect transistors which then become part of the logic circuits within the processors. Recently it was demonstrated at the Weizmann Institute in Israel that the tiny magnetic fields that penetrate Type 2 superconductors can be used for storing and retrieving digital information. It is, however, not a foregone conclusion that computers of the future will be built around superconducting devices. Competing technologies, such as quantum (DELTT) transistors, high-density molecule-scale processors, and DNA-based processing also have the potential to achieve petaflop benchmarks.

In the electronics industry, ultra-high-performance filters are now being built. Since superconducting wire has near zero resistance, even at high frequencies, many more filter stages can be employed to achive a desired frequency response. This translates into an ability to pass desired frequencies and block undesirable frequencies in high-congestion rf (radio frequency) applications such as cellular telephone systems. ISCO International and Superconductor Technologies are companies currently offering such filters.

Superconductors have also found widespread applications in the military. HTSC SQUIDS are being used by the U.S. NAVY to detect mines and submarines. And, significantly smaller motors are being built for NAVY ships using superconducting wire and "tape". In mid-July, 2001, American Superconductor unveiled a 5000-horsepower motor made with superconducting wire

Hypres Superconducting Microchip, Incorporating 6000 Josephson Junctions.

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(below). An even larger 36.5MW HTS ship propulsion motor was delivered to the U.S. Navy in late 2006. The US Air Force expects planes will be able to fire non-lethal microwave rays at enemy ground troops with the help of a new superconducting generator system. The new generators are about the size of a small beer keg and designed to produce five megawatts of power. The generators will have lightweight metal foils coated with superconducting material that carry many times more current and are more efficient, strong enough for microwave weapons and light enough for airplanes.

The newest application for HTS wire is in the degaussing of naval vessels. American Superconductor has announced the development of a superconducting degaussing cable. Degaussing of a ship's hull eliminates residual magnetic fields which might otherwise give away a ship's presence. In addition to reduced power requirements, HTS degaussing cable offers reduced size and weight.

The military is also looking at using superconductive tape as a means of reducing the length of very low frequency antennas employed on submarines. Normally, the lower the frequency, the longer an antenna must be. However, inserting a coil of wire ahead of the antenna will make it function as if it were much longer. Unfortunately, this loading coil also increases system losses by adding the resistance in the coil's wire. Using superconductive materials can significantly reduce losses in this coil. The Electronic Materials and Devices Research Group at University of Birmingham (UK) is credited with creating the first superconducting microwave antenna. Applications engineers suggest that superconducting carbon nanotubes might be an ideal nano-antenna for high-gigahertz and terahertz frequencies, once a method of achieving zero "on tube" contact resistance is perfected.

The most ignominious military use of superconductors may come with the deployment of "E-bombs". These are devices that make use of strong, superconductor-derived magnetic fields to create a fast, high-intensity electro-magnetic pulse (EMP) to disable an enemy's electronic equipment. Such a device saw its first use in wartime in March 2003 when US Forces attacked an Iraqi broadcast facility.

Among emerging technologies are stabilizing momentum wheel (gyroscope) for earth-orbiting satellites that employs the "flux-pinning" properties of imperfect superconductors to reduce friction to near zero. Superconducting x-ray detectors and ultra-fast, superconducting light detectors are being developed due to their inherent ability to detect extremely weak amounts of energy. Already Scientists at the European Space Agency (ESA) have developed what's being called the S-Cam, an optical camera of phenomenal sensitivity (see above photo). And, superconductors may

A photo of Comet 73P/Schwassmann-Wachmann 3, in the act of disintegrating , taken with the European Space Agency S-CAM.

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even play a role in Internet communications soon. In late February, 2000, Irvine Sensors Corporation received a $1 million contract to research and develop a superconducting digital router for high-speed data communications up to 160 GHz. Since Internet traffic is increasing exponentially, superconductor technology may be called upon to meet this super need. Irvine Sensors speculates this router may see use in facilitating Internet2.

According to June 2002 estimates by the Conectus consortium, the worldwide market for superconductor products is projected to grow to near US $5 billion by the year 2010 and to US $38 billion by 2020. Low-temperature superconductors are expected to continue to play a dominant role in well-established fields such as MRI and scientific research, with high-temperature superconductors enabling the newer industries. The above ISIS graph gives a rough breakdown of the various markets in which superconductors are expected to make a contribution.

All of this is, of course, contingent upon a linear growth rate. Should new superconductors with

higher transition temperatures be discovered, growth and development in this exciting field could explode virtually overnight.

Another impetus to the wider use of superconductors is political in nature. The reduction of green-house gas (GHG) emissions has becoming a topical issue due to the Kyoto Protocol which requires the European Union (EU) to reduce its emissions by 8% from 1990 levels by 2012. Physicists in Finland have calculated that the EU could reduce carbon dioxide emissions by up to 53 million tons if high-temperature superconductors were used in power plants.

The future melding of superconductors into our daily lives will also depend to a great degree on advancements in the field of cryogenic cooling. New, high-efficiency magnetocaloric-effect compounds such as gadolinium-silicon-germanium are expected to enter the marketplace soon. Such materials should make possible compact, refrigeration units to facilitate additional HTS applications.

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SUPERCONDUCTOR LINKS:

Below are various links to web sites dealing with

superconductors and/or related fields; some of which may already

be listed on the USES, NEWS or PLAY pages.

Pictures/Graphics/Animations:

� Pictures & Animations of Levitating Magnets | University of Oslo � Images of Levitating Objects | The Magnet Man � Micro-photographs of Superconducting Materials | FSU Gallery � Magneto-Optical Imaging of Superconductors | University of Oslo � Perovskite Structures | Center for Computational Materials Science � Gallery of Abrikosov Lattices in Superconductors | University of Oslo � Crystal Atomic Lattice Viewer | University of Leicester � Crystal Structures | Israel Institute of Technology

Tutorial/Research:

� Superconductivity In Its Simplest Form | Oxford University � Superconductivity Concepts Link Map | Georgia State University � Lectures on Superconductivity | University of Cambridge � High-Temperature Superconductors | Kent State University � Condensed Matter Theory | University of Michigan � Advanced Explanation of Superconductivity | Futurescience � Flux Lines in Type 2 Superconductors | Univ. of California, Santa Barbara � Hole Theory of Superconductivity | Univ. of California, San Diego � Oxide Superconductors Tutorial | by Bob Cava of Princeton University for the American

Ceramic Society

Reference:

� Physica C | Superconductivity Journal � NIST Superconducting Materials Database | National Institute of Standards and Technology � Iowa State High-Tc Searchable Database Archive | Iowa State/Ames Lab � KEIRIN Superconductivity Papers Database | Japan � Solid State Communications | Elsevier Science � Superconductor Science and Technology | Institute Of Physics � Home page for the 1913 Nobel speech by Heike Kamerlingh Onnes � Home page for the 1972 Nobel speeches | by John Bardeen, Leon Cooper, and John Schreiffer � Home page for the 1973 Nobel speech by Brian D. Josephsen � Home page for the 2003 Nobel speeches to Abrikosov, Ginzburg and Leggett � U.S., European and Japanese Patent Search | Refined Search � High School Teacher's Guide to Superconductivity | Oak Ridge National Laboratory � Books on Superconductivity | Geometry.NET Picks

YBCO Molecule

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� Books on Superconductivity | RBookShop Picks � Glossary of Superconductor Terms | Superconductors.ORG � Matter Glossary | University of Liverpool � Ionic Radii of the Elements | Environmentalchemistry.COM