Superconductivity 2012

Department of Physics, Umeå University, Sweden

DemonstrationWhat did we see?High-Tc materials(How to make superconductors)Some applications and important properties

How do we show superconductivity?

Department of Physics, Umeå University, Sweden

Superconductors 1. have an electrical resistivity that is exactly zero,2. refuse magnetic fields to enter the superconducting volume.

(Lab experiment) Let's try!

Meissner-Ochsenfeld effect

Department of Physics, Umeå University, Sweden

“Perfect“ metal Superconductor

Room temperature

Room temperature, with magnetic field

At low temperature (T<Tc), after cooling in a constant magnetic field

"Perfect conductor" effect

Department of Physics, Umeå University, Sweden

“Perfect“ metal Superconductor

Room temperature

Low temperature (T<Tc)without magnetic field

After applying a magnetic field at low temperature (T<Tc)

Why is the levitation stable?

Department of Physics, Umeå University, Sweden

When you balance things on soft springs the situation is usually unstable. So why doesn't the magnet simply fall off?

Because the field can penetrate! Take a ceramic:

Why is the levitation stable?

Department of Physics, Umeå University, Sweden

Although the grains are superconducting, the boundaries are effectively thin "normal" films. Some field lines can find ways to penetrate the ceramic, but then get "locked" in place - they cannot move without crossing grains!

Two types of superconductors: Types I and II

Department of Physics, Umeå University, Sweden

Type I Type II

Different behaviours in magnetic fields (red):

Weak B-fields are always repelled, by both types;

strong fields destroy the superconductivity in type I, but penetrate type II in "vortex tubes" containing one flux quantum each!

Superconducting materials

Department of Physics, Umeå University, Sweden

"Classical" superconductors: Metals and alloys!

Hg 4.2 KDiscovered by Heike Kammerling Onnes in 1911 (Nobel Prize 1913)Pb 7.2 K

Nb 9.2 K(0.2 T - type II element!)

NbTi 9.8 K14 T (The "standard" superconductor)

NbN 16.1 K 16 T (used in thin film applications)

Nb3Sn 18 K24 T (expensive and difficult to use)

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

MgB2

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

Some representative ”heads of families” of HiTcs:

La2-xSrxCuO4 38 K (Bednorz & Müller, 1986)

YBa2Cu3O7-d 92 K (Wu & Chu, 1987)

Bi2Ca2Sr2Cu3O10 110 K

Tl2Ba2Ca2Cu3O10 125 K

HgBa2Ca2Cu3O8 135 K

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3":

The basic structure is tetragonal,

with copper and oxygen forming a framework

into which we insert Ba and Y.

The formula is now YBa2Cu3O6, and this material is NOT superconducting!

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3":

The basic structure is tetragonal,

with copper and oxygen forming a framework

into which we insert Ba and Y.

To get a superconducting material we must add more oxygen, to obtain YBa2Cu3O7!

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3":

CuO chain

Ba spacer

CuO plane

Y spacer

CuO plane

Ba spacer

CuO chain

These are the metallic, superconducting parts!

To some extent, more CuO planes mean higher Tc!

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

How to make YBa2Cu3Ox, "Y-1-2-3":

1. Mix and grind Y2O3, BaCO3 and CuO for a long time.

2. Heat in an oven at 900-925 oC for at least 1 hour.

3. Crush, re-grind, and repeat 2. a few times.

4. Press into a cake, then heat in pure oxygen gas at 450 oC for at least 24 hours.

5. Time to test for superconductivity!

High Transition Temperature Superconductors (HiTc:s)

Department of Physics, Umeå University, Sweden

Higher values for Tc can be found for other materials, based on Bi, Hg or Tl.

These are also layered, often with many parallel internal layers of CuO:

Tl2Ba2CuO6 Tl-2201 (single CuO) 85 K

Tl2Ba2CaCu2O8 Tl-2212 2 layers 105 K

Tl2Ba2Ca2Cu3O10 Tl-2223 3 layers 125 K

(Bi-2223 110 K, Hg-1223 135 K)

A new star: MgB2

Department of Physics, Umeå University, Sweden

Superconductivity in MgB2 was discovered in 2001 with Tc = 39 K, the highest for any "classical" superconductor.

The material is cheap, easy to handle, non-poisonous, and easily formed into wires or films/tapes. Problem: The practical critical field seems to be limited to 3.5 T.

An even newer star: iron arsenides

Department of Physics, Umeå University, Sweden

In 2008, another type of layered, exotic superconductors, based on iron and arsenic, was discovered.

Takahashi et al., Nature 453, 376 (2008)

An even newer star: iron arsenides

Department of Physics, Umeå University, Sweden

In 2008, another type of layered, exotic superconductors, based on iron and arsenic, was discovered. Another family is BaxKyFe2As2.

Critical temperatures up to above 55 K have been reported when changing the La to heavier rare earths. Again, the material is cheap and fairly easy to handle, but As is clearly poisonous!

Applications for superconductors

Department of Physics, Umeå University, Sweden

There are basically two types of applications:

Power circuits and electronics/measurements. Most practical applications use type II superconductors.

Existing and future commercial devices:

Power transmission components, power storage devices, electric motors and generators, frictionless bearings, permanent magnets and electromagnets, voltage standards, fast computers and electronics, microwave filters, .........

Applications for superconductors

Department of Physics, Umeå University, Sweden

In electronics, one possible application is in fast computers. Clock pulses must be synchronized in a computer, but at 3 GHz light travels only 10 cm during one clock pulse! Shrinking a computer means more concentrated heating, killing the CPU.

The obvious solution is a cool superconducting computer!

Electronics and measurements: tunnelling

Department of Physics, Umeå University, Sweden

Tunneling between two superconductors (”SIS”) can be used as the basis for many devices.

In principle, both electrons and pairs can tunnel through a Josephson junction, so the real behaviour can be either bistable (logic 1/0!) or continuous.

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

A particularly useful device is the SQUID:

Superconducting QUantum Interference Device

or

With a SQUID it is possible to routinely measure magnetic fields down to well below 10-16 T!

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

The SQUID can be used for measurements (as a sensor).

Superconducting loop

Josephson junctions, called ”weak links”

External connections

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

Each Josephson junction has a maximum supercurrent I = I0 sin , so the maximum current that can run through the device is 2I0.

2I0

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

If we apply a very weak external magnetic field, a circulating shielding current will appear and no field will exist inside the loop!

The external current must decrease to avoid exceeding the maximum supercurrents in the junctions.

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

When the magnetic field corresponds to exactly ½ magnetic flux quantum inside the ring, the circulating current has its maximum and the external current its minimum value.

Electronics and measurements: the SQUID

Department of Physics, Umeå University, Sweden

If the field increases further, one flux quantum is admitted through a weak link, and the circulating current reverses!

It can easily be shown that the external current is a periodic function

Imax = 2I0cos(/0)

But how do you make ceramic "wires"?

Department of Physics, Umeå University, Sweden

There are two ways:

1. Thin films on a metal or ceramic substrate

2. "Powder-in-tube" technology

Stainless Deposition of Oxygen treatment Storage steel band ceramic film in hot oven

But how do you make ceramic "wires"?

Department of Physics, Umeå University, Sweden

There are two ways:

1. Thin films on a metal or ceramic substrate

2. "Powder-in-tube" technology

Fill a silver tube with superconductor powder, then draw to desired shape, then heat treat ("anneal").

But how do you make ceramic "wires"?

Department of Physics, Umeå University, Sweden

The "powder-in-tube" method is simlar to what you do to "classical" superconductors:

Basic procedure:- Make a Cu cylinder,- make a lot of holes along axis,- fill the holes with superconducting rods,- draw the whole cylinder to wire, as if it were massive Cu!This procedure works well with Nb-Ti, which is soft and ductile like copper!

But how do you make ceramic "wires"?

Department of Physics, Umeå University, Sweden

All superconductor wires have similar internal "multi-strand" structures!

NbTi wire High-Tc (BiSSC) wires

Using type II superconductors

Department of Physics, Umeå University, Sweden

An obvious application for a superconductor is to transport electric current.

What happens to electrons in a B-field ?

Current

B-field

Let us remember two laws:

Fm = qv B ("Maxwell")

F = 0 ("Newton")There will be a force on the magnetic

field lines!

Using type II superconductors

Department of Physics, Umeå University, Sweden

Is this a problem ?

A moving field ↔ changing flux; but - d/dt = E !

Current

B-field

This gives two problems:

1. A voltage appears along the current flow; "resistance"!

2. This causes dissipation of heat, since P = UI

Using type II superconductors

Department of Physics, Umeå University, Sweden

Is this a problem ?

A moving field ↔ changing flux; but - d/dt = E !

This gives two problems:

1. A voltage appears along the current flow; "resistance"!

2. This causes dissipation of heat, since P = UI

Using type II superconductors

Department of Physics, Umeå University, Sweden

Or, if we measure voltage as a function of applied current at constant temperature:

Using type II superconductors

Department of Physics, Umeå University, Sweden

Conclusion: We want to keep the flux lattice fixed in space! How do we do this?

Flux lines prefer to go through non-superconducting regions, because it requires energy to create a vortex tube! So, we should insert impurity particles into the superconductor!

This method is called flux pinning.

Using type II superconductors

Department of Physics, Umeå University, Sweden

You have already seen a magnet fly !

You can also make a really good magnetic bearing,

or ”freeze in” a field to make a permanent magnet – with a field which you can shape exactly as you want it!

Using type II superconductors

Department of Physics, Umeå University, Sweden

BUT: Flux pinning also gives problems:

There is a”friction force” that keeps them in place, and because J X B, dBz/dx Jc everywhere inside a type II superconductor! Increasing external field:

Using type II superconductors

Department of Physics, Umeå University, Sweden

BUT: Flux pinning also gives problems:

There is a”friction force” that keeps them in place, and because J X B, dBz/dx Jc everywhere inside a type II superconductor! Increasing external field:

Using type II superconductors

Department of Physics, Umeå University, Sweden

BUT: Flux pinning also gives problems:

There is a”friction force” that keeps them in place, and because J X B, dBz/dx Jc everywhere inside a type II superconductor! Decreasing external field:

Using type II superconductors

Department of Physics, Umeå University, Sweden

BUT: Flux pinning also gives problems:

There is a”friction force” that keeps them in place, and because J X B, dBz/dx Jc everywhere inside a type II superconductor! :

This leads to a magnetic hysteresis, and to energy loss (= heating!). It can be shown that the loss is proportional to the thickness a of the superconductor!

A possible novel application

Department of Physics, Umeå University, Sweden

The first practical application for high-Tc materials in power circuits is likely to be something that cannot be made without superconductivity. One such example is the superconducting current limiter:Consider a standard transformer (which you can find in any electronic device, at home or here):

U1/U2 = N1/N2 = I2/I1,

where 1 means "input" side, 2 "output" side, and N is the number of wire turns! http://

www.yourdictionary.com

A possible novel application

Department of Physics, Umeå University, Sweden

The first practical application for high-Tc materials in power circuits is likely to be something that cannot be made without superconductivity. One such example is the superconducting current limiter:Suppose we make a

transformer with N2 = 1 (a single turn).

If we short-circuit the output, U2=0,

then U1 = NU2 = 0, for all currents!

Usually this is just stupid, but what if we make the secondary one turn of superconducting wire?

A possible novel application

Department of Physics, Umeå University, Sweden

Superconducting current limiter:

Primary current I1I2 = N I1;

if the coil superconducts U1 = U2 = 0, and P = UI = 0 !

However, whenever I2 > Ic the secondary turns normal and

R1 = U1/I1 = N2U2/I2 = N2R2 !

Because N can be made large and high-Tc materials have very large normal resistivities, this works as a "fuse"!

A possible novel application

Department of Physics, Umeå University, Sweden

Superconducting current limiter:

N1 = 500

N2 = 1

Ic ≈ 85 A at 77 K (measured!)

Tc ≈ 110 K (Bi-2223)