Basic Superconductivity

5/26/2018 Basic Superconductivity

1/32 Physics Summer School: Superconductivity Ted Forgan

Birmingham Condensed Matter Physics Group

SuperconductivityProfessor Ted Forgan

Exploring the world of Low Temperatures and Quantum effectshttp://www.phy

sics.ubc.ca/~outreach/phys420/p420_

96/bruce/ybc

o.html




Superconductivity

how do we get to low temperatures?

why does it happen?

what use is it?

what is superconductivity?




300 K

200 K

100 K

0 K

liquid N2

77 K

liquid He

4.2 K

hot day

27 C

Dewar (vacuum) flask




Kamerlingh OnnesLiquefied He 1908




Temperature

Tc= 4.2 K

Resistanceof pure

mercury

Kamerlingh Onnes:Superconductivity 1911

http://www.nobel.se/physics/la

ureates/1913/onnes-bio.h

tml




-all previous theories

were unable to describe

Superconductivity!

Thats because it

involved new physics:

Quantum Mechanics




The Meissner (and Ochsenfeld) Effect

superconductors push out magnetic fields

- and keep them outwith constantly- flowingresistance-less currents

this diamagnetic property is more fundamental than zero resistance

T> Tc T< Tc

http:/

/www.p

hysics.u

bc.c

a/~ou

treac

h/phys4

20/p420

_96/bruce

/ybco.h

tm

l




Persistent Currents

SN

SN

currents flow in loops under the poles

-and repel the magnet as ifthere were an image magnet

underneath: N to N and S to S!

superconducting plate

B

supercurrent can also flowaround a ring

giving a magnetic field Bthat never dies away




Circulating Super-currents are stablebecause of Quantum effects

large B small B

small Ilarge I

?

de Broglie:

= h / pCurrents form stable wave patterns:

the numberof wavelengths cannot decaysmoothly to zero




Levitation with High-Tc Superconductors

A kg or so of fish bowl

up to 202 kg of sumo wrestler!

www.istec.or.jp




More levitation: Yogic Flying

a trick of thecamera?

orsuperconductivity!!??

TranscendentalMeditation?

SN SN SN

ht

tp://www.vedicobservato

ry.org/A7YogicFlying.html




More levitation: Flying Frog

water is slightlyrepelled bymagnetic fields. This supportsa frog or a strawberry - in ahighmagnetic field gradient

http://www.hfml.kun.nl/




Georg Bednorzand Alex Muller

received the

Nobel Prize 1987for discovery ofthe first of thecopper-oxide

superconductors

this is how itwas announced

http://www.phys.ntnu.no/brukdef/prosjek

ter/super/Profiles/bednm

ull.jpg




Possible High Tcsuperconductivity inthe Ba La Cu O

system

resistivity

(cm)

Temperature( K)

35 K

10 K

Their sample at first becamemoreresistive as it cooled!

At 35 K, when the sample was5000 x more resistive than copper,

the resistance began to fall

Only by 10 Khad the resistancefallen to (possibly) zero !


15/32

Physics Summer School: Superconductivity Ted Forgan


93 K !

77 Kliquid

nitrogen


16/32



1910 1930 1950 1970 1975 1980 19901985 20001995 2005

HgPb Nb

NbO

NbNV3Sn

NbAlSi

Nb3Sn

Ba(Pb,Bi)O3

organic materials

NbGe3Ba(K,Bi)O3

Doped buckyballsA3C60

MgB239K

Liquid N2

Liquid He0-

10-

70-

30-

20-

120-

60-

50-

40-

110-

90-

100-

80-

130-

140 -

150- Temperature(K)

conventionalsuperconductors

(La,Ba)CuOBednorz and Muller

TlBaCaCuO

Bi2Sr2Ca2Cu3Ox

HgBaCaCuO

pressur e

~ 155K

High temperaturecuprate

superconductorsYBa2Cu3O7-d

pressur e


17/32



The crystal structures of High-Tcsuperconducting materials all have

copper-oxide CuO2layers

Is a room-temperaturesuperconductor out there

waiting to be discovered ??

Have we reachedthe maximum

possible Tcin thisclass of materials?

We are still notsure exactly whythis is important!


18/32



John Bardeen, Leon Cooper and Bob Schrieffer

B. C. S.Nobel Prize 1972 for their theory of 1957 whichexplained conventional superconductors: nearly 50years after their discovery by Kamerlingh Onnes!

http://sup

erconductors.org/history

.htm#resist


19/32



Cooper Pairs

each electron in a pair does its own thing

(as the Pauli Exclusion Principle says it must)

but the CENTRES OF MASS of allthe pairs do exactly the SAME thing

(all at rest, or all carrying a steady current)

and how can weprovethey travel inpairs ?

but what force holds the electrons together?


20/32



The BCS attractive mechanismis due to electrons

slightly deformingthe crystal lattice

- but we can visualise it asthe sagging mattress

effect!

A theoretician would describe thisattraction as due to exchange of

virtual phonons

RandySimon,AndySmith,Superconductors


21/32



The Quantum of magnetic flux o

large flux

small Ilarge I

=h / p

o = h/2e

small flux

=nowith n= 8

n= 4

= nowith n= 4

n= 8

2e

- because the electrons go round in pairs!


22/32



Measurement ofthe Quantum of

magnetic flux oin YBCO by the

Birmingham Group

YBCO ring

blutak!

flux detector

calibration coil


23/32



Millikan-style results on flux quantisation

flux jump

h / 2e

-so electrons are paired in High-Tcmaterials just like BCS onesbut WHAT attracts them?


24/32



Flux lines in superconductors

For some superconductors, called Type II,the Meissner effect is not complete, andhigh enough magnetic fields can enter

each carries a fluxof o= h /2e

as flux lines

field lines run from a magnet into adisk of superconductor which is

suspended swinging under the magnet

://w

ww.fys.uio.no/super/levit

ation/

B h d d Ph G


25/32



- in Grenoble

ESRF: X-raysILL: neutrons

the most powerfulresearch reactor

in the world

We can detect flux lines with neutrons

Bi i h C d d M Ph i G


26/32



using neutron diffraction

Bi i h C d d M Ph i G


27/32



Typical results from a low-Tcmaterial

Looking at a reactor through a flux line latticeis like looking at a street lamp through an umbrella

niobium 0.2 T low resolution niobium 0.2 T high resolution

Bi i h C d d M tt Ph i G


28/32



Typical results from a high-Tcmaterial

SQUARE not triangular patterns!These data (published 2004) tell us about electron pairing in YBCO

YBCO B= 1 T YBCO B= 11 T



29/32



Superconducting QuantumInterference Devicescan

measure tiny fields such asthose due to currents flowing

in your heart muscle

The quantum of flux, o= h/ 2e,has the tiny value 2 x 10-15T.m2

The application of QuantumMechanics to measure tinymagnetic fields: SQUIDs

This is one millionth of the fluxdue to the earths field through

the hole in a polomint!



30/32



The dream - Tomorrows Superconducting World

350 mphlevitated

Intercitytrains

Underground rapid transit:Heathrow to Gatwick in 10

minutes

Computing:1000 times fastersupercomputers

Cargo-carrying

submarines,all-electricUS Navy

Energy Saving:power lineselectric motorstransformers

MedicalDiagnostics:MagneticResonanceImagingSQUID:

Brain activity

Heart function

Information Technology:much faster, wider band

communications

magnetically

launchedspaceshuttle



31/32



Some of these dreams are already reality

Japanese levitating train hassuperconducting magnets onboard

Superconducting power cableinstalled in Denmark

SQUIDmeasure-ment ofneuro-

magneticsignals

(nuclear) magneticresonance imaging of thebrain, in the field from asuperconducting magnet

www.rtr

i.or.jp/rd/maglev/html/en

glish/maglev_frame_E.html

www.lanl.gov/quarterly/q_spring03/meg_h

elmet.shtml

http://www.bestofjesse.com/projects

/indust/projec

t1.html

Bi min h m C nd n d M tt Ph i G p


32/32

Ph i S mm S h l: S d ti it T d F


Thats all Folks!

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Basic Superconductivity