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Superconductivity- An overview of science and technology

Prof Damian P. Hampshire

Durham University, UK

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Structure of the Talk

I) The fundamental building blocks - (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper-

Schrieffer theoriesThe Josephson effect Critical current and pinning (zero resistance)

II) The important materialsClassic LTS high field materials – NbTi and Nb3SnThe high temperature superconductors

- The pnictides (Superconductivity and magnetism)III) Technology – MRI, LHC, ITER and beyond..

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ii) Microscopic BCS theory – describes why materials are superconducting

There are two main theories in superconductivity:

i) Ginzburg-Landau Theory – describes the properties of superconductors in magnetic fields

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Ginzburg-Landau Theory

2 4 2- 2e ) + d 1 1f = + + (-i2 2m

α ψ β ψ ψ∇ ∫A H B

Ginzburg and Landau (G-L) postulated a Helmholtz energy density for superconductors of the form:

where α and β are constants and ψ is the wavefunction. αis of the form α’(T-TC) which changes sign at TC

High magnetic fields penetrate superconductors in units of quantised flux (fluxons)!

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A fluxon has quantised magnetic flux -its structure is like a tornado

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The Mixed State in Nb

Vortex lattice in niobium – the triangular layout can clearly be seen. (The normal regions are preferentially decorated by ferromagnetic powder).

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Reversible Magnetic Properties of ‘Perfect’ Superconductors

Below Hc, Type I superconductors are in the Meissner state: current flows in a thin layer around the edge of the superconductor, and there is no magnetic flux in the bulk of the superconductor. (Hc : Thermodynamic Critical Field.)In Type II superconductors, between the lower critical field (Hc1), and the upper critical field (Hc2), magnetic flux – fluxons - penetrates into the sample, giving a “mixed” state.

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Josephson dc. SQUID

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Josephson diffraction

The voltage across a biased SQUID as a function of field

10BCS Theory - the origin of superconductivity

Bardeen Cooper and Schrieffer derived two expressions that describe the mechanism that causes superconductivity,

where Tc is the critical temperature, Δ is a constant energy gap around the Fermi surface, N(0) is the density of states and V is the strength of the coupling.

( )12 exp0D N V

ω⎡ ⎤

Δ = −⎢ ⎥⎣ ⎦

( )11.14 exp0B c D

k TN V

ω⎡ ⎤

= −⎢ ⎥⎣ ⎦

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Model for a polycrystalline superconductor – with strong pinning

A collection of truncated octahedra

G. J. Carty and Damian P. Hampshire - Phys. Rev. B. 77 (2008) 172501 also published in Virtual journal of applications of Superconductivity 15th May 2008

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-100

102030405060708090

100110

0 1 2 3 4 5 6 7

0 20 40 60 80 100 120

0

1.0

2.0

Initial (ε = 0 %)After 1 strain cycleto ε = +0.455%

T = 4.2 K

Ec = 100 μVm-1

Ec = 10 μVm-1

12.5 T

13 T

13.5 T

14 T

14.5 T15 T

Current Density, J (108 Am-2)

Ele

ctric

Fie

ld, E

(μV

m-1

)

Vol

tage

, V ( μ

V)

Current, I (A)

Critical current (Jc) measurements

0 10 20 30 40 50 60 70-2

-1

0

1

2

3

4

5

V (n

V)

I (A)

4.2 K, variable B-field, Nb3Sn77 K, zero field YBCO

13Fluxons do not move smoothly through a polycrystalline superconductor

The motion of flux through the system takes place predominantly along the grain boundaries.TDGL movie 0.430Hc2 Psi2TDGL movie 0.430Hc2 Psi2TDGL movie 0.430Hc2 Psi2

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Structure of the Talk

I) The fundamental building blocks- (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper-

Schrieffer theoriesThe Josephson effect Critical current and pinning (zero resistance)

II) The important materialsClassic LTS high field materials – NbTi and Nb3SnThe high temperature superconductors

- The pnictides (Superconductivity and magnetism)III) Technology – MRI, LHC, ITER and beyond..

15NbTi multifilamentary wire – the workhorse for fields up to ~ 10 Tesla

Alloy - NbTi

Tc ~ 9 K BC2 ~ 14 TDuctile

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EM-LMI ITERInternal-tin Nb3Sn

Furukawa ITERBronze-route Nb3Sn

OST MJR Nb3Sn

Outokumpu Italy (OCSI)ITER Internal tin Nb3Sn

Intermetallic compound Nb3Sn

Tc ~ 18 K BC2 ~ 30 TBrittle

Nb3Sn superconducting wires- the workhorse for ITER

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-1.5 -1.0 -0.5 0.0 0.5105

106

107

108

109

Eng

inee

ring

Crit

ical

Cur

rent

Den

sity

(Am

-2)

0.1

1

10

100

1000Temperature: 4.2 K

Crit

ical

Cur

rent

(A)

23 T

Magnetic Field: 8 T

Applied Strain (%)

Why is the effect of strain on JC important ?

The critical current density (JC) depends on the magnetic field, the temperature and the strain-state of the superconductor.

Superconducting magnets: large strains due to the differential thermal contraction during cool-down and the Lorentz-forces during high-field operation.

Nb3Sn Wire

18HTS – BiSrCaCuO (BiSCCO)- Powder-in-tube fabrication- Granularity is an issue- d-wave

19HTS coated conductors

- Kilometre long single crystals

Configuration of SuperPower 2G HTS Wire™

20MgB2 - Brittle compound Tc ~ 40 K, BC2 (//c) ~ 20 T

A nodeless BCS-type gap !

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Conductors in the USA

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10

100

1000

10000

0 5 10 15 20 25 30 35 40 45

Applied Field (T)

J E(A

/mm

²)

YBCO Insert Tape (B|| Tape Plane)

YBCO Insert Tape (B⊥ Tape Plane)

MgB2 19Fil 24% Fill (HyperTech)

2212 OI-ST 28% Ceramic Filaments

NbTi LHC Production 38%SC (4.2 K)

Nb3Sn RRP Internal Sn (OI-ST)

Nb3Sn High Sn Bronze Cu:Non-Cu 0.3

YBCO B|| Tape Plane

YBCO BYBCO B⊥⊥ Tape PlaneTape Plane

2212

RRP NbRRP Nb33SnSn

BronzeBronzeNbNb33SnSnMgB2

NbNb--TiTiSuperPower tape SuperPower tape used in record used in record breaking NHMFL breaking NHMFL insert coil 2007insert coil 2007

18+1 MgB18+1 MgB22/Nb/Cu/Monel /Nb/Cu/Monel Courtesy M. Tomsic, 2007Courtesy M. Tomsic, 2007

427 filament strand with Ag alloy outer sheath tested at NHMFL

Maximal JE for entire LHC Nb-Ti strand production (CERN-T. Boutboul '07)

Complied from Complied from ASC'02 and ASC'02 and ICMC'03 papers ICMC'03 papers (J. Parrell OI(J. Parrell OI--ST)ST)

4543 filament High Sn 4543 filament High Sn BronzeBronze--16wt.%Sn16wt.%Sn--

0.3wt%Ti (Miyazaki0.3wt%Ti (Miyazaki--MT18MT18--IEEEIEEE’’04)04)

Conductors in the USA

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HTS materials and exotic materials

Phase diagram for the ferromagnet UGe2

A schematic of a high-Tc phase diagram

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The Pnictide Superconductors – the iron age revisited

Iron Man : In cinemas now from Paramount Pictures and Marvel Entertainment

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The Pnictides- the original discovery

Layered structure

Original material:Tc 3-5 K 2006 LaOFeP

26A big class of new materials (> 2000 compounds)

Re-O-TM-Pn.

Re = La+

TM =

Pn

27Comparing HTS and pnictide structure

In both cases, the superconductivity is in metallic layers, there is a charge reservoir and they are antiferromagnetic in their undoped state.

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Tc of the iron-based system is quite high

Tc 3-5 K 2006 LaOFeP

Tc 26 K, LaOFFeAs. Jun. 2008

Tc 43 K with high pressure (4 GPa) LaOFeAs. Feb. 2008Possibly the 1st 40K-class LTS superconductor

Tc 55 K NdFeAsO1-d. April/May 2008.

(Also 111 phase and 122 phase)

29Oxygen concentration is critical for superconductivity

• For the NdFeAsO1-d with different O concentration• A dome-shaped superconducting bubble has been found

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Tc ~ 42K

Point-contact spectroscopy

Page 1224

Sweep the V I - V

dI/dV - V

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A nodeless BCS-type gap !

32Does Superconductivity coexist or compete with magnetism ?

This sharp drop about 150 K is due to a SDW – confirmed using neutron diffraction - P. C. Dai Nature (2008)

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BC2 is highLarbalestier et al measured the resistance of F doped LaOFeAs at high fields up to 45 T. Nature 453 903

H.H. Wen et al measured F doped NdOFeAs. Hc2 ~ 300 T in the ab plane and ~60-70T in c axis. Arxive:cond-mat/0806.0532

Two-gap model is qualitatively consistent with their data.

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High critical currentin polycrystalline pnictides !

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Structure of the Talk

I) The fundamental building blocks - (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper-

Schrieffer theoriesThe Josephson effect Critical current and pinning (zero resistance)

II) The important materialsClassic LTS high field materials – NbTi and Nb3SnThe high temperature superconductors

- The pnictides (Superconductivity and magnetism)III) Technology – MRI, LHC, ITER and beyond..

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Applications using Superconductors

MRI Body scannersLHCITERTransportPower transmissionPublic outreach

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MRI - $1B annual market

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Large hadron collider – LHC ~ $ 6B

6000 superconducting magnets will accelerate proton beams in opposite directions around a 27 km-long ring and smash them together at energies bordering on 14 TeV.

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Some facts about the LHCProtons are accelerated to 99.999999991% of the speed of light

The LHC lets us glimpse the conditions 1/100th of a billionth of a second after the Big Bang: a travel back in time by 13.7 billion years

High energy collisions create particles that haven’t existed in nature since the Big Bang

Find out what makes the Universe tick at the most fundamental level

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ITER – Building a star on planet earth

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Picture courtesy of the SOHO/EIT collaboration

Matter becomes a plasma

At 200 million ºC,

We need extreme conditions …

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ITER – A large transformer

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The fuel for ITER is from seawater

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16 Nb3Sn toroidal field coils - each coil is ~ 290 tonnes, has 1100 strands, ~ 0.8 mm diameter to form a conductor 820

m long.

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A burning plasma

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Fusion powers the Sun and stars and has many potential attractions

• Essentially limitless fuel

• No green house gases

• Major accidents impossible

• No long-lived radioactive waste

• Could be a reality in 30 years

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Applications using SuperconductorsTransport

In Jan 08, the Central Japan Railway Company (JR Central) announced that it plans to construct the world's fastest train, a second-generation maglev

train that will run from Tokyo to central Japan.

Cost ~ 44.7 billion dollarsCompletion in 2025

Speed ~ 500 kilometers per hourLength ~ 290 kilometers

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Applications using SuperconductorsSuperconducting power transmission- currently we waste ~ 20 % of our

energy just transporting it around- potentially the next industrial

revolution

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Conclusions

Superconductivity offers excellent science, excellent technology, excellent training and the possibility of saving the planet !!

Using world-class science to produce technology is tough. It requires first class scientists, time, perserverance, creativity, luck and funding.

The many uses for superconductivity means that many of the technological tools required to exploit new materials are in place. The new materials discovered in the last 20 years were found by relatively small determined groups.

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References + Acknowledgements

Acknowledgements: Xifeng Lu + colleagues in Beijing, Mark Raine, Georg Weiglein (IPPP, Durham), Eric Hellstrom (ASC Florida), Chris Carpenter (Culham) + many others …….

Bibliography/electronic version of all talks and publications are available at: http://www.dur.ac.uk/superconductivity.durham/

Superconductivity�- An overview of science and technologyStructure of the Talkii) Microscopic BCS theory – describes why materials are superconductingGinzburg-Landau TheoryA fluxon has quantised magnetic flux - its structure is like a tornadoThe Mixed State in NbReversible Magnetic Properties of ‘Perfect’ SuperconductorsJosephson dc. SQUIDJosephson diffractionBCS Theory �- the origin of superconductivityModel for a polycrystalline superconductor – with strong pinningFluxons do not move smoothly through a polycrystalline superconductorStructure of the TalkNbTi multifilamentary wire �– the workhorse for fields up to ~ 10 TeslaNb3Sn superconducting wires�- the workhorse for ITERWhy is the effect of strain on JC important ?HTS – BiSrCaCuO (BiSCCO)�- Powder-in-tube fabrication�- Granularity is an issue�- d-waveHTS coated conductors�- Kilometre long single crystals MgB2 - Brittle compound �Tc ~ 40 K, BC2 (//c) ~ 20 T Conductors in the USAHTS materials and exotic materialsThe Pnictide Superconductors �– the iron age revisitedThe Pnictides� - the original discoveryA big class of new materials �(> 2000 compounds) Comparing HTS and pnictide structureTc of the iron-based system is quite highOxygen concentration is critical for superconductivityDoes Superconductivity coexist or compete with magnetism ?BC2 is highHigh critical current� in polycrystalline pnictides !Structure of the TalkApplications using SuperconductorsLarge hadron collider – LHC ~ $ 6BSome facts about the LHCApplications using SuperconductorsApplications using SuperconductorsConclusionsReferences + Acknowledgements