1 Superconductivity - An overview of science and technology Prof Damian P. Hampshire Durham University, UK

2 Structure of the Talk I)The fundamental building blocks - (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper- Schrieffer theories The Josephson effect Critical current and pinning (zero resistance) II)The important materials Classic LTS high field materials – NbTi and Nb 3 Sn The high temperature superconductors - The pnictides (Superconductivity and magnetism) III) Technology – MRI, LHC, ITER and beyond..

3 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

4 Ginzburg-Landau Theory 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. α is of the form α’(T-T C ) which changes sign at T C High magnetic fields penetrate superconductors in units of quantised flux (fluxons)!

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

6 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).

7 Reversible Magnetic Properties of ‘Perfect’ Superconductors Below H c, 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. (H c : Thermodynamic Critical Field.) In Type II superconductors, between the lower critical field (H c1 ), and the upper critical field (H c2 ), magnetic flux – fluxons - penetrates into the sample, giving a “mixed” state.

8 Josephson dc. SQUID

9 Josephson diffraction The voltage across a biased SQUID as a function of field

10 BCS 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.

11 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) also published in Virtual journal of applications of Superconductivity 15th May 2008

12 Critical current (J c ) measurements 4.2 K, variable B-field, Nb 3 Sn 77 K, zero field YBCO

13 Fluxons do not move smoothly through a polycrystalline superconductor

14 Structure of the Talk I)The fundamental building blocks - (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper- Schrieffer theories The Josephson effect Critical current and pinning (zero resistance) II)The important materials Classic LTS high field materials – NbTi and Nb 3 Sn The high temperature superconductors - The pnictides (Superconductivity and magnetism) III) Technology – MRI, LHC, ITER and beyond..

15 NbTi multifilamentary wire – the workhorse for fields up to ~ 10 Tesla Alloy - NbTi Tc ~ 9 K B C2 ~ 14 T Ductile

16 EM-LMI ITER Internal-tin Nb 3 Sn Furukawa ITER Bronze-route Nb 3 Sn OST MJR Nb 3 Sn Outokumpu Italy (OCSI) ITER Internal tin Nb 3 Sn Nb 3 Sn superconducting wires - the workhorse for ITER Intermetallic compound Nb 3 Sn Tc ~ 18 K B C2 ~ 30 T Brittle

17 23 T Magnetic Field: 8 T Applied Strain (%) Why is the effect of strain on J C important ? The critical current density (J C ) 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. Nb 3 Sn Wire

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

19 HTS coated conductors - Kilometre long single crystals Configuration of SuperPower 2G HTS Wire™

20 MgB 2 - Brittle compound Tc ~ 40 K, B C2 (//c) ~ 20 T A nodeless BCS-type gap !

21 Conductors in the USA

Applied Field (T) J E (A/mm²) YBCO Insert Tape (B|| Tape Plane) YBCO Insert Tape (B  Tape Plane) MgB 2 19Fil 24% Fill (HyperTech) 2212 OI-ST 28% Ceramic Filaments NbTi LHC Production 38%SC (4.2 K) Nb 3 Sn RRP Internal Sn (OI-ST) Nb 3 Sn High Sn Bronze Cu:Non-Cu 0.3 YBCO B|| Tape Plane YBCO B  Tape Plane 2212 RRP Nb 3 Sn Bronze Nb 3 Sn MgB 2 Nb-Ti SuperPower tape used in record breaking NHMFL insert coil MgB 2 /Nb/Cu/Monel Courtesy M. Tomsic, filament strand with Ag alloy outer sheath tested at NHMFL Maximal J E for entire LHC Nb­ Ti strand production (CERN- T. Boutboul '07) Complied from ASC'02 and ICMC'03 papers (J. Parrell OI-ST) 4543 filament High Sn Bronze-16wt.%Sn- 0.3wt%Ti (Miyazaki- MT18-IEEE’04) Conductors in the USA

23 HTS materials and exotic materials Phase diagram for the ferromagnet UGe2 A schematic of a high-Tc phase diagram

24 The Pnictide Superconductors – the iron age revisited Iron Man : In cinemas now from Paramount Pictures and Marvel Entertainment

25 The Pnictides - the original discovery Layered structure Original material: Tc 3-5 K 2006 LaOFeP

26 A big class of new materials (> 2000 compounds) Re-O-TM-Pn. Re = La+ TM = Pn

27 Comparing 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.

28 Tc of the iron-based system is quite high Tc 3-5 K 2006 LaOFeP Tc 26 K, LaOFFeAs. Jun Tc 43 K with high pressure (4 GPa) LaOFeAs. Feb Possibly the 1 st 40K-class LTS superconductor Tc 55 K NdFeAsO 1-d. April/May (Also 111 phase and 122 phase)

29 Oxygen concentration is critical for superconductivity For the NdFeAsO 1-d with different O concentration A dome-shaped superconducting bubble has been found

30 T c ~ 42K Point-contact spectroscopy Page 1224 Sweep the V I - V dI/dV - V

31 A nodeless BCS-type gap !

32 Does 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)

33 B C2 is high Larbalestier et al measured the resistance of F doped LaOFeAs at high fields up to 45 T. Nature 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/ Two-gap model is qualitatively consistent with their data.

34 High critical current in polycrystalline pnictides !

35 Structure of the Talk I)The fundamental building blocks - (G-L) Ginzburg-Landau and (B-C-S) Bardeen-Cooper- Schrieffer theories The Josephson effect Critical current and pinning (zero resistance) II)The important materials Classic LTS high field materials – NbTi and Nb 3 Sn The high temperature superconductors - The pnictides (Superconductivity and magnetism) III) Technology – MRI, LHC, ITER and beyond..

36 Applications using Superconductors MRI Body scanners LHC ITER Transport Power transmission Public outreach

37 MRI - $1B annual market

38 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.

39 Some facts about the LHC Protons are accelerated to % 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

40 ITER – Building a star on planet earth

41 Picture courtesy of the SOHO/EIT collaboration Matter becomes a plasma At 200 million ºC, We need extreme conditions …

42 ITER – A large transformer

43 The fuel for ITER is from seawater

44 16 Nb 3 Sn toroidal field coils - each coil is ~ 290 tonnes, has 1100 strands, ~ 0.8 mm diameter to form a conductor 820 m long.

45 A burning plasma

46 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

47 Applications using Superconductors Transport 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 dollars Completion in 2025 Speed ~ 500 kilometers per hour Length ~ 290 kilometers

48 Applications using Superconductors Superconducting power transmission - currently we waste ~ 20 % of our energy just transporting it around - potentially the next industrial revolution

49 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.

50 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: