High temperature superconductivity

Transcription

1 High temperature superconductivity Applications to the maglev industry Elsa Abreu April 30, 2009

2 Outline Historical overview of superconductivity Copper oxide high temperature superconductors Angle Resolved Photoemission Spectroscopy (ARPES) studies on cuprates Iron pnictide high temperature superconductors Recent applications in the maglev industry Concluding remarks

3 Superconductivity s first steps 1911: Onnes and Holst make the first observation in mercury (T c = 4.2K) 1935: London equations => microscopic description of the electromagnetic fields 1950: Ginzburg and Landau theory => phenomenological description of the macroscopic properties 1957: BCS theory => complete treatment (Cooper pairs, phonon mediated interaction)

4 and the increase in T c 1986: Müller and Bednorz measure T c = 30K in a La-Ba-Cu-O ceramic May 2006: T c = 150K in InSnBa 4 Tm 4 Cu 6 O 18+ => patented March 2009: T c = 233K in Tl 5 Ba 4 Ca 2 Cu 10 O x => claimed Why do we care? => ideally up to room temperature

5 Who are the cuprates? Main element: CuO 2 plane p-orbital coupling in the planes; very weak coupling between the planes Temperature lowering => rotation of the CuO 6 octahedron A. Damascelli et al., Rev. Mod. Phys. 75, (2003) La 2-x Sr x CuO 4

6 Who are the cuprates? Ceramic materials => insulators Doping => bad metals or superconductors Three main families: LSCO (e.g. La 2-x Sr x CuO 4 ), YBCO (e.g.yba 2 Cu 3 O 6+y ), BSCCO (e.g. Ba 2 Sr 2 CaCu 2 O 8+y ) Described as Mott insulator (e - -e - repulsion >> hoping energy), which become conductors upon doping. A. Damascelli et al., Rev. Mod. Phys. 75, (2003)

7 Cuprates: phase diagram A. Damascelli et al., Rev. Mod. Phys. 75, (2003) e - - doped h - doped

8 Suggested theories Anderson s view (1987) quasiparticle pairs in a spin singlet configuration, caused by spin fluctuations in a 2D system Resonating valence bond (RVB) motion carriers provided by doping move in the structure => Similar to BCS theory Stripes, due to spin-charge separation in the system No fully explanatory theory so far

9 ARPES Angle resolved photoemission spectroscopy Main technique used to study high temperature superconductors Gives information about the binding energy and the momentum of electrons in the crystal

10 Main result in cuprates A. Damascelli et al., Rev. Mod. Phys. 75, (2003) d-wave symmetry of the superconducting gap!

11 So? Conventional superconductivity has an isotropic gap; high temperature superconductivity has an anisotropic gap is there a BCS like theory for high temperature superconductors? Not all responses (spin fluctuations, charge transport) exhibit a d-wave symmetry is this evidence for spin-charge separation?

12 Iron arsenide compounds January 2008: High temperature superconductivity in LaO 1-x F x FeAs (T c = 26K) Main differences w.r.t. cuprates: Spin-density-wave metal vs. Mott insulator parent Isotropic vs. anisotropic behavior (3D d-orbital coupling vs. 2D p-orbital coupling) => great news for applications! Is the same theory capable of explaining superconductivity in cuprates and iron pnictides? Y. Kamihara et al., J. Am. Chem. Soc. 130, (2008)

13 MagLev history 1979: first maglev train licensed to passenger transport in Hamburg 3 techniques: electromagnetic suspension (EMS), electrodynamic suspension (EDS), stabilized permanent magnet suspension (SPM) 2003, Japan: 581km/h sped record in an EDS based system Conventional magnets are replaced by superconducting electromagnets to increase the levitation gap. Transrapid 05 Yamanashi MagLev Test Line

14 HTS in maglev systems 2002, China: first man loading HTS based maglev vehicle High temperature superconductors in the vehicle cause it to levitate in the field created by permanent magnets in the tracks. 3.5m long, 1.2m wide, 0.8m high 20mm levitation gap J. Wang et al., Physica C 386, (2003)

15 Conclusion The properties of high temperature superconductors, namely cuprates, have been the subject of intensive experimental study (ARPES); Some theories have emerged to explain them but none is as yet complete enough; 2008 marks the discovery of iron pnictides HTS; Many applications exist for HTS (e.g. MagLev transportation) due to the significant cost reduction associated with the substitution of liquid helium by liquid nitrogen in cooling devices.

16 Thank you for your attention! Elsa Abreu April 30, 2009