High-temperature superconductivity

Transcription

1 Superconductivity and Low temperature physics, FMI036 High-temperature superconductivity Alexey Kalabukhov Quantum Device Physics Laboratory, MC2

2 Outline Lecture I (19/2): History of discovery, phenomenology and crystal structures of cuprates, normal state properties, doping, phase diagram. Lecture II (25/02): superconducting properties of cuprates, symmetry of the order parameter, theories of HTS. Superconductivity and Low temperature physics, FMI036 2

3 High-Tc vs Low-Tc superconductivity Metals (1911-): (Nb, Al, Pb, ) Tc < 10 K, BCS theory (1957): weak electron-phonon coupling Cuprates (1986-): La 2 CuO 4 30 K < Tc < 135 K, non-bcs (?). Coupling mechanism debated. No unified theory to describe high-t c. Heavy fermion, organic superconductors and oxypnictides: unconventional superconductors Superconductivity and Low temperature physics, FMI036 3

4 Why High-Tc are important? Applications: can be cooled using liquid nitrogen (77 K), 30 times cheaper than liquid helium Physics: no theoretical description of High-T C superconductivity! Road to RTS - Room temperature superconductivity? Superconductivity and Low temperature physics, FMI036 4

5 Before 1987: 0.28 K/year GL BCS From: Warren E. Pickett, UC Davis ( Superconductivity and Low temperature physics, FMI036 5

6 Superconducting paradigm Matthias rules Superconductor = metal (alloy) Cubic symmetry Electronic density must be high Magnetism destroys the SC state Physics is well understood (BCS 1957) Maximum T C calculated ~ 30 K Superconductivity and Low temperature physics, FMI036 6

7 After 1987: 100 K/year! From: Warren E. Pickett, UC Davis ( Superconductivity and Low temperature physics, FMI036 7

8 1986: Discovery of cuprates, La 2 CuO 4 T C = 30 K K. Alex Muller and J. Georg Bednorz from IBM Zurich discovered ceramic high temperature superconductors. They won the 1987 Nobel Prize for Physics. Superconductivity and Low temperature physics, FMI036 8

9 Cuprates: evolution oft C YBa 2 Cu 3 O 7 (La,Ba)CuO 4 Tc = 93 K (CuO 2 ) x3= Bi 2 Sr 2 Ca 2 Cu 3 O 10 Tc = 110 K Tc = 40 K Y. Ichikawa. et al., (1988) Phys. Rev. B Chu, C. Wet al., (1987) Phys. Rev. Lett. 58, 405. Wu, M. et al., (1987) Phys. Rev. Lett Superconductivity and Low temperature physics, FMI036 9

10 The Woodstock of Physics APS Spring meeting, March 1987 *by MICHAEL D. LEMONICK: What stirred all the excitement at that tumultuous meeting in March was a discovery that could change the world, a startling breakthrough in achieving an esoteric phenomenon long relegated to the backwaters of science: superconductivity. That discovery, most scientists believe, could lead to incredible savings in energy; trains that speed across the countryside at hundreds of miles per hour on a cushion of magnetism; practical electric cars; powerful, yet smaller computers and particle accelerators; safer reactors operating on nuclear fusion rather than fission and a host of other rewards still undreamed of. There might even be benefits for the Strategic Defense Initiative, which could draw on efficient, superconductor power sources for its space-based weapons.... Superconductivity and Low temperature physics, FMI036 10

11 Applications of HTS Power cables, motors and SC magnets JR-Maglev: 581 km/h (RTRI Japan) HTS Electronic applications: power filters, logic elements (RSFQ) HTS SQUIDs for NDE, biomedical applications (MCG, MRI) Superconductivity and Low temperature physics, FMI036 11

12 High-Tc SQUID magnetometers Thin film YBa 2 Cu 3 O 7 SQUIDs: Magnetic field Hz : 40 ft/ Hz (1 ft = T!) White noise ~ 25 ft/ Hz 1000 DC dc Bias bias reversal ac bias SQUIDs Field (ft/ö Hz) STO YBCO Frequency (Hz) Superconductivity and Low temperature physics, FMI036 12

13 High-T C SQUID MEG MagnetoEncephaloGraphy: measures the magnetic component produced by neural currents Axon Dendrite Synapse Nucleus Neuron: processes and transmits information Typical magnetic field measured from the brain ~ cm from the skull Superconductivity and Low temperature physics, FMI036 13

14 High-T C SQUID MEG Liquid nitrogen fiberglass cryostat (77 K) Allows < 0.5 mm stand-off Compared with > 2 cm in LTS systems! Eyes closed Eyes open Spontaneous brain activity, alpha-rythm: Occipital, 8-13 Hz Attenuated with eyes open Activated with eyes closed F. Öisjöen et al., Appl. Phys. Lett. 100, (2012) Superconductivity and Low temperature physics, FMI036 14

15 CuO-based ceramics: brittle, non-metallic LSCO crystal Melt textured YBCO Floating zone method for crystal growth: BSCCO 2223 single crystal Superconductivity and Low temperature physics, FMI036 15

16 New paradigm: High-T C Brittle ceramic materials Low electronic density Magnetic and insulating ground state Maximum T C ~ 130 K BCS theory is not valid for HTS (?) Probably no electron-phonon coupling, other mechanisms involved Superconductivity and Low temperature physics, FMI036 16

17 Crystal structure: perovskite (ABO 3 ) O SrTiO 3 Ti Model oxide semiconductor Band-gap insulator (Vg ~ 3.2 ev) when stoichiometric Chemical doping: oxygen vacancies SrTiO 3-x The value of seeing nothing Jochen Mannhart and Darrell G. Schlom, Science 430, p.620 (2004) Superconductivity and Low temperature physics, FMI036 17

18 SrTiO 3 : doping and T C Superconducting below 0.5 K T C scales with carrier concentration J.F. Shooley et al., Phys.Rev. Lett., 14(9) 1965 Superconductivity and Low temperature physics, FMI036 18

19 Ba(Pb,Bi)O 3 : oxide superconductors 1980 s Hole doping by smooth variation of Bi content Seems to be BSC but T C ~ 12 K (x ~ 0.25), too high for this structures Are there more materials with even higher T C? Robert J. Cava, Oxide superconductors, J.Am.Ceram.Soc., 83 [1] 5-28, 2000 Superconductivity and Low temperature physics, FMI036 19

20 Single crystals

21 Cuprates: crystal structure Superconductivity and Low temperature physics, FMI036 21

22 Crystal structure of cuprates Superconductivity and Low temperature physics, FMI036 22

23 Crystal symmetry and superconductivity Tetragonal Non-SC Adding oxygen results in decrease of c-axis parameter Crystal phase transition is correlated with onset of superconductivity Orthorhombic SC Tetragonal phase (i.e. low oxygen concentration): semiconducting, underdoped Orthorhombic phase (high oxygen concentration): superconducting, optimally doping Increasing oxygen concentration Superconductivity and Low temperature physics, FMI036 23

24 How doping works: ionic substitutions Parent compound: La 2 Cu 1 O 4 insulator La 2-x Sr x Cu 1 O 4 (214): T C ~ 40 K Sr 2+ <->La 3+ : one hole in CuO 2 plane Robert J. Cava, Oxide superconductors, J.Am.Ceram.Soc., 83 [1] 5-28, 2000 Superconductivity and Low temperature physics, FMI036 24

25 How doping works: oxygen vacancies Parent compound: defect perovskite, insulating: Y 3+ Ba 2+ 2Cu 2+ 3O 2-66 Electrical doping: holes by increasing oxygen in CuO 2 planes Superconductivity and Low temperature physics, FMI036 25

26 T C scales with number of CuO 2 -planes Superconductivity and Low temperature physics, FMI036 26

27 Number of CuO 2 -planes and T C Quantum tunneling between the layers Charge imbalance and competing order Superconductivity destroyed at n > 3 Superconductivity and Low temperature physics, FMI036 27

28 Anisotropy of normal properties R c R c R ab R ab Superconductivity and Low temperature physics, FMI036 28

29 Anisotropy: Intrinsic Josephson effect A. Yurgens et al., Appl. Phys. Lett. 70, 1760 (1997) Superconductivity and Low temperature physics, FMI036 29

30 Building blocks of cuprates CuO 2 planes: conductivity and superconductivity Insulating layers (CuO, AO): doping (charge reservoirs) Superconductivity and Low temperature physics, FMI036 30

31 Normal state properties: resistivity Optimally doped: linear R(T) Underdoped: non-linear, semiconducting at low doping levels Overdoped: R(T) ~ R 0 +αt+ βt 2 Superconductivity and Low temperature physics, FMI036 31

32 Anomalous Hall effect Hall coefficient varies with T and doping level Does not agree with simple Drude picture Can Fermi liquid model be applied to cuprates? Superconductivity and Low temperature physics, FMI036 32

33 Mott-Hubbard insulator t : overlapping integral U : Hubbard potential H ˆ t ci c j U n n i, i, Q E 2 e Electrostatic 4 r 0 2 2m r e 2 repulsion Zero kinetic energy Orbital overlapping Electrostatic repulsion At large distances, electrostatic repulsion becomes dominant low doping limit, Hubbard-Mott insulator Superconductivity and Low temperature physics, FMI036 33

34 Low doping: AFM insulator Anti-ferromagnetic arrangement of spins Destroyed by doping! From: Robert J. Cava, Oxide superconductors, J.Am.Ceram.Soc., , 2000 Superconductivity and Low temperature physics, FMI036 34

35 Electronic phase diagram Superconductivity exists only in narrow doping range. Non-doped material is AF insulator ( parent compund ). At high doping level cuprates are normal metals. Pseudogap region in low doping levels Superconductivity and Low temperature physics, FMI036 35

36 Fermi Liquid problem in HTS Fermi liquid: Fermi gas with interactions, dressed excitations (quasi-particles, QP) QP are defined only close to p F : p-p F <<p F Non-Fermi liquid: deviations from FL assumptions (spherical symmetry, no defined QP) Difficult to separate from complex scattering mechanisms within FL model Superconductivity and Low temperature physics, FMI036 36

37 Fermi surface: ARPES Angle resolved photoemission spectroscopy Probes directly electronic structure (energy and momentum) Superconductivity and Low temperature physics, FMI036 37

38 Fermi surface: ARPES ARPES proves 4-fold symmetry in overdoped cuprates, anisotropic! Superconductivity and Low temperature physics, FMI036 38

39 Observation of quantum oscillations Shubnikov-de Haas resistivity oscillations Quantization of energy bands in magnetic fields (Landau levels) When Landau level is near Fermi energy, scattering increases -> resistivity oscillations Direct measure of the FS Superconductivity and Low temperature physics, FMI036 39

40 Summary I: normal state, cuprates Cuprate superconductors are brittle ceramic materials, poor metals and very anisotropic. Difficult to get pure phases, low reproducibility of data. Problem of normal state: does not look like a normal metal, very complilated scattering mechanims, 2D/3D crossover Recent ARPES and QHE observations: Fermi liquid with unusual scattering Superconductivity and Low temperature physics, FMI036 40