Probing Superconductors using Point Contact Andreev Reflection Pratap Raychaudhuri Tata Institute of Fundamental Research Mumbai Collaborators: Gap anisotropy in YNi 2 B 2 C G. Sheet, S. Mukhopadhyay, D. Jaiswal, S. Ramakrishnan, H. Takeya (Japan) Phys. Rev. Lett. 93, 156802 (2004). Nanostructured Nb S. Bose, P. Vasa, P. Ayyub, R. Bannerjee (Ohio)
V Scattering Centre (elementary excitation, defects): the electron loses energy K. E imparted to the electron= Mean free path Sample size Free path: the electron accelerates Lattice Ballistic Transport a<<l T » 0 • Statistically no scattering • Electrons kinetic energy=e. V • Spectroscopic Probe e V=(1/2)mv 2 e. V
Spectroscopy using Point Contact Electron phonon interaction (Au foil/ Au tip) Superconducting energy gap Normal Metal Superconductor
BCS density of states Normal Reflection Andreev reflection N(E) Fitting parameters: D-superconducting gap Z-barrier height parameter G-broadening parameter E(me. V)
Superconducting Energy Gap Fitting parameters: D-superconducting gap Z-barrier height parameter G-broadening parameter Angle resolved probe capable of probing different k directions on the Fermi surface
Superconducting gap anisotropy in YNi 2 B 2 C Unusual variation in magnetic field Angular. Vortex Symmetries evolving with Discovered in TIFR in Specific Heat temperature 1994 Thermal Conductivity Tc~14. 6 K Type II Supserconductor: k~10 -15 BCS Superconductor with conventional electron phonon coupling Izawa et al. , PRL 89, 137006 (2002) Park et al. , PRL 92, 237002 (2004)
Unusual gap function symmetry s+g (mixed angular momentum symmetry) K Maki, P Thalmeier and others Purely Geometrical with no microscopic origin
D(k) of YNi 2 B 2 C S-wave superconductivity s+g symmetry of the order parameter Multiband superconductivty? ? ?
Crystal used for this study Very low defect density
Gap anisotropy DI||c/DI||a ~ 7 at 1. 75 K
Temperature dependence
Temperature dependent s+g Yuan and Thalmeier PRB 68, 174501 (2003) Tc Two Band Superconductor Suhl et al, PRL 3, 552 (1959) No Interband scattering Weak Interband Scattering
Magnetic field dependence
Zero bias density of States
Comparison with theoretical predictions for a two band superconductor Superconducting energy gap Zero bias density of states Koshelev & Golubov, PRL 90, 177002 (2003)
Two band superconductivity in Mg. B 2 Gonnelli et al. , PRL 89, 247004 (2002).
Band Structure of YNi 2 B 2 C 3 bands crossing the FS produce 5 FS sheets Mostly fast I||a electrons: responsible Cylindrical FS for small gap Mostly slow electrons: responsible for large gap Square FS I||c Encloses only 0. 3% of the Fermi surface volume. Not important in Ellipsoidal FS PC expt.
Epilogue What is special about Mg. B 2 (or YNi 2 B 2 C)? Under what limiting condition will a multiband superconductor behave like a single band superconductor? Interband scattering The clear demonstration of multiband superconductivity in Mg. B 2 calls for a closer look at all the known superconductors.
Size effect in nanoscale superconductors d~D d Complete destruction of superconductivity Open Questions How does the superconducting properties evolve at small sizes? In Al, Sn, Tc gets enhanced by a factor of 2 before destruction of superconductivity In Pb, Nb Tc decreases monotonically Softening of the (surface) Phonon modes vs. quantum size effect? Softening of Phonon Modes increased electron phonon coupling Quantum size effect N(0) will decrease 2 D/k. BTc
Evolution of Superconducting properties in nanostructured Nb Magnetization Resistivity Mechanism of destruction of Tc
Nature of the grain boundary Weakly coupled Josephson Junction
Evolution of Energy Gap with Particle size 45 nm 10 nm 15 nm 8 nm Remains in the weak coupling limit down to the lowest size
Temperature variation of D
Summary In Nb, Tc decreases monotonically with decreasing particle size. 2 D/k. BTc remains constant down to the Anderson limit. The suppression of Tc in nanocrystalline Nb is possibly governed by quantum size effects rather than phonon softening.
YNi 2 B 2 C: Superconducting properties Angular variation in magnetic field CµH 1/2 Unusual Vortex Symmetries evolving with temperature Specific Heat Thermal Conductivity Critical fields Specific heat Tc~14. 6 K Type II Supserconductor: k~10 -15 Coherence length: BCS Superconductor with conventional electron phonon coupling Izawa et al. , PRL 89, 137006 (2002) Tuson Park et al. PRL 92, Park et al. , PRL 92, 237002(2004)
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