Unconventional superconductivity

Author: Jure KokaljMentor: prof. dr. Peter Prelovšek

Introduction

History and present status of SC (superconductivity)

Properties of SC BCS theory - attractive interaction

- Cooper pair - gap and pairing symmetry What is unconventional Experiments for probing pairing symmetry - Josephson junction

- SQUID experiments

History

Onnes, 1911SC Mercury

Meissner, 1933

BCS theory, 1957

Műller, Bednorz 1986 high Tc

Josephson, 1962

organic, heavy Fermion , Sr2RuO4

MgB2

Properties of SC

Zero resistivity Energy gap Δ in excitation spectrum

Meissner effect

example of SC

BCS theory

Attractive interaction between electrons

phonons spin fluctuation ?

Attraction → pairs (bosons) → condensation (SC)

BCS theory

Cooper pair

Fourier transformation

Schrődinger equation

Legendre polynomials and spherical harmonics

BCS theory

Cooper pair Example of interaction

Radial part of bounded state’s wave-function

BCS theory

Gap and order parameter

introduction of Δk gap equation:

What is unconventional ?

USC: gap Δk breaks symmetry of underlying lattice or goes to zero at some parts of Fermi surface (nodes)

s-wave:original BCS

p-wave:superfluid 3He, ‘Sr2RuO4

‘

d-wave:cuprates

px dx2-y2

Experimental evidence of gap symmetry

Density of states is different for gaps with nodes: influence on specific heat, density of states probing, …

singlet or triplet pairing is tested with NMR

direct probe: Josephson junction

Josephson junction

I(U) characteristic

d.c. Josephson (s-wave)

SQUID experiment

Two Josephson junctions in a ring of s-wave SC

C

s-wave and d-wave SC

large jc , high inductance

experiments

observations of half-integer magnetic flux quantum

Conclusions

no widely excepted theory for unconventional superconductivity

pairing symmetry important for understanding of USC

same pairing for all high temperature SC ? room-temperature SC ?

Thank you