Unconventional superconductivity Author: Jure Kokalj Mentor: 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, 1911 SC Mercury Meissner, 1933 BCS theory, 1957 Műller, Bednorz 1986 high T c Josephson, 1962 organic, heavy Fermion, Sr 2 RuO 4 MgB 2

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 Δ k gap equation:

What is unconventional ? USC: gap Δ k breaks symmetry of underlying lattice or goes to zero at some parts of Fermi surface (nodes) s-wave: original BCS p-wave: superfluid 3 He, ‘Sr 2 RuO 4 ‘ d-wave: cuprates pxpx d x 2 -y 2

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 j c, 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