1. Unconventionalsuperconductivity Author: Jure Kokalj Mentor: prof. dr. Peter Prelovšek

2. 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

3. BCS theory, 1957 Műller, Bednorz 1986 high Tc Meissner, 1933 organic, heavy Fermion , Sr2RuO4 MgB2 Josephson, 1962 History Onnes, 1911 SC Mercury

4. Properties of SC • Energy gap Δ in excitation spectrum • Zero resistivity • Meissner effect • example of SC

5. BCS theory • Attractive interaction between electrons phonons spin fluctuation ? • Attraction → pairs (bosons) → condensation (SC)

6. BCS theory • Schrődinger equation • Cooper pair • Legendre polynomials and spherical harmonics • Fourier transformation

7. BCS theory • Cooper pair • Example of interaction • Radial part of bounded state’s wave-function

8. BCS theory • Gap and order parameter • introduction of Δk gap equation:

9. What is unconventional ? • USC: gap Δk breaks symmetry of underlying lattice or goes to zero at some parts of Fermi surface (nodes) • s-wave: • p-wave: • d-wave: cuprates original BCS superfluid 3He, ‘Sr2RuO4‘ px dx2-y2

10. 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

11. Josephson junction • I(U) characteristic • d.c. Josephson (s-wave)

12. SQUID experiment • Two Josephson junctions in a ring of s-wave SC C • s-wave and d-wave SC large jc , high inductance

13. experiments • observations of half-integer magnetic flux quantum

14. 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 ?

15. Thank you