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Unconventional Superconductivity: Theory, Experiments, and Symmetry Analysis

Explore the history, properties, and BCS theory of unconventional superconductivity, including experiments on pairing symmetry like Josephson junctions and SQUID testing. The discussion delves into gap equations, nodal gaps, and experimental evidence of gap symmetries. Conclude with the importance of pairing symmetry in understanding unconventional superconductivity and questions about room-temperature superconductors.

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Unconventional Superconductivity: Theory, Experiments, and Symmetry Analysis

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  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

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