Unconventional superconductivity

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Unconventional superconductivity

• Author Jure Kokalj
• Mentor prof. dr. Peter Prelovšek

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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

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History
Onnes, 1911 SC Mercury
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Properties of SC

• Energy gap ? in excitation spectrum

• Zero resistivity

• Meissner effect

• example of SC

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BCS theory

• Attractive interaction between electrons

phonons
spin fluctuation ?

• Attraction ? pairs (bosons) ? condensation (SC)

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BCS theory

• Schrodinger equation

• Cooper pair

• Legendre polynomials and spherical harmonics

• Fourier transformation

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BCS theory

• Cooper pair

• Example of interaction

• Radial part of bounded states wave-function

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BCS theory

• Gap and order parameter

• introduction of ?k

• gap equation

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What is unconventional ?

• USC gap ?k 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
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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

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Josephson junction

• I(U) characteristic

• d.c. Josephson (s-wave)

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SQUID experiment

• Two Josephson junctions in a ring of s-wave SC

C

• s-wave and d-wave SC

large jc , high inductance
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experiments

• observations of half-integer magnetic flux quantum

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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 ?

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Thank you
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