Transcript Diapozitiv 1

Unconventional
superconductivity
Author: Jure Kokalj
Mentor: prof. dr. Peter Prelovšek
Introduction
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History and present status of SC (superconductivity)
Properties of SC
BCS theory
- attractive interaction
- Cooper pair
- gap and pairing symmetry
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What is unconventional
Experiments for probing pairing symmetry
- Josephson junction
- SQUID experiments
History
BCS theory, 1957
Meissner,
1933
Műller, Bednorz
1986 high Tc
organic, heavy
Fermion , Sr2RuO4
MgB2
Onnes, 1911
SC Mercury
Josephson,
1962
Properties of SC
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Zero resistivity
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Meissner effect
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Energy gap Δ in
excitation spectrum
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example of SC
BCS theory
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Attractive interaction between electrons
phonons
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spin fluctuation ?
Attraction → pairs (bosons) → condensation (SC)
BCS theory
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Cooper pair
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Fourier transformation
Schrődinger equation
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Legendre polynomials and
spherical harmonics
BCS theory
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Cooper pair
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Example of interaction
Radial part of bounded state’s wave-function
BCS theory
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Gap and order parameter
introduction of Δk
gap equation:
What is unconventional ?
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USC: gap Δk breaks symmetry of underlying lattice or
goes to zero at some parts of Fermi surface (nodes)
s-wave:
original BCS
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p-wave:
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superfluid 3He, ‘Sr2RuO4 ‘
px
d-wave:
cuprates
dx2-y2
Experimental evidence of gap symmetry
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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
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I(U) characteristic
d.c. Josephson (s-wave)
SQUID experiment
Two Josephson junctions in a ring of s-wave SC
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C
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s-wave and d-wave SC
large jc , high inductance
experiments
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observations of half-integer magnetic flux quantum
Conclusions
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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