unconventional superconductivity author: jure kokalj mentor: prof. dr. peter prelovšek
TRANSCRIPT
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Unconventional superconductivity
Author: Jure KokaljMentor: 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, 1911SC Mercury
Meissner, 1933
BCS theory, 1957
Műller, Bednorz 1986 high Tc
Josephson, 1962
organic, heavy Fermion , Sr2RuO4
MgB2
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Properties of SC
Zero resistivity Energy gap Δ in excitation spectrum
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
Cooper pair
Fourier transformation
Schrődinger equation
Legendre polynomials and spherical harmonics
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BCS theory
Cooper pair Example of interaction
Radial part of bounded state’s wave-function
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BCS theory
Gap and order parameter
introduction of Δk gap equation:
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What is unconventional ?
USC: gap Δk breaks symmetry of underlying lattice or goes to zero at some parts of Fermi surface (nodes)
s-wave:original BCS
p-wave:superfluid 3He, ‘Sr2RuO4
‘
d-wave:cuprates
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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