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Superconductors

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Superconductivity, discovered in 1911 by Heike Kamerlingh Onnes, is a phenomenon ... Lower anisotropy unlike cuprates. Larger coherence lenghts ... – PowerPoint PPT presentation

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Title: Superconductors


1
Superconductors
  • Presented by Onur KARAGÖZ

2
Content
  • Meissner Effect
  • Quantum Effects
  • Transition to Zero Resistivity

3
Super conductivity?
  • Superconductivity, discovered in 1911 by Heike
    Kamerlingh Onnes, is a phenomenon occurring in
    certain materials at extremely low temperatures.
  • It was not yet discovered whether the resistance
    remains 0, or it is exactly 0 due to experimental
    results. So we name it super not perfect
    conductor.

4
Meissner Effect
5
Meissner Effect
http//www.superlife.info/en/book/index.html
http//www.superlife.info/en/book/index.html
The hallmark of superconductivity is expulsion of
the internal B field in an applied magnetic (H)
field, unless the applied field exceeds a
critical level.
6
London Equation
A bulk superconductor is shielded completely from
an external magnetic field by a supercurrent that
flows within the penetration depth (?) at the
surface.
melectron mass eelectron charge neconcentration
µopermeability in vacuum
7
London Equation
  • ne, the concentration is given by the following
    equation

?conductors mass density NAAvogadros
number WAAtomic weight
8
Quantum Effects
  • Cooper Pairs
  • Flux Quantization

9
Cooper Pairs
Cooper pairs can have the same energy level. The
mediator that holds the two electrons is a phonon.
10
Flux Quantization
  • Deaver and Fairbank did experiments with a tiny
    superconducting cylinder made by electroplating
    tin on a copper wire. They found magnetic flux
    quantized in units of
  • such that the flux through the cylinder was given
    by
  • Related with the Type II superconductors in the
    mediated phase that creates vortices.

11
Transition to Zero Resistivity
  • Critical
  • Temperature
  • Magnetic Field
  • Current Density
  • Superconductor Types
  • Type I (soft)
  • Type II (hard)

12
Critical Temperature
13
Critical Magnetic Field Hc
http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
scbc.htmlc2
14
Critical Current Density Jc
http//www.users.qwest.net/csconductor/Experiment
_Guide/Critical20Current20Density-1.htm
15
Tc Hc and Jc of YBCO
16
BCS Theory
  • Bardeen, Cooper, and Schrieffer
  • A key conceptual element in this theory is the
    pairing of electrons close to the Fermi level
    into Cooper pairs through interaction with the
    crystal lattice.
  • This pairing results from a slight attraction
    between the electrons related to lattice
    vibrations the coupling to the lattice is called
    a phonon interaction.

17
Bandgap
  • The BCS theory predicts a bandgap of

18
Ginzburg-Landau Theory
  • Coherence length is a measure of the shortest
    distance over which superconductivity may be
    established or destroyed without excessive cost
    in energy.
  • Penetration depth

19
Types of Superconductors
  • Type I Metals and metalloids
  • Type II Metal alloys, cuprates etc.

20
Type I
  • Always rather pure metallic elements
  • Easily quenched in magnetic fields less than
    1000 gauss
  • Must exclude virtually all of an applied magnetic
    field to remain superconducting

21
Type I
http//www.superlife.info/en/book/victor/Image9.jp
g
22
Type I
  • Nb3Sn
  • Niobium stannide, Nb3Sn, is a well-established
    superconductor of the A15 (Cr3Si) structure.

http//alpha.mems.duke.edu/aiqin/sdarticle1.pdf
A15 structure
23
Type II
  • Alloys or compounds (Niobium and vanadium are
    exceptions)
  • They are able to retaintheir superconductive
    characteristics in rather intense magnetic
    fields.
  • Rather than using the energy required to
    completely expel magnetic fields, the fields are
    confined to an internal array of normal-state
    flux tubes called"vortices" since they are
    surrounded by a circulating supercurrent.
  • They are capable of carrying relatively large
    current densities.

24
Type II
Three critical magnetic fields Hc1 Hc2 Hc3
http//www.superlife.info/en/book/victor/Image10.j
pg
25
YBCO
  • YBa2Cu3O7-x was discovered to have a Tc of 92 K
    in 1987.
  • This was the first time that superconductivity
    had been observed at temperatures which could be
    conveniently attained with liquid nitrogen, and
    so created great excitement at the prospect of
    low-cost applications of superconductivity.

26
YBCO
  • There are two CuO layers.
  • CuO2 layers are responsible for the
    superconductivity

http//cst-www.nrl.navy.mil/lattice/struk.picts/hi
ghtc/1212c.s.png
27
YBCO
  • The only known stable four-element compound with
    a Tc above 77 K.
  • Includes neither toxic elements nor volatile
    compounds
  • Easy to make single-phase YBCO
  • Less anisotropic than other HTS materials,
    carries higher current densities at higher
    magnetic fields

http//www.tkk.fi/Units/AES/projects/prlaser/ybco.
jpg
28
BSCCO (Bismuth strontium calcium copper oxide)
  • Bi2Sr2CuOx
  • BSCCO can have 1, 2, or 3 CuO planes, with Tc
    increasing with the number of planes.
  • Bismuth can also be replaced with thallium or
    mercury, which results in the highest Tc material
    known (142K).

29
MgB2
30
MgB2
  • Magnesium diboride, MgB2, was first reported to
    be superconducting in 2001.
  • It is superconducting at a temperature of 38-40 K
    and it is an intermetallic material.
  • Why MgB2?
  • Cost
  • Lower anisotropy unlike cuprates
  • Larger coherence lenghts
  • Transparency of the grain boundaries to current
    flow

31
References
  • Solid State Physics, J.R. Hook H.E.
    Hall,Wiley,2000
  • Superconductivity, J.B. Ketterson S.N. Song,
    Cambridge,1999
  • Superconductivity Fundamentals and Applications,
    W. Buckel R. Kleiner, Wiley,2004
  • http//alpha.mems.duke.edu/aiqin/sdarticle1.pdf
  • http//www.ewh.ieee.org/tc/csc/News/MgB2Feb2002.ht
    ml
  • http//www.tkk.fi/Units/AES/projects/prlaser/mater
    ial.htm
  • http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
    supcon.htmlc1
  • http//www.msm.cam.ac.uk/ascg/materials/mgb2.php
  • http//hoffman.physics.harvard.edu/research/SCmate
    rials.php
  • http//www.superconductors.org/INdex.htm
  • http//www.futurescience.com/manual/sc1000.htmlC
  • http//www.superlife.info/en/book/index.html
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