Superconductors

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Superconductors

• Presented by Onur KARAGÖZ

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Content

• Meissner Effect
• Quantum Effects
• Transition to Zero Resistivity

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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.

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Meissner Effect
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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.
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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
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London Equation

• ne, the concentration is given by the following
  equation

?conductors mass density NAAvogadros
number WAAtomic weight
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Quantum Effects

• Cooper Pairs
• Flux Quantization

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Cooper Pairs
Cooper pairs can have the same energy level. The
mediator that holds the two electrons is a phonon.
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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.

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Transition to Zero Resistivity

• Critical
• Temperature
• Magnetic Field
• Current Density
• Superconductor Types
• Type I (soft)
• Type II (hard)

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Critical Temperature
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Critical Magnetic Field Hc
http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
scbc.htmlc2
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Critical Current Density Jc
http//www.users.qwest.net/csconductor/Experiment
_Guide/Critical20Current20Density-1.htm
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Tc Hc and Jc of YBCO
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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.

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Bandgap

• The BCS theory predicts a bandgap of

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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

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Types of Superconductors

• Type I Metals and metalloids
• Type II Metal alloys, cuprates etc.

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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

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Type I
http//www.superlife.info/en/book/victor/Image9.jp
g
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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
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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.

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Type II
Three critical magnetic fields Hc1 Hc2 Hc3
http//www.superlife.info/en/book/victor/Image10.j
pg
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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.

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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
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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
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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).

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MgB2
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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

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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