Superconducting Cobaltites

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Superconducting Cobaltites

• Nick Vence

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Definition

• A material which looses its electrical
  resistivity below a certain temperature (Tc)is
  said to be superconducting

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History
K
Lead Tc7K Aluminum Tc1K Uranium Tc0.2K Tung
sten Tc0.02K Carbon Tc0K SWCNT Tc15K MWCNT
Tc12K
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Magnetism

• Ferromagnetism is the strongest because of the
  aligned dipoles
• Antiferromagnetism has antiparallel spin ordering
  and has no net magnetic field
• Paramagnetism is exhibited when a magnetic field
  aligns all the orbitals in a sample causing it to
  be magnetic. These materials have a positive
  magnetic susceptibility
• Diamagnetism happens when a material cancels an
  applied magnetic field through surface currents.
  It is said to have a negative magnetic
  susceptibility

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Basic Theory

• The Meisner Effect
• Breaking the Meisner Effect
• Not are SC are broken equally

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Type II Superconductors
L
(Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 Tc138K Record
holder w/o pressure InSnBa4Tm4Cu6O18 Tc150
K Patent Pending Sept. 2005 MgB2 (Used in
industry) Tc 39 Jc 106 A/cm2 (Critical
current density) Na0.35 CoO2 1.3H2O Tc 5K
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Where we are

• The Highest Temperature SC are Cuperates
• Since a theory isnt established for type II
  superconductors, we need more experimental help
• Help comes in the form materials which have
  novel features to compare and contrast
• Lets understand the cuperates first

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Cuperates

• Copper Oxide makes a 2D square lattice
• Which has anisotropic resistivity
• YBa2Cu3O7 (like many cuperates) has an odd number
  of electrons per unit cell--thus doping is
  necessary
• Doping induces an antiferromagnetic spin ordering
  on the square planar lattice
• This causes Cu to loose its magnetic moment which
  hinders Cooper Pairs propogation

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Cobaltites

• Nax CoO2 yH2O x0.35 y1.3 (NCO) also has
  antiferromagetic ordering
• Cobaltites are the only other type II SC compound
  to share the metal oxide planar structure of the
  cuperates
• Has a triangular metal oxide lattice which can
  lead to a spin frustrated system
• Which has anisotropic resistivity with
  extra-planar resistivity 1000 x planar
  resistivity

H
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Dimensionality ofNaxCoO2 yH2O
H

• y0 doesnt exhibit SC
• y0.6 doexnt exhibit SC
• The distance between CoO2 planes doubles from y0
  to y1.3

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Spin lattice relaxation rates
J

• suggest 2D antiferromagnetic ordering
• resembles SC cuperates
• suggests non s-wave SC

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Low temperature magnetic states
G

• Spin waves
• Spin density waves
• Disordered/frustrated antiferromagnetism

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Difficulties

• Sample preparation is difficult
• Sample is sensitive to both temperature and
  humidity at ambient conditions

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Rabbits to chase

• Spin density waves in NCO
• The spin density wave transition temperature vs
  doping fraction graph exhibits the same dome
  shaped behavior as Tc vs doping concentration
• The interplanar coupling mechanism is critical
  our understanding
• Spin-glass regions

G
F
Q
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More Rabbits

• How might are p-wave and d-wave effect SC?
• Why doesnt BCS theory work for type II SC? or
  does it?
• How might a low T magnetic state (or spin system)
  effect SC?
• What role do vortex currents play in type II SC?
• Possible Resonating Valance Bond candidate

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