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Superconducting Cobaltites

Superconducting Cobaltites. Nick Vence. Definition. A material which looses its electrical resistivity below a certain temperature (Tc)is said to be superconducting. History. K. Lead Tc=7K Aluminum Tc=1K Uranium Tc=0.2K Tungsten Tc=0.02K Carbon Tc=0K SWCNT Tc=15K MWCNT Tc=12K.

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Superconducting Cobaltites

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  1. Superconducting Cobaltites • Nick Vence

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

  3. History K Lead Tc=7K Aluminum Tc=1K Uranium Tc=0.2K Tungsten Tc=0.02K Carbon Tc=0K SWCNT Tc=15K MWCNT Tc=12K

  4. 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

  5. Basic Theory • The Meisner Effect • Breaking the Meisner Effect • Not are SC are broken equally

  6. Type II Superconductors L (Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 Tc=138K Record holder w/o pressure InSnBa4Tm4Cu6O18+ Tc=150K Patent Pending Sept. 2005 MgB2 (Used in industry) Tc = 39 Jc = 10^6 A/cm^2 (Critical current density) Na0.35 CoO2 1.3H2O Tc = 5K

  7. Where we are • The Highest Temperature SC are Cuperates • Since a theory isn’t established for type II superconductors, we need more experimental help • “Help” comes in the form materials which have novel features to compare and contrast • Let’s understand the cuperates first

  8. 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

  9. Cobaltites • Nax CoO2 yH2O x=0.35 y=1.3 (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

  10. Dimensionality ofNaxCoO2 yH2O H • y=0 doesn’t exhibit SC • y=0.6 doexn’t exhibit SC • The distance between CoO2 planes doubles from y=0 to y=1.3

  11. Spin lattice relaxation rates J • suggest 2D antiferromagnetic ordering • resembles SC cuperates • suggests non s-wave SC

  12. Low temperature magnetic states G • Spin waves • Spin density waves • Disordered/frustrated antiferromagnetism

  13. Difficulties • Sample preparation is difficult • Sample is sensitive to both temperature and humidity at “ambient conditions”

  14. 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

  15. More Rabbits O • How might are p-wave and d-wave effect SC? • Why doesn’t 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 O L

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