Superconducting Cobaltites

Nick Vence

Definition

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

HistoryLead Tc=7K

Aluminum Tc=1K

Uranium Tc=0.2K

TungstenTc=0.02K

Carbon Tc=0K

SWCNT Tc=15K

MWCNT Tc=12K

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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

Basic Theory•The Meisner Effect

•Breaking the Meisner Effect

•Not are SC are broken equally

Type II Superconductors

(Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 Tc=138KRecord holder w/o pressure

InSnBa4Tm4Cu6O18+ Tc=150KPatent 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

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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

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

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

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Dimensionality ofNaxCoO2 yH2O

•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

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Spin lattice relaxation rates

•suggest 2D antiferromagnetic ordering

•resembles SC cuperates

•suggests non s-wave SC

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Low temperature magnetic states

•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”

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

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Q

More Rabbits•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

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