Superconductivity and NMR

Superconductivity and NMR

1. Spin-lattice relaxation

   • The Korringa law
     • Satisfied in normal metals but a deviation is seen at the SC transiton temperature (Tc)
   • Can be used to map Vortex structure
   • Rate of relaxation can tell us information about superconducting gap symmetry
   • Hebel-Slichter coherence peak
     • Present
       • Suggests conventional BCS model
     • Absence
       • Suggests unconventional pairing mechanism

2. Magnetic Shift

   • Knight Shift (Spin part)
     • Spin susceptibility
       • Gives information on the cooper pair state
         • Spin susceptibility vanishes at T=0. Spin is Singlet (S=0)
         • Spin susceptibility persists at T=0. Spin is triplet (S=1)
       • Gives information on superconducting energy gap.
     • Measurement of the Knight shift tells us temperature dependence of spin susceptibility
   • Chemical shift (Orbital part)
     • Lets us distinguish crystal sites e.g. The plane and chain oxygen sites in YBCO

3. Practical considerations of NMR in the superconducting state

   • Solid state NMR often needs sample rotation
     • Samples require dilution with insulators to reduce eddy currents and allow rotation
   • Superconducting transition temperature
     • Crude measurement can be done by observation of a detuning of the NMR probe
   • Meissner effect
     • The static magnetic field is expelled apart from the penetration depth
       • Work with nanoparticles to ensure higher surface