Determination of symmetry of a superconducting order parameter in nuclear magnetic resonance experiments

This is the authors' abstract. We don't add key points for this paper.

Superconductivity is a state of matter characterised by zero electrical resistance and perfect diamagnetism. This state is established when Cooper pairs condense below the critical temperature Tc. The microscopic properties of the superconducting state are encoded in the rotational symmetry of the superconducting order parameter, Δk. Nuclear magnetic resonance (NMR) experiments are a powerful tool for directly determining the momentum dependence of Δk. Both, the shift of the NMR spectrum and the spin-lattice relaxation rate, 1/T1, depend on the evolution of the density of states around the Fermi energy, thereby enabling a direct evaluation of Δk(T). In cases with s−wave symmetry, 1/T1 is initially enhanced just below Tc before adopting a thermally activated temperature dependence. The effects of magnetic fields, gap anisotropy, phonon damping, and electron correlations on the partial suppression of the so-called coherence peak are discussed. Finally, it is shown that in non-s−wave superconductors, the coherence peak is completely suppressed, and 1/T1 instead follows a power-law temperature dependence. NMR is thus one of the few experimental methods through which Δk can be directly extracted hence providing critical tests for the interactions responsible for Cooper pairing.