Temperature dependence of NMR Knight shift in pnictides: proximity to a van Hove singularity

TL;DR

This paper explains the unusual temperature dependence of the Knight shift in Fe-based superconductors by modeling the effects of a van Hove singularity near the Fermi level, highlighting the role of electronic correlations.

Contribution

It demonstrates how proximity to a van Hove singularity influences spin susceptibility and incorporates vertex corrections to show enhanced correlation effects.

Findings

Downturn in spin susceptibility near van Hove singularity

Vertex corrections amplify the Knight shift drop

Correlation effects shift characteristic temperature $T^*$

Abstract

The unconventional temperature variation of the Knight shift (static spin susceptibility) that has been observed in Fe-based superconductors AFe$_2$As$_2$ (A = K, Rb, Cs) is explained in terms of proximity to a van Hove singularity. Using the Hubbard model we show that when the Fermi energy is in the vicinity of a van Hove singularity, a downturn in spin susceptibility occurs as the temperature is lowered. This behavior is characterized by a temperature, $T^*$, which is determined by the difference in energy between the Fermi level and the van Hove singularity. When vertex corrections are taken into account in a dynamical mean-field approximation, the effect of correlations amplifies the relative drop in the Knight shift and moves $T^*$ to lower temperatures.