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

The unconventional temperature variation of the Knight shift (static spin susceptibility) that has been observed in Fe-based superconductors $A{\mathrm{Fe}}₂{\mathrm{As}}₂$ ($A=\text{K}$, 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.