Vertex Corrections and the Korringa Ratio in Strongly Correlated   Electron Materials
  We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections for the dynamic spin susceptibility are negligible and the hyperfine coupling is momentum independent. In the absence of vertex corrections we also find a Korringa behaviour for $T_1$, the nuclear spin relaxation rate, i.e., $1/T_1\propto T$, and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system. 
Eddy Yusuf, B. J. Powell, Ross H. McKenzie