Spin 1/2 systems perturbed by fluctuating, arbitrary fields; relaxation   and frequency shifts, a new approach to Redfield theory
  The usual approach to considerations of apin relaxation and frequency shifts due to fluctuating fields is through the density matrix Slichter. Here we treat the problem of the influence of fluctuating fields on a spin 1/2 system based on direct solution of the Schroedinger equation in contrast to the usual treatment. Our results are seen to be in agreement with the known results in the literature McGregor, Slichter, Red2, CSH, as they must, but our derivation directly from the Schroedinger equation allows us to see the role of the necessary assumptions in a somewhat clearer way. 
Robert Golub and Albert Steyerl
physics.atom-ph