Brownian motion of the electron and the Lamb shift at finite temperature
  By enhancing electron position fluctuations, equilibrium electromagnetic radiation modifies the potential for an electron in a Hydrogen atom. This can have significant effects for weakly bound states and especially at finite temperature. This implies a 2% correction to Bethe's value for 2S-2P Lamb shift for weak fluctuations, but the effect is an order of magnitude larger for strong fluctuations where it provides direct measure of the proton diameter. 
Eugene B. Kolomeisky
physics.atom-ph cond-mat.stat-mech quant-ph