Electronic spin resonance of a weakly interacting Bose gas

Study of the physical properties of Bose condensed gases has recently become a very active topic due to the experimental realizations of Bose-Einstein condensation in magnetically trapped gases of alkali atoms. In this paper we theoretically study the microwave electronic spin resonance of a weakly interacting Bose gas. For a Bose gas above the critical temperature for Bose-Einstein condensation, the microwave radiation induced by the driving microwave field in the electronic spin resonance exhibits the ordinary Rabi oscillation damped by the inhomogeneous Doppler dephasing. For a Bose gas with a condensate, the induced radiation is composed of two components. One is the strong coherent Rabi oscillation from the condensate. The other is a modulated Rabi oscillation due to the noncondensed part of the gas. We show that the modulation of Rabi oscillation of the noncondensed gas is directly related to the elementary collective excitation of atoms in the gas.