Remarkable display of the double Fermi level in ac response for nonequilibrium quantum dot

General formulae for calculating the ac current response to time-dependent bias for Anderson impurity model are presented basing on the nonequilibrium Green function. To calculate the lesser Green function in the Limit U --> infinity, we have combined the Keldsh contour integration technique and the method of equation of motion. With the formulae we calculate the linear re sponse to harmonic bias, and the results are very interesting and show that both the ac conductance and phase shift display the evident characteristic s of the double Fermi level at nonequilibrium state. We show that the two F ermi level can be quite accurately determined from phase-shift curve delta similar to omega, or ac conductance curve G similar to omega. (C) 2000 Else vier Science B.V. All rights reserved.