
Timedependent
transport in mesoscopic systems
 Quantum transport through a timedependent twolevel system , with Ned Wingreen. The time average of the fully nonlinear current through a double quantum dot, subject to an arbitrary combination of ac and dc voltages, is calculated exactly using the Keldysh nonequilibrium Green function technique. Important corrections to sequential tunneling are predicted to occur when the Rabi frequency of interdot oscillations becomes comparable to the tunneling rate of electrons into the system. For reverse bias, the system functions as an efficient electron pump when driven on resonance due to Rabi oscillations between the dots. For forward bias, the system can operate as a lowthreshold currentinjection laser. It is shown that the Rabi splitting can be resolved by examining the I  V characteristic of the electron pump, and it is predicted that the interdot Rabi oscillations can be timeresolved by applying microwave pulses of varying lengths.
 Dynamic magnetoconductance of mesoscopic wires and rings , with Dongzi Liu, Ben YuKuang Hu, and Sankar Das Sarma. Using a finitefrequency recursive Green's function technique, the dynamic magnetoconductance in disordered phasecoherent normal metal systems was calculated, incorporating interparticle Coulomb interactions within a selfconsistent potential method. We find universal fluctuations of order e^2/h in the imaginary part of the admittance when hf ~ E c is where f is the frequency and E c is the Thouless energy. The corrections to the admittance due to selfconsistent charging effects are also found to be of order e^2/h in this frequency range, indicating the breakdown of a noninteracting theory. The effect of finite frequency on the AharanovBohm oscillations is found to be dramatically different than that of finite temperature or ensemble averaging: The hc / e periodic part of the admittance is found to be only weakly suppressed ~ ( hf / E c )^1/2 at high frequencies, while the hc /2 e periodic part, which is robust with respect to thermal and ensemble averaging, is exponentially suppressed.
Publications
 D. Z. Liu, Ben YuKuang Hu, C. A. Stafford, and S. Das Sarma,
Dynamic MagnetoConductance Fluctuations and Oscillations in Mesoscopic Wires and Rings,
Phys. Rev. B 50, R5799 (1994).
 C. A. Stafford and Ned S. Wingreen,
Resonant PhotonAssisted Tunneling Through a Double Quantum Dot: An Electron Pump From Spatial Rabi Oscillations,
Phys. Rev. Lett. 76, 1916 (1996).
 Ned S. Wingreen and C. A. Stafford,
QuantumDot Cascade Laser: Proposal for an UltraLowThreshold Semiconductor Laser,
IEEE J. Quantum Electron. 33, 1170 (1997).
Patents
N. S. Wingreen and C. A. Stafford,
QuantumDot Cascade Laser,
U.S. Patent No. 5,692,003, Issued November 25, 1997.
