Time-dependent transport in mesoscopic systems

• Quantum transport through a time-dependent two-level 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 low-threshold current-injection 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 time-resolved by applying microwave pulses of varying lengths.
  
  
• Dynamic magnetoconductance of mesoscopic wires and rings , with Dongzi Liu, Ben Yu-Kuang Hu, and Sankar Das Sarma. Using a finite-frequency recursive Green's function technique, the dynamic magnetoconductance in disordered phase-coherent normal metal systems was calculated, incorporating interparticle Coulomb interactions within a self-consistent 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 self-consistent 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 Aharanov-Bohm 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

1. D. Z. Liu, Ben Yu-Kuang Hu, C. A. Stafford, and S. Das Sarma,
   Dynamic Magneto-Conductance Fluctuations and Oscillations in Mesoscopic Wires and Rings,
   Phys. Rev. B 50, R5799 (1994).
   
   
2. C. A. Stafford and Ned S. Wingreen,
   Resonant Photon-Assisted Tunneling Through a Double Quantum Dot: An Electron Pump From Spatial Rabi Oscillations,
   Phys. Rev. Lett. 76, 1916 (1996).
   
   
3. Ned S. Wingreen and C. A. Stafford,
   Quantum-Dot Cascade Laser: Proposal for an Ultra-Low-Threshold Semiconductor Laser,
   IEEE J. Quantum Electron. 33, 1170 (1997).
   
   

   Patents

   N. S. Wingreen and C. A. Stafford,
   Quantum-Dot Cascade Laser,
   U.S. Patent No. 5,692,003, Issued November 25, 1997.