A theory of noise characteristics of a double-barrier resonant-tunneling system is established. The noise-current power density has a Lorentzian form with a characteristic frequency given by the resonance-level broadening. The low-frequency characteristics which are dependent on structure parameters and bias voltage can be directly compared with experimental measurements.

In this Brief Report, we explicitly show the equivalence of the Feynman-Vernon influence-functional approach and the path-integral formulation of the Schwinger-Keldysh closed-time-path Green's function. The latter simplifies the practical calculations considerably with a systematic diagrammatic technique and is expected to have a broader application.

We present a molecular dynamics (MD) study of a Brownian particle in a two-dimensional periodic potential. For a separable potential, the study of the diffusion constant along the symmetry directions reduces to two one-dimensional problems. In this case, our MD study agrees with the existing analytical results on the temperature and the friction (η) dependence of the diffusion constant (D). For a nonseparable and anisotropic potential such as the adsorption potential on a bcc(110) surface, the present study predicts an alternative D∼1/${\mathrm{\eta }}^{0.5}$ dependence in the low friction regime as opposed to the D∼1/η dependence found in previous studies of one-dimensional or separable potentials. We find that the dependence of D on η in the low friction regime is directly related to the occurrence of long jumps. The probability for the long jumps depends not only sensitively on the value of the friction but also on the geometry of the surface. On the bcc(110) surface, the path connecting adjoining adsorption sites does not coincide with the direction of easy crossing at the saddle point. Consequently, the probability of deactivation is enhanced, leading to the reduction of long jumps and the different dependence of D on η. © 1996 The American Physical Society.

An analytical path-integral method based on the nonequilibrium Green’s function is developed to investigate the linear response of a double-barrier resonant-tunneling system under a small ac signal which is superimposed upon a dc bias. When the dc-bias voltage is zero, our calculated ac conductance reduces back to that of a recent Kubo-formula study. When the system is biased at the negative differential resistance point, both the real (negative) conductance and imaginary part of admittance are derived. Comparisons with previous numerical study and experiments are discussed.

In this paper we present an approach to the dynamic transport properties of a double-barrier resonant-tunneling system. Based on the nonequilibrium-Green’s-function technique and the Feynman-path-integral theory, the essential ingredients of this microstructure will be properly treated in a self-consistent way: the quantum interference across the structure, the nonequilibrium distribution of tunneling electrons driven by the applied bias voltage, and the effect of reservoirs (electrodes). The transient behavior of the tunneling current, immediately after the switching on of a dc bias voltage, is characterized by the building-up process of tunneling electrons in the quantum well. The novel negative differential conductance demonstrates itself as a function of frequency of the small ac signal superimposed upon a dc bias. The imaginary part of admittance is shown to be related to the conductance via a Kronig-Kramers relation.

The microscopic diffusional and vibrational motion of sodium atoms on a Cu(001) surface have been studied with high-resolution helium atom scattering. The data are analyzed through a Langevin dynamics simulation for the adatoms. The observed values for the temperature and parallel wave-vector dependence of the width of the quasielastic peak are well described by the theory with an appropriate choice of potential energy surface and the frictional coupling. The theoretical prediction for the position and width of the lateral frustrated translational mode using the same potential and friction are in excellent agreement with the corresponding experimental data. The simulation thus provides a complete determination of the adatom dynamics including the value of the diffusion constant.