Linear Response Conductance Research Articles

Simulations of electron transport through three-dimensional quantum structures

In this work, we report on numerical simulations of electron transport through three-dimensional (3D) quantum structures in both linear and nonlinear response regimes of transport. The structures considered include 3D quantum point contacts, quantum dot structures, quantum dot superlattices, and resonant tunneling structures. The simulations are done using scattering matrix method. For 3D quantum point contacts, the linear response conductance shows quantizations in units of 2e2/h, but conductance steps can be in multiples of 2e2/h. The resonant tunneling peaks are found in the conductance spectra for both the quantum dot structures and resonant tunneling structures. The linear response conductance of 3D quantum dot superlattices shows multiple-fold splittings, reflecting the formation of energy minibands in the systems. By examining the dependence of peak positions on the distance between barriers, the correspondence between resonant tunneling peaks and resonant states is determined. When a small but finite source-drain voltage is applied, the nonlinear conductance is found to be asymmetric for an asymmetric 3D quantum point contact.

Kondo resonance, Coulomb blockade, and Andreev transport through a quantum dot

We study resonant tunneling through an interacting quantum dot coupled to normal metallic and superconducting leads. We show that large Coulomb interaction gives rise to novel effects in Andreev transport. Adopting an exact relation for the Green's function, we find that at zero temperature, the linear response conductance is enhanced due to Kondo-Andreev resonance in the Kondo limit, while it is suppressed in the empty site limit. In the Coulomb blockaded region, on the other hand, the conductance is reduced more than the corresponding conductance with normal leads because large charging energy suppresses Andreev reflection.

Nonequilibrium Green's Function Approach to Resonant Magnetotunneling Through a Quantum Dot**The project supported by National Natural Science Foundation of China

Keldysh's nonequilibrium Green's function is applied to studying resonant magnetotunneling through a quantum dot. We propose a microscopic Anderson-impurity-like Hamiltonian in which two kinds of the on-site Coulomb interactions are introduced: the interaction of two electrons at the same Landau level with different spins, , and the interaction for the electrons between different Landau levels, . curves are obtained under different magnetic fields in asymmetrical structures which can display both the energy quantization and single-electron charging effect. The theoretical results are in good agreement with the experiments qualitatively. The linear-response conductance is also discussed.

Even-odd conductance oscillation in atomic wires

The linear response conductance is computed in a one-dimensional atomic wire between two adiabatically expanding metal electrodes. We show that the conductance behavior is significantly different from that of a mesoscopic narrow channel. Specifically, we find that the conductance of such an atomic wire oscillates periodically between quantum unit of conductance G Q for odd number of atoms in the wire and rG Q ( r < 1) for even number of atoms. This even-odd oscillation is very robust and should be observable at room temperature.

AC transport with reservoirs of finite width

The linear response conductance coefficients are calculated in the scattering approach at finite frequency, damping and magnetic field for a microstructure in which the reservoirs are modeled as quantum wire leads of infinite length but finite width. Independent of frequency, inelastic scattering causes subbands with large group velocity to contribute more strongly to the conductance than channels of comparable transmission but slower propagation. At finite frequency and magnetic fields, additional correction terms appear. © 1997 Elsevier Science Ltd. All rights reserved.

Multiple-channel resonant tunneling in a tunneling junction with an impurity cluster.

We study the conductance of a tunneling junction with a cluster of impurities by using the Hubbard Hamiltonian. Exact diagonalization is used to calculate the eigenstates of the small clusters containing up to four impurities and the linear-response conductance is then calculated as a function of the Fermi energy. We have found that a certain cluster of impurities possesses a specific peak pattern in the conductance data; that is, the conductance resonance peaks depend on the size and structure of clusters and the broadening of the resonance is related to the temperature. At low temperature, the conductance peaks form two distinct groups separated by the intraimpurity Coulomb repulsion U, which is symmetric distribution. At higher temperature, the multiple-peak features are smeared out by thermal broadening and only ``two bands'' separated by U remain visible. These indicate that in a small cluster of impurities the conductance peaks do not show the periodic structure appearing in the Coulomb islands, but show the shell structure. \textcopyright{} 1996 The American Physical Society.

Resonant tunneling through quantum-dot arrays.

We apply the Hubbard Hamiltonian to describe quantum-dot arrays weakly coupled to two contacts. Exact diagonalization is used to calculate the eigenstates of the arrays containing up to six dots and the linear-response conductance is then calculated as a function of the Fermi energy. In the atomic limit the conductance peaks form two distinct groups separated by the intradot Coulomb repulsion, while in the band limit the peaks occur in pairs. The crossover is studied. A finite interdot repulsion is found to cause interesting rearrangements in the conductance spectrum.

Conductance spectroscopy in coupled quantum dots.

We investigate the linear-response conductance through a pair of coupled quantum dots. The conductance spectrum under ideal conditions is shown to consist of two sets of twin peaks whose locations and amplitudes are determined by the interdot coupling and the intradot charging. We will show that the qualitative features of the spectrum survive against experimental nonidealities such as (1) detuning of the individual dots, (2) interdot charging, (3) inelastic scattering, and (4) multiple lateral states. The effect of higher lateral states depends strongly on the nature of the interaction potential, screening lengths, and exchange terms, but the lowest set of twin peaks is largely unaffected by these details.

Anderson model out of equilibrium: Noncrossing-approximation approach to transport through a quantum dot.

The infinite-U Anderson model is applied to transport through a quantum dot. The current and density of states are obtained via the noncrossing approximation for two spin-degenerate levels weakly coupled to two leads. At low temperatures, the Kondo peak in the equilibrium density of states strongly enhances the linear-response conductance. Application of a finite voltage bias reduces the conductance and splits the peak in the density of states. The split peaks, one at each chemical potential, are suppressed in amplitude by a finite dissipative lifetime. We estimate this lifetime perturbatively as the time to transfer an electron from the higher-chemical-potential lead to the lower-chemical-potential one. At zero magnetic field, the clearest signatures of the Kondo effect in transport through a quantum dot are the broadening, shift, and enhancement of the linear-response conductance peaks at low temperatures, and a peak in the nonlinear differential conductance around zero bias.

Electron-molecular vibration coupling in charge-transfer crystalline complexes

Analysis of the effect of the electron-molecular vibration (EMV) coupling on the conductivity spectra of charge-transfer crystalline complexes is presented. Discussion is limited to the case of n-merized compounds, for which the localized electronic states should be treated explicitly. Theoretical conductivity spectra based on the linear response and vibronic adiabatic Mulliken theories are compared with the experimental data. An alternative derivation of the linear response conductivity expression is presented. In addition, a simple method of calculation of the EMV coupling constants is proposed.