Effect of electron-optical phonon interaction on resonant tunneling in coupled quantum wells

The spatial distribution of the wave functions for electrons in a coupled-quantum-well system of GaAs/Al _x Ga_1?x As with triple barriers is discussed. Within the framework of the dielectric continuum model, the dispersion relations of interface optical phonon modes are given. Furthermore, the interaction between an electron and optical phonons and the ternary mixed crystal effect in these structures are investigated in detail. The optical phonon-assisted tunneling (PAT) is studied using the Fermi golden rule to obtain numerically the PAT currents. The results reveal that the interface optical phonons are more important than the confined longitudinal optical phonons. Only one PAT peak does appear when the middle barrier is wide enough or its Al component is high enough, and the peak moves to the higher applied voltage direction, whereas two PAT peaks do appear when the middle barrier is narrow enough or its Al component is low enough.     

High frequency properties of resonant tunneling diode

The small signal analysis for the resonant tunneling diode (RTD) is carried out by using a semiclassical transport theory. Multiple scattering effects are accounted for in an optical approximation by using a complex mean free path. An analytical expression for the conduction current is given. The results show that the negative differential conductance prevails up to the frequency f₀ limited by the quantum well transit time. The imaginary part of the admittance can be presented by a series inductance as has been recently found experimentally. In addition, the equivalent circuit has a capacitor in parallel with the conductance-inductance branch. Above f₀ the admittance shows an oscillatory behaviour. The oscillations are associated with the quantum well transit time resonances.     

Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy

Using low-temperature scanning tunneling spectroscopy applied to the Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe electron-electron interaction effects in the quantum Hall regime. The 2DES is decoupled from bulk states and exhibits spreading resistance within the insulating quantum Hall phases. In quantitative agreement with calculations we find an exchange enhancement of the spin splitting. Moreover, we observe that both the spatially averaged as well as the local density of states feature a characteristic Coulomb gap at the Fermi level. These results show that electron-electron interaction can be probed down to a resolution below all relevant length scales.     

Macroscopic resonant tunneling in the presence of low frequency noise

We develop a theory of macroscopic resonant tunneling of flux in a double-well potential in the presence of realistic flux noise with a significant low-frequency component. The rate of incoherent flux tunneling between the wells exhibits resonant peaks, the shape and position of which reflect qualitative features of the noise, and can thus serve as a diagnostic tool for studying the low-frequency flux noise in SQUID qubits. We show, in particular, that the noise-induced renormalization of the first resonant peak provides direct information on the temperature of the noise source and the strength of its quantum component.     

Effect of electron-electron interaction on thermoelectric power in impure metals

A new kinetic phenomenon related to the effect of electron-electron scattering on the thermoelectric coefficient η a conductor with a small electron mean free path l is considered. The effect is proportional to the electron-hole asymmetry factor (ε _Fτ)⁻¹ and the real part of the diffusion-enhanced Coulomb propagator with characteristic wave vectors of up to l ⁻¹. Unlike weak localization effects, in the two-dimensional case this effect results in a logarithmic temperature dependence of η and yields the major contribution to the differential thermoelectric power. Zh. éksp. Teor. Fiz. 111, 1738–1747 (May 1997)     

On resonant tunneling

Localized electron states in oxides adjacent to metals hybridize with conduction electron states forming interface states, which at the localized site have an amplitude resonantly enhanced over the amplitude of the conduction electron states. The interface states mediate a continuous transition between the metal and the semiconducting or insulating oxide. Resonant tunneling via these interface states to an opposing metal surface can dominate over direct and intermediate-state tunneling. Resonant tunneling is obstructed by the correlation (Coulomb) energy which causes voltage, temperature and time dependencies. The obstruction increases with distance of the localized state from the metal and this increased obstruction causes the transition from resonant to intermediate-state tunneling. This corresponds to a space-wise metal-insulator transition. In oxides, like Nb₂O₅, the correlation energy is small and the hybridization is strong and thus resonant tunneling through localized states at the Fermi energy can account for various tunnel anomalies observed in the normal or superconducting state.     

Inelastic resonant tunneling

A general expression for the resonant contribution to a tunneling current has been obtained and analyzed in the tunneling Hamiltonian approximation. Two types of resonant tunneling structures are considered: structures with a random impurity distribution and double-barrier structures, where the resonant level results from size quantization. The effect of temperature on the current-voltage curves of tunneling structures is discussed. The study of the effect of potential barrier profile on the d ² I/dV ² line shape is of interest for experiments in inelastic tunneling spectroscopy. Various experimental situations where the inelastic component of the tunneling current can become comparable to the elastic one are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1151–1155 (June 1998)     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

Superconductivity-induced macroscopic resonant tunneling

We show analytically and by numerical simulations that the conductance through pi-biased chaotic Josephson junctions is enhanced by several orders of magnitude in the short-wavelength regime. We identify the mechanism behind this effect as macroscopic resonant tunneling through a macroscopic number of low-energy quasidegenerate Andreev levels.     

Experimental analysis of resonant tunneling transit time using high-frequency characteristics of resonant tunneling transistors

We systematically estimate the resonant tunneling transit time from the high-frequency characteristics of resonant tunneling transistors. It is found that the transit time across an InGaAs/InAlAs resonant tunneling structure is more than one order of magnitude shorter than that for GaAs/AlAs with the same barrier layer thickness. In addition, the obtained times agree reasonably well with calculated phase time. It is probably the elastic resonance width and not the inelastic scattering effect that mainly determines the resonant tunneling transit time.     

Density of states effect on resonant tunneling

We study the effect of a gaussian type of density of states distribution on the conductance of a thin amorphous junction in the presence of localized states. The absolute value of the conductance is strongly affected by the width of the distribution but is sensitive to the center of the distribution only in the case of narrow distributions. All these effects disappear once we normalize the conductance to its zero field value.     

Theory of resonant tunneling through quantum dots in the presence of electron-phonon interaction

Structures where the electrons of a two-dimensional electron gas are confined to disconnected regions can be fabricated by the use of appropriate gate geometries. The transport between these electrostatically defined quantum dots takes place by tunneling. Using the tunneling Hamiltonian approach we present a theoretical model of the system including electron-phonon interaction. The relevant coupling constants are determined from realistic wave functions for the expected confinement potentials. The phonon part of the Hamiltonian is diagonalized using a canonical transformation. Starting from the determination of the transmission matrix for the interacting system we calculate the current-voltage characteristics for different temperatures and phonon coupling strengths.