Wannier-Stark states in finite superlattices

Individual Wannier-Stark states are resolved in a current experiment over a wide electric-field range for a 5 and 4 period finite superlattice utilizing a hot-electron transistor. The observed field dependence of the tunneling transmission through the various states directly resembles the progressive localization of the wave functions. The basic transport through Wannier-Stark states in short-period superlattices is identified to be coherent. By tuning the Wannier-Stark state splitting with electric field into the optical phonon energy, the opening of new LO-phonon mediated transport paths is observed.     

New Wannier-Stark localization effects in natural 6H-SiC superlattice

A premature electric breakdown caused by the formation of a strong-field domain under conditions of negative differential conductivity in the 6H-SiC n⁺-n^?-n⁺ structure optimized for ultrahigh-frequency measurements was observed in the range of electric fields corresponding to the Bloch oscillation regime in a natural 6H-SiC superlattice. The experimental results and ensuing estimates indicate that this domain is mobile and, hence, oscillating, allowing the microwave oscillations that are rapidly damped under conditions of avalanche break-down in a natural 6H-SiC superlattice to be forecasted. Crystal perfectness of a natural 6H-SiC superlattice made it possible to directly observe the Wannier-Stark localization up to electric breakdown, i.e., during the natural crystal lifetime. This was accomplished by the optical photoelectric transformation method in the multiplication regime for a photocurrent created by photons with above-bandgap energy. It was shown that the Wannier-Stark localization, which involves only electrons, occurs in natural 6H-SiC superlattice up to fields that are almost equal to the breakdown field in 6H-SiC, unresponsively to band mixing, i.e., to the fundamental destroyer of the Wannier-Stark localization.     

Fano resonances in semiconductor superlattices

We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier–Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter – the coupling strength Γ between the discrete state and the degenerate continuum – by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances.     

Type-I and type-II Wannier-Stark effect in InGaAs/InGaAs superlattices

The formation of minibands is demonstrated in photocurrent experiments on shallow In_0.53Ga_0.47As/In_0.40Ga_0.60 As superlattices grown by low-pressure metal-organic vapor-phase epitaxy. Field-dependent variations of the spectral shape are attributed to Wanner-Stark localization. Both type-I transitions between electrons and heavy holes and type-II transitions involving light holes confined in the In_0.40Ga_0.60 As layers are observed and distinguished by their characteristic field dependence.     

Terahertz radiation induced by the Wannier-Stark localization of electrons in a natural silicon carbide superlattice

Intense terahertz electroluminescence from SiC structures with a miniband electron spectrum caused by the natural superlattice has been observed. The shape of the terahertz radiation line, the linear dependence of the position of its maximum on the bias voltage, the typical value of the field required to induce the radiation, and the prevailing polarization of the radiation along the superlattice axis indicate that the observed radiation results from to the excitation of stationary Bloch oscillations of electrons in the natural silicon carbide superlattice.     

Aharonov-bohm effect on multiwall carbon nanotubes under conditions close to strong carrier localization

The Aharonov-Bohm effect on multiwall carbon nanotubes has been studied under conditions of resistance with decreasing temperature as an inverse power function, which precede strong carrier localization. A periodic contribution with a period of 18 T corresponding to the magnetic flux quantum ħc/e per nanotube cross section has been revealed in the longitudinal magnetoresistance. The result points to the possibility of the ballistic motion of the carriers over the sample perimeter under conditions close to their strong localization in the longitudinal direction.     

Effect of photoelasticity enhancement stimulated by carrier localization in a quantum well near interband resonances in superlattices

An analytical expression is obtained for the resonant permittivity and linear photoelasticity coefficients in multiple quantum well structures near interband resonances. It is shown that the resonant photoelasticity in these structures is considerably higher than that in the bulk case and can exceed the photoelasticity near the resonance of a bulk exciton. It is noted that this result is associated with localization of noninteracting electrons and holes in the layer with a quantum well. Similar to the exciton in a bulk crystal, this system determines the elastooptic properties of the superlattice in the vicinity of the interband resonance.     

Relaxation under conditions of strong interaction

A method is developed for considering a nonequilibrium transition in an isolated system when the correlation time [2] and relaxation time are of the same order. The perturbation V in the Hamiltonian of (1.1) is not restricted in any way. The method is based on the assumption [4] that it is possible to use a representation in which the indeterminacy of the energy of a nonstationary system appears explicitly.