N. V. Demarina

Realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3 heterostructures.

Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p–n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future.

Features of electron gas in InAs nanowires imposed by interplay between nanowire geometry, doping and surface states

We present a study of electron gas properties in InAs nanowires determined by interaction between nanowire geometry, doping and surface states. The electron gas density and space distribution are calculated via self-consistent solution of coupled Schroedinger and Poisson equations in the nanowires with a hexagonal cross-section. We show that the density of surface states and the nanowire width define the spatial distribution of the electrons. Three configurations can be distinguished, namely the electrons are localized in the center of the wire, or they are arranged in a uniform tubular distribution, or finally in a tubular distribution with additional electron accumulation at the corners of the nanowire. The latter one is dominating for most experimentally obtained nanowires. N-type doping partly suppresses electron accumulation at the nanowire corners. The electron density calculated for both, various nanowire widths and different positions of the Fermi level at the nanowire surface, is compared with the experimental data for intrinsic InAs nanowires. Suitable agreement is obtained by assuming a Fermi level pinning at 60 to 100 meV above the conduction band edge, leading to a tubular electron distribution with accumulation along the corners of the nanowire.

Examining the terahertz signal from a photoexcited biased semiconductor superlattice for evidence of gain

We present a careful analysis of the coherent terahertz emission from an undoped biased semiconductor superlattice excited via an ultrashort optical pulse. We use both a semiclassical model and a fully quantum-mechanical model that includes the excitation process and excitonic effects to analyze emission. We conclude that, in contrast to what has recently been claimed, it is not possible to deduce whether there is terahertz gain from the analysis of the emitted coherent terahertz pulse.

Time-resolved photocurrent spectroscopy of optically excited superlattices and the prospects for Bloch gain

We report on experiment and theory of the evolution of the electric field in undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. We performed time-resolved pump-probe experiments and measured the photocurrent generated by spectrally narrowed and wavelength-tuned probe pulses as a function of delay time, pump power and bias field. The drift of the charge carriers, subsequent to the optical excitation, leads to the buildup of an inhomogeneity of the electric field which was traced via the temporal changes of the Wannier-Stark spectra. Although the photocurrent spectra by themselves only yield information on the absorption integrated spatially over the superlattice, we extract information on the local electric fields and the charge-carrier densities by a comparison of the measured data with the results of Monte-Carlo simulations. We find that at moderate excitation densities (1016-cm-3 range) the superlattice within a few picoseconds splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. The largest field differences are found just when the last electrons are swept out after 10-30 ps, the exact time depending on the superlattice parameters and excitation conditions. The initial homogeneous field is restored on a much longer time scale of hundreds of picoseconds which is defined basically by the drift of the heavy holes. Our calculations show that Bloch gain in optically excited semiconductor superlattice is expected in spite of the inhomogeneous field if the field in the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep-out of the electrons.

Electron ensemble coherence and terahertz radiation amplification in a cascade superlattice structure

We report on an ensemble Monte Carlo study of the electron dynamics in a biased GaAs/AlGaAs cascade superlattice structure with domains of a strong electric field. We simulate the electron transport in a single superlattice, serving as an active region of the structure, and verify Kroemers criterion which specifies over which distance in a superlattice a stable domain forms. We show that at room temperature in a superlattice with electrons subject to scattering at optical and acoustic phonons, the deviation of the superlattice length critical for domain development from that which is determined by Esaki-Tsu formula, varies from 10% to 300%. In a superlattice with the length larger than the critical one due to the electric field non-homogeneity, an electron ensemble becomes incoherent and its interaction with a terahertz field causes only the damping of the latter.

Anisotropic phase coherence in GaAs/InAs core/shell nanowires

Low-temperature transport in nanowires is accompanied by phase-coherent effects, which are observed as modulation of the conductance in an external magnetic field. In the GaAs/InAs core/shell nanowires investigated here, these are h/e flux periodic oscillations in a magnetic field aligned parallel to the nanowire axis and aperiodic universal conductance fluctuations in a field aligned perpendicularly to the nanowire axis. Both electron interference effects are used to analyse the phase coherence of the system. Temperature-dependent measurements are carried out, in order to derive the phase coherence lengths in the cross-sectional plane as well as along the nanowire sidewalls. It is found that these values show a strong anisotropy, which can be explained by the crystal structure of the GaAs/InAs core/shell nanowire. For nanowires with a radius as low as 45 nm, flux periodic oscillations were observed up to a temperature of 55 K.