Terahertz plasmons in doped HgTe quantum well heterostructures: dispersion, losses, and amplification.

We have calculated two-dimensional plasmon energy spectra in HgTe/CdHgTe quantum wells with normal, gapless, and inverted energy spectra with different electron concentrations, taking into account spatial dispersion of electron polarizability and plasmon interaction with the optical phonons. The spectra of the absorption coefficients of two-dimensional plasmons are found. It is shown that an increase of electron concentration in a quantum well leads to a decrease in the plasmon absorption coefficient. We have calculated the probabilities to recombine via the plasmon emission for nonequilibrium holes. The threshold concentrations of the nonequilibrium holes, above which the plasmon amplification is possible, have been calculated for various electron concentrations. It is shown that the presence of equilibrium electrons can significantly reduce the threshold hole concentration required for amplification of plasmon in the terahertz wavelength region. The dependencies of threshold hole concentration on electron concentration for different quantum wells are discussed. Gain spectra of the two-dimension plasmon are calculated.

Temperature-Induced Topological Phase Transition in HgTe Quantum Wells.

We report a direct observation of temperature-induced topological phase transition between the trivial and topological insulator states in an HgTe quantum well. By using a gated Hall bar device, we measure and represent Landau levels in fan charts at different temperatures, and we follow the temperature evolution of a peculiar pair of "zero-mode" Landau levels, which split from the edge of electronlike and holelike subbands. Their crossing at a critical magnetic field B_{c} is a characteristic of inverted band structure in the quantum well. By measuring the temperature dependence of B_{c}, we directly extract the critical temperature T_{c} at which the bulk band gap vanishes and the topological phase transition occurs. Above this critical temperature, the opening of a trivial gap is clearly observed.

Magneto-Optical Signature of Massless Kane Electrons in Cd_{3}As_{2}.

We report on optical reflectivity experiments performed on Cd_{3}As_{2} over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

Nanometer size field effect transistors for terahertz detectors.

Nanometer size field effect transistors can operate as efficient resonant or broadband terahertz detectors, mixers, phase shifters and frequency multipliers at frequencies far beyond their fundamental cut-off frequency. This work is an overview of some recent results concerning the application of nanometer scale field effect transistors for the detection of terahertz radiation.

Room temperature detection of sub-terahertz radiation in double-grating-gate transistors.

Room temperature photovoltaic non-resonant detection by large area double-grating-gate InGaP/InGaAs/GaAs heterostructures was investigated in sub-THz range (0.24 THz). Semi-quantitative estimation of the characteristic detection length combined with self-consistent calculations of the electric fields excited in the structure by incoming terahertz radiation allowed us to interpret quantitatively the results and conclude that this detection takes place mainly in the regions of strong oscillating electric field excited in depleted portions of the channel.

Landau level spectroscopy of valence bands in HgTe quantum wells: effects of symmetry lowering.

The Landau level spectroscopy technique has been used to explore the electronic structure of the valence band in a series of p-type HgTe/HgCdTe quantum wells with both normal and inverted ordering of bands. We find that the standard axial-symmetric 4-band Kane model, which is nowadays widely applied in physics of HgTe-based topological materials, does not fully account for the complex magneto-optical response observed in our experiments-notably, for the unexpected avoided crossings of excitations and for the appearance of transitions that are electric-dipole forbidden within this model. Nevertheless, reasonable agreement with experiments is achieved when the standard model is expanded to include effects of bulk and interface inversion asymmetries. These remove the axial symmetry, and among other, profoundly modify the shape of valence bands.

Plasma wave oscillations in nanometer field effect transistors for terahertz detection and emission.

The channel of a field effect transistor can act as a resonator for plasma waves propagating in a two-dimensional electron gas. The plasma frequency increases with reduction of the channel length and can reach the terahertz (THz) range for nanometer size transistors. Recent experimental results show these transistors can be potential candidates for a new class of THz detectors and emitters. This work gives an overview of our recent relevant experimental results. We also outline unresolved problems and questions concerning THz detection and emission by nanometer transistors.

Radiative recombination in narrow gap HgTe/CdHgTe quantum well heterostructures for laser applications.

Radiative recombination is studied in CdHgTe/HgTe QWs with bandgap in the 40-140 meV range using four-band Kane model. Calculated radiative lifetimes agree well with the photoconductivity kinetics measurements. We show that the side maxima in the valence band hinder the radiative recombination at high carrier concentrations and discuss how to overcome this effect for the development of long-wavelength lasers.

Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond.

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the "single-valley" analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this "bilayer graphene" phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.

Temperature-driven massless Kane fermions in HgCdTe crystals.

It has recently been shown that electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in condensed matter systems. These single valley relativistic states, massless Kane fermions, cannot be described by any other relativistic particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying cadmium content or temperature. At critical concentration or temperature, the bandgap collapses as the system undergoes a semimetal-to-semiconductor topological phase transition between the inverted and normal alignments. Here, using far-infrared magneto-spectroscopy we explore the continuous evolution of band structure of bulk HgCdTe as temperature is tuned across the topological phase transition. We demonstrate that the rest mass of Kane fermions changes sign at critical temperature, whereas their velocity remains constant. The velocity universal value of (1.07±0.05) × 10(6) m s(-1) remains valid in a broad range of temperatures and Cd concentrations, indicating a striking universality of the pseudo-relativistic description of the Kane fermions in HgCdTe.