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.
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.
We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in a temperature range from 4.2 up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau ...levels, which split from the bottom conduction and upper valence sub-bands and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such a phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.
We have performed a detailed investigation of the photoluminescence features taken at 2
K on a series of Ga
x
In
1−
x
N alloys grown by metal-organic vapour-phase epitaxy through the whole ...composition range. The evolution of the photoluminescence lineshape of GaInN alloys in the indium-rich region is dominated by doping effects rather than by band-gap tailing effects correlated to existence of random chemical crystal inhomogeneities. The lineshape of the photoluminescence indicates a residual electron concentration of about 10
18–10
19
cm
−3 in the bulk part of the epilayers. The value we get for the bowing parameter is
b=2.8
eV.
Due to their specific physical properties, HgCdTe-based heterostructures are expected to play an important role in terahertz photonic systems. Here, focusing on gated devices presenting inverted band ...ordering, we evidence an enhancement of the terahertz photoconductive response close to the charge neutrality point and at the magnetic field driven topological phase transition. We also show the ability of these heterostructures to be used as terahertz imagers. Regarding terahertz emitters, we present results on stimulated emission of HgCdTe heterostructures in their conventional semiconductor state above 30 THz, discussing the physical mechanisms involved and promising routes towards the 5–15 THz frequency domain.
Quantum spin Hall insulators (QSHIs) based on HgTe and three-layer InAs/GaSb quantum wells (QWs) have comparable bulk band gaps of about 10-18 meV. The former, however, features a band gap vanishing ...with temperature, while the gap in InAs/GaSb QSHIs is rather temperature independent. Here, we report on the realization of a large inverted band gap in strained three-layer InAs/GaInSb QWs. By temperature-dependent magnetotransport measurements of gated Hall bar devices, we extract a gap as high as 45 meV. By combining local and nonlocal measurements, we detect edge conductivity at temperatures up to 40 K, possibly of topological origin, with equilibrium lengths of a few micrometers. Our results pave the way for the manipulation of topological edge states at high temperatures in QW heterostructures.
We report on the temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness dc. Our results, obtained in magnetic fields up to 16 T and s ...temperature range from 2 to 150 K, clearly indicate a change in the band-gap energy with temperature. A quantum well wider than dc evidences a temperature-driven transition from topological insulator to semiconductor phases. At a critical temperature of 90 K, the merging of inter- and intraband transitions in weak magnetic fields clearly specifies the formation of a gapless state, revealing the appearance of single-valley massless Dirac fermions with a velocity of 5.6×105ms−1. For both quantum wells, the energies extracted from the experimental data are in good agreement with calculations on the basis of the eight-band Kane Hamiltonian with temperature-dependent parameters.
PDF
