We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) ...radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (-0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov-de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity nu = 7.2 x 10 super(5) m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.
The magnetic field and temperature dependences of longitudinal magnetoresistance and the Hall effect have been measured in order to determine the energy spectrum of the valence band in HgTe quantum ...wells with the width
d
QW
= 20–200 nm. The comparison of hole densities determined from the period of Shubnikov–de Haas oscillations and the Hall effect shows that states at the top of the valence band are doubly degenerate in the entire
d
QW
range, and the cyclotron mass
determined from the temperature dependence of the amplitude of Shubnikov–de Haas oscillation increases monotonically from
to
(
is the mass of the free electron) with increasing hole density
from
to
cm
–2
. The determined dependence has been compared to theoretical dependences
calculated within the four-band
k
P
model. These calculations predict an approximate stepwise increase in
owing to the pairwise merging of side extrema with increasing hole density, which should be observed at
and 4 × 10
10
cm
–2
for
d
QW
= 20 and 200 nm, respectively. The experimental dependences are strongly inconsistent with this prediction. It has been shown that the inclusion of additional factors (electric field in the quantum well, strain) does not remove the contradiction between the experiment and theory. Consequently, it is doubtful that the mentioned
k
P
calculations adequately describe the valence band at all
d
QW
values.
Abstract
We report on observation of strong non-local photoconducitivity induced by terahertz laser pulses in non-zero magnetic field in heterostructures based on Hg
1−
x
Cd
x
Te films being in the ...topological phase. While the zero-field non-local photoconductivity is negligible, it is strongly enhanced in magnetic fields ~ 0.05 T resulting in appearance of an edge photocurrent that exceeds the respective dark signal by orders of magnitude. This photocurrent is chiral, and the chirality changes every time the magnetic field or the electric bias is reversed. Appearance of the non-local terahertz photoconductivity is attributed to features of the interface between the topological film and the trivial buffer.
The transport response of a CdHgTe quantum well with a thickness of 11.5 nm is investigated. The behavior of the local and nonlocal resistance in the temperature range from 0.1 to 20 K is examined. ...It is shown that the system under study is a two-dimensional topological insulator. In comparison with traditional two-dimensional topological insulators implemented in 8-nm-thick HgTe quantum wells, the investigated one is characterized by a significantly smaller energy gap and, at the same time, a higher carrier mobility. The data are analyzed using computer simulations taking into account the actual geometry of the sample, as well as scattering between edge and bulk carrier states. It is shown that the backscattering probability of topological electrons within the edge states is nearly independent of temperature. In contrast, the probability of scattering from the edge channels into the bulk depends exponentially on the temperature, and fitting this dependence with a standard activation formula is the most accurate way to determine the mobility gap in the system under study. Even at the highest temperature, the probability of scattering between the counter-propagating states of the same edge exceeds the probability of scattering into the bulk by an order of magnitude. Therefore, this mechanism is dominant and determines the mean free path of edge electrons.
Abstract
We show that the terahertz (THz) photoconductivity in the topological phase of Hg
1–
x
Cd
x
Te-based structures exhibits the apparent
PT
- (parity-time) symmetry whereas the
P
-symmetry and ...the
T
-symmetry, separately, are not conserved. Moreover, it is demonstrated that the
P
- and
T
-symmetry breaking may not be related to any type of the sample anisotropy. This result contradicts the apparent symmetry arguments and means that there exists an external factor that interacts with the sample electronic system and breaks the symmetry. We show that deviations from the ideal experimental geometry may not be such a factor.
Spectral studies of the photoconductivity in the temperature range of
T
= 5–70 K, as well as studies of the magneto-absorption and magnetotransport at
T
= 4.2 K, have been performed in a HgTe/CdHgTe ...heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.
Two waveguide heterostructures with an array of 10 HgCdTe/CdHgTe quantum wells grown within a single technological series and designed to generate stimulated emission near 20 and 30 μm are studied. ...At a temperature of 10 K, stimulated emission in the “short-wavelength” structure is obtained at a wavelength of ∼23.9 μm, while in the “long-wavelength” structure stimulated emission is not observed. Calculations of optical absorption in the passive layers for both structures are carried out and it is demonstrated that its level is higher in the long-wavelength structure. Approaches are proposed to minimize its effect on the generation of stimulated emission.
We present experimental results of non-linear transport in HgTe-based 2D topological insulators, where the conductance is dominated by Dirac-like helical edge states when the Fermi level is pinned to ...the bulk insulating gap. We find that hot carrier's energy relaxation is faster close to the charge neutrality point (CNP) which can be attributed to localized nature and incompressibility of charge puddles resulting from inhomogeneous charge distribution near CNP. The tunnel-coupling of these puddles (quantum dots) to 1D edge channels can randomize phase memory leading to incoherent inelastic processes. Hot edge carriers, excited by the electric field, relax to equilibrium via thermalization in multiple puddles resulting in the emission of phonons in the puddles. At relatively low temperature (T ≤ 10 K), the energy relaxation time shows strong temperature dependence (τε ∝ (Te−5)), which is interpreted as small angle scattering, consistent with resistance saturation at low temperatures.
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•Electric field induced non-equilibrium phenomena in 2D topological insulators•Energy relaxation of hot carriers is possible only due to inelastic processes since the 1D edge channels are helical in nature.•Tunnel-coupling of 1D channels to charge puddles resulting from charge inhomogenity•Fast energy relaxation in multiple puddles close to CNP•Strong temperature dependence at relatively low temperatures
A clinical observation is presented of a patient with pulmonary sarcoidosis, who was diagnosed with kidney damage after three years with the development of chronic renal failure and the need for ...replacement therapy. A histological examination of the renal biopsy revealed a granulomatous process in the interstitial tissue, which was regarded as an extrapulmonary manifestation of sarcoidosis. Pulse therapy with glucocorticoids was prescribed and an attempt was made to reduce hemodialysis sessions.
The magnetic susceptibility of two-dimensional systems in the limit of low fields can hardly be detected by magnetometric measurements because it is difficult to separate the contributions from the ...substrate and two-dimensional gas for the magnetization not oscillating with the field. The derivative of the magnetization with respect to the carrier density in two-dimensional systems in narrow (thickness <7 nm) HgTe quantum wells has been measured in this work by means of the modulation of the chemical potential by a magnetic field perpendicular to the system plane. It was previously established that the spectrum of the valence band of such quantum wells contains not only easy Dirac states in the center of the Brillouin zone but also heavy hole valleys with maxima shifted from the center in the 331 direction. It has been found that the magnetic susceptibility drops sharply as the Fermi level leaves these heavy valleys with the addition of electrons to the system. This behavior can be interpreted either as the weakening of paramagnetism or as the enhancement of diamagnetism. The estimates obtained show that the observed effect is due primarily to the paramagnetism of states in the heavy valleys of the valence band.
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