Nanowires with helical surface states represent key prerequisites for observing and exploiting phase-coherent topological conductance phenomena, such as spin-momentum locked quantum transport or ...topological superconductivity. We demonstrate in a joint experimental and theoretical study that gated nanowires fabricated from high-mobility strained HgTe, known as a bulk topological insulator, indeed preserve the topological nature of the surface states, that moreover extend phase-coherently across the entire wire geometry. The phase-coherence lengths are enhanced up to 5μm when tuning the wires into the bulk gap, so as to single out topological transport. The nanowires exhibit distinct conductance oscillations, both as a function of the flux due to an axial magnetic field and of a gate voltage. The observed h/e-periodic Aharonov-Bohm-type modulations indicate surface-mediated quasiballistic transport. Furthermore, an in-depth analysis of the scaling of the observed gate-dependent conductance oscillations reveals the topological nature of these surface states. To this end we combined numerical tight-binding calculations of the quantum magnetoconductance with simulations of the electrostatics, accounting for the gate-induced inhomogeneous charge carrier densities around the wires. We find that helical transport prevails even for strongly inhomogeneous gating and is governed by flux-sensitive high-angular momentum surface states that extend around the entire wire circumference.
We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction ...of the photocurrent reverses by switching the radiation polarization from a right-handed to a left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a ≈3μm distance at CNP approaches 2e2/h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with experimental data.
We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear ...energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency
ω
and momentum relaxation time
τ
l
larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to
exp
(
−
E
0
2
/
E
2
)
, with the radiation electric field amplitude
E
and the characteristic field parameter
E
0
. As observed in experiment, it exhibits a strong frequency dependence for
ω
τ
≫ 1 characterized by the characteristic field
E
0
linearly increasing with the radiation frequency
ω
.
Abstract
In this paper, we show that electron states formed in topological insulators at the interfaces topological phase–trivial phase and topological phase–vacuum may possess different properties. ...This is demonstrated on an example of heterostructures based on thick topological Hg
1−
x
Cd
x
Te films, in which the
PT
-symmetric terahertz photoconductivity is observed. It is shown that the effect originates from features of the interface topological film–trivial buffer/cap layer. The
PT
-symmetric terahertz photoconductivity is not provided by electron states formed at the interface topological film–vacuum.
Results of the Hall-effect studies of surface properties of n–type HgCdTe films modified with arsenic ion implantation and thermal annealing are reported on. A complete annihilation of ...implantation-induced extended defects (dislocation loops), quasi-point defects and related donor centers was observed as a result of a two-stage arsenic activation annealing. A high degree of activation of implanted arsenic was achieved with the annealing. In some cases, the annealing was found to lead to the modification of the properties of the ‘base’ layers not affected by implantation due to activation of uncontrolled acceptor defects and resulting changes in the degree of electrical compensation.
•Modification of the surface of HgCdTe films with arsenic implantation and annealing was studied.•Annealing was shown to annihilate structural defects induced by implantation.•High degree of activation of arsenic as a result of post-implantation annealing was achieved.•Activation of residual acceptor impurities by post-implantation annealing was discovered.
Variations in the surface chlorophyll
a
concentration (Chl-a) measured during 25 cruises conducted from 1984 to 2020 were investigated in the Barents Sea and adjacent Svalbard waters. Seasonal maps ...of the mean annual Chl-a estimations were created for the Barents Sea. Significant seasonal and temporal variations of Chl-a were found in different water masses. The surface Chl-a content reached a maximum in the spring (46% of the annual estimation). In the summer, it tended to decrease accounting for 28% of the total annual value, while in the autumn and winter, Chl-a was found to be 21 and 5% of the total annual stock, respectively. Atlantic Water and Arctic Water had the highest contribution to the total annual Chl-a biomass. In Arctic Water, peaks of the Chl-a concentrations were registered in spring and these decreased in summer and autumn. The summer season was the most productive period in Atlantic Water. The Chl-a dynamics and distribution in relation to climatic factors were investigated. Application of generalized linear and non-linear models to predict Chl-a variations showed the great significance of the indices associated with the global atmospheric circulation, mean annual temperature, and salinity anomalies, as well as the extent of ice in the Barents Sea.
An atomically clean and structurally ordered surface of the CdTe layer of a (013)-oriented GaAs/ZnTe/CdTe substrate after storage in air is obtained by treatment in isopropyl alcohol saturated with ...hydrochloric-acid vapors, with subsequent thermal annealing in ultrahigh vacuum. It is shown that chemical treatment of the CdTe surface results in the removal of native oxides and in enrichment of the surface with an elemental Te layer. During heating in vacuum, two stages of change in the state of the surface (at ~125 and ≤250°C) are observed. At the temperature
T
> 250°C, elemental tellurium is desorbed, and a Te-stabilized (1 × 1) CdTe(013) structure is formed.
Metal–insulator–semiconductor (MIS) structures based on HgCdTe were fabricated after various stages of pn junction formation using As+ implantation and activation annealing. The energy of As+ ions ...was 200 keV with the fluence of 1014 cm−2. Heteroepitaxial HgCdTe films with near-surface graded-gap layers were grown by molecular beam epitaxy (MBE) on silicon substrates. It was shown that the electron concentration in the near-surface semiconductor layer increases after implantation to values of about 1017 cm−3, and after implantation and annealing in the near-surface semiconductor layer, a p+ layer appears with a hole concentration of more than 1.5 × 1018 cm−3. The generation rate of minority charge carriers in the space charge region after implantation is low, which indicates a low defectiveness of the thin near-surface MBE HgCdTe layer. After implantation and annealing, the generation rate increases significantly, which may be due to high defectiveness near the interface between Al2O3 and MBE HgCdTe. Dopant profiles were constructed in the near-surface HgCdTe layers after various technological procedures. It was shown that after implantation in films, the dopant concentration increases with distance from the interface to the depth of 0.1 μm.
•Surface HgCdTe properties were modified by As implantation and thermal annealing.•Metal–insulator–semiconductor structures were created and their admittance studied.•Concentration profiles in HgCdTe after various procedures were determined.•After As implantation, a low-defect layer appears near the interface HgCdTe-Al2O3.•Activation annealing leads to a decrease in the density of slow states.
A magnetic field parallel to the layers of a double quantum well with conventional semiconductor constituents leads to a relative shift of the conduction band spectra of the constituent layers along ...the wave vector perpendicular to the field. If the states of the layers are tunnel-coupled, a tunneling gap is formed at the intersection of the single-layer spectra and is shifted upward with increasing field. This leads to striking features in the magnetoresistance caused by intersections of the Fermi level with the edges of the tunneling gap. Similar studies of transformations of the spectrum of the double quantum well in a HgTe/CdHgTe heterosystem, which has a
p
-type conductivity and HgTe layers with a gapless inverse energy spectrum, are reported in this work. Our experiments and corresponding calculations in the eight-band
kp
approach indicate that the evolution of the magnetoresistance with the variation of the in-plane field here has a much more complex and diverse character depending qualitatively on the thickness of the layers.
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