174 research outputs found
Large gap quantum spin Hall insulator, massless Dirac fermions and bilayer graphene analogue in InAs/Ga(In)Sb heterostructures
The quantum spin Hall insulator (QSHI) state has been demonstrated in two
semiconductor systems - HgTe/CdTe quantum wells (QWs) and InAs/GaSb QW
bilayers. Unlike the HgTe/CdTe QWs, the inverted band gap in InAs/GaSb QW
bilayers does not open at the point of the Brillouin zone, preventing
the realization of massless Dirac fermions. Here, we propose a new class of
semiconductor systems based on InAs/Ga(In)Sb multilayers, hosting a QSHI state,
a graphene-like phase and a bilayer graphene analogue, depending on their layer
thicknesses and geometry. The QSHI gap in the novel structures can reach up to
60 meV for realistic design and parameters. This value is twice as high as the
thermal energy at room temperature and significantly extends the application
potential of III-V semiconductor-based topological devices.Comment: 5 pages, 4 figure
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
Tunable plasma wave resonant detection of optical beating in high electron mobility transistor
We report on tunable terahertz resonant detection of two 1.55 µm
cw-lasers beating by plasma waves in AlGaAs/InGaAs/InP high-electron-mobility
transistor. We show that the fundamental plasma resonant frequency and its odd
harmonics can be tuned with the applied gate-voltage in the range 75-490 GHz.
The observed frequency dependence on gate-bias is found to be in good agreement
with the theoretical plasma waves dispersion law.Comment: Applied Physics Letters to be published (2006) -
Pressure and temperature driven phase transitions in HgTe quantum wells
We present theoretical investigations of pressure and temperature driven
phase transitions in HgTe quantum wells grown on CdTe buffer. Using the 8-band
\textbf{kp} Hamiltonian we calculate evolution of energy band structure
at different quantum well width with hydrostatic pressure up to 20 kBar and
temperature ranging up 300 K. In particular, we show that in addition to
temperature, tuning of hydrostatic pressure allows to drive transitions between
semimetal, band insulator and topological insulator phases. Our realistic band
structure calculations reveal that the band inversion under hydrostatic
pressure and temperature may be accompanied by non-local overlapping between
conduction and valence bands. The pressure and temperature phase diagrams are
presented.Comment: 9 pages, 8 figures + Supplemental material (5 pages
Terahertz Radiation Detection by Field Effect Transistor in Magnetic Field
We report on terahertz radiation detection with InGaAs/InAlAs Field Effect
Transistors in quantizing magnetic field. The photovoltaic detection signal is
investigated at 4.2 K as a function of the gate voltage and magnetic field.
Oscillations analogous to the Shubnikov-de Haas oscillations, as well as their
strong enhancement at the cyclotron resonance, are observed. The results are
quantitatively described by a recent theory, showing that the detection is due
to rectification of the terahertz radiation by plasma waves related
nonlinearities in the gated part of the channel.Comment: 4 pages, 3 figure
Current-induced enhancement of photo-response in graphene THz radiation detectors
Thermoelectric readout in a graphene terahertz (THz) radiation detector requires a p-n junction across the graphene channel. Even without an intentional p-n junction, two latent junctions can exist in the vicinity of the electrodes/antennas through the proximity to the metal. In a symmetrical structure, these junctions are connected back-to-back and therefore counterbalance each other with regard to rectification of the ac signal. Because of the Peltier effect, a small dc current results in additional heating in one and cooling in another p-n junction, thereby breaking the symmetry. The p-n junctions then no longer cancel, resulting in a greatly enhanced rectified signal. This allows simplifying the design and controlling the sensitivity of THz radiation detectors
Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications
Resonant frequencies of the two-dimensional plasma in FETs increase with the
reduction of the channel dimensions and can reach the THz range for sub-micron
gate lengths. Nonlinear properties of the electron plasma in the transistor
channel can be used for the detection and mixing of THz frequencies. At
cryogenic temperatures resonant and gate voltage tunable detection related to
plasma waves resonances, is observed. At room temperature, when plasma
oscillations are overdamped, the FET can operate as an efficient broadband THz
detector. We present the main theoretical and experimental results on THz
detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape
Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells
We investigate experimentally transport in gated microsctructures containing
a band-inverted HgTe/Hg_{0.3}Cd_{0.7}Te quantum well. Measurements of nonlocal
resistances using many contacts prove that in the depletion regime the current
is carried by the edge channels, as expected for a two-dimensional topological
insulator. However, high and non-quantized values of channel resistances show
that the topological protection length (i.e. the distance on which the carriers
in helical edge channels propagate without backscattering) is much shorter than
the channel length, which is ~100 micrometers. The weak temperature dependence
of the resistance and the presence of temperature dependent reproducible
quasi-periodic resistance fluctuations can be qualitatively explained by the
presence of charge puddles in the well, to which the electrons from the edge
channels are tunnel-coupled.Comment: 8 pages, 4 figures, published versio
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