615 research outputs found
Cavity-mediated coupling of antiferromagnetic spin waves
Coupling of space-separated oscillators is interesting for quantum and
communication technologies. In this work, it is shown that two
antiferromagnetic oscillators placed inside an electromagnetic cavity couple
cooperatively to its terahertz modes and, in effect, hybridized
magnon-polariton modes are formed. This is supported by a systematic study of
reflection spectra from two parallel-plane slabs of hematite
(-FeO), measured as a function of their temperatures and
separation distance, and modeled theoretically. The mediating cavity was formed
by the crystals themselves and the experiment was performed in a practical
distance range of a few millimetres and above room temperature. Cavity-mediated
coupling allows for engineering of complex resonators controlled by their
geometry and by sharing properties of their components
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
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
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
Weak antilocalization in a 2D electron gas with the chiral splitting of the spectrum
Motivated by the recent observation of the metal-insulator transition in
Si-MOSFETs we consider the quantum interference correction to the conductivity
in the presence of the Rashba spin splitting. For a small splitting, a
crossover from the localizing to antilocalizing regime is obtained. The
symplectic correction is revealed in the limit of a large separation between
the chiral branches. The relevance of the chiral splitting for the 2D electron
gas in Si-MOSFETs is discussed.Comment: 7 pages, REVTeX. Mistake corrected; in the limit of a large chiral
splitting the correction to the conductivity does not vanish but approaches
the symplectic valu
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
Plasmons and Coulomb drag in Dirac/Schroedinger hybrid electron systems
We show that the plasmon spectrum of an ordinary two-dimensional electron gas
(2DEG) hosted in a GaAs heterostructure is significantly modified when a
graphene sheet is placed on the surface of the semiconductor in close proximity
to the 2DEG. Long-range Coulomb interactions between massive electrons and
massless Dirac fermions lead to a new set of optical and acoustic intra-subband
plasmons. Here we compute the dispersion of these coupled modes within the
Random Phase Approximation, providing analytical expressions in the
long-wavelength limit that shed light on their dependence on the Dirac velocity
and Dirac-fermion density. We also evaluate the resistivity in a Coulomb-drag
transport setup. These Dirac/Schroedinger hybrid electron systems are
experimentally feasible and open new research opportunities for fundamental
studies of electron-electron interaction effects in two spatial dimensions.Comment: 7 pages, 4 figure
Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors
We report on the observation of a radiation helicity sensitive photocurrent
excited by terahertz (THz) radiation in dual-grating-gate (DGG)
InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a
circular polarization the current measured between source and drain contacts
changes its sign with the inversion of the radiation helicity. For elliptically
polarized radiation the total current is described by superposition of the
Stokes parameters with different weights. Moreover, by variation of gate
voltages applied to individual gratings the photocurrent can be defined either
by the Stokes parameter defining the radiation helicity or those for linear
polarization. We show that artificial non-centrosymmetric microperiodic
structures with a two-dimensional electron system excited by THz radiation
exhibit a dc photocurrent caused by the combined action of a spatially periodic
in-plane potential and spatially modulated light. The results provide a proof
of principle for the application of DGG HEMT for all-electric detection of the
radiation's polarization state.Comment: 7 pages, 4 figure
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