394 research outputs found
Quantum transport in weakly coupled superlattices at low temperature
We report on the study of the electrical current flowing in weakly coupled
superlattice (SL) structures under an applied electric field at very low
temperature, i.e. in the tunneling regime. This low temperature transport is
characterized by an extremely low tunneling probability between adjacent wells.
Experimentally, I(V) curves at low temperature display a striking feature, i.e
a plateau or null differential conductance. A theoretical model based on the
evaluation of scattering rates is developed in order to understand this
behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The
dominant interaction in usual experimental conditions such as ours is found to
be the electron-ionized donors scattering. The existence of the plateau in the
I(V) characteristics is physically explained by a competition between the
electric field localization of the Wannier-Stark electron states in the weakly
coupled quantum wells and the electric field assisted tunneling between
adjacent wells. The influence of the doping concentration and profile as well
as the presence of impurities inside the barrier are discussed
A study of long range order in certain two-dimensional frustrated lattices
We have studied the Heisenberg antiferromagnets on two-dimensional frustrated
lattices, triangular and kagome lattices using linear spin-wave theory. A
collinear ground state ordering is possible if one of the three bonds in each
triangular plaquette of the lattice becomes weaker or frustrated. We study
spiral order in the Heisenberg model along with Dzyaloshinskii-Moriya (DM)
interaction and in the presence of a magnetic field. The quantum corrections to
the ground state energy and sublattice magnetization are calculated
analytically in the case of triangular lattice with nearesr-neighbour
interaction. The corrections depend on the DM interaction strength and the
magnetic field. We find that the DM interaction stabilizes the long-range
order, reducing the effect of quantum fluctuations. Similar conclusions are
reached for the kagome lattice. We work out the linear spin-wave theory at
first with only nearest-neighbour (nn) terms for the kagome lattice. We find
that the nn interaction is not sufficient to remove the effects of low energy
fluctuations. The flat branch in the excitation spectrum becomes dispersive on
addition of furthet neighbour interactions. The ground state energy and the
excitation spectrum have been obtained for various cases.Comment: 18 pages, 9 figure
Extended Quantum Dimer Model and novel valence-bond phases
We extend the quantum dimer model (QDM) introduced by Rokhsar and Kivelson so
as to construct a concrete example of the model which exhibits the first-order
phase transition between different valence-bond solids suggested recently by
Batista and Trugman and look for the possibility of other exotic dimer states.
We show that our model contains three exotic valence-bond phases (herringbone,
checkerboard and dimer smectic) in the ground-state phase diagram and that it
realizes the phase transition from the staggered valence-bond solid to the
herringbone one. The checkerboard phase has four-fold rotational symmetry,
while the dimer smectic, in the absence of quantum fluctuations, has massive
degeneracy originating from partial ordering only in one of the two spatial
directions. A resonance process involving three dimers resolves this massive
degeneracy and dimer smectic gets ordered (order from disorder).Comment: 20 pages, 13 figures, accepted for publication in J. Stat. Mec
A proposed search for a fourth neutrino with a PBq antineutrino source
Several observed anomalies in neutrino oscillation data can be explained by a
hypothetical fourth neutrino separated from the three standard neutrinos by a
squared mass difference of a few eV^2. We show that this hypothesis can be
tested with a PBq (ten kilocurie scale) 144Ce or 106Ru antineutrino beta-source
deployed at the center of a large low background liquid scintillator detector.
In particular, the compact size of such a source could yield an
energy-dependent oscillating pattern in event spatial distribution that would
unabiguously determine neutrino mass differences and mixing angles.Comment: 4 pages ; 1 table ; 4 figures - Add energy spectrum shape only
analysis + referee comments/suggestion
High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt
The Cretaceous Normal Superchron (CNS, 84–121 Ma) is a singular period of the geodynamo's history, identified by a prolonged absence of polarity reversals. To better characterize the paleosecular variation (PSV) of the geomagnetic field at the end of this interval, we sampled seven continuous sequences of lava flows from the Okhotsk-Chukotka Volcanic Belt, emplaced 84–89 Ma in the vicinity of the Kupol ore deposit (NE Russia). From a collection of 1,024 paleomagnetic cores out of 82 investigated lava flows, we successfully determined the paleodirections of 78 lava flows, which led to 57 directional groups after removing the serial correlations. The resulting paleomagnetic pole is located at 170.0°E, 76.8°N (A95 = 5.2°, N = 57), in good agreement with previous estimates for north-eastern Eurasia. Aiming at quantifying PSV at a reconstructed paleolatitude (λ) of ∼80°N, we obtained a virtual geomagnetic pole (VGP) scatter
, the value of which was corrected for within-site dispersion and is little dependent on the choice of the selection criteria. Compared to previous paleodirectional data sets characterizing PSV at various paleolatitudes during the CNS, our Sb estimate confirms a relative latitudinal increase Sb(λ = 90°)/Sb(λ = 0°) on the order of 2–2.5. Focusing on PSV at high paleolatitude within the 70°–90° range, we show that Sb was ∼15% lower at the end of the CNS than during the past 10 Myr, confirming that the singular polarity regime of the geodynamo observed during the CNS is likely accompanied with reduced PSV
Intermittence and peculiarities of a statistic characteristic of the geomagnetic field in geodynamo models
© 2017, Pleiades Publishing, Ltd. The analysis of the statistical characteristics of the geomagnetic field generated in the numerical geodynamo models has shown that the distribution of the spherical harmonic coefficients in some cases is not Gaussian but, instead, has much in common with the Laplace distribution. The shape of the corresponding histograms depends on the time scale, which allows interpreting the obtained data in terms of a mixture of Gaussian distributions. The similar effects associated with the intermittence were observed in the experiments in a turbulent fluid flow. Hence, the behavior of secular variations in the magnetic field of the Earth should perhaps be described in terms of a mixture of several Gaussian stationary processes corresponding to switching between the different regimes of geodynamo generation
Ultimate performance of Quantum Well Infrared Photodetectors in the tunneling regime
Thanks to their wavelength diversity and to their excellent uniformity,
Quantum Well Infrared Photodetectors (QWIP) emerge as potential candidates for
astronomical or defense applications in the very long wavelength infrared
(VLWIR) spectral domain. However, these applications deal with very low
backgrounds and are very stringent on dark current requirements. In this paper,
we present the full electro-optical characterization of a 15 micrometer QWIP,
with emphasis on the dark current measurements. Data exhibit striking features,
such as a plateau regime in the IV curves at low temperature (4 to 25 K). We
show that present theories fail to describe this phenomenon and establish the
need for a fully microscopic approach
Modified spin-wave theory with ordering vector optimization I: frustrated bosons on the spatially anisotropic triangular lattice
We investigate a system of frustrated hardcore bosons, modeled by an XY
antiferromagnet on the spatially anisotropic triangular lattice, using
Takahashi's modified spin-wave (MSW) theory. In particular we implement
ordering vector optimization on the ordered reference state of MSW theory,
which leads to significant improvement of the theory and accounts for quantum
corrections to the classically ordered state. The MSW results at zero
temperature compare favorably to exact diagonalization (ED) and projected
entangled-pair state (PEPS) calculations. The resulting zero-temperature phase
diagram includes a 1D quasi-ordered phase, a 2D Neel ordered phase, and a 2D
spiraling ordered phase. We have strong indications that the various ordered or
quasi-ordered phases are separated by spin-liquid phases with short-range
correlations, in analogy to what has been predicted for the Heisenberg model on
the same lattice. Within MSW theory we also explore the finite-temperature
phase diagram. We find that the zero-temperature long-range-ordered phases turn
into quasi-ordered phases (up to a Berezinskii-Kosterlitz-Thouless
temperature), while zero-temperature quasi-ordered phases become short-range
correlated at finite temperature. These results show that modified spin-wave
theory is very well suited for describing ordered and quasi-ordered phases of
frustrated XY spins (or, equivalently, of frustrated lattice bosons) both at
zero and finite temperatures. While MSW theory, just as other theoretical
methods, cannot describe spin-liquid phases, its breakdown provides a fast
method for singling out Hamiltonians which may feature these intriguing quantum
phases. We thus suggest a tool for guiding our search for interesting systems
whose properties are necessarily studied with a physical quantum simulator.Comment: 40 pages, 16 figure
Properties and Detection of Spin Nematic Order in Strongly Correlated Electron Systems
A spin nematic is a state which breaks spin SU(2) symmetry while preserving
translational and time reversal symmetries. Spin nematic order can arise
naturally from charge fluctuations of a spin stripe state. Focusing on the
possible existence of such a state in strongly correlated electron systems, we
build a nematic wave function starting from a t-J type model. The nematic is a
spin-two operator, and therefore does not couple directly to neutrons. However,
we show that neutron scattering and Knight shift experiments can detect the
spin anisotropy of electrons moving in a nematic background. We find the mean
field phase diagram for the nematic taking into account spin-orbit effects.Comment: 13 pages, 11 figures. (v2) References adde
On the origin of the reactor antineutrino anomalies in light of a new summation model with parameterized transitions
We investigate the possible origins of the norm and shape reactor
antineutrino anomalies in the framework of a summation model (SM) where
transitions are simulated by a phenomenological Gamow-Teller
-decay strength model. The general trends of the discrepancies to the
Huber-Mueller model on the antineutrino side can be reproduced both in norm and
shape. From the exact electron-antineutrino correspondence of the SM model, we
predict similar distortions in the electron spectra, suggesting that biases on
the reference fission-electron spectra could be at the origin of the anomalies
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