9,566 research outputs found
Infinite boundary conditions for matrix product state calculations
We propose a formalism to study dynamical properties of a quantum many-body
system in the thermodynamic limit by studying a finite system with infinite
boundary conditions (IBC) where both finite size effects and boundary effects
have been eliminated. For one-dimensional systems, infinite boundary conditions
are obtained by attaching two boundary sites to a finite system, where each of
these two sites effectively represents a semi-infinite extension of the system.
One can then use standard finite-size matrix product state techniques to study
a region of the system while avoiding many of the complications normally
associated with finite-size calculations such as boundary Friedel oscillations.
We illustrate the technique with an example of time evolution of a local
perturbation applied to an infinite (translationally invariant) ground state,
and use this to calculate the spectral function of the S=1 Heisenberg spin
chain. This approach is more efficient and more accurate than conventional
simulations based on finite-size matrix product state and density-matrix
renormalization-group approaches.Comment: 10 page
Dynamical windows for real-time evolution with matrix product states
We propose the use of a dynamical window to investigate the real-time
evolution of quantum many-body systems in a one-dimensional lattice. In a
recent paper [H. Phien et al, arxiv:????.????], we introduced infinite boundary
conditions (IBC) in order to investigate real-time evolution of an infinite
system under a local perturbation. This was accomplished by restricting the
update of the tensors in the matrix product state to a finite window, with left
and right boundaries held at fixed positions. Here we consider instead the use
of a dynamical window, namely a window where the positions of left and right
boundaries are allowed to change in time. In this way, all simulation efforts
can be devoted to the space-time region of interest, which leads to a
remarkable reduction in computational costs. For illustrative purposes, we
consider two applications in the context of the spin-1 antiferromagnetic
Heisenberg model in an infinite spin chain: one is an expanding window, with
boundaries that are adjusted to capture the expansion in time of a local
perturbation of the system; the other is a moving window of fixed size, where
the position of the window follows the front of a propagating wave
Harvesting Excitons Through Plasmonic Strong Coupling
Exciton harvesting is demonstrated in an ensemble of quantum emitters coupled
to localized surface plasmons. When the interaction between emitters and the
dipole mode of a metallic nanosphere reaches the strong coupling regime, the
exciton conductance is greatly increased. The spatial map of the conductance
matches the plasmon field intensity profile, which indicates that transport
properties can be tuned by adequately tailoring the field of the plasmonic
resonance. Under strong coupling, we find that pure dephasing can have
detrimental or beneficial effects on the conductance, depending on the
effective number of participating emitters. Finally, we show that the exciton
transport in the strong coupling regime occurs on an ultrafast timescale given
by the inverse Rabi splitting (fs), orders of magnitude faster than
transport through direct hopping between the emitters.Comment: 5 pages, 3 figure
Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy
We present a Green's function approach based on a LCAO scheme to compute the
elastic propagation of electrons injected from a STM tip into a metallic film.
The obtained 2D current distribution in real and reciprocal space furnish a
good representation of the elastic component of Ballistic Electron Emission
Microscopy (BEEM) currents. Since this component accurately approximates the
total current in the near threshold region, this procedure allows --in contrast
to prior analyses-- to take into account effects of the metal band structure in
the modeling of these experiments. The Au band structure, and in particular its
gaps appearing in the [111] and [100] directions provides a good explanation
for the previously irreconcilable results of nanometric resolution and
similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).Comment: 12 pages, 9 postscript figures, revte
Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents
Using a Green's function approach, we investigate band structure effects in
the BEEM current distribution in reciprocal space. In the elastic limit, this
formalism provides a 'parameter free' solution to the BEEM problem. At low
temperatures, and for thin metallic layers, the elastic approximation is enough
to explain the experimental I(V) curves at low voltages. At higher voltages
inelastic effects are approximately taken into account by introducing an
effective RPA-electron lifetime, much in similarity with LEED theory. For thick
films, however, additional damping mechanisms are required to obtain agreement
with experiment.Comment: 4 pages, 3 postscript figures, revte
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Surface and bulk band-structure effects on CoSi<sub>2</sub>/Si(111) ballistic-electron emission experiments
A theoretical model of ballistic-electron-emission microscopy (BEEM) based on linear combination of atomic orbitals Hamiltonians and Keldysh Green’s functions is applied to analyze experimental data obtained for CoSi2/Si(111) contacts. Hot electrons injected from a scanning tunneling microscope tip into the silicide film form a highly focused beam, which even after propagation through films of moderate thickness is narrow enough to allow the observed atomic resolution of interfacial point defects. On (2×1) reconstructed domains a certain fraction of the initial current is injected into localized surface states, leading to the reported contrast in BEEM images, reflecting the topography at the surface. These results confirm that band-structure effects, both in the bulk and at the surface of the metallic overlayer, intricately influence the interface-related information contained in BEEM data. It is found that for a careful analysis of experimental results, a theoretical model going beyond the ballistic hypotesis is required
Applicability of Ammonia Sensors for Controlling Environmental Parameters in Accommodations for Lamb Fattening
Electrochemical ammonia sensors were used to analyse the existing relationship between the ammonia concentration and ambient levels of both temperature and relative humidity in commercial lamb fattening housing equipped with mechanical ventilation and straw-bedded pens. In the first stage of the experiment, sensors were placed over straw beds covered in lamb urine and analysed under laboratory conditions in order to determine ammonia emission evolution over time; three control temperatures (25, 35, and 50 degrees C) were used. A HOBO H8 temperature and relative humidity logger and a Drager NH3LC-6809680 electrochemical ammonia sensor placed in a Drager Polytron 7000 gas detector were utilized as sensors. A positive correlation was established between both ammonia emission time and emitted amount with temperature. Additionally, tests were performed in a commercial lamb housing to determine ammonia concentration variation with respect to height from the ground; three ammonia sensors placed at 50, 90, and 135 cm above the ground were used simultaneously. The ammonia concentration significantly decreased as height increased. A 90 cm height was selected, and three ammonia probes were placed in three different pens inside the livestock housing, along with temperature and relative humidity sensors; four different housing ventilation rates were then tested under real conditions over a time period of 4 months. An adjustment polynomial equation between the housing ambient temperature and the ammonia concentration was obtained with R-2 = 0.632. In conclusion, a relationship can be established between temperature and ammonia concentration in commercial lamb housing under certain handling conditions, which in turn allows for estimating the ammonia concentration adequately based on the ambient internal temperature
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Matrix product decomposition and classical simulation of quantum dynamics in the presence of a symmetry
We propose a refined matrix product state representation for many-body
quantum states that are invariant under SU(2) transformations, and indicate how
to extend the time-evolving block decimation (TEBD) algorithm in order to
simulate time evolution in an SU(2) invariant system. The resulting algorithm
is tested in a critical quantum spin chain and shown to be significantly more
efficient than the standard TEBD.Comment: 5 pages, 4 figure
- …