1,779 research outputs found
Quantum Monte Carlo method using phase-free random walks with Slater determinants
We develop a quantum Monte Carlo method for many fermions that allows the use
of any one-particle basis. It projects out the ground state by random walks in
the space of Slater determinants. An approximate approach is formulated to
control the phase problem with a trial wave function . Using
plane-wave basis and non-local pseudopotentials, we apply the method to Si
atom, dimer, and 2, 16, 54 atom (216 electrons) bulk supercells. Single Slater
determinant wave functions from density functional theory calculations were
used as with no additional optimization. The calculated binding
energy of Si2 and cohesive energy of bulk Si are in excellent agreement with
experiments and are comparable to the best existing theoretical results.Comment: 5 pages, Latex, with 1 fi
Pressure-induced diamond to beta-tin transition in bulk silicon: a near-exact quantum Monte Carlo study
The pressure-induced structural phase transition from diamond to beta-tin in
silicon is an excellent test for theoretical total energy methods. The
transition pressure provides a sensitive measure of small relative energy
changes between the two phases (one a semiconductor and the other a semimetal).
Experimentally, the transition pressure is well characterized.
Density-functional results have been unsatisfactory. Even the generally much
more accurate diffusion Monte Carlo method has shown a noticeable fixed-node
error. We use the recently developed phaseless auxiliary-field quantum Monte
Carlo (AFQMC) method to calculate the relative energy differences in the two
phases. In this method, all but the error due to the phaseless constraint can
be controlled systematically and driven to zero. In both structural phases we
were able to benchmark the error of the phaseless constraint by carrying out
exact unconstrained AFQMC calculations for small supercells. Comparison between
the two shows that the systematic error in the absolute total energies due to
the phaseless constraint is well within 0.5 mHa/atom. Consistent with these
internal benchmarks, the transition pressure obtained by the phaseless AFQMC
from large supercells is in very good agreement with experiment.Comment: 9 pages, 5 figure
Microscopic calculation of the phonon dynamics of SrRuO compared with LaCuO
The phonon dynamics of the low-temperature superconductor SrRuO
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor LaCuO. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic -dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for SrRuO reveals important differences from the lattice
dynamics of - and -doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
-mode with a very steep dispersion coupling strongly with the
electrons is missing in SrRuO. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for SrRuO which has been
shown recently to be realistic for LaCuO. In contrast to
LaCuO in SrRuO the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference
Experimental chronic noise is related to elevated fecal corticosteroid metabolites in lekking male greater Sage-Grouse (Centrocercus urophasianus).
There is increasing evidence that individuals in many species avoid areas exposed to chronic anthropogenic noise, but the impact of noise on those who remain in these habitats is unclear. One potential impact is chronic physiological stress, which can affect disease resistance, survival and reproductive success. Previous studies have found evidence of elevated stress-related hormones (glucocorticoids) in wildlife exposed to human activities, but the impacts of noise alone are difficult to separate from confounding factors. Here we used an experimental playback study to isolate the impacts of noise from industrial activity (natural gas drilling and road noise) on glucocorticoid levels in greater sage-grouse (Centrocercus urophasianus), a species of conservation concern. We non-invasively measured immunoreactive corticosterone metabolites from fecal samples (FCMs) of males on both noise-treated and control leks (display grounds) in two breeding seasons. We found strong support for an impact of noise playback on stress levels, with 16.7% higher mean FCM levels in samples from noise leks compared with samples from paired control leks. Taken together with results from a previous study finding declines in male lek attendance in response to noise playbacks, these results suggest that chronic noise pollution can cause greater sage-grouse to avoid otherwise suitable habitat, and can cause elevated stress levels in the birds who remain in noisy areas
Modeling of the electronic state of the High-Temperature Superconductor LaCuO: Phonon dynamics and charge response
A modeling of the normal state of the p-doped high-temperature
superconductors (HTSC's) is presented. This is achieved starting from a more
conventional metallic phase for optimal- and overdoping and passing via the
underdoped to the insulating state by consecutive orbital selective
compressibility-incompressibility transitions in terms of sum rules for the
charge response. The modeling is substantiated by corresponding phonon
calculations. Extending investigations of the full dispersion and in particular
of the strongly doping dependent anomalous phonon modes in LaCuO, which so far
underpin our treatment of the density response of the electrons in the p-doped
HTSC's, gives additional support for the modeling of the electronic state,
compares well with recent experimental data and predicts the dispersion for the
overdoped regime. Moreover, phonon densities of states have been calculated and
compared for the insulating, underdoped, optimally doped and overdoped state of
LaCuO. From our modeling of the normal state a consistent picture of the
superconducting phase also can be extracted qualitatively pointing in the
underdoped regime to a phase ordering transition. On the other hand, the
modeling of the optimal and overdoped state is consistent with a quasi-particle
picture with a well defined Fermi surface. Thus, in the latter case a Fermi
surface instability with an evolution of pairs of well defined quasiparticles
is possible and can lead to a BCS-type ordering. So, it is tempting to
speculate that optimal in the HTSC's marks a crossover region between
these two forms of ordering.Comment: 18 RevTex pages, 10 figures, revised version, references updated,
accepted for publication in Physical Review
High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding
A first principles embedded cluster approach is used to calculate O chemical
shielding tensors, sigma, in prototypical transition metal oxide ABO_3
perovskite crystals. Our principal findings are 1) a large anisotropy of sigma
between deshielded sigma_x ~ sigma_y and shielded sigma_z components (z along
the Ti-O bond); 2) a nearly linear variation, across all the systems studied,
of the isotropic sigma_iso and uniaxial sigma_ax components, as a function of
the B-O-B bond asymmetry. We show that the anisotropy and linear variation
arise from large paramagnetic contributions to sigma_x and sigma_y due to
virtual transitions between O(2p) and unoccupied B(nd) states. The calculated
isotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreement
with recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3
and PbZrO_3, calculated delta_iso are also in good agreement with NMR powder
spectrum measurements. In PbZrO_3, delta_iso calculations of the five
chemically distinct sites indicate a correction of the experimental
assignments. The strong dependence of sigma on covalent O(2p)-B(nd)
interactions seen in our calculations indicates that 17O NMR spectroscopy,
coupled with first principles calculations, can be an especially useful tool to
study the local structure in complex perovskite alloys.Comment: 12 pages, 3 figures, and 3 Table
Kinetic Monte Carlo Simulations of Crystal Growth in Ferroelectric Alloys
The growth rates and chemical ordering of ferroelectric alloys are studied
with kinetic Monte Carlo (KMC) simulations using an electrostatic model with
long-range Coulomb interactions, as a function of temperature, chemical
composition, and substrate orientation. Crystal growth is characterized by
thermodynamic processes involving adsorption and evaporation, with
solid-on-solid restrictions and excluding diffusion. A KMC algorithm is
formulated to simulate this model efficiently in the presence of long-range
interactions. Simulations were carried out on Ba(Mg_{1/3}Nb_{2/3})O_3 (BMN)
type materials. Compared to the simple rocksalt ordered structures, ordered BMN
grows only at very low temperatures and only under finely tuned conditions. For
materials with tetravalent compositions, such as (1-x)Ba(Mg_{1/3}Nb_{2/3})O_3 +
xBaZrO_3 (BMN-BZ), the model does not incorporate tetravalent ions at
low-temperature, exhibiting a phase-separated ground state instead. At higher
temperatures, tetravalent ions can be incorporated, but the resulting crystals
show no chemical ordering in the absence of diffusive mechanisms.Comment: 13 pages, 16 postscript figures, submitted to Physics Review B
Journa
Non-equilibrium phase transitions in biomolecular signal transduction
We study a mechanism for reliable switching in biomolecular
signal-transduction cascades. Steady bistable states are created by system-size
cooperative effects in populations of proteins, in spite of the fact that the
phosphorylation-state transitions of any molecule, by means of which the switch
is implemented, are highly stochastic. The emergence of switching is a
nonequilibrium phase transition in an energetically driven, dissipative system
described by a master equation. We use operator and functional integral methods
from reaction-diffusion theory to solve for the phase structure, noise
spectrum, and escape trajectories and first-passage times of a class of minimal
models of switches, showing how all critical properties for switch behavior can
be computed within a unified framework
Assessing weak hydrogen binding on Ca+ centers: An accurate many-body study with large basis sets
Weak H2 physisorption energies present a significant challenge to even the
best correlated theoretical many-body methods. We use the phaseless
auxiliary-field quantum Monte Carlo (AFQMC) method to accurately predict the
binding energy of Ca+ - 4H2. Attention has recently focused on this model
chemistry to test the reliability of electronic structure methods for H2
binding on dispersed alkaline earth metal centers. A modified Cholesky
decomposition is implemented to realize the Hubbard-Stratonovich transformation
efficiently with large Gaussian basis sets. We employ the largest
correlation-consistent Gaussian type basis sets available, up to cc-pCV5Z for
Ca, to accurately extrapolate to the complete basis limit. The calculated
potential energy curve exhibits binding with a double-well structure.Comment: 10 pages, 7 figures. Submitted to JC
Low-energy renormalization of the electron dispersion of high-T superconductors
High-resolution ARPES studies in cuprates have detected low-energy changes in
the dispersion and absorption of quasi-particles at low temperatures, in
particular, in the superconducting state. Based on a new 1/N expansion of the
t-J-Holstein model, which includes collective antiferromagnetic fluctuations
already in leading order, we argue that the observed low-energy structures are
mainly caused by phonons and not by spin fluctuations, at least, in the optimal
and overdoped regime.Comment: 6 pages, 3 figure
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