18,519 research outputs found
Effects of nanoscale spatial inhomogeneity in strongly correlated systems
We calculate ground-state energies and density distributions of Hubbard
superlattices characterized by periodic modulations of the on-site interaction
and the on-site potential. Both density-matrix renormalization group and
density-functional methods are employed and compared. We find that small
variations in the on-site potential can simulate, cancel, or even
overcompensate effects due to much larger variations in the on-site interaction
. Our findings highlight the importance of nanoscale spatial inhomogeneity
in strongly correlated systems, and call for reexamination of model
calculations assuming spatial homogeneity.Comment: 5 pages, 1 table, 4 figures, to appear in PR
Density-functionals not based on the electron gas: Local-density approximation for a Luttinger liquid
By shifting the reference system for the local-density approximation (LDA)
from the electron gas to other model systems one obtains a new class of density
functionals, which by design account for the correlations present in the chosen
reference system. This strategy is illustrated by constructing an explicit LDA
for the one-dimensional Hubbard model. While the traditional {\it ab initio}
LDA is based on a Fermi liquid (the electron gas), this one is based on a
Luttinger liquid. First applications to inhomogeneous Hubbard models, including
one containing a localized impurity, are reported.Comment: 4 pages, 4 figures (final version, contains additional applications
and discussion; accepted by Phys. Rev. Lett.
Ecological Effects of Fear: How Spatiotemporal Heterogeneity in Predation Risk Influences Mule Deer Access to Forage in a Sky‐Island System
Forage availability and predation risk interact to affect habitat use of ungulates across many biomes. Within sky‐island habitats of the Mojave Desert, increased availability of diverse forage and cover may provide ungulates with unique opportunities to extend nutrient uptake and/or to mitigate predation risk. We addressed whether habitat use and foraging patterns of female mule deer (Odocoileus hemionus) responded to normalized difference vegetation index (NDVI), NDVI rate of change (green‐up), or the occurrence of cougars (Puma concolor). Female mule deer used available green‐up primarily in spring, although growing vegetation was available during other seasons. Mule deer and cougar shared similar habitat all year, and our models indicated cougars had a consistent, negative effect on mule deer access to growing vegetation, particularly in summer when cougar occurrence became concentrated at higher elevations. A seemingly late parturition date coincided with diminishing NDVI during the lactation period. Sky‐island populations, rarely studied, provide the opportunity to determine how mule deer respond to growing foliage along steep elevation and vegetation gradients when trapped with their predators and seasonally limited by aridity. Our findings indicate that fear of predation may restrict access to the forage resources found in sky islands
Probing the two-scale-factor universality hypothesis by exact rotation symmetry-breaking mechanism
We probe the two-scale factor universality hypothesis by evaluating, firstly
explicitly and analytically at the one-loop order, the loop quantum corrections
to the amplitude ratios for O() scalar field theories with
rotation symmetry-breaking in three distinct and independent methods in which
the rotation symmetry-breaking mechanism is treated exactly. We show that the
rotation symmetry-breaking amplitude ratios turn out to be identical in the
three methods and equal to their respective rotation symmetry-breaking ones,
although the amplitudes themselves, in general, depend on the method employed
and on the rotation symmetry-breaking parameter. At the end, we show that all
these results can be generalized, through an inductive process based on a
general theorem emerging from the exact calculation, to any loop level and
physically interpreted based on symmetry ideas.Comment: 17 pages, 3 figure
Predicting Psi-BN: computational insights into its mechanical, electronic, and optical characteristics
Computational materials are pivotal in advancing our understanding of
distinct material classes and their properties, offering valuable insights in
predicting novel structures and complementing experimental approaches. In this
context, Psi-graphene is a stable two-dimensional carbon allotrope composed of
5-6-7 carbon rings theoretically predicted recently. Using density functional
theory (DFT) calculations, we explored its boron nitride counterpart's
mechanical, electronic, and optical characteristics (Psi-BN). Our results
indicate that Psi-BN possesses a band gap of 4.59 eV at the HSE06 level. Phonon
calculations and ab initio molecular dynamics simulations demonstrated that
this material has excellent structural and dynamic stability. Moreover, its
formation energy is -7.48 eV. Psi-BN exhibited strong ultraviolet activity,
suggesting its potential as an efficient UV collector. Furthermore, we
determined critical mechanical properties of Psi-BN, such as the elastic
stiffness constants, Young's modulus (250-300 GPa), and Poisson ratio (0.7),
providing valuable insights into its mechanical behavior.Comment: 13 pages, 7 figure
TODD-Graphene: A Novel Porous 2D Carbon Allotrope for High-Performance Lithium-Ion Batteries
The class of 2D carbon allotropes has garnered significant attention due to
its exceptional optoelectronic and mechanical properties, crucial for diverse
device applications, such as energy storage. This study employs density
functional theory calculations, ab initio molecular dynamics (AIMD), and
classical reactive (ReaxFF) molecular dynamics (MD) simulations to introduce
TODD-Graphene, a novel 2D planar carbon allotrope with a porous structure
composed of 3-8-10-12 carbon rings. TODD-G exhibits intrinsic metallic
properties with low formation energy and demonstrates exceptional dynamic,
thermal, and mechanical stability. Calculations reveal a high theoretical
capacity for adsorbing Li atoms by showing a low average diffusion barrier of
0.83 eV and a metallic framework boasting excellent conductivity, emerging as a
promising anode material for lithium-ion batteries. We also calculated the
charge carrier mobility for electrons and holes in TOOD-G, and the values
surpassed the graphene ones. Classical reactive MD simulation results suggested
its structural integrity with no bond reconstructions at 1800 K.Comment: 21 pages, 11 figure
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
On Matrix Superpotential and Three-Component Normal Modes
We consider the supersymmetric quantum mechanics (SUSY QM) with three-
component normal modes for the Bogomol'nyi-Prasad-Sommerfield (BPS) states. An
explicit form of the SUSY QM matrix superpotential is presented and the
corresponding three-component bosonic zero-mode eigenfunction is investigated.Comment: 17 pages, no figure. Paper accepted for publication in Journal of
Physics A: Mathematical and Theoretica
Magnetic phases evolution in the LaMn1-xFexO3+y system
We have investigated the crystal structure and magnetic properties for
polycrystalline samples of LaMn1-xFexO3+y, in the whole range x=0.0 to x=1.0,
prepared by solid state reaction in air. All samples show the ORT-2
orthorhombic structure that suppresses the Jahn-Teller distortion, thus
favoring a ferromagnetic (FM) superexchange (SE) interaction between
Mn^{3+}-O-Mn^{3+}. For x=0.0 the oxygen excess (y ~ 0.09) produces vacancies in
the La and Mn sites and generates a fraction around 18% of Mn^{4+} ions and 82%
of the usual Mn^{3+} ions, with possible double exchange interaction between
them. The Fe doping in this system is known to produce only stable Fe^{3+}
ions. We find an evolution from a fairly strong FM phase with a Curie
temperature T_{C} ~ 160 K, for x=0.0, to an antiferromagnetic (AFM) phase with
T_{N} = 790 K, for x=1.0, accompanied by clear signatures of a cluster-glass
behavior. For intermediate Fe contents a mixed-phase state occurs, with a
gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic
transition broadening for 0.2 < x < 0.7. A model based on the expected exchange
interaction among the various magnetic-ion types, accounts very well for the
saturation-magnetization dependence on Fe doping.Comment: 27 pages, 9 figure
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