3,160 research outputs found
Algorithmic differentiation and the calculation of forces by quantum Monte Carlo
We describe an efficient algorithm to compute forces in quantum Monte Carlo
using adjoint algorithmic differentiation. This allows us to apply the space
warp coordinate transformation in differential form, and compute all the 3M
force components of a system with M atoms with a computational effort
comparable with the one to obtain the total energy. Few examples illustrating
the method for an electronic system containing several water molecules are
presented. With the present technique, the calculation of finite-temperature
thermodynamic properties of materials with quantum Monte Carlo will be feasible
in the near future.Comment: 32 pages, 4 figure, to appear in The Journal of Chemical Physic
Ising transition in the two-dimensional quantum Heisenberg model
We study the thermodynamics of the spin- two-dimensional quantum
Heisenberg antiferromagnet on the square lattice with nearest () and
next-nearest () neighbor couplings in its collinear phase (),
using the pure-quantum self-consistent harmonic approximation. Our results show
the persistence of a finite-temperature Ising phase transition for every value
of the spin, provided that the ratio is greater than a critical value
corresponding to the onset of collinear long-range order at zero temperature.
We also calculate the spin- and temperature-dependence of the collinear
susceptibility and correlation length, and we discuss our results in light of
the experiments on LiVOSiO and related compounds.Comment: 4 page, 4 figure
Using compound earcons to represent hierarchies
Previous research on non-speech audio messages called <i>earcons</i> showed that they could provide powerful navigation cues in menu hierarchies. This work used <i>hierarchical</i> earcons. In this paper we suggest <i>compound</i> earcons provide a more flexible method for presenting this information. A set of sounds was created to represent the numbers 0-4 and dot. Sounds could then be created for any node in a hierarchy by concatenating these simple sounds. A hierarchy of four levels and 27 nodes was constructed. An experiment was conducted in which participants had to identify their location in the hierarchy by listening to an earcon. Results showed that participants could identify their location with over 97% accuracy, significantly better than with hierarchical earcons. Participants were also able to recognise previously unheard earcons with over 97% accuracy. These results showed that compound earcons are an effective way of representing hierarchies in sound
Effect of local charge fluctuations on spin physics in the Neel state of LaCuO
We explore the effect of local charge fluctuations on the spin response of a
Mott insulator by deriving an effective spin model, and studying it using
Schwinger boson mean field theory. Applying this to LaCuO, we show that
an accurate fit to the magnon dispersion relation, measured by Coldea {\em et
al.} [Phys. Rev. Lett. {\bf 86}, 5377 (2001)] is obtained with Hubbard model
parameters , and . These parameters lead
to estimates of the staggered magnetization (), spin wave
velocity (-\AA), and spin stiffness (). In particular the staggered moment as well as the effective local moment
are renormalized to smaller values compared to the Heisenberg model due to
local charge fluctuations in the Hubbard model. The dynamical structure factor
shows considerable weight in the continuum along the zone boundary as well as
secondary peaks that may be observed in high resolution neutron scattering
experiments.Comment: Manuscript considerably revised following referee comments. Also
added a brief discussion of sum rules. 8 pages, 6 eps figure
Spin-lattice coupling in frustrated antiferromagnets
We review the mechanism of spin-lattice coupling in relieving the geometrical
frustration of pyrochlore antiferromagnets, in particular spinel oxides. The
tetrahedral unit, which is the building block of the pyrochlore lattice,
undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom
are coupled to the antiferromagnetism. By restricting our considerations to
distortions which preserve the translational symmetries of the lattice, we
present a general theory of the collective spin-Jahn-Teller effect in the
pyrochlore lattice. One of the predicted lattice distortions breaks the
inversion symmetry and gives rise to a chiral pyrochlore lattice, in which
frustrated bonds form helices with a definite handedness. The chirality is
transferred to the spin system through spin-orbit coupling, resulting in a
long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit
models of spin-lattice coupling using local phonon modes, and their
applications in other frustrated magnets.Comment: 23 pages, 6 figures. Lecture notes for Trieste Summer School, August
2007. To appear as a chapter in "Highly Frustrated Magnetism", Eds. C.
Lacroix, P. Mendels, F. Mil
Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice
The classical XXZ triangular-lattice antiferromagnet (TAF) shows both an
Ising and a BKT transition, related to the chirality and the in-plane spin
components, respectively. In this paper the quantum effects on the
thermodynamic quantities are evaluated by means of the pure-quantum
self-consistent harmonic approximation (PQSCHA), that allows one to deal with
any spin value through classical MC simulations. We report the internal energy,
the specific heat, and the in-plane correlation length of the quantum XX0 TAF,
for S=1/2, 1, 5/2. The quantum transition temperatures turn out to be smaller
the smaller the spin, and agree with the few available theoretical and
numerical estimates.Comment: 4 pages,3 postscript figure
Model-free moments: predictability of STOXX Europe 600 Oil & Gas future returns
The relationship between prices and volatility of energy assets (primarily oil and gas) is of paramount importance for investors and policy makers. We construct a volatility index for the European oil and gas market based on a model-free approach to obtain a European counterpart of US volatility indices for the energy market, such as the CBOE Crude Oil Volatility Index (OVX). Given that investors are averse to volatility of losses, but appreciate volatility of gains, we also derive risk measures that focus on positive and negative returns and their imbalance. We assess whether the constructed
indices have predictive power on future returns. We show that in the medium term all the risk indices behave as market greed indicators, whereas in the short term they behave as fear indicators since rises in risk indices are linked with negative returns. The implications for investors and policy-makers are outlined
Spiral order by disorder and lattice nematic order in a frustrated Heisenberg antiferromagnet on the honeycomb lattice
Motivated by recent experiments on BiMnO(NO), we study a
frustrated - Heisenberg model on the two dimensional (2D) honeycomb
lattice. The classical - Heisenberg model on the two dimensional (2D)
honeycomb lattice has N\'eel order for , it
exhibits a one-parameter family of degenerate incommensurate spin spiral ground
states where the spiral wave vector can point in any direction. Spin wave
fluctuations at leading order lift this accidental degeneracy in favor of
specific wave vectors, leading to spiral order by disorder. For spin ,
quantum fluctuations are, however, likely to be strong enough to melt the
spiral order parameter over a wide range of . Over a part of this
range, we argue that the resulting state is a valence bond solid (VBS) with
staggered dimer order - this VBS is a nematic which breaks lattice rotational
symmetry. Our arguments are supported by comparing the spin wave energy with
the energy of the dimer solid obtained using a bond operator formalism. Turning
to the effect of thermal fluctuations on the spiral ordered state, any nonzero
temperature destroys the magnetic order, but the discrete rotational symmetry
of the lattice remains broken resulting in a thermal analogue of the nematic
VBS. We present arguments, supported by classical Monte Carlo simulations, that
this nematic transforms into the high temperature symmetric paramagnet via a
thermal phase transition which is in the universality class of the classical
3-state Potts (clock) model in 2D. We discuss the possible relevance of our
results for honeycomb magnets, such as BiMO(NO) (with
M=Mn,V,Cr), and bilayer triangular lattice magnets.Comment: Slightly revise
Inhomogeneity Induces Resonance Coherence Peaks in Superconducting BSCCO
In this paper we analyze, using scanning tunneling spectroscopy, the density
of electronic states in nearly optimally doped BSCCO in zero field. Focusing on
the superconducting gap, we find patches of what appear to be two different
phases in a background of some average gap, one with a relatively small gap and
sharp large coherence peaks and one characterized by a large gap with broad
weak coherence peaks. We compare these spectra with calculations of the local
density of states for a simple phenomenological model in which a 2 xi_0 * 2
xi_0 patch with an enhanced or supressed d-wave gap amplitude is embedded in a
region with a uniform average d-wave gap.Comment: 4 pages, 3 figure
Climate risk definition and measures: asset pricing models and stock returns
The aim of this study is to examine the literature on climate risk definition and measures and the impact of climate risk on stock returns. We review how asset pricing models (and their testable implications) consider climate risk as a residual systemic risk driver in excess of either standard market risk factors or latent factors identified with business and financial cycles. Firms less exposed to transition risk, in equilibrium, should face a lower cost of equity financing, given an expected return lower than the one associated with pollutant firms. The existence of a recent outperformance of realized returns on green stocks can be reconciled with unexpected shifts in investors tastes for green assets. Finally, we identify some issues regarding the empirical approach and suggest several potential areas for future research
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