2,876 research outputs found
Inverse pressure-induced Mott transition in TiPO
TiPO shows interesting structural and magnetic properties as temperature
and pressure are varied, such as a spin-Peierls phase transition and the
development of incommensurate modulations of the lattice. Recently, high
pressure experiments for TiPO reported two new structural phases appearing
at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem.
Int. Ed. 55, 15053]. The latter was shown to include the first example of
5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work
we characterize the electronic structure and other physical properties of these
new phases by means of ab-initio calculations, and investigate the structural
transition. We find that the appearance of phases IV and V coincides with a
collapse of the Mott insulating gap and quenching of magnetism in phase III as
pressure is applied. Remarkably, our calculations show that in the high
pressure phase V, these features reappear, leading to an antiferromagnetic Mott
insulating phase, with robust local moments
A Variational Approach for Minimizing Lennard-Jones Energies
A variational method for computing conformational properties of molecules
with Lennard-Jones potentials for the monomer-monomer interactions is
presented. The approach is tailored to deal with angular degrees of freedom,
{\it rotors}, and consists in the iterative solution of a set of deterministic
equations with annealing in temperature. The singular short-distance behaviour
of the Lennard-Jones potential is adiabatically switched on in order to obtain
stable convergence. As testbeds for the approach two distinct ensembles of
molecules are used, characterized by a roughly dense-packed ore a more
elongated ground state. For the latter, problems are generated from natural
frequencies of occurrence of amino acids and phenomenologically determined
potential parameters; they seem to represent less disorder than was previously
assumed in synthetic protein studies. For the dense-packed problems in
particular, the variational algorithm clearly outperforms a gradient descent
method in terms of minimal energies. Although it cannot compete with a careful
simulating annealing algorithm, the variational approach requires only a tiny
fraction of the computer time. Issues and results when applying the method to
polyelectrolytes at a finite temperature are also briefly discussed.Comment: 14 pages, uuencoded compressed postscript fil
Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4
The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5%
and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While
this system is considered as the archetypal quantum spin glass, the detailed
analysis of our magnetization data indicates the absence of a phase transition,
not only in a transverse applied magnetic field, but also without field. A
zero-Kelvin phase transition is also unlikely, as the magnetization seems to
follow a non-critical exponential dependence on the temperature. Our analysis
thus unmasks the true, short-ranged nature of the magnetic properties of the
LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations
suggesting the lack of phase transition in this system.Comment: 5 pages, 4 figure
Constraining dark matter halo properties using lensed SNLS supernovae
This paper exploits the gravitational magnification of SNe Ia to measure
properties of dark matter haloes. The magnification of individual SNe Ia can be
computed using observed properties of foreground galaxies and dark matter halo
models. We model the dark matter haloes of the galaxies as truncated singular
isothermal spheres with velocity dispersion and truncation radius obeying
luminosity dependent scaling laws. A homogeneously selected sample of 175 SNe
Ia from the first 3-years of the Supernova Legacy Survey (SNLS) in the redshift
range 0.2 < z < 1 is used to constrain models of the dark matter haloes
associated with foreground galaxies. The best-fitting velocity dispersion
scaling law agrees well with galaxy-galaxy lensing measurements. We further
find that the normalisation of the velocity dispersion of passive and star
forming galaxies are consistent with empirical Faber-Jackson and Tully-Fisher
relations, respectively. If we make no assumption on the normalisation of these
relations, we find that the data prefer gravitational lensing at the 92 per
cent confidence level. Using recent models of dust extinction we deduce that
the impact of this effect on our results is very small. We also investigate the
brightness scatter of SNe Ia due to gravitational lensing. The gravitational
lensing scatter is approximately proportional to the SN Ia redshift. We find
the constant of proportionality to be B = 0.055 +0.039 -0.041 mag (B < 0.12 mag
at the 95 per cent confidence level). If this model is correct, the
contribution from lensing to the intrinsic brightness scatter of SNe Ia is
small for the SNLS sample.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Existence of a phase transition under finite magnetic field in the long-range RKKY Ising spin glass DyYRuSi
A phase transition of a model compound of the long-range Ising spin glass
(SG) DyYRuSi, where spins interact via the RKKY
interaction, has been investigated. The static and the dynamic scaling analyses
reveal that the SG phase transition in the model magnet belongs to the
mean-field universality class. Moreover, the characteristic relaxation time in
finite magnetic fields exhibits a critical divergent behavior as well as in
zero field, indicating a stability of the SG phase in finite fields. The
presence of the SG phase transition in field in the model magnet strongly
syggests that the replica symmetry is broken in the long-range Ising SG.Comment: 4 pages, 4 figures, to be published in JPSJ (2010
Optical response of two-dimensional electron fluids beyond the Kohn regime: strong non-parabolic confinement and intense laser light
We investigate the linear and non-linear optical response of two-dimensional
(2D) interacting electron fluids confined by a strong non-parabolic potential.
We show that such fluids may exhibit higher-harmonic spectra under realistic
experimental conditions. Higher harmonics arise as the electrons explore
anharmonicities of the confinement potential (electron-electron interactions
reduce this non-linear effect). This opens the possibility of controlling the
optical functionality of such systems by engineering the confinement potential.
Our results were obtained within time-dependent density-functional theory,
employing the adiabatic local-density approximation. A classical hydrodynamical
model is in good agreement with the quantum-mechanical results.Comment: 4 pages, 4 figure
Extended atomic data for oxygen abundance analyses
As the most abundant element in the universe after hydrogen and helium,
oxygen plays a key role in planetary, stellar, and galactic astrophysics. Its
abundance is especially influential on stellar structure and evolution, and as
the dominant opacity contributor at the base of the Sun's convection zone it is
central to the discussion around the solar modelling problem. However,
abundance analyses require complete and reliable sets of atomic data. We
present extensive atomic data for O I, by using the multiconfiguration
Dirac-Hartree-Fock and relativistic configuration interaction methods.
Lifetimes and transition probabilities for radiative electric dipole
transitions are given and compared with results from previous calculations and
available measurements. The accuracy of the computed transition rates is
evaluated by the differences between the transition rates in Babushkin and
Coulomb gauges, as well as by a cancellation factor analysis. Out of the 989
computed transitions in this work, 205 are assigned to the accuracy classes
AA-B, that is, with uncertainties less than 10%, following the criteria defined
by the National Institute of Standards and Technology Atomic Spectra Database.
We discuss the influence of the new log(gf) values on the solar oxygen
abundance and ultimately advocate .Comment: 13 pages, 5 figures; Accepted for publication in Astronomy &
Astrophysic
The implications of maintaining Earth's hemispheric albedo symmetry for shortwave radiative feedbacks
The Earth's albedo is observed to be symmetric between the hemispheres on the annual mean timescale, despite the clear-sky albedo being asymmetrically higher in the Northern Hemisphere due to more land area and aerosol sources; this is because the mean cloud distribution currently compensates for the clear-sky asymmetry almost exactly. We investigate the evolution of the hemispheric difference in albedo in the Coupled Model Intercomparison Project Phase 6 (CMIP6) coupled model simulations following an abrupt quadrupling of CO2 concentrations, to which all models respond with an initial decrease of albedo in the Northern Hemisphere (NH) due to loss of Arctic sea ice. Models disagree over whether the net effect of NH cloud responses is to reduce or amplify initial NH albedo reductions. After the initial response, the evolution of the hemispheric albedo difference diverges among models, with some models remaining stably at their new hemispheric albedo difference and others returning towards their pre-industrial difference primarily through a reduction in SH cloud cover. Whereas local increases in cloud cover contribute to negative shortwave cloud feedback, the cross-hemispheric communicating mechanism found to be primarily responsible for restoring hemispheric symmetry in the models studied implies positive shortwave cloud feedback.</p
Spin Glasses: Model systems for non-equilibrium dynamics
Spin glasses are frustrated magnetic systems due to a random distribution of
ferro- and antiferromagnetic interactions. An experimental three dimensional
(3d) spin glass exhibits a second order phase transition to a low temperature
spin glass phase regardless of the spin dimensionality. In addition, the low
temperature phase of Ising and Heisenberg spin glasses exhibits similar
non-equilibrium dynamics and an infinitely slow approach towards a
thermodynamic equilibrium state. There are however significant differences in
the detailed character of the dynamics as to memory and rejuvenation phenomena
and the influence of critical dynamics on the behaviour. In this article, some
aspects of the non-equilibrium dynamics of an Ising and a Heisenberg spin glass
are briefly reviewed and some comparisons are made to other glassy systems that
exhibit magnetic non-equilibrium dynamics.Comment: To appear in J. Phys.: Condens. Matter, Proceedings from HFM2003,
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