11,486 research outputs found
Magnetic properties of the spin-1 two-dimensional Heisenberg model on a triangular lattice
Motivated by the recent experiment in NiGaS, the spin-1 Heisenberg
model on a triangular lattice with the ferromagnetic nearest- and
antiferromagnetic third-nearest-neighbor exchange interactions,
and , is studied in the range of the parameter . Mori's projection operator technique is used as a method, which retains the
rotation symmetry of spin components and does not anticipate any magnetic
ordering. For zero temperature several phase transitions are observed. At the ground state is transformed from the ferromagnetic order into
a disordered state, which in its turn is changed to an antiferromagnetic
long-range ordered state with the incommensurate ordering vector at . With growing the ordering vector moves along the line to the
commensurate point , which is reached at . The
final state with the antiferromagnetic long-range order can be conceived as
four interpenetrating sublattices with the spin structure on each of
them. Obtained results offer a satisfactory explanation for the experimental
data in NiGaS.Comment: 2 pages, 3 figure
Phase diagram and binding energy of interacting Bose gases
From the many-body T-matrix the condition for a medium-dependent bound state
and its binding energy is derived for a homogeneous interacting Bose gas. This
condition provides the critical line in the phase diagram in terms of the
medium-dependent scattering length. Separating the Bose pole from the
distribution function the influence of a Bose condensate is discussed and a
thermal minimum of the critical scattering length is found
Phase diagram of the three-dimensional Anderson model of localization with random hopping
We examine the localization properties of the three-dimensional (3D) Anderson
Hamiltonian with off-diagonal disorder using the transfer-matrix method (TMM)
and finite-size scaling (FSS). The nearest-neighbor hopping elements are chosen
randomly according to . We find that the
off-diagonal disorder is not strong enough to localize all states in the
spectrum in contradistinction to the usual case of diagonal disorder. Thus for
any off-diagonal disorder, there exist extended states and, consequently, the
TMM converges very slowly. From the TMM results we compute critical exponents
of the metal-insulator transitions (MIT), the mobility edge , and study
the energy-disorder phase diagram.Comment: 4 pages, 5 EPS figures, uses annalen.cls style [included]; presented
at Localization 1999, to appear in Annalen der Physik [supplement
White dwarf masses in cataclysmic variables
The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has
recently been found to dramatically disagree with the predictions of the
standard CV formation model. The high mean WD mass among CVs is not imprinted
in the currently observed sample of CV progenitors and cannot be attributed to
selection effects. Two possibilities have been put forward: either the WD grows
in mass during CV evolution, or in a significant fraction of cases, CV
formation is preceded by a (short) phase of thermal time-scale mass transfer
(TTMT) in which the WD gains a sufficient amount of mass. We investigate if
either of these two scenarios can bring theoretical predictions and
observations into agreement. We employed binary population synthesis models to
simulate the present intrinsic CV population. We incorporated aspects specific
to CV evolution such as an appropriate mass-radius relation of the donor star
and a more detailed prescription for the critical mass ratio for dynamically
unstable mass transfer. We also implemented a previously suggested wind from
the surface of the WD during TTMT and tested the idea of WD mass growth during
the CV phase by arbitrarily changing the accretion efficiency. We compare the
model predictions with the characteristics of CVs derived from observed
samples. We find that mass growth of the WDs in CVs fails to reproduce the
observed WD mass distribution. In the case of TTMT, we are able to produce a
large number of massive WDs if we assume significant mass loss from the surface
of the WD during the TTMT phase. However, the model still produces too many CVs
with helium WDs. Moreover, the donor stars are evolved in many of these
post-TTMT CVs, which contradicts the observations. We conclude that in our
current framework of CV evolution neither TTMT nor WD mass growth can fully
explain either the observed WD mass or the period distribution in CVs.Comment: 15 pages, 7 figures, 1 table, accepted for publication in A&A.
Replaced and added a reference, corrected typo
Femtosecond formation of collective modes due to meanfield fluctuations
Starting from a quantum kinetic equation including the mean field and a
conserving relaxation-time approximation we derive an analytic formula which
describes the time dependence of the dielectric function in a plasma created by
a short intense laser pulse. This formula reproduces universal features of the
formation of collective modes seen in recent experimental data of femtosecond
spectroscopy. The presented formula offers a tremendous simplification for the
description of the formation of quasiparticle features in interacting systems.
Numerical demanding treatments can now be focused on effects beyond these gross
features found here to be describable analytically.Comment: 4 pages 3 figures, PRB in pres
The influence of self-citation corrections on Egghe's g index
The g index was introduced by Leo Egghe as an improvement of Hirsch's index h
for measuring the overall citation record of a set of articles. It better takes
into account the highly skewed frequency distribution of citations than the h
index. I propose to sharpen this g index by excluding the self-citations. I
have worked out nine practical cases in physics and compare the h and g values
with and without self-citations. As expected, the g index characterizes the
data set better than the h index. The influence of the self-citations appears
to be more significant for the g index than for the h index.Comment: 9 pages, 2 figures, submitted to Scientometric
Insights on star formation histories and physical properties of Herschel-detected galaxies
We test the impact of using variable star forming histories (SFHs) and the
use of the IR luminosity (LIR) as a constrain on the physical parameters of
high redshift dusty star-forming galaxies. We explore in particular the stellar
properties of galaxies in relation with their location on the SFR-M* diagram.
We perform SED fitting of the UV-NIR and FIR emissions of a large sample of
GOODS-Herschel galaxies, for which rich multi-wavelength observations are
available. We test different SFHs and imposing energy conservation in the SED
fitting process, to face issues like the age-extinction degeneracy and produce
SEDs consistent with observations. Our models work well for the majority of the
sample, with the notable exception of the high LIR end, for which we have
indications that our simple energy conservation approach cannot hold true. We
find trends in the SFHs fitting our sources depending on stellar mass M* and z.
Trends also emerge in the characteristic timescales of the SED models depending
on the location on the SFR-M* diagram. We show that whilst using the same
available observational data, we can produce galaxies less star-forming than
usually inferred, if we allow declining SFHs, while properly reproducing their
observables. These sources can be post-starbursts undergoing quenching, and
their SFRs are potentially overestimated if inferred from their LIR. Fitting
without the IR constrain leads to a strong preference for declining SFHs, while
its inclusion increases the preference of rising SFHs, more so at high z, in
tentative agreement with the cosmic star formation history. Keeping in mind
that the sample is biased towards high LIR, the evolution shaped by our model
appears as both bursty (initially) and steady-lasting (later on). The global
SFH of the sample follows the cosmic SFH with a small scatter, and is
compatible with the "downsizing" scenario of galaxy evolution.Comment: 28 pages, 26 figures, one appendix, Accepted for publication in
Astronomy & Astrophysic
Thickness dependence of linear and quadratic magneto-optical Kerr effect in ultrathin Fe(001) films
Magneto-optical Kerr effect (MOKE) magnetometry is one of the most widely
employed techniques for the characterization of ferromagnetic thin-film
samples. Some information, such as coercive fields or anisotropy strengths can
be obtained without any knowledge of the optical and magneto-optical (MO)
properties of the material. On the other hand, a quantitative analysis, which
requires a precise knowledge of the material's index of refraction n and the MO
coupling constants K and G is often desirable, for instance for the comparison
of samples, which are different with respect to ferromagnetic layer
thicknesses, substrates, or capping layers. While the values of the parameters
n and the linear MO coupling parameter K reported by different authors usually
vary considerably, the relevant quadratic MO coupling parameters G of Fe are
completely unknown. Here, we report on measurements of the thickness dependence
(0-60nm) of the linear and quadratic MOKE in epitaxial bcc-Fe(001) wedge-type
samples performed at a commonly used laser wavelength of 670nm. By fitting the
thickness dependence we are able to extract a complete set of parameters n, K,
(G11 - G12), and G44 for the quantitative description of the MOKE of
bcc-Fe(001). We find sizable different n, K, and G parameters for films thinner
than about 10nm as compared to thicker films, which is indicative of a
thickness dependence of the electronic properties or of surface contributions
to the MOKE. The effect size of the quadratic MOKE is found to be about a third
of the record values recently reported for Co2FeSi.Comment: 8 pages, 5 figure
Impact of Sodium Layer variations on the performance of the E-ELT MCAO module
Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may
exploit Natural Guide Stars to solve intrinsic limitations of artificial
beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the
impact of the sodium layer structure and variability. The sodium layer may also
have transverse structures leading to differential effects among Laser Guide
Stars. Starting from the analysis of the input perturbations related to the
Sodium Layer variability, modeled directly on measured sodium layer profiles,
we analyze, through a simplified end-to-end simulation code, the impact of the
low/medium orders induced on global performance of the European Extremely Large
Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin
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