15,158 research outputs found
Spin-Current Relaxation Time in Spin-Polarized Heisenberg Paramagnets
We study the spatial Fourier transform of the spin correlation function
G_q(t) in paramagnetic quantum crystals by direct simulation of a 1d lattice of
atoms interacting via a nearest-neighbor Heisenberg exchange Hamiltonian. Since
it is not practical to diagonalize the s=1/2 exchange Hamiltonian for a lattice
which is of sufficient size to study long-wavelength (hydrodynamic)
fluctuations, we instead study the s -> infinity limit and treat each spin as a
vector with a classical equation of motion. The simulations give a detailed
picture of the correlation function G_q(t) and its time derivatives. At high
polarization, there seems to be a hierarchy of frequency scales: the local
exchange frequency, a wavelength-independent relaxation rate 1/tau that
vanishes at large polarization P ->1, and a wavelength-dependent spin-wave
frequency proportional to q^2. This suggests a form for the correlation
function which modifies the spin diffusion coefficients obtained in a moments
calculation by Cowan and Mullin, who used a standard Gaussian ansatz for the
second derivative of the correlation function.Comment: 6 pages, 3 figure
Composite Fermions and quantum Hall systems: Role of the Coulomb pseudopotential
The mean field composite Fermion (CF) picture successfully predicts angular
momenta of multiplets forming the lowest energy band in fractional quantum Hall
(FQH) systems. This success cannot be attributed to a cancellation between
Coulomb and Chern-Simons interactions beyond the mean field, because these
interactions have totally different energy scales. Rather, it results from the
behavior of the Coulomb pseudopotential V(L) (pair energy as a function of pair
angular momentum) in the lowest Landau level (LL). The class of short range
repulsive pseudopotentials is defined that lead to short range Laughlin like
correlations in many body systems and to which the CF model can be applied.
These Laughlin correlations are described quantitatively using the formalism of
fractional parentage. The discussion is illustrated with an analysis of the
energy spectra obtained in numerical diagonalization of up to eleven electrons
in the lowest and excited LL's. The qualitative difference in the behavior of
V(L) is shown to sometimes invalidate the mean field CF picture when applied to
higher LL's. For example, the nu=7/3 state is not a Laughlin nu=1/3 state in
the first excited LL. The analysis of the involved pseudopotentials also
explains the success or failure of the CF picture when applied to other systems
of charged Fermions with Coulomb repulsion, such as the Laughlin quasiparticles
in the FQH hierarchy or charged excitons in an electron-hole plasma.Comment: 27 pages, 23 figures, revised version (significant changes in text
and figures), submitted to Phil. Mag.
Clusters under strong VUV pulses: A quantum-classical hybrid-description incorporating plasma effects
The quantum-classical hybrid-description of rare-gas clusters interacting
with intense light pulses which we have developed is described in detail. Much
emphasis is put on the treatment of screening electrons in the cluster which
set the time scale for the evolution of the system and form the link between
electrons strongly bound to ions and quasi-free plasma electrons in the
cluster. As an example we discuss the dynamics of an Ar147 cluster exposed to a
short VUV laser pulse of 20eV photon energy.Comment: 8 pages, 9 figure
Temperature-dependent Fermi surface evolution in heavy fermion CeIrIn5
In Cerium-based heavy electron materials, the 4f electron's magnetic moments
bind to the itinerant quasiparticles to form composite heavy quasiparticles at
low temperature. The volume of the Fermi surfacein the Brillouin zone
incorporates the moments to produce a "large FS" due to the Luttinger theorem.
When the 4f electrons are localized free moments, a "small FS" is induced since
it contains only broad bands of conduction spd electrons. We have addressed
theoretically the evolution of the heavy fermion FS as a function of
temperature, using a first principles dynamical mean-field theory (DMFT)
approach combined with density functional theory (DFT+DMFT). We focus on the
archetypical heavy electrons in CeIrIn5, which is believed to be near a quantum
critical point. Upon cooling, both the quantum oscillation frequencies and
cyclotron masses show logarithmic scaling behavior (~ ln(T_0/T)) with different
characteristic temperatures T_0 = 130 and 50 K, respectively. The resistivity
coherence peak observed at T ~ 50 K is the result of the competition between
the binding of incoherent 4f electrons to the spd conduction electrons at Fermi
level and the formation of coherent 4f electrons.Comment: 5 pages main article,3 figures for the main article, 2 page
Supplementary information, 2 figures for the Supplementary information.
Supplementary movie 1 and 2 are provided on the
webpage(http://www-ph.postech.ac.kr/~win/supple.html
Systematic computation of crystal field multiplets for X-ray core spectroscopies
We present a new approach to computing multiplets for core spectroscopies,
whereby the crystal field is constructed explicitly from the positions and
charges of surrounding atoms. The simplicity of the input allows the
consideration of crystal fields of any symmetry, and in particular facilitates
the study of spectroscopic effects arising from low symmetry environments. The
interplay between polarization directions and crystal field can also be
conveniently investigated. The determination of the multiplets proceeds from a
Dirac density functional atomic calculation, followed by the exact
diagonalization of the Coulomb, spin-orbit and crystal field interactions for
the electrons in the open shells. The eigenstates are then used to simulate
X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering spectra.
In examples ranging from high symmetry down to low symmetry environment,
comparisons with experiments are done with unadjusted model parameters as well
as with semi-empirically optimized ones. Furthermore, predictions for the RIXS
of low-temperature MnO and for Dy in a molecular complex are proposed.Comment: Accepted for publication in Phys. Rev.
Augmented collisional ionization via excited states in XUV cluster interactions
The impact of atomic excited states is investigated via a detailed model of
laser-cluster interactions, which is applied to rare gas clusters in intense
femtosecond pulses in the extreme ultraviolet (XUV). This demonstrates the
potential for a two-step ionization process in laser-cluster interactions, with
the resulting intermediate excited states allowing for the creation of high
charge states and the rapid dissemination of laser pulse energy. The
consequences of this excitation mechanism are demonstrated through simulations
of recent experiments in argon clusters interacting with XUV radiation, in
which this two-step process is shown to play a primary role; this is consistent
with our hypothesis that XUV-cluster interactions provide a unique window into
the role of excited atomic states due to the relative lack of photoionization
and laser field-driven phenomena. Our analysis suggests that atomic excited
states may play an important role in interactions of intense radiation with
materials in a variety of wavelength regimes, including potential implications
for proposed studies of single molecule imaging with intense X-rays.Comment: 4 pages, 2 figure
The Environment of Galaxies at Low Redshift
We compare environmental effects in two analogous samples of galaxies, one
from the Sloan Digital Sky Survey (SDSS) and the other from a semi-analytic
model (SAM) based on the Millennium Simulation (MS), to test to what extent
current SAMs of galaxy formation are reproducing environmental effects. We
estimate the large-scale environment of each galaxy using a Bayesian density
estimator based on distances to all ten nearest neighbors and compare
broad-band photometric properties of the two samples as a function of
environment. The feedbacks implemented in the semi-analytic model produce a
qualitatively correct galaxy population with similar environmental dependence
as that seen in SDSS galaxies. In detail, however, the colors of MS galaxies
exhibit an exaggerated dependence on environment: the field contains too many
blue galaxies while clusters contain too many red galaxies, compared to the
SDSS sample. We also find that the MS contains a population of highly
clustered, relatively faint red galaxies with velocity dispersions comparable
to their Hubble flow. Such high-density galaxies, if they exist, would be
overlooked in any low-redshift survey since their membership to a cluster
cannot be determined due to the "Fingers of God" effect.Comment: 15 pages, 5 figures, ApJL accepted, fixed author listing
Empirically Derived Integrated Stellar Yields of Fe-Peak Elements
We present here the initial results of a new study of massive star yields of
Fe-peak elements. We have compiled from the literature a database of carefully
determined solar neighborhood stellar abundances of seven iron-peak elements,
Ti, V, Cr, Mn, Fe, Co, and Ni and then plotted [X/Fe] versus [Fe/H] to study
the trends as functions of metallicity. Chemical evolution models were then
employed to force a fit to the observed trends by adjusting the input massive
star metallicity-sensitive yields of Kobayashi et al. Our results suggest that
yields of Ti, V, and Co are generally larger as well as anticorrelated with
metallicity, in contrast to the Kobayashi et al. predictions. We also find the
yields of Cr and Mn to be generally smaller and directly correlated with
metallicity compared to the theoretical results. Our results for Ni are
consistent with theory, although our model suggests that all Ni yields should
be scaled up slightly. The outcome of this exercise is the computation of a set
of integrated yields, i.e., stellar yields weighted by a slightly flattened
time-independent Salpeter initial mass function and integrated over stellar
mass, for each of the above elements at several metallicity points spanned by
the broad range of observations. These results are designed to be used as
empirical constraints on future iron-peak yield predictions by stellar
evolution modelers. Special attention is paid to the interesting behavior of
[Cr/Co] with metallicity -- these two elements have opposite slopes -- as well
as the indirect correlation of [Ti/Fe] with [Fe/H]. These particular trends, as
well as those exhibited by the inferred integrated yields of all iron-peak
elements with metallicity, are discussed in terms of both supernova
nucleosynthesis and atomic physics.Comment: 27 pages, 6 figures; Accepted for Publication in the Astrophysical
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