12,460 research outputs found
Excited-state quantum phase transitions in a two-fluid Lipkin model
Background: Composed systems have became of great interest in the framework
of the ground state quantum phase transitions (QPTs) and many of their
properties have been studied in detail. However, in these systems the study of
the so called excited-state quantum phase transitions (ESQPTs) have not
received so much attention.
Purpose: A quantum analysis of the ESQPTs in the two-fluid Lipkin model is
presented in this work. The study is performed through the Hamiltonian
diagonalization for selected values of the control parameters in order to cover
the most interesting regions of the system phase diagram. [Method:] A
Hamiltonian that resembles the consistent-Q Hamiltonian of the interacting
boson model (IBM) is diagonalized for selected values of the parameters and
properties such as the density of states, the Peres lattices, the
nearest-neighbor spacing distribution, and the participation ratio are
analyzed.
Results: An overview of the spectrum of the two-fluid Lipkin model for
selected positions in the phase diagram has been obtained. The location of the
excited-state quantum phase transition can be easily singled out with the Peres
lattice, with the nearest-neighbor spacing distribution, with Poincar\'e
sections or with the participation ratio.
Conclusions: This study completes the analysis of QPTs for the two-fluid
Lipkin model, extending the previous study to excited states. The ESQPT
signatures in composed systems behave in the same way as in single ones,
although the evidences of their presence can be sometimes blurred. The Peres
lattice turns out to be a convenient tool to look into the position of the
ESQPT and to define the concept of phase in the excited states realm
Solar energy storage using a Cu2O-TiO photocathode in a lithium battery
A Cu2O-TiO2 photoelectrode is pr+oposed for simultaneous solar light energy harvesting and storing of electrochemical energy in an adapted lithium coin cell. The p-type Cu2O semiconductor layer is the light harvester component of the photoelectrode and the TiO2 film performs as the capacitive layer. The rationale of the energy scheme shows that the photocharges generated in the Cu2O semiconductor induce lithiation/delithiation processes in the TiO2 film as a function of the applied bias voltage and light power. A photorechargeable lithium button cell drilled on one side recharges with visible white light in ≈9 h in open circuit. It provides an energy density of ≈150 mAh g−1 at 0.1 C discharge current in dark, and the overall efficiency is 0.29%. This work draws a new approach for the photoelectrode role to advance in monolithic rechargeable batteries
Interstitial Fractionalization and Spherical Crystallography
Finding the ground states of identical particles packed on spheres has
relevance for stabilizing emulsions and a venerable history in the literature
of theoretical physics and mathematics. Theory and experiment have confirmed
that defects such as disclinations and dislocations are an intrinsic part of
the ground state. Here we discuss the remarkable behavior of vacancies and
interstitials in spherical crystals. The strain fields of isolated
disclinations forced in by the spherical topology literally rip interstitials
and vacancies apart, typically into dislocation fragments that combine with the
disclinations to create small grain boundary scars. The fractionation is often
into three charge-neutral dislocations, although dislocation pairs can be
created as well. We use a powerful, freely available computer program to
explore interstitial fractionalization in some detail, for a variety of power
law pair potentials. We investigate the dependence on initial conditions and
the final state energies, and compare the position dependence of interstitial
energies with the predictions of continuum elastic theory on the sphere. The
theory predicts that, before fragmentation, interstitials are repelled from
5-fold disclinations and vacancies are attracted. We also use vacancies and
interstitials to study low energy states in the vicinity of "magic numbers"
that accommodate regular icosadeltahedral tessellations.Comment: 21 pages, 9 figure
The soft X-ray and narrow-line emission of Mrk573 on kiloparcec scales
We present a study of the circumnuclear region of the nearby Seyfert galaxy
Mrk573 using Chandra, XMM-Newton and HST data. The X-ray morphology shows a
biconical region extending up to 12 arcsecs (4 kpc) in projection from the
nucleus. A strong correlation between the X-rays and the highly ionized gas
seen in the [O III] image is reported. Moreover, we have studied the line
intensities detected with the RGS/XMM-Newton and used them to fit the low
resolution EPIC/XMM-Newton and ACIS/Chandra spectra. The RGS spectrum is
dominated by emission lines of C VI, O VII, O VIII, Fe XVII, and Ne IX, among
others. A good fit is obtained using these emission lines found in the RGS
spectrum as a template for Chandra spectra of the nucleus and extended
emission. The photoionization model Cloudy provides a reasonable fit for both
the nuclear region and the cone-like structures. For the nucleus the emission
is modelled using two phases: a high ionization [log(U)=1.23] and a low
ionization [log(U)=0.13]. For the high ionization phase the transmitted and
reflected component are in a ratio 1:2, whereas for the low ionization the
reflected component dominates. For the extended emission, we successfully
reproduced the emission with two phases. The first phase shows a higher
ionization parameter for the NW (log(U)=0.9) than for the SE cone (log(U)=0.3).
The second phase shows a low ionization parameter (log(U)=-3) and is rather
uniform for NW and SE cones. In addition, the nuclear optical/infrared SED has
been modeled by a clumpy torus model. The torus bolometric luminosity agrees
with the AGN luminosity inferred from the observed hard X-ray spectrum. The
optical depth along the line of sight derived from the SED fit indicates a high
neutral column density in agreement with the classification of the nucleus as a
Compton-thick AGN.Comment: 15 pages, 14 figures, final version of the paper submitted to Ap
Expansion for the solutions of the Bogomolny equations on the torus
We show that the solutions of the Bogomolny equations for the Abelian Higgs
model on a two-dimensional torus, can be expanded in powers of a quantity
epsilon measuring the departure of the area from the critical area. This allows
a precise determination of the shape of the solutions for all magnetic fluxes
and arbitrary position of the Higgs field zeroes. The expansion is carried out
to 51 orders for a couple of representative cases, including the unit flux
case. We analyse the behaviour of the expansion in the limit of large areas, in
which case the solutions approach those on the plane. Our results suggest
convergence all the way up to infinite area.Comment: 26 pages, 8 figures, slightly revised version as published in JHE
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