1,247 research outputs found
A non trivial extension of the two-dimensional Ising model: the d-dimensional "molecular" model
A recently proposed molecular model is discussed as a non-trivial extension
of the Ising model. For d=2 the two models are shown to be equivalent, while
for d>2 the molecular model describes a peculiar second order transition from
an isotropic high temperature phase to a low-dimensional anisotropic low
temperature state. The general mean field analysis is compared with the results
achieved by a variational Migdal-Kadanoff real space renormalization group
method and by standard Monte Carlo sampling for d=3. By finite size scaling the
critical exponent has been found to be 0.44\pm 0.02 thus establishing that the
molecular model does not belong to the universality class of the Ising model
for d>2.Comment: 25 pages, 5 figure
Atomic structure at 2.5 Å resolution of uridine phosphorylase from E. coli as refined in the monoclinic crystal lattice
AbstractUridine phosphorylase from E. coli (Upase) has been crystallized using vapor diffusion technique in a new monoclinic crystal form. The structure was determined by the molecular replacement method at 2.5 Å resolution. The coordinates of the trigonal crystal form were used as a starting model and the refinement by the program XPLOR led to the R-factor of 18.6%. The amino acid fold of the protein was found to be the same as that in the trigonal crystals. The positions of flexible regions were refined. The conclusion about the involvement in the active site is in good agreement with the results of the biochemical experiments
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
GRB 081028 and its late-time afterglow re-brightening
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical SocietySwift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure Eγ,iso measurements.Peer reviewe
Non-initiation of hepatitis C virus antiviral therapy in patients with human immunodeficiency virus/hepatitis C virus co-infection
AIM: To assess whether reasons for hepatitis C virus (HCV) therapy non-initiation differentially affect racial and ethnic minorities with human immunodeficiency virus (HIV)/HCV co-infection. METHODS: Analysis included co-infected HCV treatment- naïve patients in the University of North Carolina CFAR HIV Clinical Cohort (January 1, 2004 and December 31, 2011). Medical records were abstracted to document non-modifiable medical (e.g. , hepatic decompensation, advanced immunosuppression), potentially modifiable medical (e.g. , substance abuse, severe depression, psychiatric illness), and non-medical (e.g. , personal, social, and economic factors) reasons for non-initiation. Statistical differences in the prevalence of reasons for non-treatment between racial/ethnic groups were assessed using the two-tailed Fisher's exact test. Three separate regression models were fit for each reason category. Odds ratios and their 95%CIs (Wald's) were computed. RESULTS: One hundred and seventy-one patients with HIV/HCV co-infection within the cohort met study inclusion. The study sample was racially and ethnically diverse; most patients were African-American (74%), followed by Caucasian (19%), and Hispanic/other (7%). The median age was 46 years (interquartile range = 39-50) and most patients were male (74%). Among the 171 patients, reasons for non-treatment were common among all patients, regardless of race/ethnicity (50% with ≥ 1 non-modifiable medical reason, 66% with ≥ 1 potentially modifiable medical reason, and 66% with ≥ 1 non-medical reason). There were no significant differences by race/ethnicity. Compared to Caucasians, African-Americans did not have increased odds of nonmodifiable [adjusted odds ratio (aOR) = 1.47, 95%CI: 0.57-3.80], potentially modifiable (aOR = 0.72, 95%CI: 0.25-2.09) or non-medical (aOR = 0.90, 95%CI: 0.32-2.52) reasons for non-initiation. CONCLUSION: Race/ethnicity alone is not predictive of reasons for HCV therapy non-initiation. Targeted interventions are needed to improve access to therapy for all co-infected patients, including minorities
Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study
A recent experiment by Shimizu et al. has provided evidence of a
superconducting phase in hcp Fe under pressure. To study the
pressure-dependence of this superconducting phase we have calculated the phonon
frequencies and the electron-phonon coupling in hcp Fe as a function of the
lattice parameter, using the linear response (LR) scheme and the full potential
linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the
Eliashberg functions indicate that conventional s-wave
electron-phonon coupling can definitely account for the appearance of the
superconducting phase in hcp Fe. However, the observed change in the transition
temperature with increasing pressure is far too rapid compared with the
calculated results. For comparison with the linear response results, we have
computed the electron-phonon coupling also by using the rigid muffin-tin (RMT)
approximation. From both the LR and the RMT results it appears that
electron-phonon interaction alone cannot explain the small range of volume over
which superconductivity is observed. It is shown that
ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from
magnetic impurities (spin-ordered clusters) can account for the observed values
of the transition temperatures but cannot substantially improve the agreeemnt
between the calculated and observed presure/volume range of the superconducting
phase. A simplified treatment of p-wave pairing leads to extremely small ( K) transition temperatures. Thus our calculations seem to rule out
both - and - wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR
The Spectrum of Electromagnetic Jets from Kerr Black Holes and Naked Singularities in the Teukolsky Perturbation Theory
We give a new theoretical basis for examination of the presence of the Kerr
black hole (KBH) or the Kerr naked singularity (KNS) in the central engine of
different astrophysical objects around which astrophysical jets are typically
formed: X-ray binary systems, gamma ray bursts (GRBs), active galactic nuclei
(AGN), etc. Our method is based on the study of the exact solutions of the
Teukolsky master equation for electromagnetic perturbations of the Kerr metric.
By imposing original boundary conditions on the solutions so that they describe
a collimated electromagnetic outflow, we obtain the spectra of possible {\em
primary jets} of radiation, introduced here for the first time. The theoretical
spectra of primary electromagnetic jets are calculated numerically. Our main
result is a detailed description of the qualitative change of the behavior of
primary electromagnetic jet frequencies under the transition from the KBH to
the KNS, considered here as a bifurcation of the Kerr metric. We show that
quite surprisingly the novel spectra describe linearly stable primary
electromagnetic jets from both the KBH and the KNS. Numerical investigation of
the dependence of these primary jet spectra on the rotation of the Kerr metric
is presented and discussed.Comment: 18 pages, 35 figures, LaTeX file. Final version. Accepted for
publication in Astrophysics and Space Science. Amendments. Typos corrected.
Novel notion -"primary jet" is introduced. New references and comments adde
Theory of charge transport in diffusive normal metal / unconventional singlet superconductor contacts
We analyze the transport properties of contacts between unconventional
superconductor and normal diffusive metal in the framework of the extended
circuit theory. We obtain a general boundary condition for the Keldysh-Nambu
Green's functions at the interface that is valid for arbitrary transparencies
of the interface. This allows us to investigate the voltage-dependent
conductance (conductance spectrum) of a diffusive normal metal (DN)/
unconventional singlet superconductor junction in both ballistic and diffusive
cases. For d-wave superconductor, we calculate conductance spectra numerically
for different orientations of the junctions, resistances, Thouless energies in
DN, and transparencies of the interface. We demonstrate that conductance
spectra exhibit a variety of features including a -shaped gap-like
structure, zero bias conductance peak (ZBCP) and zero bias conductance dip
(ZBCD). We show that two distinct mechanisms: (i) coherent Andreev reflection
(CAR) in DN and (ii) formation of midgap Andreev bound state (MABS) at the
interface of d-wave superconductors, are responsible for ZBCP, their relative
importance being dependent on the angle between the interface normal
and the crystal axis of d-wave superconductors. For , the ZBCP is due
to CAR in the junctions of low transparency with small Thouless energies, this
is similar to the case of diffusive normal metal / insulator /s-wave
superconductor junctions. With increase of from zero to , the
MABS contribution to ZBCP becomes more prominent and the effect of CAR is
gradually suppressed. Such complex spectral features shall be observable in
conductance spectra of realistic high- junctions at very low temperature
Dynamic screening in solar and stellar nuclear reactions
In the hot, dense plasma of solar and stellar interiors, the Coulomb
interaction is screened by the surrounding plasma. Although the standard
Salpeter approximation for static screening is widely accepted and used in
stellar modeling, the question of dynamic screening has been revisited. In
particular, Shaviv and Shaviv apply the techniques of molecular dynamics to the
conditions in the solar core in order to numerically determine the dynamic
screening effect. By directly calculating the motion of ions and electrons due
to Coulomb interactions, they compute the effect of screening without the
mean-field assumption inherent in the Salpeter approximation. Here we reproduce
their numerical analysis of the screening energy in the plasma of the solar
core and conclude that the effects of dynamic screening are relevant and should
be included in the treatment of the plasma, especially in the computation of
stellar nuclear reaction rates.Comment: Astrophysics and Space Science, Special Issue Solar & Stellar
Modelling Corrected sign error. Now consistent with final published versio
Tomography of pairing symmetry from magnetotunneling spectroscopy -- a case study for quasi-1D organic superconductors
We propose that anisotropic -, -, or -wave pairing symmetries can be
distinguished from a tunneling spectroscopy in the presence of magnetic fields,
which is exemplified here for a model organic superconductor .
The shape of the Fermi surface (quasi-one-dimensional in this example) affects
sensitively the pairing symmetry, which in turn affects the shape (U or V) of
the gap along with the presence/absence of the zero-bias peak in the tunneling
in a subtle manner. Yet, an application of a magnetic field enables us to
identify the symmetry, which is interpreted as an effect of the Doppler shift
in Andreev bound states.Comment: 4 papegs, 4 figure
- …