1,249 research outputs found
Anomalous temperature dependence of the single-particle spectrum in the organic conductor TTF-TCNQ
The angle-resolved photoemission spectrum of the organic conductor TTF-TCNQ
exhibits an unusual transfer of spectral weight over a wide energy range for
temperatures 60K<T<260K. In order to investigate the origin of this finding,
here we report numerical results on the single-particle spectral weight
A(k,omega) for the one-dimensional (1D) Hubbard model and, in addition, for the
1D extended Hubbard and the 1D Hubbard-Holstein models. Comparisons with the
photoemission data suggest that the 1D Hubbard model is not sufficient for
explaining the unusual T dependence, and the long-range part of the Coulomb
repulsion also needs to be included.Comment: 4 pages, 4 figure
d_{x^2-y^2} Symmetry and the Pairing Mechanism
An important question is if the gap in the high temperature cuprates has
d_{x^2-y^2} symmetry, what does that tell us about the underlying interaction
responsible for pairing. Here we explore this by determining how three
different types of electron-phonon interactions affect the d_{x^2-y^2} pairing
found within an RPA treatment of the 2D Hubbard model. These results imply that
interactions which become more positive as the momentum transfer increases
favor d_{x^2-y^2} pairing in a nearly half-filled band.Comment: 9 pages and 2 eps figs, uses revtex with epsf, in press, PR
Temperature dependence of spinon and holon excitations in one-dimensional Mott insulators
Motivated by the recent angle-resolved photoemission spectroscopy (ARPES)
measurements on one-dimensional Mott insulators, SrCuO and
NaVO, we examine the single-particle spectral weight
of the one-dimensional (1D) Hubbard model at half-filling. We are particularly
interested in the temperature dependence of the spinon and holon excitations.
For this reason, we have performed the dynamical density matrix renormalization
group and determinantal quantum Monte Carlo (QMC) calculations for the
single-particle spectral weight of the 1D Hubbard model. In the QMC data, the
spinon and holon branches become observable at temperatures where the
short-range antiferromagnetic correlations develop. At these temperatures, the
spinon branch grows rapidly. In the light of the numerical results, we discuss
the spinon and holon branches observed by the ARPES experiments on
SrCuO. These numerical results are also in agreement with the
temperature dependence of the ARPES results on NaVO.Comment: 8 pages, 8 figure
Effects of spin fluctuations in the t-J model
Recent experiments on the Fermi surface and the electronic structure of the
cuprate-supercondutors showed the importance of short range antiferromagnetic
correlations for the physics in these systems. Theoretically, features like
shadow bands were predicted and calculated mainly for the Hubbard model. In our
approach we calculate an approximate selfenergy of the - model. Solving
the Hubbard model in the Dynamical Mean Field Theory (DMFT) yields a
selfenergy that contains most of the local correlations as a starting point.
Effects of the nearest neighbor spin interaction are then included in a
heuristical manner. Formally like in -perturbation theory all ring diagrams,
with the single bubble assumed to be purely local, are summed to get a
correction to the DMFT-self engergy This procedure causes new bands and can
furnish strong deformation of quasiparticle bands. % Our results are finally
compared with %former approaches to the Hubbard model.Comment: 3 Pages, Latex, 2 Postscript-Figures submitted to Physica
Anomalous magnetic properties near Mott transition in Kagom\'e lattice Hubbard model
We investigate the characteristics of the metallic phase near the Mott
transition in the Kagom\'e lattice Hubbard model using the cellular dynamical
mean field theory. By calculating the specific heat and spin correlation
functions, we demonstrate that the quasiparticles show anomalous properties in
the metallic phase close to the Mott transition. We find clear evidence for the
multi-band heavy quasiparticles in the specific heat, which gives rise to
unusual temperature dependence of the spin correlation functions.Comment: 2 pages, 3 figures, accepted for publication in J. Mag. Mag. Mater.
(Proceedings of the ICM, Kyoto, Japan, August 2006
Neutron scattering in a d_{x^2-y^2}-wave superconductor with strong impurity scattering and Coulomb correlations
We calculate the spin susceptibility at and below T_c for a d_{x^2-y^2}-wave
superconductor with resonant impurity scattering and Coulomb correlations. Both
the impurity scattering and the Coulomb correlations act to maintain peaks in
the spin susceptibility, as a function of momentum, at the Brillouin zone edge.
These peaks would otherwise be suppressed by the superconducting gap. The
predicted amount of suppression of the spin susceptibility in the
superconducting state compared to the normal state is in qualitative agreement
with results from recent magnetic neutron scattering experiments on
La_{1.86}Sr_{0.14}CuO_4 for momentum values at the zone edge and along the zone
diagonal. The predicted peak widths in the superconducting state, however, are
narrower than those in the normal state, a narrowing which has not been
observed experimentally.Comment: 24 pages (12 tarred-compressed-uuencoded Postscript figures), REVTeX
3.0 with epsf macros, UCSBTH-94-1
Randomised field trial to evaluate serological response after foot-and-mouth disease vaccination in Turkey
AbstractDespite years of biannual mass vaccination of cattle, foot-and-mouth disease (FMD) remains uncontrolled in Anatolian Turkey. To evaluate protection after mass vaccination we measured post-vaccination antibodies in a cohort of cattle (serotypes O, A and Asia-1). To obtain results reflecting typical field protection, participants were randomly sampled from across Central and Western Turkey after routine vaccination. Giving two-doses one month apart is recommended when cattle are first vaccinated against FMD. However, due to cost and logistics, this is not routinely performed in Turkey, and elsewhere. Nested within the cohort, we conducted a randomised trial comparing post-vaccination antibodies after a single-dose versus a two-dose primary vaccination course.Four to five months after vaccination, only a third of single-vaccinated cattle had antibody levels above a threshold associated with protection. A third never reached this threshold, even at peak response one month after vaccination. It was not until animals had received three vaccine doses in their lifetime, vaccinating every six months, that most (64% to 86% depending on serotype) maintained antibody levels above this threshold. By this time cattle would be >20 months old with almost half the population below this age. Consequently, many vaccinated animals will be unprotected for much of the year. Compared to a single-dose, a primary vaccination course of two-doses greatly improved the level and duration of immunity. We concluded that the FMD vaccination programme in Anatolian Turkey did not produce the high levels of immunity required. Higher potency vaccines are now used throughout Turkey, with a two-dose primary course in certain areas.Monitoring post-vaccination serology is an important component of evaluation for FMD vaccination programmes. However, consideration must be given to which antigens are present in the test, the vaccine and the field virus. Differences between these antigens affect the relationship between antibody titre and protection
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Single- and multi-band Hubbard models have been found to describe many of the
complex phenomena that are observed in the cuprate and iron-based
high-temperature superconductors. Simulations of these models therefore provide
an ideal framework to study and understand the superconducting properties of
these systems and the mechanisms responsible for them. Here we review recent
dynamic cluster quantum Monte Carlo simulations of these models, which provide
an unbiased view of the leading correlations in the system. In particular, we
discuss what these simulations tell us about superconductivity in the
homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this
behavior. We then describe recent simulations of a bilayer Hubbard model, which
provides a simple model to study the type and nature of pairing in systems with
multiple Fermi surfaces such as the iron-based superconductors.Comment: Published as part of Superstripes 2011 (Rome) conference proceeding
Slow release and delivery of antisense oligonucleotide drug by self-assembled peptide amphiphile nanofibers
Cataloged from PDF version of article.Antisense oligonucleotides provide a promising therapeutic approach for several disorders including cancer. Chemical stability, controlled release, and intracellular delivery are crucial factors determining their efficacy. Gels composed of nanofibrous peptide network have been previously suggested as carriers for controlled delivery of drugs to improve stability and to provide controlled release, but have not been used for oligonucleotide delivery. In this work, a self-assembled peptide nanofibrous system is formed by mixing a cationic peptide amphiphile (PA) with Bcl-2 antisense oligodeoxynucleotide (ODN), G3139, through electrostatic interactions. The self-assembly of PA-ODN gel was characterized by circular dichroism, rheology, atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM and SEM images revealed establishment of the nanofibrous PA-ODN network. Due to the electrostatic interactions between PA and ODN, ODN release can be controlled by changing PA and ODN concentrations in the PA-ODN gel. Cellular delivery of the ODN by PA-ODN nanofiber complex was observed by using fluorescently labeled ODN molecule. Cells incubated with PA-ODN complex had enhanced cellular uptake compared to cells incubated with naked ODN. Furthermore, Bcl-2 mRNA amounts were lower in MCF-7 human breast cancer cells in the presence of PA-ODN complex compared to naked ODN and mismatch ODN evidenced by quantitative RT-PCR studies. These results suggest that PA molecules can control ODN release, enhance cellular uptake and present a novel efficient approach for gene therapy studies and oligonucleotide based drug deliver
Charge transfer fluctuation, wave superconductivity, and the Raman phonon in the Cuprates: A detailed analysis
The Raman spectrum of the phonon in the superconducting cuprate
materials is investigated theoretically in detail in both the normal and
superconducting phases, and is contrasted with that of the phonon. A
mechanism involving the charge transfer fluctuation between the two oxygen ions
in the CuO plane coupled to the crystal field perpendicular to the plane is
discussed and the resulting electron-phonon coupling is evaluated. Depending on
the symmetry of the phonon the weight of different parts of the Fermi surface
in the coupling is different. This provides the opportunity to obtain
information on the superconducting gap function at certain parts of the Fermi
surface. The lineshape of the phonon is then analyzed in detail both in the
normal and superconducting states. The Fano lineshape is calculated in the
normal state and the change of the linewidth with temperature below T is
investigated for a pairing symmetry. Excellent agreement is
obtained for the phonon lineshape in YBaCuO. These
experiments, however, can not distinguish between and a
highly anisotropic -wave pairing.Comment: Revtex, 21 pages + 4 postscript figures appended, tp
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