3,723 research outputs found
Charge and Spin Structures of a Superconductor in the Proximity of an Antiferromagnetic Mott Insulator
To the Hubbard model on a square lattice we add an interaction, , which
depends upon the square of a near-neighbor hopping. We use zero temperature
quantum Monte Carlo simulations on lattice sizes up to , to show
that at half-filling and constant value of the Hubbard repulsion, the
interaction triggers a quantum transition between an antiferromagnetic Mott
insulator and a superconductor. With a combination of finite
temperature quantum Monte Carlo simulations and the Maximum Entropy method, we
study spin and charge degrees of freedom in the superconducting state. We give
numerical evidence for the occurrence of a finite temperature
Kosterlitz-Thouless transition to the superconducting state.
Above and below the Kosterlitz-Thouless transition temperature, , we
compute the one-electron density of states, , the spin relaxation
rate , as well as the imaginary and real part of the spin susceptibility
. The spin dynamics are characterized by the vanishing of
and divergence of in the low
temperature limit. As is approached develops a pseudo-gap
feature and below shows a peak
at finite frequency.Comment: 46 pages (latex) including 14 figures in encapsulated postscript
format. Submitted for publication in Phys. Rev.
Entropic Interactions in Suspensions of Semi-Flexible Rods: Short-Range Effects of Flexibility
We compute the entropic interactions between two colloidal spheres immersed
in a dilute suspension of semi-flexible rods. Our model treats the
semi-flexible rod as a bent rod at fixed angle, set by the rod contour and
persistence lengths. The entropic forces arising from this additional
rotational degree of freedom are captured quantitatively by the model, and
account for observations at short range in a recent experiment. Global fits to
the interaction potential data suggest the persistence length of fd-virus is
about two to three times smaller than the commonly used value of .Comment: 4 pages, 5 figures, submitted to PRE rapid communication
The Red-Sequence Luminosity Function in Galaxy Clusters since z~1
We use a statistical sample of ~500 rich clusters taken from 72 square
degrees of the Red-Sequence Cluster Survey (RCS-1) to study the evolution of
~30,000 red-sequence galaxies in clusters over the redshift range 0.35<z<0.95.
We construct red-sequence luminosity functions (RSLFs) for a well-defined,
homogeneously selected, richness limited sample. The RSLF at higher redshifts
shows a deficit of faint red galaxies (to M_V=> -19.7) with their numbers
increasing towards the present epoch. This is consistent with the `down-sizing`
picture in which star-formation ended at earlier times for the most massive
(luminous) galaxies and more recently for less massive (fainter) galaxies. We
observe a richness dependence to the down-sizing effect in the sense that, at a
given redshift, the drop-off of faint red galaxies is greater for poorer (less
massive) clusters, suggesting that star-formation ended earlier for galaxies in
more massive clusters. The decrease in faint red-sequence galaxies is
accompanied by an increase in faint blue galaxies, implying that the process
responsible for this evolution of faint galaxies is the termination of
star-formation, possibly with little or no need for merging. At the bright end,
we also see an increase in the number of blue galaxies with increasing
redshift, suggesting that termination of star-formation in higher mass galaxies
may also be an important formation mechanism for higher mass ellipticals. By
comparing with a low-redshift Abell Cluster sample, we find that the
down-sizing trend seen within RCS-1 has continued to the local universe.Comment: ApJ accepted. 11 pages, 5 figure
Effect of Finite Impurity Mass on the Anderson Orthogonality Catastrophe in One Dimension
A one-dimensional tight-binding Hamiltonian describes the evolution of a
single impurity interacting locally with electrons. The impurity spectral
function has a power-law singularity
with the same exponent
that characterizes the logarithmic decay of the quasiparticle weight
with the number of electrons , . The exponent
is computed by (1) perturbation theory in the interaction strength and
(2) numerical evaluations with exact results for small systems and variational
results for larger systems. A nonanalytical behavior of is observed in
the limit of infinite impurity mass. For large interaction strength, the
exponent depends strongly on the mass of the impurity in contrast to the
perturbative result.Comment: 26 pages, RevTeX, 7 figures included, to be published in Phys. Rev.
Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model
We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure
A Multi-Component Measurement of the Cosmic Ray Composition Between 10^{17} eV and 10^{18} eV
The average mass composition of cosmic rays with primary energies between
eV and eV has been studied using a hybrid detector consisting
of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array.
Measurements have been made of the change in the depth of shower maximum,
, and in the change in the muon density at a fixed core location,
, as a function of energy. The composition has also been
evaluated in terms of the combination of and . The
results show that the composition is changing from a heavy to lighter mix as
the energy increases.Comment: 14 pages including 3 figures in revtex epsf style, submited to PR
Synthesis, structural and antioxidant studies of some novel n-ethyl phthalimide esters
A series of N-ethyl phthalimide esters 4(a-n) were synthesized and characterized by spectroscopic studies. Further, the molecular structure of majority of compounds were analysed by single crystal X-ray diffraction studies. The X-ray analysis revealed the importance of substituents on the crystal stability and molecular packing. All the synthesized compounds were tested for in vitro antioxidant activity by DPPH radical scavenging, FRAP and CUPRAC methods. Few of them have shown good antioxidant activity
Measurement of the Cosmic Ray Energy Spectrum and Composition from 10^{17} to 10^{18.3} eV Using a Hybrid Fluorescence Technique
We study the spectrum and average mass composition of cosmic rays with
primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector
consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon
array. Measurements have been made of the change in the depth of shower maximum
as a function of energy. A complete Monte Carlo simulation of the detector
response and comparisons with shower simulations leads to the conclusion that
the cosmic ray intensity is changing f rom a heavier to a lighter composition
in this energy range. The spectrum is consistent with earlier Fly's Eye
measurements and supports the previously found steepening near 4 \times 10^{17}
eV .Comment: 39 pages, 15 figures, in revtex4 epsf style, submited to AP
Boojums and the Shapes of Domains in Monolayer Films
Domains in Langmuir monolayers support a texture that is the two-dimensional
version of the feature known as a boojum. Such a texture has a quantifiable
effect on the shape of the domain with which it is associated. The most
noticeable consequence is a cusp-like feature on the domain boundary. We report
the results of an experimental and theoretical investigation of the shape of a
domain in a Langmuir monolayer. A further aspect of the investigation is the
study of the shape of a ``bubble'' of gas-like phase in such a monolayer. This
structure supports a texture having the form of an inverse boojum. The
distortion of a bubble resulting from this texture is also studied. The
correspondence between theory and experiment, while not perfect, indicates that
a qualitative understanding of the relationship between textures and domain
shapes has been achieved.Comment: replaced with published version, 10 pages, 13 figures include
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