1,105 research outputs found
PANDA FACIES
A case of bilateral peri orbital ecchymosis without any identifiablepathology,which subsided on its own, in due course.
Spontaneous breaking of the Fermi surface symmetry in the t-J model: a numerical study
We present a variational Monte Carlo (VMC) study of spontaneous Fermi surface
symmetry breaking in the t-J model. We find that the variational energy of a
Gutzwiller projected Fermi sea is lowered by allowing for a finite asymmetry
between the x- and the y-directions. However, the best variational state
remains a pure superconducting state with d-wave symmetry, as long as the
underlying lattice is isotropic. Our VMC results are in good overall agreement
with slave boson mean field theory (SBMFT) and renormalized mean field theory
(RMFT), although apparent discrepancies do show up in the half-filled limit,
revealing some limitations of mean field theories. VMC and complementary RMFT
calculations also confirm the SBMFT predictions that many-body interactions can
enhance any anisotropy in the underlying crystal lattice. Thus, our results may
be of consequence for the description of strongly correlated superconductors
with an anisotropic lattice structure.Comment: 6 pages, 7 figures; final versio
Microphase separation in polyelectrolytic diblock copolymer melt : weak segregation limit
We present a generalized theory of microphase separation for charged-neutral
diblock copolymer melt. Stability limit of the disordered phase for salt-free
melt has been calculated using Random Phase Approximation (RPA) and
self-consistent field theory (SCFT). Explicit analytical free energy
expressions for different classical ordered microstructures (lamellar, cylinder
and sphere) are presented. We demonstrate that chemical mismatch required for
the onset of microphase separation () in charged-neutral
diblock melt is higher and the period of ordered microstructures is lower than
those for the corresponding neutral-neutral diblock system. Theoretical
predictions on the period of ordered structures in terms of Coulomb
electrostatic interaction strength, chain length, block length, and the
chemical mismatch between blocks are presented. SCFT has been used to go beyond
the stability limit, where electrostatic potential and charge distribution are
calculated self-consistently. Stability limits calculated using RPA are in
perfect agreement with the corresponding SCFT calculations. Limiting laws for
stability limit and the period of ordered structures are presented and
comparisons are made with an earlier theory. Also, transition boundaries
between different morphologies have been investigated
Quasiparticles as composite objects in the RVB superconductor
We study the nature of the superconducting state, the origin of d-wave
pairing, and elementary excitations of a resonating valence bond (RVB)
superconductor. We show that the phase string formulation of the t-J model
leads to confinement of bare spinon and holon excitations in the
superconducting state, though the vacuum is described by the RVB state. Nodal
quasiparticles are obtained as composite excitations of spinon and holon
excitations. The d-wave pairing symmetry is shown to arise from short range
antiferromagnetic correlations
Bosonic resonating valence bond wave function for doped Mott insulators
We propose a new class of ground states for doped Mott insulators in the
electron second-quantization representation. They are obtained from a bosonic
resonating valence bond (RVB) theory of the t-J model. At half filling, the
ground state describes spin correlations of the S=1/2 Heisenberg model very
accurately. Its spin degrees of freedom are characterized by RVB pairing of
spins, the size of which decreases continuously as holes are doped into the
system. Charge degrees of freedom emerge upon doping and are described by
twisted holes in the RVB background. We show that the twisted holes exhibit an
off diagonal long range order (ODLRO) in the pseudogap ground state, which has
a finite pairing amplitude, but is short of phase coherence. Unpaired spins in
such a pseudogap ground state behave as free vortices, preventing
superconducting phase coherence. The existence of nodal quasiparticles is also
ensured by such a hidden ODLRO in the ground state, which is
non-Fermi-liquid-like in the absence of superconducting phase coherence. Two
distinct types of spin excitations can also be constructed. The superconducting
instability of the pseudogap ground state is discussed and a d-wave
superconducting ground state is obtained. This class of pseudogap and
superconducting ground states unifies antiferromagnetism, pseudogap,
superconductivity, and Mott physics into a new state of matter.Comment: 28 pages, 5 figures, final version to appear in Phys. Rev.
Efficient Set Sharing Using ZBDDs
Set sharing is an abstract domain in which each concrete object is represented by the set of local variables from which it might be reachable. It is a useful abstraction to detect parallelism opportunities, since it contains definite information about which variables do not share in memory, i.e., about when the memory regions reachable from those variables are disjoint. Set sharing is a more precise alternative to pair sharing, in which each domain element is a set of all pairs of local variables from which a common object may be reachable. However, the exponential complexity of some set sharing operations has limited its wider application. This work introduces an efficient implementation of the set sharing domain using Zero-suppressed Binary Decision Diagrams (ZBDDs). Because ZBDDs were designed to represent sets of combinations (i.e., sets of sets), they naturally represent elements of the set sharing domain. We show how to synthesize the operations needed in the set sharing transfer functions from basic ZBDD operations. For some of the operations, we devise custom ZBDD algorithms that perform better in practice. We also compare our implementation of the abstract domain with an efficient, compact, bit set-based alternative, and show that the ZBDD version scales better in terms of both memory usage and running time
Electronic structure of strongly correlated d-wave superconductors
We study the electronic structure of a strongly correlated d-wave
superconducting state. Combining a renormalized mean field theory with direct
calculation of matrix elements, we obtain explicit analytical results for the
nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum
distribution, n_k, as a function of hole doping in a Gutzwiller projected
d-wave superconductor. We calculate the energy dispersion, E_k, and spectral
weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral
weight associated with the quasiparticle excitation at the antinodal point
shows a non monotonic behavior as a function of doping. Results are compared to
angle resolved photoemission spectroscopy (ARPES) of the high temperature
superconductors.Comment: final version, comparison to experiments added, 4+ pages, 4 figure
Prediction of Algal Bloom Using Genetic Programming
In this study, an attempt was made to mathematically model and predict algal blooms in Tolo Harbor (Hong Kong) using genetic programming (GP). Chlorophyll plays a vital role in blooms and was used in this model as a measure of algal bloom biomass, and eight other variables were used as input for its prediction. It has been observed that GP evolves multiple models with almost the same values of errors-of-measure. Previous studies on GP modeling have primarily focused on comparing GP results with actual values. In contrast, in this study, the main aim was to propose a systematic procedure for identifying the most appropriate GP model from a list of feasible models (with similar error-of-measure) using a physical understanding of the process aided by data interpretation. Evaluation of the GP-evolved equations shows that they correctly identify the ecologically significant variables. Analysis of the final GP-evolved mathematical model indicates that, of the eight variables assumed to affect algal blooms, the most significant effects are due to chlorophyll, total inorganic nitrogen and dissolved oxygen for a 1-week prediction. For longer lead predictions (biweekly), secchi-disc depth and temperature appear to be significant variables, in addition to chlorophyll
Determining the underlying Fermi surface of strongly correlated superconductors
The notion of a Fermi surface (FS) is one of the most ingenious concepts
developed by solid state physicists during the past century. It plays a central
role in our understanding of interacting electron systems. Extraordinary
efforts have been undertaken, both by experiment and by theory, to reveal the
FS of the high temperature superconductors (HTSC), the most prominent strongly
correlated superconductors. Here, we discuss some of the prevalent methods used
to determine the FS and show that they lead generally to erroneous results
close to half filling and at low temperatures, due to the large superconducting
gap (pseudogap) below (above) the superconducting transition temperature. Our
findings provide a perspective on the interplay between strong correlations and
superconductivity and highlight the importance of strong coupling theories for
the characterization as well as the determination of the underlying FS in ARPES
experiments
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