12,206 research outputs found
Grand-canonical variational approach for the t-J model
Gutzwiller-projected BCS wave function or the resonating-valence-bond (RVB)
state in the 2D extended t-J model is investigated by using the variational
Monte Carlo technique. We show that the results of ground-state energy and
excitation spectra calculated in the grand-canonical scheme allowing particle
number to fluctuate are essentially the same as previous results obtained by
fixing the number of particle in the canonical scheme if the grand
thermodynamic potential is used for minimization. To account for the effect of
Gutzwiller projection, a fugacity factor proposed by Laughlin and Anderson few
years ago has to be inserted into the coherence factor of the BCS state.
Chemical potential, particle number fluctuation, and phase fluctuation of the
RVB state, difficult or even impossible to be calculated in the canonical
ensemble, have been directly measured in the grand-canonical picture. We find
that except for La-214 materials, the doping dependence of chemical potential
is consistent with experimental findings on several cuprates. Similar to what
has been reported by scanning tunneling spectroscopy experiments, the tunneling
asymmetry becomes much stronger as doping decreases. We found a very large
enhancement of phase fluctuation in the underdoped regime.Comment: 9 pages, 6 figure
The infrared conductivity of NaCoO: evidence of gapped states
We present infrared ab-plane conductivity data for the layered cobaltate
NaCoO at three different doping levels (, and 0.75). The
Drude weight increases monotonically with hole doping, . At the lowest
hole doping level =0.75 the system resembles the normal state of underdoped
cuprate superconductors with a scattering rate that varies linearly with
frequency and temperature and there is an onset of scattering by a bosonic mode
at 600 \cm. Two higher hole doped samples ( and 0.25) show two
different-size gaps (110 \cm and 200 \cm, respectively) in the optical
conductivities at low temperatures and become insulators. The spectral weights
lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks
at 200 \cm and 800 \cm, respectively. We propose that the two gapped states of
the two higher hole doped samples (=0.50 and 0.25) are pinned charge ordered
states.Comment: 4 pages, 3 figure
Magic-Angle Semimetals with Chiral Symmetry
We construct and solve a two-dimensional, chirally symmetric model of Dirac
cones subjected to a quasiperiodic modulation. In real space, this is realized
with a quasiperiodic hopping term. This hopping model, as we show, at the Dirac
node energy has a rich phase diagram with a semimetal-to-metal phase transition
at intermediate amplitude of the quasiperiodic modulation, and a transition to
a phase with a diverging density of states and sub-diffusive transport when the
quasiperiodic hopping is strongest. We further demonstrate that the
semimetal-to-metal phase transition can be characterized by the multifractal
structure of eigenstates in momentum space and can be considered as a unique
"unfreezing" transition. This unfreezing transition in momentum space generates
flat bands with a dramatically renormalized bandwidth in the metallic phase
similar to the phenomena of the band structure of twisted bilayer graphene at
the magic angle. We characterize the nature of this transition numerically as
well as analytically in terms of the formation of a band of topological zero
modes. For pure quasiperiodic hopping, we provide strong numerical evidence
that the low-energy density of states develops a divergence and the eigenstates
exhibit Chalker (quantum-critical) scaling despite the model not being random.
At particular commensurate limits the model realizes higher-order topological
insulating phases. We discuss how these systems can be realized in experiments
on ultracold atoms and metamaterials.Comment: 20+4 pages, 24 figures, published versio
Analysis and representation of statistical performance of JPEG2000 encoded image over wireless channels
JPEG2000 is a new coming image standard. In this paper we analyze the performance of error resilience tools in JPEG2000, and present an analytical model to estimate the quality of JPEG2000 encoded image transmitted over wireless channels. The effectiveness of the analytical model is validated by simulation results. Furthermore, analytical model is utilized by the base station to design efficient unequal error protection schemes for JPEG2000 transmission. In the design, a utility function is defined to make a tradeoff between the image quality and the cost for transmitting the image over a wireless channel
Radiative and Semileptonic B Decays Involving the Tensor Meson K_2^*(1430) in the Standard Model and Beyond
We study semileptonic and radiative B decays involving the strange tensor
meson K_2^*(1430) in the final state. Using the large energy effective theory
(LEET) techniques, we formulate the B \to K_2^* transition form factors in
large recoil region. All the form factors can be parametrized in terms of two
independent LEET parameters \zeta_\perp and \zeta_\parallel. The magnitude of
\zeta_\perp is estimated from the data for Br(B \to K_2^*(1430)\gamma).
Assuming a dipole q^2-dependence for the LEET parameters and
\zeta_\parallel/\zeta_\perp = 1.0 \pm 0.2, we investigate the decays B \to
K_2^* \ell^+ \ell^- and B \to K_2^* \nu \bar{\nu}, where the contributions due
to $\zeta_\parallel are suppressed by m_{K_2^*}/m_B. For the B \to K_2^* \ell^+
\ell^- decay, in the large recoil region where the hadronic uncertainties are
considerably reduced, the longitudinal distribution d F_L/ds is reduced by
20-30 % due to the flipped sign of c_7^eff compared with the standard model
result. Moreover, the forward-backward asymmetry zero is about 3.4 GeV^2 in the
standard model, but changing the sign of c_7^eff yields a positive asymmetry
for all values of the invariant mass of the lepton pair. We calculate the
branching fraction for B \to K_2^* \nu \bar{\nu} in the standard model. Our
result exhibits the impressed resemblance between B \to K_2^*(1430)
\ell^+\ell^-, \nu \bar{\nu} and B \to K^*(892) \ell^+ \ell^-, \nu\bar{\nu}.Comment: 21 pages, 4 figures (v2) comments adde
Anomalous Magnetic and Thermal Behavior in Some RMn2O5 Oxides
The RMn2O5 (R=Pr, Nd, Sm, and Eu) oxides showing magnetoelectric (ME)
behavior have been prepared in polycrystalline form by a standard citrate
route. The lattice parameters, obtained from the powder XRD analysis, follow
the rare-earth contraction indicating the trivalent character of the R ions.
Cusp-like anomalies in the magnetic susceptibility curve and sharp peaks in the
specific heat were reported at the corresponding temperatures in RMn2O5 (R=Pr,
Nd, Sm, and Eu) indicating the magnetic or electric ordering transitions.Comment: 2 pages, 1 table, 3 figures, will be published in the Proceedings of
the 24th International Conference on Low Temperature Physic
Mathematical models for vulnerable plaques
A plaque is an accumulation and swelling in the artery walls and typically consists of cells, cell debris, lipids, calcium deposits and fibrous connective tissue. A person is likely to have many plaques inside his/her body even if they are healthy. However plaques may become "vulnerable", "high-risk" or "thrombosis-prone" if the person engages in a high-fat diet and does not exercise regularly.
In this study group, we proposed two mathematical models to describe plaque growth and rupture.
The first model is a mechanical one that approximately treats the plaque as an inflating elastic balloon. In this model, the pressure inside the core increases and then decreases suggesting that plaque stabilization and prevention of rupture is possible.
The second model is a biochemical one that focuses on the role of MMPs in degrading the fibrous plaque cap. The cap stress, MMP concentration, plaque volume and cap thickness are coupled together in a system of phenomenological equations. The equations always predict an eventual rupture since the volume, stresses and MMP concentrations generally grow without bound. The main weakness of the model is that many of the important parameters that control the behavior of the plaque are unknown.
The two simple models suggested by this group could serve as a springboard for more realistic theoretical studies. But most importantly, we hope they will motivate more experimental work to quantify some of the important mechanical and biochemical properties of vulnerable plaques
Momentum Distribution for Bosons with Positive Scattering Length in a Trap
The coordinate-momentum double distribution function is calculated in the local density approximation for bosons with
positive scattering length in a trap. The calculation is valid to the first
order of . To clarify the meaning of the result, it is compared for a
special case with the double distribution function of
Wigner.Comment: Latex fil
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