Statics and dynamics of charge fluctuations in the t-J model

The equation for the charge vertex $\gamma$ of the $t-J$ model is derived and solved in leading order of an 1/N expansion, working directly in terms of Hubbard operators. Various quantities which depend crucially on $\gamma$ are then calculated, such as the life time and the transport life time of electrons due to a charge coupling to other degrees of freedom and the charge-charge correlation function. Our results show that the static screening of charges and the dynamics of charge fluctuations depend only weakly on $J$ and are mainly determined by the constraint of having no double occupancies of sites.Comment: 10 latex pages, 4 figures as post-script file

Angle-resolved photoemission in high Tc cuprates from theoretical viewpoints

The angle-resolved photoemission (ARPES) technique has been developed rapidly over the last decay, accompanied by the improvement of energy and momentum resolutions. This technique has been established as the most powerful tool to investigate the high Tc cuprate superconductors. We review recent ARPES data on the cuprates from a theoretical point of view, with emphasis on the systematic evolution of the spectral weight near the momentum (pi,0) from insulator to overdoped systems. The effects of charge stripes on the ARPES spectra are also reviewed. Some recent experimental and theoretical efforts to understand the superconducting state and the pseudogap phenomenon are discussed.Comment: Review, 25 pages, with 22 GIF figures. To appear in Supercond. Sci. Technol. Vol. 13 April 2000. A version including PS figures can be found at http://www.maekawa-lab.imr.tohoku.ac.jp/TOHYAMA/tohyama.ps.g

Electronic States in the Antiferromagnetic Phase of Electron-Doped High-Tc Cuprates

We investigate the electronic states in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for a t-t'-t''-J model. When AF correlation develops with decreasing temperature, a gaplike behavior emerges in the optical conductivity. Simultaneously, the coherent motion of carriers due to the same sublattice hoppings is enhanced. We propose that the phase is characterized as an AF state with small Fermi surface around the momentum k=(\pi,0) and (0,\pi). This is a remarkable contrast to the behavior of hole-doped cuprates.Comment: RevTeX, 5 pages, 4 figures, to appear in Phys. Rev. B Brief Report

Scissors mode of trapped dipolar gases

We study the scissors modes of dipolar boson and fermion gases trapped in a spherically symmetric potential. We use the harmonic oscillator states to solve the time-dependent Gross-Pitaevskii equation for bosons and the time-dependent Hartree-Fock equation for fermions. It is pointed out that the scissors modes of bosons and fermions can be of quite different nature

Node of Ranvier length as a potential regulator of myelinated axon conduction speed

Myelination speeds conduction of the nerve impulse, enhancing cognitive power. Changes of white matter structure contribute to learning, and are often assumed to reflect an altered number of myelin wraps. We now show that, in rat optic nerve and cerebral cortical axons, the node of Ranvier length varies over a 4.4-fold and 8.7-fold range respectively and that variation of the node length is much less along axons than between axons. Modelling predicts that these node length differences will alter conduction speed by ~20%, similar to the changes produced by altering the number of myelin wraps or the internode length. For a given change of conduction speed, the membrane area change needed at the node is >270-fold less than that needed in the myelin sheath. Thus, axon-specific adjustment of node of Ranvier length is potentially an energy-efficient and rapid mechanism for tuning the arrival time of information in the CNS

Phase diagram of a Bose gas near a wide Feshbach resonance

In this paper, we study the phase diagram of a homogeneous Bose gas with a repulsive interaction near a wide Feshbach resonance at zero temperature. The Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the scattering length $a$ exceeds a critical value depending on the atom density $n$, $na^3>0.035$, the molecular excitation energy is imaginary and the atomic BEC state is dynamically unstable against molecule formation. The BEC state of diatomic molecules has lower energy, where the atomic excitation is gapped and the molecular excitation is gapless. However when the scattering length is above another critical value, $na^3>0.0164$, the molecular BEC state becomes a unstable coherent mixture of atoms and molecules. In both BEC states, the binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure

Qualitative understanding of the sign of t' asymmetry in the extended t-J Model and relevance for pairing properties

Numerical calculations illustrate the effect of the sign of the next nearest-neighbor hopping term t' on the 2-hole properties of the t-t'-J model. Working mainly on 2-leg ladders, in the -1.0 < t'/t < 1.0 regime, it is shown that introducing t' in the t-J model is equivalent to effectively renormalizing J, namely t' negative (positive) is equivalent to an effective t-J model with smaller (bigger) J. This effect is present even at the level of a 2x2 plaquette toy model, and was observed also in calculations on small square clusters. Analyzing the transition probabilities of a hole-pair in the plaquette toy model, it is argued that the coherent propagation of such hole-pair is enhanced by a constructive interference between both t and t' for t'>0. This interference is destructive for t'<0.Comment: 5 pages, 4 figures, to appear in PRB as a Rapid Communicatio

Temperature Dependence of Spin Correlation and Charge Dynamics in the Stripe Phase of High-T_c Superconductors

We examine the temperature dependence of the electronic states in the stripe phase of high-Tc cuprates by using the t-J model with a potential that stabilizes vertical charge stripes. Charge and spin-correlation functions and optical conductivity are calculated by using finite-temperature Lanczos method. At zero temperature, the antiferromagnetic correlation between a spin in a charge stripe and that in a spin domain adjacent to the stripe is weak, since the charge stripe and the spin domain are almost separated. With increasing temperature, the correlation increases and then decreases toward high temperature. This is in contrast to other correlations that decrease monotonically. From the examination of the charge dynamics, we find that this anomalous temperature dependence of the correlation is the consequence of a crossover from one-dimensional electronic states to two-dimensional ones.Comment: 7 pages in two-column format, 6 figures, to be published in Phys. Rev.

Pseudo-gap behavior in dynamical properties of high-Tc cuprates

Dynamical properties of 2D antiferromagnets with hole doping are investigated to see the effects of short range local magnetic order on the temperature dependence of the dynamical magnetic susceptibility. We show the pseudo-gap like behavior of the temperature dependence of the NMR relaxation rate. We also discuss implications of the results in relations to the observed spin gap like behavior of low-doped copper oxide high-$T_c$ superconductors.Comment: 3 pages, Revtex, with 2 eps figures, to appear in J.Phys.Soc.Jpn. Vol.67 No.