501 research outputs found
Comment on "Spin Transport properties of the quantum one-dimensional non-linear sigma model"
In a recent preprint (cond-mat/9905415), Fujimoto has used the Bethe ansatz
to compute the finite temperature, zero frequency Drude weight of spin
transport in the quantum O(3) non-linear sigma model in a magnetic field . We show here that, contrary to his claims, the results are in accord
with earlier semiclassical results (Sachdev and Damle, cond-mat/9610115). We
also comment on his 1/N expansion, and show that it does not properly describe
the long-time correlations.Comment: 4 page
Non-equilibrium Gross-Pitaevskii dynamics of boson lattice models
Motivated by recent experiments on trapped ultra-cold bosonic atoms in an
optical lattice potential, we consider the non-equilibrium dynamic properties
of such bosonic systems for a number of experimentally relevant situations.
When the number of bosons per lattice site is large, there is a wide parameter
regime where the effective boson interactions are strong, but the ground state
remains a superfluid (and not a Mott insulator): we describe the conditions
under which the dynamics in this regime can be described by a discrete
Gross-Pitaevskii equation. We describe the evolution of the phase coherence
after the system is initially prepared in a Mott insulating state, and then
allowed to evolve after a sudden change in parameters places it in a regime
with a superfluid ground state. We also consider initial conditions with a "pi
phase" imprint on a superfluid ground state (i.e. the initial phases of
neighboring wells differ by pi), and discuss the subsequent appearance of
density wave order and "Schrodinger cat" states.Comment: 16 pages, 11 figures; (v2) added reference
Spin-3/2 random quantum antiferromagnetic chains
We use a modified perturbative renormalization group approach to study the
random quantum antiferromagnetic spin-3/2 chain. We find that in the case of
rectangular distributions there is a quantum Griffiths phase and we obtain the
dynamical critical exponent as a function of disorder. Only in the case of
extreme disorder, characterized by a power law distribution of exchange
couplings, we find evidence that a random singlet phase could be reached. We
discuss the differences between our results and those obtained by other
approaches.Comment: 4 page
Conductance of a phenylene-vinylene molecular wire: Contact gap and tilt angle dependence
Charge transport through a molecular junction comprising an oligomer of p-phenylene-vinylene between gold contacts has been investigated using density-functional theory and the nonequilibrium Green's function method. The influence of the contact gap geometry on the transport has been studied for elongated and contracted gaps, as well as various molecular conformations. The calculated current-voltage characteristics show an unusual increase in the low bias conductance with the contact separation. In contrast, for compressed junctions the conductance displays only a very weak dependence on both the separation and related molecular conformation. However, if the contraction of the gap between the electrodes is accommodated by tilting the molecule, the conductance will increase with the tilting angle, in line with experimental observations. It is demonstrated that the effect of tilting on transport can be interpreted in a similar way to the case of the stretching the junction with a molecule in an upright position. The lowest conductance was observed for the equilibrium gap geometry. With the dominant transport contribution arising from the π system of the frontier junction orbitals, all the predicted increases in the conductance arise simply from the better band alignment between relevant frontier orbitals at the nonequilibrium geometries at the expense of weaker coupling with the contacts
Multicritical crossovers near the dilute Bose gas quantum critical point
Many zero temperature transitions, involving the deviation in the value of a
conserved charge from a quantized value, are described by the dilute
Bose gas quantum critical point. On such transitions, we study the consequences
of perturbations which break the symmetry down to in spatial
dimensions. For the case , , we obtain exact, finite temperature,
multicritical crossover functions by a mapping to an integrable lattice model.Comment: 10 pages, REVTEX 3.0, 2 EPS figure
Balloon observations of hard X-rays from some galactic X-ray sources
An X-ray telescope consisting of 400 cm phoswich detectors (NaI(T1)/CsI(Na)) was flown from Hyderabad (India) on 18 December 1984. The field of view was 5 deg x 5 deg FWHM. In a 10 hour float at 4 MB several galactic X-ray sources were tracked by the telescope using an on-board microprocessor. Fluxes and spectra in 18-120 keV X-rays for SCO X-1, GX 1+4, Gx 5-1, GX 17+2, SCT X-1, CYC X-1 an CYG X-3 will be presented
Integer quantum Hall effect of interacting electrons: dynamical scaling and critical conductivity
We report on a study of interaction effects on the polarization of a
disordered two-dimensional electron system in a strong magnetic field. Treating
the Coulomb interaction within the time-dependent Hartree-Fock approximation we
find numerical evidence for dynamical scaling with a dynamical critical
exponent z=1 at the integer quantum Hall plateau transition in the lowest
Landau level. Within the numerical accuracy of our data the conductivity at the
transition and the anomalous diffusion exponent are given by the values for
non-interacting electrons, independent of the strength of the interaction.Comment: Minor changes. Final version to be published in Phys. Rev. Lett. June
2
Translational Symmetry Breaking in the Superconducting State of the Cuprates: Analysis of the Quasiparticle Density of States
Motivated by the recent STM experiments of J.E. Hoffman et.al. and C. Howald
et.al., we study the effects of weak translational symmetry breaking on the
quasiparticle spectrum of a d-wave superconductor. We develop a general
formalism to discuss periodic charge order, as well as quasiparticle scattering
off localized defects. We argue that the STM experiments in
cannot be explained using a simple charge density
wave order parameter, but are consistent with the presence of a periodic
modulation in the electron hopping or pairing amplitude. We review the effects
of randomness and pinning of the charge order and compare it to the impurity
scattering of quasiparticles. We also discuss implications of weak
translational symmetry breaking for ARPES experiments.Comment: 12 pages, 9 figs; (v2) minor corrections to formalism, discussions of
dispersion, structure factors and sum rules added; (v3) discussion of
space-dependent normalization added. To be published in PR
A neutron scattering study of two-magnon states in the quantum magnet copper nitrate
We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using
inelastic neutron scattering we have measured the one and two magnon excitation
spectra in a large single crystal. The data are in excellent agreement with a
perturbative expansion of the alternating Heisenberg Hamiltonian from the
strongly dimerized limit. The expansion predicts a two-magnon bound state for q
~ (2n+1)pi*d which is consistent with the neutron scattering data.Comment: 11 pages of revtex style with 6 figures include
Low temperature spin diffusion in the one-dimensional quantum nonlinear -model
An effective, low temperature, classical model for spin transport in the
one-dimensional, gapped, quantum non-linear -model is developed.
Its correlators are obtained by a mapping to a model solved earlier by Jepsen.
We obtain universal functions for the ballistic-to-diffusive crossover and the
value of the spin diffusion constant, and these are claimed to be exact at low
temperatures. Implications for experiments on one-dimensional insulators with a
spin gap are noted.Comment: 4 pages including 3 eps-figures, Revte
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