8,970 research outputs found
Comment on the Coupling of Zero Sound to the Modes of He-B
Features in the zero sound attenuation near the pair-breaking edge in
superfluid He-B have been observed in large magnetic fields. Schopohl and
Tewordt [{\sl J. Low Temp. Phys.} {\bf 57}, 601 (1984)] claim that the order-parameter collective modes couple to zero sound as a result of
the distortion of the equilibrium order parameter by a magnetic field; they
identify the new features with these modes. However, we show that, when the
effect of gap distortion on the collective modes is properly taken into
account, the collective mode equations of Schopohl and Tewordt yield no direct
coupling of zero sound to the modes. Thus, the identification of the
absorption features reported by Ling, Saunders and Dobbs [{\sl Phys. Rev.
Lett.} {\bf 59}, 461 (1987)] near the pair-breaking edge with the modes
is not clearly established.Comment: 6 pages (Tex with jnl.tex
Mott transition in two-dimensional frustrated compounds
The phase diagrams of isotropic and anisotropic triangular lattices with
local Coulomb interactions are evaluated within cluster dynamical mean field
theory. As a result of partial geometric frustration in the anisotropic
lattice, short range correlations are shown to give rise to reentrant behavior
which is absent in the fully frustrated isotropic limit. The qualitative
features of the phase diagrams including the critical temperatures are in good
agreement with experimental data for the layered organic charge transfer salts
kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl and kappa-(BEDT-TTF)_2Cu_2(CN)_3.Comment: 4 pages, 4 figure
Dynamical properties of a strongly correlated model for quarter-filled layered organic molecular crystals
The dynamical properties of an extended Hubbard model, which is relevant to
quarter-filled layered organic molecular crystals, are analyzed. We have
computed the dynamical charge correlation function, spectral density, and
optical conductivity using Lanczos diagonalization and large-N techniques. As
the ratio of the nearest-neighbour Coulomb repulsion, V, to the hopping
integral, t, increases there is a transition from a metallic phase to a charge
ordered phase. Dynamical properties close to the ordering transition are found
to differ from the ones expected in a conventional metal. Large-N calculations
display an enhancement of spectral weight at low frequencies as the system is
driven closer to the charge ordering transition in agreement with Lanczos
calculations. As V is increased the charge correlation function displays a
plasmon-like mode which, for wavevectors close to (pi,pi), increases in
amplitude and softens as the charge ordering transition is approached. We
propose that inelastic X-ray scattering be used to detect this mode. Large-N
calculations predict superconductivity with dxy symmetry close to the ordering
transition. We find that this is consistent with Lanczos diagonalization
calculations, on lattices of 20 sites, which find that the binding energy of
two holes becomes negative close to the charge ordering transition.Comment: 22 pages, 16 eps figures; caption of Fig. 5 correcte
Sensitivity of the photo-physical properties of organometallic complexes to small chemical changes
We investigate an effective model Hamiltonian for organometallic complexes
that are widely used in optoelectronic devices. The two most important
parameters in the model are , the effective exchange interaction between the
and orbitals of the ligands, and , the renormalized
energy gap between the highest occupied orbitals on the metal and on the
ligand. We find that the degree of metal-to-ligand charge transfer (MLCT)
character of the lowest triplet state is strongly dependent on the ratio
. is purely a property of the complex and can be
changed significantly by even small variations in the complex's chemistry, such
as replacing substituents on the ligands. We find that that small changes in
can cause large changes in the properties of the complex,
including the lifetime of the triplet state and the probability of injected
charges (electrons and holes) forming triplet excitations. These results give
some insight into the observed large changes in the photophysical properties of
organometallic complexes caused by small changes in the ligands.Comment: Accepted for publication in J. Chem. Phys. 14 pages, 9 figures,
Supplementary Info: 15 pages, 17 figure
How linear features alter predator movement and the functional\ud response
In areas of oil and gas exploration, seismic lines have been reported to alter the movement patterns of wolves (Canis lupus). We developed a mechanistic first passage time model, based on an anisotropic elliptic partial differential equation, and used this to explore how wolf movement responses to seismic lines influence the encounter rate of the wolves with their prey. The model was parametrized using 5 min GPS location data. These data showed that wolves travelled faster on seismic lines and had a higher probability of staying on a seismic line once they were on it. We simulated wolf movement on a range of seismic line densities and drew implications for the rate of predator–prey interactions as described by the functional response. The functional response exhibited a more than linear increase with respect to prey density (type III) as well as interactions with seismic line density. Encounter rates were significantly higher in landscapes with high seismic line density and were most pronounced at low prey densities. This suggests that prey at low population densities are at higher risk in environments with a high seismic line density unless they learn to avoid them
The Poisson geometry of SU(1,1)
We study the natural Poisson structure on the Lie group SU(1,1) and related
questions. In particular, we give an explicit description of the
Ginzburg-Weinstein isomorphism for the sets of admissible elements. We also
establish an analogue of Thompson's conjecture for this group.Comment: 11 pages, minor correction
Ferromagnetism, paramagnetism and a Curie-Weiss metal in an electron doped Hubbard model on a triangular lattice
Motivated by the unconventional properties and rich phase diagram of NaxCoO2
we consider the electronic and magnetic properties of a two-dimensional Hubbard
model on an isotropic triangular lattice doped with electrons away from
half-filling. Dynamical mean-field theory (DMFT) calculations predict that for
negative inter-site hopping amplitudes (t<0) and an on-site Coulomb repulsion,
U, comparable to the bandwidth, the system displays properties typical of a
weakly correlated metal. In contrast, for t>0 a large enhancement of the
effective mass, ferromagnetism and a Curie-Weiss magnetic susceptibility are
found in a broad electron doping range. Our observation of Nagaoka
ferromagnetism is consistent with the A-type antiferromagnetism (i.e.
ferromagnetic layers stacked antiferromagnetically) observed in neutron
scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase
observed in NaxCoO2 is a consequence of the crossover from ``bad metal'' with
incoherent quasiparticles at temperatures T>T* and Fermi liquid behavior with
enhanced parameters below T*, where T* is a low energy coherence scale induced
by strong local Coulomb electron correlations. We propose a model which
contains the charge ordering phenomena observed in the system which, we
propose, drives the system close to the Mott insulating phase even at large
dopings.Comment: 24 pages, 15 figures; accepted for publication in Phys. Rev.
Magic angle effects in the interlayer magnetoresistance of quasi-one-dimensional metals due to interchain incoherence
The dependence of the magnetoresistance of quasi-one-dimensional metals on
the direction of the magnetic field show dips when the field is tilted at the
so called magic angles determined by the structural dimensions of the
materials. There is currently no accepted explanation for these magic angle
effects. We present a possible explanation. Our model is based on the
assumption that, the intralayer transport in the second most conducting
direction has a small contribution from incoherent electrons. This incoherence
is modelled by a small uncertainty in momentum perpendicular to the most
conducting (chain) direction. Our model predicts the magic angles seen in
interlayer transport measurements for different orientations of the field. We
compare our results to predictions by other models and to experiment.Comment: 7 pages, 3 figures, Submitted To Phys. Rev.
Universal subgap optical conductivity in quasi-one-dimensional Peierls systems
Quasi-one-dimensional Peierls systems with quantum and thermal lattice
fluctuations can be modeled by a Dirac-type equation with a Gaussian-correlated
off-diagonal disorder. A powerful new method gives the exact disorder-averaged
Green function used to compute the optical conductivity. The strong subgap tail
of the conductivity has a universal scaling form. The frequency and temperature
dependence of the calculated spectrum agrees with experiments on KCP(Br) and
trans-polyacetylene.Comment: 11 pages (+ 3 figures), LATEX (REVTEX 3.0
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