287 research outputs found
On the Verdet constant and Faraday rotation for graphene-like materials
We present a rigorous and rather self-contained analysis of the Verdet
constant in graphene- like materials. We apply the gauge-invariant magnetic
perturbation theory to a nearest- neighbour tight-binding model and obtain a
relatively simple and exactly computable formula for the Verdet constant, at
all temperatures and all frequencies of sufficiently large absolute value.
Moreover, for the standard nearest neighbour tight-binding model of graphene we
show that the transverse component of the conductivity tensor has an asymptotic
Taylor expansion in the external magnetic field where all the coefficients of
even powers are zero.Comment: 23 pages, 4 figures, revised versio
Optical Hall conductivity in bulk and nanostructured graphene beyond the Dirac approximation
We present a perturbative method for calculating the optical Hall
conductivity in a tight-binding framework based on the Kubo formalism. The
method involves diagonalization only of the Hamiltonian in absence of the
magnetic field, and thus avoids the computational problems usually arising due
to the huge magnetic unit cells required to maintain translational invariance
in presence of a Peierls phase. A recipe for applying the method to numerical
calculations of the magneto-optical response is presented. We apply the
formalism to the case of ordinary and gapped graphene in a next-nearest
neighbour tight-binding model as well as graphene antidot lattices. In both
case, we find unique signatures in the Hall response, that are not captured in
continuum (Dirac) approximations. These include a non-zero optical Hall
conductivity even when the chemical potential is at the Dirac point energy.
Numerical results suggest that this effect should be measurable in experiments.Comment: 7 pages, 4 figures, accepted in Physical Review
Neutron diffraction studies of magnetostrictive Fe–Ga alloy ribbons
Melt-spun Fe–Ga ribbons were prepared and some ribbons were annealed at 1000 °C for 1 h then
slowly cooled to room temperature. X-ray diffraction patterns revealed no evidence of texture and
only bcc phase in the as-quenched ribbons. However, high-resolution neutron diffraction patterns
gave more information on the structure of these ribbons. Only diffractions from the disordered bcc
A2 phase were found in as-quenched ribbons with 15, 17.5, and 19.5 at. % Ga content, without any
trace of satellite peaks or splitting peaks from the proposed Ga–Ga pairing superlattice structure.
The broadening of the base of the �110� peaks for all samples except the as-quenched 15 at. % Ga
ribbon might indicate the existence of some kind of short range ordering. Ribbons developed L12
phase after annealing especially in the Fe 19.5 at. % Ga ribbon where the formation of L12 phase
reduced the Ga content in the remaining A2 phase and decreased its lattice parameter dramatically.
D03 phase formed in the as-quenched 22.5 at. % Ga ribbon and the following annealing treatment
transformed more A2 phase into D03 phase
How stress affects functional near-infrared spectroscopy (fNIRS) measurements of mental workload
Recent work has demonstrated that functional Near-Infrared Spectroscopy has the potential to measure changes in Mental Workload with increasing ecological validity. It is not clear, however, whether these measurements are affected by anxiety and stress of the workload, where our informal observations see some participants enjoying the workload and succeeding in tasks, while others worry and struggle with the tasks. This research evaluated the effects of stress on fNIRS measurements and performance, using the Montreal Imaging Stress Task to manipulate the experience of stress. While our results largely support this hypothesis, our conclusions were undermined by data from the Rest condition, which indicated that Mental Workload and Stress were often higher than during tasks. We hypothesize that participants were experiencing anxiety in anticipation of subsequent stress tasks. We discuss this hypothesis and present a revised study designed to better control for this result
Fidelity for Multimode Thermal Squeezed States
In the theory of quantum transmission of information the concept of fidelity
plays a fundamental role. An important class of channels, which can be
experimentally realized in quantum optics, is that of Gaussian quantum
channels. In this work we present a general formula for fidelity in the case of
two arbitrary Gaussian states. From this formula one can get a previous result
(H. Scutaru, J. Phys. A: Mat. Gen {\bf 31}, 3659 (1998)), for the case of a
single mode; or, one can apply it to obtain a closed compact expression for
multimode thermal states.Comment: 5 pages, RevTex, submitted to Phys. Rev.
Diamagnetism of quantum gases with singular potentials
We consider a gas of quasi-free quantum particles confined to a finite box,
subjected to singular magnetic and electric fields. We prove in great
generality that the finite volume grand-canonical pressure is jointly analytic
in the chemical potential ant the intensity of the external magnetic field. We
also discuss the thermodynamic limit
Bures distance between two displaced thermal states
The Bures distance between two displaced thermal states and the corresponding
geometric quantities (statistical metric, volume element, scalar curvature) are
computed. Under nonunitary (dissipative) dynamics, the statistical distance
shows the same general features previously reported in the literature by
Braunstein and Milburn for two--state systems. The scalar curvature turns out
to have new interesting properties when compared to the curvature associated
with squeezed thermal states.Comment: 3 pages, RevTeX, no figure
On the Lipschitz continuity of spectral bands of Harper-like and magnetic Schroedinger operators
We show for a large class of discrete Harper-like and continuous magnetic
Schrodinger operators that their band edges are Lipschitz continuous with
respect to the intensity of the external constant magnetic field. We generalize
a result obtained by J. Bellissard in 1994, and give examples in favor of a
recent conjecture of G. Nenciu.Comment: 15 pages, accepted for publication in Annales Henri Poincar
Oxygen adsorption on Au clusters and a rough Au(111) surface: The role of surface flatness, electron confinement, excess electrons, and band gap
It has been shown recently that while bulk gold is chemically inert, small Au clusters are catalytically active. The reasons for this activity and its dramatic dependence on cluster size are not understood. We use density functional theory to study O2 binding to Au clusters and to a Au(111) surface modified by adsorption of Au clusters on it. We find that O2 does not bind to a flat face of a planar Au cluster, even though it binds well to its edge. Moreover, O2 binds to Au clusters deposited on a Au(111) surface, even though it does not bind to Au(111). This indicates that a band gap is not an essential factor in binding O2, but surface roughness is. Adding electrons to the surface of a Au(111) slab, on which one has deposited a Au cluster, increases the binding energy of O2. However, adding electrons to a flat Ausurface has no effect on O2binding energy. These observations have a simple explanation: in clusters and in the rough surface, the highest occupied molecular orbital (HOMO) is localized and its charge density sticks out in the vacuum. This facilitates charge transfer into the π* orbital of O2, which induces the molecule to bind to gold. A flat face of a cluster or a flat bulk surface tends to delocalize the HOMO, diminishing the ability of the surface to bind O2. The same statements are true for the LUMO orbital, which is occupied by the additional electron given to the system to charge the system negatively
Adiabatic non-equilibrium steady states in the partition free approach
Consider a small sample coupled to a finite number of leads, and assume that
the total (continuous) system is at thermal equilibrium in the remote past. We
construct a non-equilibrium steady state (NESS) by adiabatically turning on an
electrical bias between the leads. The main mathematical challenge is to show
that certain adiabatic wave operators exist, and to identify their strong limit
when the adiabatic parameter tends to zero. Our NESS is different from, though
closely related with the NESS provided by the Jak{\v s}i{\'c}-Pillet-Ruelle
approach. Thus we partly settle a question asked by Caroli {\it et al} in 1971
regarding the (non)equivalence between the partitioned and partition-free
approaches
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