1,838 research outputs found
Islands, craters, and a moving surface step on a hexagonally reconstructed (100) noble metal surface
Deposition/removal of metal atoms on the hex reconstructed (100) surface of
Au, Pt and Ir should present intriguing aspects, since a new island implies hex
-> square deconstruction of the substrate, and a new crater the square -> hex
reconstruction of the uncovered layer. To obtain a microscopic understanding of
how islands/craters form in these conditions, we have conducted simulations of
island and crater growth on Au(100), whose atomistic behavior, including the
hex reconstruction on top of the square substrate, is well described by mean s
of classical many-body forces. By increasing/decreasing the Au coverage on
Au(100), we find that island/craters will not grow unless they exceed a
critical size of about 8-10 atoms. This value is close to that which explains
the nonlinear coverage dependence observed in molecular adsorption on the
closely related surface Pt (100). This threshold size is rationalized in terms
of a transverse step correlation length, measuring the spatial extent where
reconstruction of a given plane is disturbed by the nearby step.Comment: 11 pages, 5 figures, accepted for publication in Surface Science
(ECOSS-18
Geometry of mixed states for a q-bit and the quantum Fisher information tensor
After a review of the pure state case, we discuss from a geometrical point of
view the meaning of the quantum Fisher metric in the case of mixed states for a
two-level system, i.e. for a q-bit, by examining the structure of the fiber
bundle of states, whose base space can be identified with a co-adjoint orbit of
the unitary group. We show that the Fisher Information metric coincides with
the one induced by the metric of the manifold of SU(2), i.e. the 3-dimensional
sphere , when the mixing coefficients are varied. We define the notion of
Fisher Tensor and show that its anti-symmetric part is intrinsically related to
the Kostant Kirillov Souriau symplectic form that is naturally defined on
co-adjoint orbits, while the symmetric part is nontrivially again represented
by the Fubini Study metric on the space of mixed states, weighted by the mixing
coefficients.Comment: 20 pages; Abstract and Introduction modified, references added. Final
published versio
Bent surface free energy differences from simulation
We present a calculation of the change of free energy of a solid surface upon
bending of the solid. It is based on extracting the surface stress through a
molecular dynamics simulation of a bent slab by using a generalized stress
theorem formula, and subsequent integration of the stress with respect to
strain as a function of bending curvature. The method is exemplified by
obtaining and comparing free energy changes with curvature of various
reconstructed Au(001) surfaces.Comment: 14 pages, 2 figures, accepted for publication in Surface Science
(ECOSS-19
Continuum in the Excitation Spectrum of the S=1 Compound CsNiCl_3
Recent neutron scattering experiments on CsNiCl_3 reveal some features which
are not well described by the nonlinear sigma model nor by numerical
simulations on isolated S=1 spin chains. In particular, in real systems the
intensity of the continuum of multiparticle excitations, at T=6K, is about 5
times greater than predicted. Also the gap is slightly higher and the
correlation length is smaller. We propose a theoretical scenario where the
interchain interaction is approximated by a staggered magnetic field, yielding
to a correct prediction of the observed quantities.Comment: 4 pages, 2 figures (.eps), RevTe
From the Equations of Motion to the Canonical Commutation Relations
The problem of whether or not the equations of motion of a quantum system
determine the commutation relations was posed by E.P.Wigner in 1950. A similar
problem (known as "The Inverse Problem in the Calculus of Variations") was
posed in a classical setting as back as in 1887 by H.Helmoltz and has received
great attention also in recent times. The aim of this paper is to discuss how
these two apparently unrelated problems can actually be discussed in a somewhat
unified framework. After reviewing briefly the Inverse Problem and the
existence of alternative structures for classical systems, we discuss the
geometric structures that are intrinsically present in Quantum Mechanics,
starting from finite-level systems and then moving to a more general setting by
using the Weyl-Wigner approach, showing how this approach can accomodate in an
almost natural way the existence of alternative structures in Quantum Mechanics
as well.Comment: 199 pages; to be published in "La Rivista del Nuovo Cimento"
(www.sif.it/SIF/en/portal/journals
Detecting a many-body mobility edge with quantum quenches
The many-body localization (MBL) transition is a quantum phase transition
involving highly excited eigenstates of a disordered quantum many-body
Hamiltonian, which evolve from "extended/ergodic" (exhibiting extensive
entanglement entropies and fluctuations) to "localized" (exhibiting area-law
scaling of entanglement and fluctuations). The MBL transition can be driven by
the strength of disorder in a given spectral range, or by the energy density at
fixed disorder - if the system possesses a many-body mobility edge. Here we
propose to explore the latter mechanism by using "quantum-quench spectroscopy",
namely via quantum quenches of variable width which prepare the state of the
system in a superposition of eigenstates of the Hamiltonian within a
controllable spectral region. Studying numerically a chain of interacting
spinless fermions in a quasi-periodic potential, we argue that this system has
a many-body mobility edge; and we show that its existence translates into a
clear dynamical transition in the time evolution immediately following a quench
in the strength of the quasi-periodic potential, as well as a transition in the
scaling properties of the quasi-stationary state at long times. Our results
suggest a practical scheme for the experimental observation of many-body
mobility edges using cold-atom setups.Comment: v2: references added v3: minor revisions, added reference
Edge States in Gauge Theories: Theory, Interpretations and Predictions
Gauge theories on manifolds with spatial boundaries are studied. It is shown
that observables localized at the boundaries (edge observables) can occur in
such models irrespective of the dimensionality of spacetime. The intimate
connection of these observables to charge fractionation, vertex operators and
topological field theories is described. The edge observables, however, may or
may not exist as well-defined operators in a fully quantized theory depending
on the boundary conditions imposed on the fields and their momenta. The latter
are obtained by requiring the Hamiltonian of the theory to be self-adjoint and
positive definite. We show that these boundary conditions can also have nice
physical interpretations in terms of certain experimental parameters such as
the penetration depth of the electromagnetic field in a surrounding
superconducting medium. The dependence of the spectrum on one such parameter is
explicitly exhibited for the Higgs model on a spatial disc in its London limit.
It should be possible to test such dependences experimentally, the above Higgs
model for example being a model for a superconductor. Boundary conditions for
the 3+1 dimensional system confined to a spatial ball are studied. Their
physical meaning is clarified and their influence on the edge states of this
system (known to exist under certain conditions) is discussed. It is pointed
out that edge states occur for topological solitons of gauge theories such as
the 't Hooft-Polyakov monopoles.Comment: 36 pages, LATEX File (revised because figures had problems
Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem
Due to particle conservation, Canonical Molecular Dynamics (MD) simulations
fail in the description of surface phase transitions involving coverage or
lateral density changes. However, a step on the surface can act effectively as
a source or a sink of atoms, in the simulation as well as in real life. A
single surface step can be introduced by suitably modifying planar Periodic
Boundary Conditions (PBC), to accommodate the generally inequivalent stacking
of two adjacent layers. We discuss here how, through the introduction of two
orthogonal surface steps, particle number conservation may no longer represent
a fatal constraint for the study of these surface transitions. As an example,
we apply the method for estimating temperature-induced lateral density increase
of the reconstructed
Au (001) surface; the resulting anisotropic cell change is consistent with
experimental observations. Moreover, we implement this kind of scheme in
conjunction with the variable curvature MD method, recently introduced by our
group.Comment: 9 pages, 5 figures, accepted for publication in Surface Science
(ECOSS-19
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