1,151 research outputs found
Dimerized ground states in spin-S frustrated systems
We study a family of frustrated anti-ferromagnetic spin- systems with a
fully dimerized ground state. This state can be exactly obtained without the
need to include any additional three-body interaction in the model. The
simplest members of the family can be used as a building block to generate more
complex geometries like spin tubes with a fully dimerized ground state. After
present some numerical results about the phase diagram of these systems, we
show that the ground state is robust against the inclusion of weak disorder in
the couplings as well as several kinds of perturbations, allowing to study some
other interesting models as a perturbative expansion of the exact one. A
discussion on how to determine the dimerization region in terms of quantum
information estimators is also presented. Finally, we explore the relation of
these results with a the case of the a 4-leg spin tube which recently was
proposed as the model for the description of the compound
CuClDCSO, delimiting the region of the parameter space
where this model presents dimerization in its ground state.Comment: 10 pages, 9 figure
Phase diagram study of a dimerized spin-S zig-zag ladder
The phase diagram of a frustrated spin- zig-zag ladder is studied through
different numerical and analytical methods. We show that for arbitrary ,
there is a family of Hamiltonians for which a fully-dimerized state is an exact
ground state, being the Majumdar-Ghosh point a particular member of the family.
We show that the system presents a transition between a dimerized phase to a
N\'eel-like phase for , and spiral phases can appear for large . The
phase diagram is characterized by means of a generalization of the usual Mean
Field Approximation (MFA). The novelty in the present implementation is to
consider the strongest coupled sites as the unit cell. The gap and the
excitation spectrum is analyzed through the Random Phase Approximation (RPA).
Also, a perturbative treatment to obtain the critical points is discussed.
Comparisons of the results with numerical methods like DMRG are also presented.Comment: 14 pages, 6 figures. Some typos were corrected, and notation was
clarifie
Measurements, quantum discord and parity in spin 1 systems
We consider the evaluation of the quantum discord and other related measures
of quantum correlations in a system formed by a spin 1 and a complementary spin
system. A characterization of general projective measurements in such system in
terms of spin averages is thereby introduced, which allows to easily visualize
their deviation from standard spin measurements. It is shown that the
measurement optimizing these measures corresponds in general to a non-spin
measurement. The important case of states that commute with the total
spin parity is discussed in detail, and the general stationary measurements for
such states (parity preserving measurements) are identified. Numerical and
analytical results for the quantum discord, the geometric discord and the one
way information deficit in the relevant case of a mixture of two aligned spin 1
states are also presented.Comment: 6 pages, 2 figures, References adde
kmos: A lattice kinetic Monte Carlo framework
Kinetic Monte Carlo (kMC) simulations have emerged as a key tool for
microkinetic modeling in heterogeneous catalysis and other materials
applications. Systems, where site-specificity of all elementary reactions
allows a mapping onto a lattice of discrete active sites, can be addressed
within the particularly efficient lattice kMC approach. To this end we describe
the versatile kmos software package, which offers a most user-friendly
implementation, execution, and evaluation of lattice kMC models of arbitrary
complexity in one- to three-dimensional lattice systems, involving multiple
active sites in periodic or aperiodic arrangements, as well as site-resolved
pairwise and higher-order lateral interactions. Conceptually, kmos achieves a
maximum runtime performance which is essentially independent of lattice size by
generating code for the efficiency-determining local update of available events
that is optimized for a defined kMC model. For this model definition and the
control of all runtime and evaluation aspects kmos offers a high-level
application programming interface. Usage proceeds interactively, via scripts,
or a graphical user interface, which visualizes the model geometry, the lattice
occupations and rates of selected elementary reactions, while allowing
on-the-fly changes of simulation parameters. We demonstrate the performance and
scaling of kmos with the application to kMC models for surface catalytic
processes, where for given operation conditions (temperature and partial
pressures of all reactants) central simulation outcomes are catalytic activity
and selectivities, surface composition, and mechanistic insight into the
occurrence of individual elementary processes in the reaction network.Comment: 21 pages, 12 figure
Coherent control of quantum systems as a resource theory
Control at the interface between the classical and the quantum world is
fundamental in quantum physics. In particular, how classical control is
enhanced by coherence effects is an important question both from a theoretical
as well as from a technological point of view. In this work, we establish a
resource theory describing this setting and explore relations to the theory of
coherence, entanglement and information processing. Specifically, for the
coherent control of quantum systems the relevant resources of entanglement and
coherence are found to be equivalent and closely related to a measure of
discord. The results are then applied to the DQC1 protocol and the precision of
the final measurement is expressed in terms of the available resources.Comment: 9 pages, 4 figures, final version. Discussions were improved and some
points were clarified. The title was slightly changed to agree with the
published versio
Factorization and entanglement in general XYZ spin arrays in non-uniform transverse fields
We determine the conditions for the existence of a pair of degenerate parity
breaking separable eigenstates in general arrays of arbitrary spins connected
through couplings of arbitrary range and placed in a transverse field,
not necessarily uniform. Sufficient conditions under which they are ground
states are also provided. It is then shown that in finite chains, the
associated definite parity states, which represent the actual ground state in
the immediate vicinity of separability, can exhibit entanglement between any
two spins regardless of the coupling range or separation, with the reduced
state of any two subsystems equivalent to that of pair of qubits in an
entangled mixed state. The corresponding concurrences and negativities are
exactly determined. The same properties persist in the mixture of both definite
parity states. These effects become specially relevant in systems close to the
limit. The possibility of field induced alternating separable solutions
with controllable entanglement side limits is also discussed. Illustrative
numerical results for the negativity between the first and the
spin in an open spin chain for different values of and are as well
provided.Comment: 6 pages, figures adde
Generalized mean field description of entanglement in dimerized spin systems
We discuss a generalized self-consistent mean field (MF) treatment, based on
the selection of an arbitrary subset of operators for representing the system
density matrix, and its application to the problem of entanglement evaluation
in composite quantum systems. As a specific example, we examine in detail a
pair MF approach to the ground state (GS) of dimerized spin 1/2 systems with
anisotropic ferromagnetic-type XY and XYZ couplings in a transverse field,
including chains and arrays with first neighbor and also longer range
couplings. The approach is fully analytic and able to capture the main features
of the GS of these systems, in contrast with the conventional single spin MF.
Its phase diagram differs significantly from that of the latter, exhibiting
(Sz) parity breaking just in a finite field window if the coupling between
pairs is sufficiently weak, together with a fully dimerized phase below this
window and a partially aligned phase above it. It is then shown that through
symmetry restoration, the approach is able to correctly predict not only the
concurrence of a pair, but also its entanglement with the rest of the chain,
which shows a pronounced peak in the parity breaking window. Perturbative
corrections allow to reproduce more subtle observables like the entanglement
between weakly coupled spins and the low lying energy spectrum. All predictions
are tested against exact results for finite systems.Comment: 13 pages, 9 figures. Final versio
Thermal entanglement in fully connected spin systems and its RPA description
We examine the thermal pairwise entanglement in a symmetric system of
spins fully connected through anisotropic -type couplings embedded in a
transverse magnetic field. We consider both the exact evaluation together with
that obtained with the static path + random phase approximation (RPA) and the
ensuing mean field + RPA. The latter is shown to provide an accurate analytic
description of both the parallel and antiparallel thermal concurrence in large
systems. We also analyze the limit temperature for pairwise entanglement, which
is shown to increase for large fields and to decrease logarithmically with
increasing . Special finite size effects are as well discussed.Comment: 9 pages, 5 figure
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