5,588 research outputs found
Spin-phonon induced magnetic order in Kagome ice
We study the effects of lattice deformations on the Kagome spin ice, with
Ising spins coupled by nearest neighbor exchange and long range dipolar
interactions, in the presence of in-plane magnetic fields. We describe the
lattice energy according to the Einstein model, where each site distortion is
treated independently. Upon integration of lattice degrees of freedom,
effective quadratic spin interactions arise. Classical MonteCarlo simulations
are performed on the resulting model, retaining up to third neighbor
interactions, under different directions of the magnetic field. We find that,
as the effect of the deformation is increased, a rich plateau structure appears
in the magnetization curves.Comment: 7 pages, 8 figure
Equilibrium and Disorder-induced behavior in Quantum Light-Matter Systems
We analyze equilibrium properties of coupled-doped cavities described by the
Jaynes-Cummings- Hubbard Hamiltonian. In particular, we characterize the
entanglement of the system in relation to the insulating-superfluid phase
transition. We point out the existence of a crossover inside the superfluid
phase of the system when the excitations change from polaritonic to purely
photonic. Using an ensemble statistical approach for small systems and
stochastic-mean-field theory for large systems we analyze static disorder of
the characteristic parameters of the system and explore the ground state
induced statistics. We report on a variety of glassy phases deriving from the
hybrid statistics of the system. On-site strong disorder induces insulating
behavior through two different mechanisms. For disorder in the light-matter
detuning, low energy cavities dominate the statistics allowing the excitations
to localize and bunch in such cavities. In the case of disorder in the light-
matter coupling, sites with strong coupling between light and matter become
very significant, which enhances the Mott-like insulating behavior. Inter-site
(hopping) disorder induces fluidity and the dominant sites are strongly coupled
to each other.Comment: about 10 pages, 12 figure
Non-Abelian fractional quantum Hall states and chiral coset conformal field theories
We propose an effective Lagrangian for the low energy theory of the Pfaffian
states of the fractional quantum Hall effect in the bulk in terms of
non-Abelian Chern-Simons (CS) actions. Our approach exploits the connection
between the topological Chern-Simons theory and chiral conformal field
theories. This construction can be used to describe a large class of
non-Abelian FQH states.Comment: Revised manuscript, 17 pages; new section discusses parafermion
state
Fractal Fidelity as a signature of Quantum Chaos
We analyze the fidelity of a quantum simulation and we show that it displays
fractal fluctuations iff the simulated dynamics is chaotic. This analysis
allows us to investigate a given simulated dynamics without any prior
knowledge. In the case of integrable dynamics, the appearance of fidelity
fractal fluctuations is a signal of a highly corrupted simulation. We
conjecture that fidelity fractal fluctuations are a signature of the appearance
of quantum chaos. Our analysis can be realized already by a few qubit quantum
processor.Comment: 5 pages, 5 figure
Engineering fidelity echoes in Bose-Hubbard Hamiltonians
We analyze the fidelity decay for a system of interacting bosons described by
a Bose-Hubbard Hamiltonian. We find echoes associated with "non-universal"
structures that dominate the energy landscape of the perturbation operator.
Despite their classical origin, these echoes persist deep into the quantum
(perturbative) regime and can be described by an improved random matrix
modeling. In the opposite limit of strong perturbations (and high enough
energies), classical considerations reveal the importance of self-trapping
phenomena in the echo efficiency.Comment: 6 pages, use epl2.cls class, 5 figures Cross reference with nlin,
quant-phy
Bond-impurity induced bound states in disordered spin-1/2 ladders
We discuss the effect of weak bond-disorder in two-leg spin ladders on the
dispersion relation of the elementary triplet excitations with a particular
focus on the appearance of bound states in the spin gap. Both the cases of
modified exchange couplings on the rungs and the legs of the ladder are
analyzed. Based on a projection on the single-triplet subspace, the
single-impurity and small cluster problems are treated analytically in the
strong-coupling limit. Numerically, we study the problem of a single impurity
in a spin ladder by exact diagonalization to obtain the low-lying excitations.
At finite concentrations and to leading order in the inter-rung coupling, we
compare the spectra obtained from numerical diagonalization of large systems
within the single-triplet subspace with the results of diagrammatic techniques,
namely low-concentration and coherent-potential approximations. The
contribution of small impurity clusters to the density of states is also
discussed.Comment: 9 pages REVTeX4 including 7 figures, final version; Fig. 5 modifie
Charge and spin transport in strongly correlated one-dimensional quantum systems driven far from equilibrium
We study the charge conductivity in one-dimensional prototype models of
interacting particles, such as the Hubbard and the t-V spinless fermion model,
when coupled to some external baths injecting and extracting particles at the
boundaries. We show that, if these systems are driven far from equilibrium, a
negative differential conductivity regime can arise. The above electronic
models can be mapped into Heisenberg-like spin ladders coupled to two magnetic
baths, so that charge transport mechanisms are explained in terms of quantum
spin transport. The negative differential conductivity is due to oppositely
polarized ferromagnetic domains which arise at the edges of the chain, and
therefore inhibit spin transport: we propose a qualitative understanding of the
phenomenon by analyzing the localization of one-magnon excitations created at
the borders of a ferromagnetic region. We also show that negative differential
conductivity is stable against breaking of integrability. Numerical simulations
of non-equilibrium time evolution have been performed by employing a
Monte-Carlo wave function approach and a matrix product operator formalism.Comment: 20 pages, 19 figures. Published versio
Ground state fidelity and quantum phase transitions in free Fermi systems
We compute the fidelity between the ground states of general quadratic
fermionic hamiltonians and analyze its connections with quantum phase
transitions. Each of these systems is characterized by a real
matrix whose polar decomposition, into a non-negative and a unitary
, contains all the relevant ground state (GS) information. The boundaries
between different regions in the GS phase diagram are given by the points of,
possibly asymptotic, singularity of . This latter in turn implies a
critical drop of the fidelity function. We present general results as well as
their exemplification by a model of fermions on a totally connected graph.Comment: 4 pages, 2 figure
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