338 research outputs found
Detecting two-site spin-entanglement in many-body systems with local particle-number fluctuations
We derive experimentally measurable lower bounds for the two-site
entanglement of the spin-degrees of freedom of many-body systems with local
particle-number fluctuations. Our method aims at enabling the spatially
resolved detection of spin-entanglement in Hubbard systems using
high-resolution imaging in optical lattices. A possible application is the
observation of entanglement generation and spreading during spin impurity
dynamics, for which we provide numerical simulations. More generally, the
scheme can simplify the entanglement detection in ion chains, Rydberg atoms, or
similar atomic systems
Anyonic tight-binding models of parafermions and of fractionalized fermions
Parafermions are emergent quasi-particles which generalize Majorana fermions
and possess intriguing anyonic properties. The theoretical investigation of
effective models hosting them is gaining considerable importance in view of
present-day condensed-matter realizations where they have been predicted to
appear. Here we study the simplest number-conserving model of particle-like
Fock parafermions, namely a one-dimensional tight-binding model. By means of
numerical simulations based on exact diagonalization and on the density-matrix
renormalization group, we prove that this quadratic model is nonintegrable and
displays bound states in the spectrum, due to its peculiar anyonic properties.
Moreover, we discuss its many-body physics, characterizing anyonic correlation
functions and discussing the underlying Luttinger-liquid theory at low
energies. In the case when Fock parafermions behave as fractionalized fermions,
we are able to unveil interesting similarities with two counter-propagating
edge modes of two neighboring Laughlin states at filling 1/3.Comment: 13 pages, 11 figures. Updated version after publication in PR
Energy transport in Heisenberg chains beyond the Luttinger liquid paradigm
We study the energy transport between two interacting spin chains which are
initially separated, held at different temperatures and subsequently put in
contact. We consider the spin-1/2 XXZ model in the gapless regime and exploit
its integrability properties to formulate an analytical Ansatz for the
non-equilibrium steady state even at temperatures where the low-energy
Luttinger liquid description is not accurate. We apply our method to compute
the steady energy current and benchmark it both with the known low-energy limit
and at higher temperatures with numerical simulations. We find an excellent
agreement even at high temperatures, where the Luttinger liquid prediction is
shown to fail.Comment: 5 pages + 3 suppl. mat., 5 figure
Measurement of rheological properties of soft biological tissue with a novel torsional resonator device
A new device for measuring the rheological properties of soft biological tissues is presented. The mechanical response is characterized for harmonic shear deformations at high frequencies (up to 10kHz) and small strains (up to 0.2%). Experiments are performed using a cylindrical rod excited to torsional resonance. One extremity of the rod is in contact with the soft tissue and adherence is ensured by vacuum clamping. The damping characteristics and the resonance frequency of the vibrating system are inferred from the control variables of a phase stabilization loop. Due to the contact with the soft tissue, and depending on the rheological properties of the tissue, changes occur in the Q-factor and in the resonance frequency of the system. The shear modulus of the soft tissue is determined from the experimental results with an analytical model. The reliability of the proposed technique is evaluated through repeatability tests and comparative measurements with synthetic materials. The results of measurements on bovine organs demonstrate the suitability of the experimental procedure for the characterization of biological tissues and provide some insight in their rheological properties at frequencies in the range 1-10kH
Photon transport in a dissipative chain of nonlinear cavities
We analyze a chain of coupled nonlinear optical cavities driven by a coherent
source of light localized at one end and subject to uniform dissipation. We
characterize photon transport by studying the populations and the photon
correlations as a function of position. When complemented with input-output
theory, these quantities provide direct information about photon transmission
through the system. The position of single- and multi-photon resonances
directly reflect the structure of the many-body energy levels. This shows how a
study of transport along a coupled cavity array can provide rich information
about the strongly correlated (many-body) states of light even in presence of
dissipation. By means of a numerical algorithm based on the time-evolving block
decimation scheme adapted to mixed states, we are able to simulate arrays up to
sixty cavities.Comment: 12 pages, 14 figure
Destruction of string order after a quantum quench
We investigate the evolution of string order in a spin-1 chain following a
quantum quench. After initializing the chain in the Affleck-Kennedy-Lieb-Tasaki
state, we analyze in detail how string order evolves as a function of time at
different length scales. The Hamiltonian after the quench is chosen either to
preserve or to suddenly break the symmetry which ensures the presence of string
order. Depending on which of these two situations arises, string order is
either preserved or lost even at infinitesimal times in the thermodynamic
limit. The fact that non-local order may be abruptly destroyed, what we call
string-order melting, makes it qualitatively different from typical order
parameters in the manner of Landau. This situation is thoroughly characterized
by means of numerical simulations based on matrix product states algorithms and
analytical studies based on a short-time expansion for several simplified
models.Comment: 14 pages, 6 figures. Changes after publication on PR
The XYZ chain with Dzyaloshinsky-Moriya interactions: from spin-orbit-coupled lattice bosons to interacting Kitaev chains
Using the density-matrix renormalization-group algorithm (DMRG) and a
finite-size scaling analysis, we study the properties of the one-dimensional
completely-anisotropic spin-1/2 XYZ model with Dzyaloshinsky-Moriya (DM)
interactions. The model shows a rich phase diagram: depending on the value of
the coupling constants, the system can display different kinds of ferromagnetic
order and Luttinger-liquid behavior. Transitions from ferromagnetic to
Luttinger-liquid phases are first order. We thoroughly discuss the transition
between different ferromagnetic phases, which, in the absence of DM
interactions, belongs to the XX universality class. We provide evidence that
the DM exchange term turns out to split this critical line into two separated
Ising-like transitions and that in between a disordered phase may appear. Our
study sheds light on the general problem of strongly-interacting
spin-orbit-coupled bosonic gases trapped in an optical lattice and can be used
to characterize the topological properties of superconducting nanowires in the
presence of an imposed supercurrent and of interactions.Comment: 18 pages, 8 figure
Multivariate meta-analysis of QTL mapping studies
A large number of quantitative trait loci (QTLs) for milk production and quality traits in dairy
cattle has been reported in literature. The large amount of information available could be exploited
by meta-analyses to draw more general conclusions from results obtained in different experimental
conditions (animals, statistical methodologies). QTL meta-analyses have been carried out to estimate
the distribution of QTL effects in livestock and to find consensus on QTL position. In this study, multivariate
dimension reduction techniques are used to analyse a database of dairy cattle QTL published
results, in order to extract latent variables able to characterise the research. A total of 92 papers by 72
authors were found on 25 scientific Journals for the period January 1995-February 2008. More than
thirty parameters were picked up from the articles. To overcome the problem of different map location,
the flanking markers were mapped on release 4.1 of the Bos taurus genome sequence (www.ensembl.
org). Their position was retrieved from public databases and, when absent, was calculated in silico
by blasting (http://blast.wustl.edu/) the markers’ nucleotide sequence against the genomic sequence.
Records were discarded if flanking markers or P-values were not available. After these edits, the final
archive consisted of 1,162 records. Seven selected variables were analysed both with the Factor Analysis
(FA), combined with the varimax rotation technique, and Principal Component Analysis (PCA). FA
was able to explain 68% of the original variability with 3 latent factors: the first factor extracted was
highly associated (factor loading of 0.98) to marker location along the chromosome and could be considered
as a marker map index; the second factor showed factor loadings of 0.74 and 0.84 related to the
variable number of animals involved and year of the experiment, respectively, and it can be regarded
as an indicator of the dimension of the study; the third factor was correlated to the significance level
of the statistical test (0.78), number of families (0.63), and, negatively, to the marker density (-0.43). It
can be named as index of power of the experiment. Same patterns can be observed in the eigenvectors
of PCA. Four PCs were able to explain about 80% of the original variance. The first two PCs basically
underlined accurately the same structure found with the first two factors in FA, whereas PC3 and PC4
summarized the structure of F3. The score that each QTL gets on each Factor or PC could be useful
to classify the original QTL records and make them more comparable once that the redundancy of
information has been removed
Energy transport between two integrable spin chains
We study the energy transport in a system of two half-infinite XXZ chains
initially kept separated at different temperatures, and later connected and let
free to evolve unitarily. By changing independently the parameters of the two
halves, we highlight, through bosonisation and time-dependent
matrix-product-state simulations, the different contributions of low-lying
bosonic modes and of fermionic quasi-particles to the energy transport. In the
simulations we also observe that the energy current reaches a finite value
which only slowly decays to zero. The general pictures that emerges is the
following. Since integrability is only locally broken in this model, a
pre-equilibration behaviour may appear. In particular, when the sound
velocities of the bosonic modes of the two halves match, the low-temperature
energy current is almost stationary and described by a formula with a
non-universal prefactor interpreted as a transmission coefficient.
Thermalisation, characterized by the absence of any energy flow, occurs only on
longer time-scales which are not accessible with our numerics.Comment: 15 pages, 14 figure
Out-of-equilibrium dynamics and thermalization of string order
We investigate the equilibration dynamics of string order in one-dimensional
quantum systems. After initializing a spin-1 chain in the Haldane phase, the
time evolution of non-local correlations following a sudden quench is studied
by means of matrix-product-state-based algorithms. Thermalization occurs only
for scales up to a horizon growing at a well defined speed, due to the finite
maximal velocity at which string correlations can propagate, related to a
Lieb-Robinson bound. The persistence of string ordering at finite times is
non-trivially related to symmetries of the quenched Hamiltonian. A
qualitatively similar behavior is found for the string order of the Mott
insulating phase in the Bose-Hubbard chain. This paves the way towards an
experimental testing of our results in present cold-atom setups.Comment: 6 pages, 5 figures. Several changes; slightly extended version of PRB
paper (non-symmetric Hamiltonians also addressed
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