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Wendenbaum P.,CNRS Jean Lamour Institute | Collura M.,DellUniversity Pisa | Karevski D.,CNRS Jean Lamour Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We study the quantum evolution of a cloud of hard-core bosons loaded on a one-dimensional optical lattice after its sudden release from a harmonic trap. Just after the trap has been removed, a linear ramp potential is applied, mimicking the so-called Galileo ramp experiment. The nonequilibrium expansion of the bosonic cloud is elucidated through a hydrodynamical description which is compared to the exact numerical evolution obtained by exact diagonalization on finite lattice sizes. The system is found to exhibit a rich behavior, showing, in particular, Bloch oscillations of a self-trapped condensate and an ejected particle density leading to two diverging entangled condensates. Depending on the initial density of the gas different regimes of Josephson-like oscillations are observed. At low densities, the trapped part of the cloud is in a superfluid phase that oscillates in time as a whole. At higher densities, the trapped condensate is in a mixed superfluid-Mott-insulator phase that show a breathing regime for steep enough potential ramps. © 2013 American Physical Society.

Collura M.,DellUniversity Pisa | Karevski D.,CNRS Jean Lamour Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We consider a quantum quench in a noninteracting fermionic one-dimensional field theory. The system of size L is initially prepared into two halves L ([-L/2,0]) and R ([0,L/2]), each of them thermalized at two different temperatures TL and TR, respectively. At a given time, the two halves are joined together by a local coupling and the whole system is left to evolve unitarily. For an infinitely extended system (L→), we show that the time evolution of the particle and energy densities is well described via a hydrodynamic approach which allows us to evaluate the correspondent stationary currents. We show, in such a case, that the two-point correlation functions are deduced, at large times, from a simple nonequilibrium steady state. Otherwise, whenever the boundary conditions are retained (in a properly defined thermodynamic limit), any current is suppressed at large times, and the stationary state is described by a generalized Gibbs ensemble, which is diagonal and depends only on the post-quench mode occupation. © 2014 American Physical Society.

Strumia A.,CERN | Strumia A.,DellUniversity Pisa | Strumia A.,National Institute of Nuclear Physics, Italy
Journal of High Energy Physics | Year: 2010

We reconsider thermal production of axinos in the early universe, adding: a) missed terms in the axino interaction; b) production via gluon decays kinematically allowed by thermal masses; c) a precise modeling of reheating. We find an axino abunance a few times larger than previous computations. © SISSA 2010.

D'Elia M.,DellUniversity Pisa | Mariti M.,DellUniversity Pisa | Negro F.,DellUniversity Genoa
Physical Review Letters | Year: 2013

We investigate two flavor quantum chromodynamics (QCD) in the presence of CP-odd electromagnetic background fields and determine, by means of lattice QCD simulations, the induced effective θ term to first order in E→·B→. We employ a rooted staggered discretization and study lattice spacings down to 0.1 fm and Goldstone pion masses around 480 MeV. In order to deal with a positive measure, we consider purely imaginary electric fields and real magnetic fields, and then exploit the analytic continuation. Our results are relevant to a description of the effective pseudoscalar quantum electrodynamics - QCD interactions. © 2013 American Physical Society.

D'Elia M.,DellUniversity Pisa | Negro F.,DellUniversity Genoa
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We study the phase diagram of non-Abelian pure gauge theories in the presence of a topological θ term. The dependence of the deconfinement temperature on θ is determined on the lattice both by analytic continuation and by reweighting, obtaining consistent results. The general structure of the diagram is discussed on the basis of large-N considerations and of the possible analogies and dualities existing with the phase diagram of QCD in the presence of an imaginary baryon chemical potential. © 2013 American Physical Society.

Fagotti M.,DellUniversity Pisa | Fagotti M.,University of Oxford
EPL | Year: 2012

We study the entanglement of two disjoint blocks in spin- 1/2 chains obtained by merging solvable models, such as XX and quantum Ising models. We focus on the universal quantities that can be extracted from the Rényi entropies S α. The most important information is encoded in some functions denoted by F α. We compute F 2and we show that F α ?1 and Fv.N., corresponding to the von Neumann entropy, can be negative, in contrast to what observed in all models examined so far. An exact relation between the entanglement of disjoint subsystems in the XX model and that in a chain embodying two quantum Ising models is a by-product of our investigations. © 2012 Europhysics Letters Association.

Campostrini M.,DellUniversity Pisa | Pelissetto A.,University of Rome La Sapienza | Vicari E.,DellUniversity Pisa
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We develop the finite-size scaling (FSS) theory at quantum transitions. We consider various boundary conditions, such as open and periodic boundary conditions, and characterize the corrections to the leading FSS behavior. Using renormalization-group (RG) theory, we generalize the classical scaling ansatz to describe FSS in the quantum case, classifying the different sources of scaling corrections. We identify nonanalytic corrections due to irrelevant (bulk and boundary) RG perturbations and analytic contributions due to regular backgrounds and analytic expansions of the nonlinear scaling fields. To check the general predictions, we consider the quantum XY chain in a transverse field. For this model exact or numerically accurate results can be obtained by exploiting its fermionic quadratic representation. We study the FSS of several observables, such as the free energy, the energy differences between low-energy levels, correlation functions of the order parameter, etc., confirming the general predictions in all cases. Moreover, we consider bipartite entanglement entropies, which are characterized by the presence of additional scaling corrections, as predicted by conformal field theory. © 2014 American Physical Society.

Crecchi F.,University of Chicago | Crecchi F.,Normal School of Pisa | Vicari E.,DellUniversity Pisa
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We investigate the effects of a trapping space-dependent potential on the low-temperature quasi-long-range order phase of two-dimensional particle systems with a relevant U(1) symmetry, such as quantum atomic gases. We characterize the universal features of the trap-size dependence using scaling arguments. The resulting scenario is supported by numerical Monte Carlo simulations of a classical two-dimensional XY model with a space-dependent hopping parameter, the inhomogeneity of which is analogous to that arising from the trapping potential in experiments of atomic gases. © 2011 American Physical Society.

Collura M.,DellUniversity Pisa | Calabrese P.,DellUniversity Pisa
Journal of Physics A: Mathematical and Theoretical | Year: 2013

We study the out-of-equilibrium time evolution after a local quench connecting two anisotropic spin-1/2 XXZ Heisenberg open chains via an impurity bond. The dynamics is obtained by means of the adaptive time-dependent density-matrix renormalization group. We show that the entanglement entropies (von Neumann and Rényi) in the presence of a weakened bond depend on the sign of the bulk interaction. For an attractive interaction (Δ < 0), the defect turns out to be irrelevant and the evolution is asymptotically equivalent to the one without defect obtained by conformal field theory. For a repulsive interaction (Δ > 0), the defect is relevant and the entanglement saturates to a finite value. This out-of-equilibrium behavior generalizes the well-known results for the ground-state entanglement entropy of the model. © 2013 IOP Publishing Ltd.

Campostrini M.,DellUniversity Pisa | Vicari E.,DellUniversity Pisa
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We develop a trap-size scaling theory for trapped particle systems at quantum transitions. As a theoretical laboratory, we consider a quantum XY chain in an external transverse field acting as a trap for the spinless fermions of its quadratic Hamiltonian representation. We discuss trap-size scaling at the Mott insulator to superfluid transition in the Bose-Hubbard model. We present exact and accurate numerical results for the XY chain and for the low-density Mott transition in the hard-core limit of the one-dimensional Bose-Hubbard model. Our results are relevant for systems of cold atomic gases in optical lattices. © 2010 The American Physical Society.

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