arXiv subjects

V. Penna

Publications and source records attributed to V. Penna.

34 records · Page 2Linked to original sources

arXiv2006

Dynamical Instability in a Trimeric Chain of Interacting Bose-Einstein Condensates

We analyze thoroughly the mean-field dynamics of a linear chain of three coupled Bose-Einstein condensates, where both the tunneling and the central-well relative depth are adjustable parameters. Owing to its nonintegrability, entailing a complex dynamics with chaos occurrence, this system is a paradigm for longer arrays whose simplicity still allows a thorough analytical study.We identify the set of dynamics fixed points, along with the associated proper modes, and establish their stability character depending on the significant parameters. As an example of the remarkable operational value of our analysis, we point out some macroscopic effects that seem viable to experiments.

cond-mat.other↗

arXiv2006

Discrete Wigner distribution for two qubits: a characterization of entanglement properties

We study the properties of the discrete Wigner distribution for two qubits introduced by Wotters. In particular, we analyze the entanglement properties within the Wigner distribution picture by considering the negativity of the Wigner function (WF) and the correlations of the marginal distribution. We show that a state is entangled if at least one among the values assumed by the corresponding discrete WF is smaller than a certain critical (negative) value. Then, based on the Partial Transposition criterion, we establish the relation between the separability of a density matrix and the non-negativity of the WF's relevant both to such a density matrix and to the partially transposed thereof. Finally, we derive a simple inequality --involving the covariance-matrix elements of a given WF-- which appears to provide a separability criterion stronger than the one based on the Local Uncertainty Relations.

quant-ph↗

arXiv2006

Ground-state Properties of Small-Size Nonlinear Dynamical Lattices

We investigate the ground state of a system of interacting particles in small nonlinear lattices with M > 2 sites, using as a prototypical example the discrete nonlinear Schroedinger equation that has been recently used extensively in the contexts of nonlinear optics of waveguide arrays, and Bose-Einstein condensates in optical lattices. We find that, in the presence of attractive interactions, the dynamical scenario relevant to the ground state and the lowest-energy modes of such few-site nonlinear lattices reveals a variety of nontrivial features that are absent in the large/infinite lattice limits: the single-pulse solution and the uniform solution are found to coexist in a finite range of the lattice intersite coupling where, depending on the latter, one of them represents the ground state; in addition, the single-pulse mode does not even exist beyond a critical parametric threshold. Finally, the onset of the ground state (modulational) instability appears to be intimately connected with a non-standard (``double transcritical'') type of bifurcation that, to the best of our knowledge, has not been reported previously in other physical systems.

cond-mat.soft↗

arXiv2005

Mean-field algebraic approach to the dynamics of fermions in a 1D optical lattice

We consider a one-dimensional optical lattice of three-dimensional Harmonic Oscillators which are loaded with neutral fermionic atoms trapped into two hyperfine states. By means of a standard variational coherent-state procedure, we derive an effective Hamiltonian for this quantum model and the hamiltonian equations describing its evolution. To this end, we identify the algebra $\mathcal L$ of two-fermion operators --describing the relevant microscopic quantum processes of our model-- whereby the natural choice for the trial state appears to be a so(2r) coherent state. The coherent-state parameters, playing the role of dynamical variables for the effective Hamiltonian, are shown to identify with the $\mathcal L$-operator expectation values thus providing a clear physical interpretation of this algebraic mean-field picture.

cond-mat.other↗

arXiv2005

An algebraic approach to the study of weakly excited states for a condensate in a ring geometry

We determine the low-energy spectrum and the eigenstates for a two-bosonic mode nonlinear model by applying the Inönü-Wigner contraction method to the Hamiltonian algebra. This model is known to well represent a Bose-Einstein condensate rotating in a thin torus endowed with two angular-momentum modes as well as a condensate in a double-well potential characterized by two space modes. We consider such a model in the presence of both an attractive and a repulsive boson interaction and investigate regimes corresponding to different values of the inter-mode tunneling parameter. We show that the results ensuing from our approach are in many cases extremely satisfactory. To this end we compare our results with the ground state obtained both numerically and within a standard semiclassical approximation based on su(2) coherent states.

cond-mat.mes-hall↗

arXiv2004

Topology-induced confined superfluidity in inhomogeneous arrays

We report the first study of the zero-temperature phase diagram of the Bose-Hubbard model on topologically inhomogeneous arrays. We show that the usual Mott-insulator and superfluid domains, in the paradigmatic case of the comb lattice, are separated by regions where the superfluid behaviour of the bosonic system is confined along the comb backbone. The existence of such {\it confined superfluidity}, arising from topological inhomogeneity, is proved by different analytical and numerical techniques which we extend to the case of inhomogeneous arrays. We also discuss the relevance of our results to real system exhibiting macroscopic phase coherence, such as coupled Bose condensates and Josephson arrays.

cond-mat.soft↗

arXiv2004

Strong-coupling expansions for the topologically inhomogeneous Bose-Hubbard model

We consider a Bose-Hubbard model with an arbitrary hopping term and provide the boundary of the insulating phase thereof in terms of third-order strong coupling perturbative expansions for the ground state energy. In the general case two previously unreported terms occur, arising from triangular loops and hopping inhomogeneities, respectively. Quite interestingly the latter involves the entire spectrum of the hopping matrix rather than its maximal eigenpair, like the remaining perturbative terms. We also show that hopping inhomogeneities produce a first order correction in the local density of bosons. Our results apply to ultracold bosons trapped in confining potentials with arbitrary topology, including the realistic case of optical superlattices with uneven hopping amplitudes. Significant examples are provided. Furthermore, our results can be extented to magnetically tuned transitions in Josephson junction arrays.

cond-mat.soft↗

arXiv2004

Analytical mean-field approach to the phase-diagram of ultracold bosons in optical superlattices

We report a multiple-site mean-field analysis of the zero-temperature phase diagram for ultracold bosons in realistic optical superlattices. The system of interacting bosons is described by a Bose-Hubbard model whose site-dependent parameters reflect the nontrivial periodicity of the optical superlattice. An analytic approach is formulated based on the analysis of the stability of a fixed-point of the map defined by the self-consistency condition inherent in the mean-field approximation. The experimentally relevant case of the period-2 one-dimensional superlattice is briefly discussed. In particular, it is shown that, for a special choice of the superlattice parameters, the half-filling insulator domain features an unusual loophole shape that the single-site mean-field approach fails to capture.

cond-mat.stat-mech↗

arXiv2004

Persistence of mean-field features in the energy spectrum of small arrays of Bose-Einstein condensates

The Bose-Hubbard Hamiltonian capturing the essential physics of the arrays of interacting Bose-Einstein condensates is addressed, focusing on arrays consisting of two (dimer) and three (trimer) sites. In the former case, some results concerning the persistence of mean-field features in the energy spectrum of the symmetric dimer are extended to the asymmetric version of the system, where the two sites are characterized by different on-site energies. Based on a previous systematic study of the mean-field limit of the trimer, where the dynamics is exhaustively described in terms of its fixed points for every choice of the significant parameters, an interesting mapping between the dimer and the trimer is emphasized and used as a guide in investigating the persistence of mean-field features in the rather complex energy spectrum of the trimer. These results form the basis for the systematic investigation of the purely quantum trimer extending and completing the existing mean-field analysis. In this respect we recall that, similar to larger arrays, the trimer is characterized by a non-integrable mean-field dynamics featuring chaotic trajectories. Hence, the correspondence between mean-field fixed points and quantum energy levels emphasized in the present work may provide a key to investigate the quantum counterpart of classical instability.

cond-mat.mes-hall↗

arXiv2004

Dynamics of twin-condensate configurations in an open chain of three Bose-Einstein condensates

The dynamics of a mean-field model of three coupled condensates is studied for initial states characterized by twin condensates. Condensates occupy an open three-well chain, where the central potential-well depth is an independent parameter. Despite its diversity from the closed-chain symmetric model, such model is shown to have an integrable regime involving a predictable evolution for twin-condensate initial states. After establishing the dependence of the phase-space topology on the model parameters, the latter are shown to allow the control of various macroscopic effects such as rapid population inversions, pulsed phase changes, self-trapping and long-duration periodic oscillations. Finally, based on quantizing the system in terms of suitable ``microscopic'' canonical variables, the stability conditions of the integrable regime are related to the su(1,1) structure of the Hamiltonian spectrum.

cond-mat.mes-hall↗

arXiv2003

Spectral Statistics of the Triaxial Rigid Rotator: Semiclassical Origin of their Pathological Behavior

In this paper we investigate the local and global spectral properties of the triaxial rigid rotator. We demonstrate that, for a fixed value of the total angular momentum, the energy spectrum can be divided into two sets of energy levels, whose classical analog are librational and rotational motions. By using diagonalization, semiclassical and algebric methods, we show that the energy levels follow the anomalous spectral statistics of the one-dimensional harmonic oscillator.

cond-mat.stat-mech↗

arXiv2002

Quantum Dynamics of Coupled Bosonic Wells within the Bose-Hubbard Picture

We relate the quantum dynamics of the Bose-Hubbard model (BHM) to the semiclassical nonlinear equations that describe an array of interacting Bose condensates by implementing a standard variational procedure based on the coherent state method. We investigate the dynamics of the two-site BHM from the purely quantum viewpoint by recasting first the model within a spin picture and using then the related dynamical algebra. The latter allows us to study thoroughly the energy spectrum structure and to interpret quantally the classical symmetries of the two-site dynamics. The energy spectrum is also evaluated through various approximations relying on the coherent state approach.

cond-mat.mes-hall↗

arXiv2000

String Limit of Vortex Current Algebra

The Poisson structure generating the hamiltonian dynamics of string vortices is reconstructed within the current algebra picture as a limiting case of the standard brackets associated to fluids with a smooth vorticity field. The approach implemented bypasses the use of Dirac's procedure. The fine structure of the dynamical algebra is derived for planar fluids by implementing an appropriate spatial fragmentation of the vorticity field, and the limit to the point vortex gas is effected. The physical interpretation of the resulting local currents is provided. Nontrivial differences characterizing the canonical quantization of point vortices and the current algebra quantization are also illustrated.

cond-mat.mes-hall↗

arXiv1999

Quantum dynamics of two-dimensional vortex pairs with arbitrary total vorticity

Quantum dynamics of a vortex pair is investigated by considering the pair Hamiltonian within various, unequivalent algebraic frameworks. First the vortex pair spectrum is constructed in the standard contest of the e(2)-like dynamical symmetry and its degeneracy is thoroughly examined. Then the Berry phase phenomenon is studied through an su(1,1) realization of the pair Hamiltonian when its parameters are assumed to be time-dependent, whereas the Feynman- Onsager quantization conditions are recovered by means of symmetry arguments within a third approach based on a magneticlike description of the vortex pair. Finally, it is shown how recasting the dynamical algebra in terms of two-particle realization of both su(2) and su(1,1) provides the correct approach for the quantization of the model Hamiltonian accounting for the pair scattering from a disklike obstacle.

cond-mat.mes-hall↗

arXiv1999

On the Structure of the Bose-Einstein Condensate Ground State

We construct a macroscopic wave function that describes the Bose-Einstein condensate and weakly excited states, using the su(1,1) structure of the mean-field hamiltonian, and compare this state with the experimental values of second and third order correlation functions.

cond-mat↗

arXiv1998

Dynamical Mean Field Theory of the Bose-Hubbard Model

Quantum dynamics of the Bose-Hubbard Model is investigated through a semiclassical hamiltonian picture provided by the Time-Dependent Variational Principle method. The system is studied within a factorized slow/fast dynamics. The semiclassical requantization procedure allows one to account for the strong quantum nature of the system when $t/U\ll 1$. The phase diagram is in good agreement with Quantum Monte Carlo results and third order strong coupling perturbative expansion.

cond-mat.supr-con↗