arXiv subjects

Michele Modugno

Publications and source records attributed to Michele Modugno.

At least 55 records · Page 3Linked to original sources

arXiv2012

Maximally localized Wannier functions for ultracold atoms in one-dimensional double-well periodic potentials

We discuss a method for constructing generalized Wannier functions that are maximally localized at the minima of a one-dimensional periodic potential with a double-well per unit cell. By following the approach of (Marzari M and Vanderbilt D 1997 Phys. Rev. B 56, 12847), we consider a set of band-mixing Wannier functions with minimal spread, and design a specific two-step gauge transformation of the Bloch functions for a composite two band system. This method is suited to efficiently compute the tight-binding coefficients needed for mapping the continuous system to a discrete lattice model. Their behaviour is analyzed here as a function of the symmetry properties of the double-well (including the possibility of parity-breaking), in a range of feasible experimental parameters.

cond-mat.quant-gas↗

arXiv2011

Localization from quantum interference in one-dimensional disordered potentials

We show that the tails of the asymptotic density distribution of a quantum wave packet that localizes in the the presence of random or quasiperiodic disorder can be described by the diagonal term of the projection over the eingenstates of the disordered potential. This is equivalent of assuming a phase randomization of the off-diagonal/interference terms. We demonstrate these results through numerical calculations of the dynamics of ultracold atoms in the one-dimensional speckle and quasiperiodic potentials used in the recent experiments that lead to the observation of Anderson localization for matter waves [Billy et al., Nature 453, 891 (2008); Roati et al., Nature 453, 895 (2008)]. For the quasiperiodic case, we also discuss the implications of using continuos or discrete models.

cond-mat.quant-gas↗

arXiv2010

Density of states in an optical speckle potential

We study the single particle density of states of a one-dimensional speckle potential, which is correlated and non-Gaussian. We consider both the repulsive and the attractive cases. The system is controlled by a single dimensionless parameter determined by the mass of the particle, the correlation length and the average intensity of the field. Depending on the value of this parameter, the system exhibits different regimes, characterized by the localization properties of the eigenfunctions. We calculate the corresponding density of states using the statistical properties of the speckle potential. We find good agreement with the results of numerical simulations.

cond-mat.dis-nn↗

arXiv2010

Anomalous Bloch oscillations in one dimensional parity-breaking periodic potentials

We investigate the dynamics of a wave packet in a parity-breaking one-dimensional periodic potential slowly varied in time and perturbed by a linear potential. Parity is broken by considering an asymmetric double well per unit cell. By comparing the prediction of the semiclassical dynamics with the full Schrödinger solution, we show that Bloch oscillations are strongly affected by anomalous velocity corrections related to Berry's phase. We characterize how these effects depend on the degree of parity breaking of the potential and on the modulation parameters. We also discuss how to measure the effects of the anomalous velocity in current experiments with non-interacting Bose-Einstein condensates in bichromatic optical lattices, under the effect of gravity.

cond-mat.quant-gas↗

arXiv2010

Correlated bosons in a one-dimensional optical lattice: Effects of the trapping potential and of quasiperiodic disorder

We investigate the effect of the trapping potential on the quantum phases of strongly correlated ultracold bosons in one-dimensional periodic and quasiperiodic optical lattices. By means of a decoupling meanfield approach, we characterize the ground state of the system and its behavior under variation of the harmonic trapping, as a function of the total number of atoms. For a small atom number the system shows an incompressible Mott-insulating phase, as the size of the cloud remains unaffected when the trapping potential is varied. When the quasiperiodic potential is added the system develops a metastable-disordered phase which is neither compressible nor Mott insulating. This state is characteristic of quasidisorder in the presence of a strong trapping potential.

cond-mat.quant-gas↗

arXiv2009

Effects of interaction on the diffusion of atomic matter waves in one-dimensional quasi-periodic potentials

We study the behaviour of an ultracold atomic gas of bosons in a bichromatic lattice, where the weaker lattice is used as a source of disorder. We numerically solve a discretized mean-field equation, which generalizes the one-dimensional Aubry-Andrè model for particles in a quasi-periodic potential by including the interaction between atoms. We compare the results for commensurate and incommensurate lattices. We investigate the role of the initial shape of the wavepacket as well as the interplay between two competing effects of the interaction, namely self-trapping and delocalization. Our calculations show that, if the condensate initially occupies a single lattice site, the dynamics of the interacting gas is dominated by self-trapping in a wide range of parameters, even for weak interaction. Conversely, if the diffusion starts from a Gaussian wavepacket, self-trapping is significantly suppressed and the destruction of localization by interaction is more easily observable.

cond-mat.quant-gas↗

arXiv2009

Instabilities of a matter wave in a matter grating

We investigate the stability of Bloch waves for a Bose-Einstein condensate moving through a periodic lattice created by another condensate modulated by an optical lattice. We show that the coupling of phonon-antiphonon modes of the two species give rise to a very rich structure of the regimes for dynamical instability, with significant differences with respect to the case of a single condensate in an optical lattice. We characterize the relative weight of each condensate in the mixing and discuss an analytic limit that accounts for the bare structure of the instability diagrams.

cond-mat.quant-gas↗

arXiv2009

Exponential localization in one-dimensional quasiperiodic optical lattices

We investigate the localization properties of a one-dimensional bichromatic optical lattice in the tight binding regime, by discussing how exponentially localized states emerge upon changing the degree of commensurability. We also review the mapping onto the discrete Aubry-Andre' model, and provide evidences on how the momentum distribution gets modified in the crossover from extended to exponentially localized states. This analysis is relevant to the recent experiment on Anderson localization of a noninteracting Bose-Einstein condensate in a quasiperiodic optical lattice [G. Roati et al., Nature 453, 895 (2008)].

cond-mat.other↗

arXiv2007

Atom interferometry with a weakly-interacting Bose Einstein condensate

We demonstrate the operation of an atom interferometer based on a weakly interacting Bose-Einstein condensate. We strongly reduce the interaction induced decoherence that usually limits interferometers based on trapped condensates by tuning the s-wave scattering length almost to zero via a magnetic Feshbach resonance. We employ a $^{39}$K condensate trapped in an optical lattice, where Bloch oscillations are forced by gravity. With a control of the scattering length better that 0.1 $a_0$ we achieve coherence times of several hundreds of ms. The micrometric sizes of the atomic sample make our sensor an ideal candidate for measuring forces with high spatial resolution. Our technique can be in principle extended to other measurement schemes opening new possibilities in the field of trapped atom interferometry.

cond-mat.other↗

arXiv2006

A multiband envelope function model for quantum transport in a tunneling diode

We present a simple model for electron transport in semiconductor devices that exhibit tunneling between the conduction and valence bands. The model is derived within the usual Bloch-Wannier formalism by a k-expansion, and is formulated in terms of a set of coupled equations for the electron envelope functions. Its connection with other models present in literature is discussed. As an application we consider the case of a Resonant Interband Tunneling Diode, demonstrating the ability of the model to reproduce the expected behaviour of the current as a function of the applied voltage

cond-mat.mes-hall↗

arXiv2005

Collective dynamics and expansion of a Bose-Einstein condensate in a random potential

We investigate the dynamics of a Bose-Einstein condensate in the presence of a random potential created by optical speckles. We first consider the effect of a weak disorder on the dipole and quadrupole collective oscillations, finding uncorrelated frequency shifts of the two modes with respect to the pure harmonic case. This behaviour, predicted by a sum rules approach, is confirmed by the numerical solution of the Gross-Pitaevskii equation. Then we analyze the role of disorder on the one-dimensional expansion in an optical guide, discussing possible localization effects. Our theoretical analysis provides a useful insight into the recent experiments performed at LENS [J. Lye et al., Phys. Rev. Lett. 95, 070401 (2005); C. Fort et al., cond-mat/0507144].

cond-mat.dis-nn↗

arXiv2004

Velocity of sound in a Bose-Einstein condensate in the presence of an optical lattice and transverse confinement

We study the effect of the transverse degrees of freedom on the velocity of sound in a Bose-Einstein condensate immersed in a one-dimensional optical lattice and radially confined by a harmonic trap. We compare the results of full three-dimensional calculations with those of an effective 1D model based on the equation of state of the condensate. The perfect agreement between the two approaches is demonstrated for several optical lattice depths and throughout the full crossover from the 1D mean-field to the Thomas Fermi regime in the radial direction.

cond-mat.stat-mech↗

arXiv2003

Finite temperature effects on the collapse of trapped Bose-Fermi mixtures

By using the self-consistent Hartree-Fock-Bogoliubov-Popov theory, we present a detailed study of the mean-field stability of spherically trapped Bose-Fermi mixtures at finite temperature. We find that, by increasing the temperature, the critical particle number of bosons (or fermions) and the critical attractive Bose-Fermi scattering length increase, leading to a significant stabilization of the mixture.

cond-mat.stat-mech↗

arXiv2003

Expansion of a quantum degenerate boson-fermion mixture

We study the expansion of an ultracold boson-fermion mixture released from an elongated magnetic trap, in the context of a recent experiment at LENS (G. Roati et al., Phys. Rev. Lett. 89, 150403 (2002)). We discuss in some details the role of the boson-fermion interaction on the evolution of the radial-to-axial aspect ratio of the condensate, and show that it depends crucially on the relative dynamics of the condensate and degenerate Fermi gas in the radial direction, which is characterized by the ratio between the trapping frequency for fermions and for bosons. Our numerical simulations are in reasonable agreement with the experiment.

cond-mat↗

arXiv2002

1D model for the dynamics and expansion of elongated Bose-Einstein condensates

We present a 1D effective model for the evolution of a cigar-shaped Bose-Einstein condensate in time dependent potentials whose radial component is harmonic. We apply this model to investigate the dynamics and expansion of condensates in 1D optical lattices, by comparing our predictions with recent experimental data and theoretical results. We also discuss negative-mass effects which could be probed during the expansion of a condensate moving in an optical lattice.

cond-mat↗

arXiv2002

Bose-Einstein condensates with a bent vortex in rotating traps

We consider a 3D dilute Bose-Einstein condensate at thermal equilibrium in a rotating harmonic trap. The condensate wavefunction is a local minimum of the Gross-Pitaevskii energy functional and we determine it numerically with the very efficient conjugate gradient method. For single vortex configurations in a cigar-shaped harmonic trap we find that the vortex line is bent, in agreement with the numerical prediction of Garcia-Ripoll and Perez-Garcia, Phys.Rev.A 63, 041603 (2001). We derive a simple energy functional for the vortex line in a cigar-shaped condensate which allows to understand physically why the vortex line bends and to predict analytically the minimal rotation frequency required to stabilize the bent vortex line. This analytical prediction is in excellent agreement with the numerical results. It also allows to find in a simple way a saddle point of the energy, where the vortex line is in a stationary configuration in the rotating frame but not a local minimum of energy. Finally we investigate numerically the effect of thermal fluctuations on the vortex line for a condensate with a straight vortex: we can predict what happens in a single realization of the experiment by a Monte Carlo sampling of an atomic field quasi-distribution function of the density operator of the gas at thermal equilibrium in the Bogoliubov approximation.

cond-mat.mes-hall↗

arXiv2002

Topology of the ground state of two interacting Bose-Einstein condensates

We investigate the spatial patterns of the ground state of two interacting Bose-Einstein condensates. We consider the general case of two different atomic species (with different mass and in different hyperfine states) trapped in a magnetic potential whose eigenaxes can be tilted with respect to the vertical direction, giving rise to a non trivial gravitational sag. Despite the complicated geometry, we show that within the Thomas-Fermi approximations and upon appropriate coordinate transformations, the equations for the density distributions can be put in a very simple form. Starting from this expressions we give explicit rules to classify the different spatial topologies which can be produced, and we discuss how the behavior of the system is influenced by the inter-atomic scattering length. We also compare explicit examples with the full numeric Gross-Pitaevskii calculation.

cond-mat↗

arXiv2000

Testing the Symmetrization Postulate on Molecules with Three Identical Nuclei

We propose an experiment to look for possible small violations of the symmetrization postulate of Quantum Mechanics, in systems composed by three identical particles. Such violations could be detected by investigating the population of particular roto-vibrational states of symmetrical molecules containing three identical nuclei. We discuss the symmetry properties of such states, and the implications of the symmetrization postulate and of the spin-statistics. A high sensitivity spectroscopic investigation on simple molecules such as SO3, BH3 and NH3 could lead to the first test of the symmetrization postulate for spin-0 and spin-1/2 nuclei.

quant-ph↗