arXiv subjects

C. Tozzo

Publications and source records attributed to C. Tozzo.

8 recordsLinked to original sources

arXiv2006

Detecting phonons and persistent currents in toroidal Bose-Einstein condensates by means of pattern formation

We theoretically investigate the dynamic properties of a Bose-Einstein condensate in a toroidal trap. A periodic modulation of the transverse confinement is shown to produce a density pattern due to parametric amplification of phonon pairs. By imaging the density distribution after free expansion one obtains i) a precise determination of the Bogoliubov spectrum and ii) a sensitive detection of quantized circulation in the torus. The parametric amplification is also sensitive to thermal and quantum fluctuations.

cond-mat.other↗

arXiv2006

Stability and excitations of solitons in 2D Bose-Einstein condensates

The small oscillations of solitons in 2D Bose-Einstein condensates are investigated by solving the Kadomtsev-Petviashvili equation which is valid when the velocity of the soliton approaches the speed of sound. We show that the soliton is stable and that the lowest excited states obey the same dispersion law as the one of the stable branch of excitations of a 1D gray soliton in a 2D condensate. The role of these states in thermodynamics is discussed.

cond-mat.other↗

arXiv2005

Vortex lattices in Bose-Einstein condensates: from the Thomas-Fermi to the lowest Landau level regime

We consider a periodic vortex lattice in a rotating Bose-Einstein condensed gas, where the centrifugal potential is exactly compensated by the external harmonic trap. By introducing a gauge transformation which makes the Hamiltonian periodic, we solve numerically the 2D Gross-Pitaevskii equation finding the exact mean field ground state. In particular, we explore the crossover between the Thomas-Fermi regime, holding for large values of the coupling constant, and the lowest Landau level limit, corresponding to the weakly interacting case. Explicit results are given for the equation of state, the vortex core size, as well as the elastic shear modulus, which is crucial for the calculation of the Tkachenko frequencies.

cond-mat.mes-hall↗

arXiv2005

Stability diagram and growth rate of parametric resonances in Bose-Einstein condensates in one-dimensional optical lattices

A Bose-Einstein condensate in an optical lattice exhibits parametric resonances when the intensity of the lattice is periodically modulated in time. These resonances correspond to an exponential growth of the population of counter-propagating Bogoliubov excitations. A suitable linearization of the Gross-Pitaevskii (GP) equation is used to calculate the stability diagram and the growth rates of the unstable modes. The results agree with the ones extracted from time-dependent GP simulations, supporting our previous claim (M. Kraemer et al., Phys. Rev. A (2005) in press) concerning the key role of parametric resonances in the response observed by Stoeferle et al. (Phys. Rev. Lett. 92, 130403 (2004)) in the superfluid regime. The role of the seed excitations required to trigger the parametric amplification is discussed. The possible amplification of the quantum fluctuations present in the quasiparticle vacuum, beyond GP theory, is also addressed, finding interesting analogies with similar processes in nonlinear quantum optics and with the dynamic Casimir effect. Our results can be used in exploiting parametric instabilities for the purpose of spectroscopy, selective amplification of a particular excitation mode and for establishing a new type of thermometry.

cond-mat.stat-mech↗

arXiv2005

Parametric excitation of a Bose-Einstein condensate in a 1D optical lattice

We study the response of a Bose-Einstein condensate to a periodic modulation of the depth of an optical lattice. Using Gross-Pitaevskii theory, we show that a modulation at frequency Omega drives the parametric excitation of Bogoliubov modes with frequency Omega/2. The ensuing nonlinear dynamics leads to a rapid broadening of the momentum distribution and a consequent large increase of the condensate size after free expansion. We show that this process does not require the presence of a large condensate depletion. Our results reproduce the main features of the spectrum measured in the superfluid phase by Stoeferle et al., Phys. Rev. Lett. 92, 130403 (2004).

cond-mat.other↗

arXiv2004

Role of transverse excitations in the instability of Bose-Einstein condensates moving in optical lattices

The occurrence of energetic and dynamical instabilities in a Bose-Einstein condensate moving in a one-dimensional (1D) optical lattice is analyzed by means of the Gross-Pitaevskii theory. Results of full 3D calculations are compared with those of an effective 1D model, the nonpolynomial Schrodinger equation, pointing out the role played by transverse degrees of freedom. The instability thresholds are shown to be scarcely affected by transverse excitations, so that they can be accurately predicted by effective 1D models. Conversely, transverse excitations turn out to be important in characterizing the stability diagram and the occurrence of a complex radial dynamics above the threshold for dynamical instability. This analysis provides a realistic framework to discuss the dissipative dynamics observed in recent experiments.

cond-mat.soft↗

arXiv2004

High sensitivity phonon spectroscopy of Bose-Einstein condensates using matter-wave interference

We study low momentum excitations of a Bose-Einstein condensate using a novel matter-wave interference technique. In time-of-flight expansion images we observe strong matter-wave fringe patterns. The fringe contrast is a sensitive spectroscopic probe of in-trap phonons and is explained by use of a Bogoliubov excitation projection method applied to the rescaled order parameter of the expanding condensate. Gross-Pitaevskii simulations agree with the experimental data and confirm the validity of the theoretical interpretation. We show that the high sensitivity of this detection scheme gives access to the quantized quasiparticle regime.

cond-mat.soft↗

arXiv2002

Bragg spectroscopy of the multi-branch Bogoliubov spectrum of elongated Bose-Einstein condensates

We measure the response of an elongated Bose-Einstein condensate to a two-photon Bragg pulse. If the duration of the pulse is long, the total momentum transferred to the condensate exhibits a nontrivial behavior which reflects the structure of the underlying Bogoliubov spectrum. It is thus possible to perform a spectroscopic analysis in which axial phonons with different number of radial nodes are resolved. The local density approximation is shown to fail in this regime, while the observed data agrees well with the results of simulations based on the numerical solution of the Gross-Pitaevskii equation.

cond-mat.soft↗