Two-body anticorrelation in a harmonically trapped ideal Bose gas

We predict the existence of a dip below unity in the second-order coherence function of a partially condensed ideal Bose gas in harmonic confinement, signaling the anticorrelation of density fluctuations in the sample. The dip in the second-order coherence function is revealed in a canonical-ensemble calculation, corresponding to a system with fixed total number of particles. In a grand-canonical ensemble description, this dip is obscured by the occupation-number fluctuation catastrophe of the ideal Bose gas. The anticorrelation is most pronounced in highly anisotropic trap geometries containing small particle numbers. We explain the fundamental physical mechanism which underlies this phenomenon, and its relevance to experiments on interacting Bose gases.Comment: 10 pages, 5 figures. v2: Minor changes and corrections to figures and text. To appear in PR

Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)

The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of (Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect (MOKE). A transient MOKE signal followed by low amplitude oscillations was evidenced, with a strong dependence on applied magnetic field, temperature and ps-ASP amplitude. Careful interferometric measurement of the layer's thickness variation induced by the ps-ASP allowed us to model very accurately the resulting signal, and interpret it as the strain modulated reflectivity (differing for $\sigma_{\pm}$ probe polarizations), independently from dynamic magnetization effects.Comment: 6 pages, 5 figure

Optimal estimates of the diffusion coefficient of a single Brownian trajectory

Modern developments in microscopy and image processing are revolutionizing areas of physics, chemistry and biology as nanoscale objects can be tracked with unprecedented accuracy. The goal of single particle tracking is to determine the interaction between the particle and its environment. The price paid for having a direct visualization of a single particle is a consequent lack of statistics. Here we address the optimal way of extracting diffusion constants from single trajectories for pure Brownian motion. It is shown that the maximum likelihood estimator is much more efficient than the commonly used least squares estimate. Furthermore we investigate the effect of disorder on the distribution of estimated diffusion constants and show that it increases the probability of observing estimates much smaller than the true (average) value.Comment: 8 pages, 5 figure

Updated results on prototype chalcogenide fibers for 10-um wavefront spatial filtering

The detection of terrestrial planets by Darwin/TPF missions will require extremely high quality wavefronts. Single-mode fibers have proven to be powerful beam cleaning components in the near-infrared, but are currently not available in the mid-infrared where they would be critically needed for Darwin/TPF. In this paper, we present updated measurements on the prototype chalcogenide fibers we are developing for the purpose of mid-infrared spatial filtering. We demonstrate the guiding property of our 3rd generation component and we characterize its filtering performances on a 4 mm length: the far-field radiation pattern matches a Gaussian profile at the level of 3% rms and 13% pk-pk.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference "Toward Other Earths, Darwin/TPF and the search for extrasolar terrestrial planets", held in Heidelberg, Germany, 22-25 April 2003, ESA SP-53

Sub-Terahertz Monochromatic Transduction with Semiconductor Acoustic Nanodevices

We demonstrate semiconductor superlattices or nanocavities as narrow band acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics experiments in the transmission geometry with pump and probe incident on opposite sides of the thick substrate, phonon generation and detection processes are fully decoupled. Generating with the semiconductor device and probing on the metal, we show that both superlattices and nanocavities generate spectrally narrow wavepackets of coherent phonons with frequencies in the vicinity of the zone center and time durations in the nanosecond range, qualitatively different from picosecond broadband pulses usually involved in picosecond acoustics with metal generators. Generating in the metal and probing on the nanoacoustic device, we furthermore evidence that both nanostructured semiconductor devices may be used as very sensitive and spectrally selective detectors

A case study of spin-11 Heisenberg model in a triangular lattice

We study the spin-$1$ model in a triangular lattice in presence of a uniaxial anisotropy field using a Cluster Mean-Field approach (CMF). The interplay between antiferromagnetic exchange, lattice geometry and anisotropy forces Gutzwiller mean-field approaches to fail in a certain region of the phase diagram. There, the CMF yields two supersolid (SS) phases compatible with those present in the spin-$1/2$ XXZ model onto which the spin-$1$ system maps. Between these two SS phases, the three-sublattice order is broken and the results of the CMF depend heavily on the geometry and size of the cluster. We discuss the possible presence of a spin liquid in this region.Comment: 7 pages, 4 figures, RevTeX 4. The abstract and conclusions have been modified and the manuscript has been extende

Hanbury Brown and Twiss correlations in atoms scattered from colliding condensates

Low energy elastic scattering between clouds of Bose condensed atoms leads to the well known s-wave halo with atoms emerging in all directions from the collision zone. In this paper we discuss the emergence of Hanbury Brown and Twiss coincidences between atoms scattered in nearly parallel directions. We develop a simple model that explains the observations in terms of an interference involving two pairs of atoms each associated with the elementary s wave scattering process.Comment: Minor corrections. reference update

Collective Atomic Recoil Laser as a synchronization transition

We consider here a model previously introduced to describe the collective behavior of an ensemble of cold atoms interacting with a coherent electromagnetic field. The atomic motion along the self-generated spatially-periodic force field can be interpreted as the rotation of a phase oscillator. This suggests a relationship with synchronization transitions occurring in globally coupled rotators. In fact, we show that whenever the field dynamics can be adiabatically eliminated, the model reduces to a self-consistent equation for the probability distribution of the atomic "phases". In this limit, there exists a formal equivalence with the Kuramoto model, though with important differences in the self-consistency conditions. Depending on the field-cavity detuning, we show that the onset of synchronized behavior may occur through either a first- or second-order phase transition. Furthermore, we find a secondary threshold, above which a periodic self-pulsing regime sets in, that is immediately followed by the unlocking of the forward-field frequency. At yet higher, but still experimentally meaningful, input intensities, irregular, chaotic oscillations may eventually appear. Finally, we derive a simpler model, involving only five scalar variables, which is able to reproduce the entire phenomenology exhibited by the original model