Anisotropic excitation spectrum of a dipolar quantum Bose gas

We measure the excitation spectrum of a dipolar Chromium Bose Einstein Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy which originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation, and, at large excitation wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of soundComment: 3 figure

Control of dipolar relaxation in external fields

We study dipolar relaxation in both ultra-cold thermal and Bose-condensed chromium atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from 3D to 2D. Although dipolar relaxation generally increases as a function of a static magnetic field intensity, we find a range of non-zero magnetic field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the S=6 scattering length of chromium atoms: $a_6 = 103 \pm 4 a_0$. We compare this new measurement to another new determination of $a_6$, which we perform by analysing the precise spectroscopy of a Feshbach resonance in d-wave collisions, yielding $a_6 = 102.5 \pm 0.4 a_0$. These two measurements provide by far the most precise determination of $a_6$ to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave $l=0$ for large enough magnetic field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultra-cold chromium gases in a 1D optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to 7 compared to the 3D case. Finally, we study dipolar relaxation in presence of radio-frequency (rf) oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of rf frequency and Rabi frequency.Comment: 25 pages, 17 figure

Averaging out magnetic forces with fast rf-sweeps in an optical trap for metastable chromium atoms

We introduce a novel type of time-averaged trap, in which the internal state of the atoms is rapidly modulated to modify magnetic trapping potentials. In our experiment, fast radiofrequency (rf) linear sweeps flip the spin of atoms at a fast rate, which averages out magnetic forces. We use this procedure to optimize the accumulation of metastable chomium atoms into an optical dipole trap from a magneto-optical trap. The potential experienced by the metastable atoms is identical to the bare optical dipole potential, so that this procedure allows for trapping all magnetic sublevels, hence increasing by up to 80 percent the final number of accumulated atoms.Comment: 4 pages, 4 figure

Potential barrier heights at metal on oxygen-terminated diamond interfaces

International audienceElectrical properties of metal-semiconductor (M/SC) and metal/oxide/SC structures built with Zr or ZrO_2 deposited on oxygen-terminated surfaces of (001)-oriented diamond films, comprising a stack of lightly p-doped diamond on a heavily doped layer itself homoepitaxially grown on a Ib substrate, are investigated experimentally and compared to different models. In Schottky barrier diodes, the interfacial oxide layer evidenced by high resolution transmission electron microscopy and electron energy losses spectroscopy before and after annealing, and barrier height inhomogeneities accounts for the measured electrical characteristics until flat bands are reached, in accordance with a model which generalizes that of R.T. Tung [Phys. Rev. B 45, 13509 (1992)] and permits to extract physically meaningful parameters of the three kinds of interface: (a) unannealed ones; (b) annealed at 350°C; (c) annealed at 450°C, with characteristic barrier heights of 2.2-2.5 V in case (a) while as low as 0.96 V in case (c). Possible models of potential barriers for several metals deposited on well defined oxygen-terminated diamond surfaces are discussed and compared to experimental data. It is concluded that interface dipoles of several kinds present at these compound interfaces and their chemical evolution due to annealing are the suitable ingredients able to account for the Mott-Schottky behavior when the effect of the metal work function is ignored, and to justify the reverted slope observed regarding metal work function, in contrast to the trend always reported for all other metal-semiconductor interfaces.Les propriétés électriques et structurales d'interfaces métal/diamant et métal/oxyde/diamant où le métal est le Zirconium et le semi-conducteur comporte un empilement de couches faiblement et fortement dopées au bore sur substrat Ib, sont étudiées expérimentalement et comparées à différents modèles. Dans le barrière de Schottky, une inter-couche d'oxyde d'environ 2 couches atomiques, mise en évidence par diverses techniques de microscopie électronique à transmission, est présente et ajoutée à la présence d'inhomogénéités de barrière de potentiel, est corrélée aux propriétés électriques simulées par un modèle qui généralise celui de R. T. Tung [Phys. Rev. B 45, 13509 (1992)] . Les paramètres physiquement caractéristiques des interfaces (a) non recuites, (b) recuite à 350°C et (c) recuite à 450°C peuvent ainsi être extraits, en particulier des hauteurs de barrière de 2.2-2.5 V dans le cas (a) et aussi basses que 0.96 V dans le cas (c). Les modèles possibles de fixation du niveau de Fermi aux interfaces métal/diamant sont examinés et confrontés aux données récemment publiées pour différents métaux sur la surface oxygénée du diamant. On conclue que les quantités physiques judicieuses sont l'affinité électronique du diamant, fonction de son état de surface, pour justifier l'allure générale conforme au modèle de Mott-Schottky et la force du dipole d'interface, dépendante des liaisons chimiques à l'interface, pour expliquer la pente de la variation de la barrière en fonction du travail de sortie du métal, qui est inversée par rapport à tous les autres semi-conducteurs

Deformation of the N=Z nucleus 76Sr using beta-decay studies

A novel method of deducing the deformation of the N=Z nucleus 76Sr is presented. It is based on the comparison of the experimental Gamow-Teller strength distribution B(GT) from its beta decay with the results of QRPA calculations. This method confirms previous indications of the strong prolate deformation of this nucleus in a totally independent way. The measurement has been carried out with a large Total Absorption gamma Spectrometer, "Lucrecia", newly installed at CERN-ISOLDE.Comment: Accepted in Phys. Rev. Letter

Plant RNases T2, but not Dicer-like proteins, are major players of tRNA-derived fragments biogenesis

RNA fragments deriving from tRNAs (tRFs) exist in all branches of life and the repertoire of their biological functions regularly increases. Paradoxically, their biogenesis remains unclear. The human RNase A, Angiogenin, and the yeast RNase T2, Rny1p, generate long tRFs after cleavage in the anticodon region. The production of short tRFs after cleavage in the D or T regions is still enigmatic. Here, we show that the Arabidopsis Dicer-like proteins, DCL1-4, do not play a major role in the production of tRFs. Rather, we demonstrate that the Arabidopsis RNases T2, called RNS, are key players of both long and short tRFs biogenesis. Arabidopsis RNS show specific expression profiles. In particular, RNS1 and RNS3 are mainly found in the outer tissues of senescing seeds where they are the main endoribonucleases responsible of tRNA cleavage activity for tRFs production. In plants grown under phosphate starvation conditions, the induction of RNS1 is correlated with the accumulation of specific tRFs. Beyond plants, we also provide evidence that short tRFs can be produced by the yeast Rny1p and that, in vitro, human RNase T2 is also able to generate long and short tRFs. Our data suggest an evolutionary conserved feature of these enzymes in eukaryotes