Connecting strongly correlated superfluids by a quantum point contact

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. Here we study resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a non-linear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.Comment: 5 pages, 4 figures, 7 pages supplementar

Connecting strongly correlated superfluids by a quantum point contact

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. Here we study resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a non-linear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.Comment: 5 pages, 4 figures, 7 pages supplementar

Band and correlated insulators of cold fermions in a mesoscopic lattice

We investigate the transport properties of neutral, fermionic atoms passing through a one-dimensional quantum wire containing a mesoscopic lattice. The lattice is realized by projecting individually controlled, thin optical barriers on top of a ballistic conductor. Building an increasingly longer lattice, one site after another, we observe and characterize the emergence of a band insulating phase, demonstrating control over quantum-coherent transport. We explore the influence of atom-atom interactions and show that the insulating state persists as contact interactions are tuned from moderately to strongly attractive. Using bosonization and classical Monte-Carlo simulations we analyze such a model of interacting fermions and find good qualitative agreement with the data. The robustness of the insulating state supports the existence of a Luther-Emery liquid in the one-dimensional wire. Our work realizes a tunable, site-controlled lattice Fermi gas strongly coupled to reservoirs, which is an ideal test bed for non-equilibrium many-body physics.Comment: 8 + 10 pages, 5 + 7 figure

Metal-insulator transition in the one-dimensional SU(N) Hubbard model

We investigate the metal-insulator transition of the one-dimensional SU(N) Hubbard model for repulsive interaction. Using the bosonization approach a Mott transition in the charge sector at half-filling (k_F=\pi/Na_0) is conjectured for N > 2. Expressions for the charge and spin velocities as well as for the Luttinger liquid parameters and some correlation functions are given. The theoretical predictions are compared with numerical results obtained with an improved zero-temperature quantum Monte Carlo approach. The method used is a generalized Green's function Monte Carlo scheme in which the stochastic time evolution is partially integrated out. Very accurate results for the gaps, velocities, and Luttinger liquid parameters as a function of the Coulomb interaction U are given for the cases N=3 and N=4. Our results strongly support the existence of a Mott-Hubbard transition at a {\it non-zero} value of the Coulomb interaction. We find $U_c \sim 2.2$ for N=3 and $U_c \sim 2.8$ for N=4.Comment: 22 pages, 9 Fig

Direct observation of Anderson localization of matter-waves in a controlled disorder

We report the observation of exponential localization of a Bose-Einstein condensate (BEC) released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle . We operate in a regime allowing AL: i) weak disorder such that localization results from many quantum reflections of small amplitude; ii) atomic density small enough that interactions are negligible. We image directly the atomic density profiles vs time, and find that weak disorder can lead to the stopping of the expansion and to the formation of a stationary exponentially localized wave function, a direct signature of AL. Fitting the exponential wings, we extract the localization length, and compare it to theoretical calculations. Moreover we show that, in our one-dimensional speckle potentials whose noise spectrum has a high spatial frequency cut-off, exponential localization occurs only when the de Broglie wavelengths of the atoms in the expanding BEC are larger than an effective mobility edge corresponding to that cut-off. In the opposite case, we find that the density profiles decay algebraically, as predicted in [Phys. Rev. Lett. 98, 210401 (2007)]. The method presented here can be extended to localization of atomic quantum gases in higher dimensions, and with controlled interactions

Effets physico-chimiques de l'immersion accidentelle de charbon

Ce mémoire présente l étude des effets physico-chimiques de l immersion accidentelle d une cargaison de charbon. Le charbon est une roche sédimentaire riche en matière organique (> 50 % en masse). La phase organique contient des substances humiques, des hydrocarbures aromatiques polycycliques (HAP) et des composés soufrés. La phase inorganique contient des éléments majoritaires (> 0,5 % en masse : Al, Ca, Fe, Si ...), minoritaires (0,02 - 0,5 % Mg, Na ...) et traces ( 50 M %). The organic part contains humic matter, polycyclic aromatic hydrocarbons (PAHs) and sulfur compounds. The inorganic part contains major (> 0,5 %: Al, Ca, Fe, Si...), minor (0,02 -0,5%: Mg, Na ) and trace elements (<0,02%: Cd, Zn, Cr, Cu, Mn...), some of the latter being heavy metals. The PAH contents in the various coals used were determined by several extraction methods: they are 0f the order 0f 1 mg kg for the 16 most toxic PAH5. The inorganic phase was analyzed by sequential extraction apart from manganese, which is present in gypsum, the studied metals (AI, Cr, Cu, Fe, Ni, Pb and Zn) are present in insoluble minerals such as kaolinite. After mixing with seawater, fluorescence analyses by xenon and LASER excitation, along with gas chromatography and mass spectrometry were used to identify organic compounds dissolved from coal onto seawater: they are humic and fulvic acids. Neither PAHs nor sulfur compounds could be detected. The concentration in humic acid equivalent increases with increasing coal mass and agitation time and intensity, and decreases with increasing grain size. Dissolved inorganic compounds were analyzed by electrothermal atomic absorption spectroscopy: since they are included in insoluble minerals, most of the analyzed elements were not dissolved, apart from Mn which is dissolved as a sulfate. The Mn concentration increases with increasing Goal mass and agitation time and intensity, and decreases with increasing grain size. It is, in batch, higher than 1.5 g L-1, which can be considered as an environmental limit. Several experimentations were eventually conducted in open media this displayed the elimination of thin suspended particles preventing photosynthesis and the dilution of humic acids and Mn due to the continuous seawater renewal. The spreading of a coal pile submitted to a seawater flow was also studied, and it occurs that the tiniest coal particles move faster: they can be dispersed over a distance of 1 km in 10 hours. The environmental impact after an accidental coal immersion remains on the whole limited.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Determination of iron in seawater by electrothermal atomic absorption spectrometry and atomic fluorescence spectrometry: a comparative study.

International audienceTwo methods available for direct determination of total Fe in seawater at low concentration level have been examined: electrothermal atomization atomic absorption spectrometry (ETAAS) and electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS). In a first part, we have optimized experimental conditions of ETAAS (electrothermal program, matrix chemical modification) for the determination of Fe in seawater by minimizing the chemical interference effects and the magnitude of the simultaneous background absorption signal. By using the best experimental conditions, a detection limit of 80 ng L(-1) (20 microL, 3sigma) for total Fe concentration was obtained by ETAAS. Using similar experimental conditions (electrothermal program, chemical modification), we have optimized experimental conditions for the determination of Fe by LEAFS. The selected experimental conditions for ETA-LEAFS: excitation wavelength (296.69 nm), noise attenuation and adequate background correction led to a detection limit (3sigma) of 3 ng L(-1) (i.e. 54 pM) for total Fe concentration with the use a 20 microL seawater sample. For the two methods, concentration values obtained for the analysis of Fe in a NASS-5 (0.2 microg L(-1)) seawater sample were in good agreement with the certified values

A study of marine pollution caused by the release of metals into seawater following acid spills.

International audienceThis study examined the potential metal pollution induced by the accidental spill of different acids into seawater. The acids sink to the bottom according to their densities and subsequently react with marine sediments. The acids selected for this study were acetic, hydrochloric, nitric, sulfuric, and phosphoric acids; the metallic elements selected were Cr, Cu, Fe, Mn, Pb and Zn. The sediment was collected in Brest Harbour. The percentages of metals released from this sediment in the presence of various concentrations of acids in seawater were important; concentrations of approximately 7 mg L(-1) for Mn and 60 mg L(-1) for Zn were observed under our experimental conditions. We also examined the rate of release of these metals from the sediment into the seawater in the presence of the different acids and under different experimental conditions. We found that most of the metallic elements were released from the sediments into the seawater during the first fifteen minutes of exposure. After this time, a high degree of pollution was induced if acids leached into seawater were not rapidly diluted

Polyethylene glycol aggregates in water formed through hydrophobic helical structures

International audienceThe present study was aimed at elucidating the mechanism of aggregation in water of hydroxyl-terminated polyethylene glycol (PEG) of low molecular weight (600 g/mol). The results from fluorescence spectroscopy at different temperatures were consistent with surface tension measurements, suggesting aggregate formation. Indeed, the process of aggregation is accompanied by an increase in the fluorescence emission of a hydrophobic probe. So, PEG aggregates in the form of internal hydrated helices covered with CH2 groups are shown to yield hydrophobic regions. These regions created upon PEG aggregation in water and at a temperature close to 35 °C result from a balance between H bonding and entropic effects. By providing the first experimental evidence for hydrophobic mediation of aggregation with OH-terminated oxy-ethylene chains of low molecular weight, this study highlights their surfactant-like behaviour

Study of trace metal leaching from coals into seawater

International audienceThe behaviour of three South African coals in water and, particularly in seawater, was examined. A sequential speciation procedure used to evaluate trace metal partitioning in coal has shown that trace metals will not be easily released from these coals into environmental ecosystems. Only a few trace elements are slightly leached from these coals into water or seawater at pH around 8. On the other hand, Mn is highly leached from these coals into water or seawater. It has been clearly shown that Mn concentrations are highly correlated to sulfate and calcium concentrations indicating that Mn is mainly solubilized into water simultaneously to gypsum; the leaching efficiency being severely reduced for coal having a high calcite content. The leaching percentage of Mn into seawater is enhanced by the presence of seawater salts that increases gypsum solubility. The leaching process of Mn from coal into water or seawater is governed by gypsum solubilization and is relatively rapid during the first thirty minutes, then very slow. In this study, it has been also shown that, depending on their physico-chemical properties, trace metals may be removed from seawater solutions in the presence of coal having a high calcite content. In this work, it has been also shown that some elements, particularly Fe, are greatly solubilized into seawater in the presence of a strong chelating agent like EDTA. Like for Mn, the leaching rate of metals from coal in the presence of EDTA is relatively rapid during the first 30 min then much slower, suggesting a solubilization process simultaneously to gypsum or/and calcite solubilization