Multiple Reflections and Diffuse Scattering in Bragg Scattering at Optical Lattices

We study Bragg scattering at 1D atomic lattices. Cold atoms are confined by optical dipole forces at the antinodes of a standing wave generated inside a laser-driven cavity. The atoms arrange themselves into an array of lens-shaped layers located at the antinodes of the standing wave. Light incident on this array at a well-defined angle is partially Bragg-reflected. We measure reflectivities as high as 30%. In contrast to a previous experiment devoted to the thin grating limit [S. Slama, et al., Phys. Rev. Lett. 94, 193901 (2005)] we now investigate the thick grating limit characterized by multiple reflections of the light beam between the atomic layers. In principle multiple reflections give rise to a photonic stop band, which manifests itself in the Bragg diffraction spectra as asymmetries and minima due to destructive interference between different reflection paths. We show that close to resonance however disorder favors diffuse scattering, hinders coherent multiple scattering and impedes the characteristic suppression of spontaneous emission inside a photonic band gap

Dimensional Crossover in Bragg Scattering from an Optical Lattice

We study Bragg scattering at 1D optical lattices. Cold atoms are confined by the optical dipole force at the antinodes of a standing wave generated inside a laser-driven high-finesse cavity. The atoms arrange themselves into a chain of pancake-shaped layers located at the antinodes of the standing wave. Laser light incident on this chain is partially Bragg-reflected. We observe an angular dependence of this Bragg reflection which is different to what is known from crystalline solids. In solids the scattering layers can be taken to be infinitely spread (3D limit). This is not generally true for an optical lattice consistent of a 1D linear chain of point-like scattering sites. By an explicit structure factor calculation we derive a generalized Bragg condition, which is valid in the intermediate regime. This enables us to determine the aspect ratio of the atomic lattice from the angular dependance of the Bragg scattered light.Comment: 4 pages, 5 figure

Tracking the origin of invasive Rosa rubiginosa populations in Argentina

The exact geographic origin of invasive species populations is rarely known; however, such knowledge is vital to understanding species’ invasion success, spread, and evolution as well as for assessing any biological control options. We investigated the shrub Rosa rubiginosa L., focusing on the presumed European origin of invasive populations in Argentina. We analyzed eight polymorphic microsatellite loci among 102 native (European) and 29 invasive (mainly central Argentinean and Patagonian) populations. Genetic diversity in the invasive range was clearly lower than in the native range, possibly because of a low number of introductions. Contrary to earlier hypotheses, the interpretation of principal coordinate analysis results and Jaccard dissimilarities contradicts the idea of the Argentinean populations having a Spanish origin. Instead, we found a close similarity between Argentinean samples and those from Germany, the Czech Republic, and Austria. We therefore assume that these neighboring countries are the most probable source regions for the Argentinean populations, which in some cases may also have arrived via Chile. According to historic information, emigrants from these regions may have introduced R. rubiginosa to South America in the nineteenth century on at least two occasions, either for food or as rootstock material for propagating living fences.Fil: Hirsch, Heidi. Martin-Luther-University Halle-Wittenberg; AlemaniaFil: Zimmermann, Heike. Martin-Luther-University Halle-Wittenberg; AlemaniaFil: Ritz, Christiane M.. Senckenberg Museum of Natural History Goerlitz; AlemaniaFil: Wissemann, Volker. Justus-Liebig-University Giessen; AlemaniaFil: von Wehrden, Henrik. Martin-Luther-University Halle-Wittenberg; Alemania. Leuphana University, Institute of Ecology and Environmental Chemistry; AlemaniaFil: Renison, Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones Biológicas y Tecnológicas. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Investigaciones Biológicas y Tecnológicas; ArgentinaFil: Wesche, Karsten. Senckenberg Museum of Natural History Goerlitz; AlemaniaFil: Welk, Erik. Martin-Luther-University Halle-Wittenberg; AlemaniaFil: Hensen, Isabell. Martin-Luther-University Halle-Wittenberg; Alemani

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process

Pressure-Induced Rotational Symmetry Breaking in URu2_2Si2_2

Phase transitions and symmetry are intimately linked. Melting of ice, for example, restores translation invariance. The mysterious hidden order (HO) phase of URu$_2$Si$_2$ has, despite relentless research efforts, kept its symmetry breaking element intangible. Here we present a high-resolution x-ray diffraction study of the URu$_2$Si$_2$ crystal structure as a function of hydrostatic pressure. Below a critical pressure threshold $p_c\approx3$ kbar, no tetragonal lattice symmetry breaking is observed even below the HO transition $T_{HO}=17.5$ K. For $p>p_c$, however, a pressure-induced rotational symmetry breaking is identified with an onset temperatures $T_{OR}\sim 100$ K. The emergence of an orthorhombic phase is found and discussed in terms of an electronic nematic order that appears unrelated to the HO, but with possible relevance for the pressure-induced antiferromagnetic (AF) phase. Existing theories describe the HO and AF phases through an adiabatic continuity of a complex order parameter. Since none of these theories predicts a pressure-induced nematic order, our finding adds an additional symmetry breaking element to this long-standing problem.Comment: 6 pages, 4 figures and supplemental material

Transfer after process-based object-location memory training in healthy older adults

A substantial part of age-related episodic memory decline has been attributed to the decreasing ability of older adults to encode and retrieve associations among simultaneously processed information units from long-term memory. In addition, this ability seems to share unique variance with reasoning. In this study, we therefore examined whether process-based training of the ability to learn and remember associations has the potential to induce transfer effects to untrained episodic memory and reasoning tasks in healthy older adults (60-75 years). For this purpose, the experimental group (n = 36) completed 30 sessions of process-based objectlocation memory training, while the active control group (n = 31) practiced visual perception on the same material. Near (spatial episodic memory), intermediate (verbal episodic memory), and far transfer effects (reasoning) were each assessed with multiple tasks at four measurements (before, midway through, immediately after, and 4 months after training). Linear mixed-effects models revealed transfer effects on spatial episodic memory and reasoning that were still observed 4 months after training. These results provide first empirical evidence that process-based training can enhance healthy older adults' associative memory performance and positively affect untrained episodic memory and reasoning abilities

Strong enhancement of spin fluctuations in the low-temperature-tetragonal phase of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4

Measurements of the static magnetization, susceptibility and ESR of Gd spin probes have been performed to study the properties of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature tetragonal structure. According to the static magnetic measurements the CuO_2 planes are magnetically decoupled in this structural phase. The ESR study reveals strong magnetic fluctuations at the ESR frequency which are not present in the orthorhombic phase. It is argued that this drastic enhancement of the spin fluctuations is due to a considerable weakening of the interlayer exchange and a pronounced influence of hole motion on the antiferromagnetic properties of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at small hole doping is obtained in the present study.Comment: 10 pages, LaTeX, 3 EPS figures; to be published in Journal of Physics: Condensed Matte

Cold atoms in a high-Q ring-cavity

We report the confinement of large clouds of ultra-cold 85-Rb atoms in a standing-wave dipole trap formed by the two counter-propagating modes of a high-Q ring-cavity. Studying the properties of this trap we demonstrate loading of higher-order transverse cavity modes and excite recoil-induced resonances.Comment: 4 pages, 4 figure

Substrate-induced strain effects on Pr_{0.6}Ca_{0.4}MnO_{3} films

We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk material, and the low-temperature ordering is found to be more robust under compressive-- as opposed to tensile-- strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO_{3}, which is the substrate that provides the least amount of nominal and measured, but tensile, strain. This result suggests the importance of the role played by the Mn--O--Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a connection between the lattice distortion associated with orbital ordering and the onset of antiferromagnetism is reported.Comment: 12 pages, 7 figure