Dimensional and Temperature Crossover in Trapped Bose Gases

We investigate the long-range phase coherence of homogeneous and trapped Bose gases as a function of the geometry of the trap, the temperature, and the mean-field interactions in the weakly interacting limit. We explicitly take into account the (quasi)condensate depletion due to quantum and thermal fluctuations, i.e., we include the effects of both phase and density fluctuations. In particular, we determine the phase diagram of the gas by calculating the off-diagonal one-particle density matrix and discuss the various crossovers that occur in this phase diagram and the feasibility of their experimental observation in trapped Bose gases.Comment: One figure added, typos corrected, refernces adde

Accretion-powered chromospheres in classical T Tauri stars

(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion. From high-resolution spectroscopy of DR Tauri we found that the amount of veiling in this star varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. This effect can be shown to exist in several other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.Comment: 11 pages, 13 figure

Mean field effects in a trapped classical gas

In this article, we investigate mean field effects for a bosonic gas harmonically trapped above the transition temperature in the collisionless regime. We point out that those effects can play also a role in low dimensional system. Our treatment relies on the Boltzmann equation with the inclusion of the mean field term. The equilibrium state is first discussed. The dispersion relation for collective oscillations (monopole, quadrupole, dipole modes) is then derived. In particular, our treatment gives the frequency of the monopole mode in an isotropic and harmonic trap in the presence of mean field in all dimensions.Comment: 4 pages, no figure submitted to Phys. Rev.

Extension of Bogoliubov theory to quasi-condensates

We present an extension of the well-known Bogoliubov theory to treat low dimensional degenerate Bose gases in the limit of weak interactions and low density fluctuations. We use a density-phase representation and show that a precise definition of the phase operator requires a space discretisation in cells of size $l$. We perform a systematic expansion of the Hamiltonian in terms of two small parameters, the relative density fluctuations inside a cell and the phase change over a cell. The resulting macroscopic observables can be computed in one, two and three dimensions with no ultraviolet or infrared divergence. Furthermore this approach exactly matches Bogoliubov's approach when there is a true condensate. We give the resulting expressions for the equation of state of the gas, the ground state energy, the first order and second order correlations functions of the field. Explicit calculations are done for homogeneous systems.Comment: 32 pages, 2 figures; typos corrected in revised versio

A low-voltage retarding-field Mott polarimeter for photocathode characterization

Nuclear physics experiments at Thomas Jefferson National Accelerator Facility's CEBAF rely on high polarization electron beams. We describe a recently commissioned system for prequalifying and studying photocathodes for CEBAF with a load-locked, low-voltage polarized electron source coupled to a compact retarding-field Mott polarimeter. The polarimeter uses simplified electrode structures and operates from 5 to 30 kV. The effective Sherman function for this device has been calibrated by comparison with the CEBAF 5 MeV Mott polarimeter. For elastic scattering from a thick gold target at 20 keV, the effective Sherman function is 0.201(5). Its maximum efficiency at 20 keV, defined as the detected count rate divided by the incident particle current, is 5.4(2) x 10-4, yielding a figure-of-merit, or analyzing power squared times efficiency, of 1.0(1) x 10-5. The operating parameters of this new polarimeter design are compared to previously published data for other compact Mott polarimeters of the retarding-field type.Comment: 9 figure

Low-dimensional Bose gases

We present an improved many-body T-matrix theory for partially Bose-Einstein condensed atomic gases by treating the phase fluctuations exactly. The resulting mean-field theory is valid in arbitrary dimensions and able to describe the low-temperature crossover between three, two and one-dimensional Bose gases. When applied to a degenerate two-dimensional atomic hydrogen gas, we obtain a reduction of the three-body recombination rate which compares favorably with experiment. Supplementing the mean-field theory with a renormalization-group approach to treat the critical fluctuations, we also incorporate into the theory the Kosterlitz-Thouless transition that occurs in a homogeneous Bose gas in two dimensions. In particular, we calculate the critical conditions for the Kosterlitz-Thouless phase transition as a function of the microscopic parameters of the theory. The proposed theory is further applied to a trapped one-dimensional Bose gas, where we find good agreement with exact numerical results obtained by solving a nonlinear Langevin field equation.Comment: 14 pages, 13 figures, revte

Archaeal Communities of Frozen Quaternary Sediments of Marine Origin on the Coast of Western Spitsbergen

The archaeal composition of permafrost samples taken during the drilling of frozen marine sediments in the area of the Barentsburg coal mine on the east coast of Grønfjord Bay of Western Spitsbergen has been studied. This study is based on an analysis of the V4 region of the 16S rRNA gene, carried out using next-generation sequencing. The general phyla of the Archaea domain are Euryarchaeota, Bathyarchaeota, Thaumarchaeota, and Asgardarchaea. As a result of a phylogenetic analysis of the dominant operational taxonomic units, representatives of methanogenic and methane- and ammonium-oxidizing archaea, as well as heterotrophic archaea, are found. The methanogenic archaea of Euryarchaeota phylum, Methanobacteria class, are found in permafrost with controversial genesis, while the methane-oxidizing archaea of Methanomicrobia class Methanosarcinales order are found in the marine permafrost at Cape Finneset: the ANME-2a, -2b group in layers of 8.6 and 11.7 m and the ANME-2d group (Candidatus Methanoperedens) in a layer of 6.5 m. Ammonium-oxidizing archaea of phylum Thaumarchaeota is present in all types of permafrost, while the order of Nitrososphaerales is found in permafrost with controversial genesis and the order Nitrosopumilales is in permafrost with marine and controversial genesis. Representatives of phylum Bathyarchaeota are found stratigraphically in the most ancient samples under study. Asgardarchaeota superfylum is excluded in the layers of permafrost with marine genesis and is represented by the phyla Lokiarchaeota, Thorarchaeota, and an unclassified group belonging to this superphylum. The presence of methane, ethylene, and ethane in the permafrost of the first sea terrace of Cape Finneset at a depth of 11.7 m, as well as the composition of the archaeal community, give us reason to assume that, before freezing, microbiological processes of anaerobic methane oxidation took place in it, probably received from Tertiary rocks. The results of both this and previous works present the Spitsbergen permafrost as a rich archive of genetic information of little-studied prokaryotic groups

Bacterial Communities of Frozen Quaternary Sediments of Marine Origin on the Coast of Western Spitsbergen

The bacterial composition of permafrost samples taken during drilling of frozen marine sediments in the area of Barentsburg coal mine on the east coast of Grønfjord Bay of Western Spitsbergen has been studied. The study was based on the analysis of the V4 region of the 16S rRNA gene, carried out using next generation sequencing, as well as using classical microbiological methods (direct luminescence microscopy and aerobic cultivation).The total cell number in permafrost samples ranges from 6.73 ± 0.73 × 106 to 3.37 ± 0.19 107 cells per g. The number of cultivable aerobic bacteria in frozen samples on 1/5 TSA and R2A media ranges from 0 to 6.20 ± 0.45 × 104 CFU/g. Isolates of aerobic bacteria were identified by 16S rRNA gene analysis as representatives of the genera Arthrobacter, Pseudarthrobacter, Psychrobacter, and Rhodoferax. The dominant phyla of the domain Bacteria were Actinobacteria, Proteobacteria, Chloroflexi, Nitrospirae and Firmicutes. As a result of phylogenetic analysis of the dominant operational taxonomic units, representatives of methane oxidizing, sulfate reducing bacteria, as well as heterotrophic bacteria involved in the transformation of organic matter were found