213 research outputs found
Finite temperature phase diagram of spin-1/2 bosons in two-dimensional optical lattice
We study a two-species bosonic Hubbard model on a two-dimensional square
lattice by means of quantum Monte Carlo simulations and focus on finite
temperature effects. We show in two different cases, ferro- and
antiferromagnetic spin-spin interactions, that the phase diagram is composed of
solid Mott phases, liquid phases and superfluid phases. In the
antiferromagnetic case, the superfluid (SF) is polarized while the Mott
insulator (MI) and normal Bose liquid (NBL) phases are not. On the other hand,
in the ferromagnetic case, none of the phases is polarized. The
superfluid-liquid transition is of the Berezinsky-Kosterlitz-Thouless type
whereas the solid-liquid passage is a crossover.Comment: 9 pages, 13 figure
Numerical Investigation of Strongly Interacting Bosons at Zero Temperature
We review some numerical works carried out within the department for Quantum
Optics and Statistics at the University of Freiburg’s Institute of Physics, between
September 2016 and June 2018. Our activities focus on quantum properties of
matter at zero temperature, i.e., a regime where the thermal energy kBT is negligible
with respect to the other energy scales of the considered system. This
area of research, related to ultracold gases, has attracted a great deal of interest,
both experimentally and theoretically, since the first realization of a Bose-Einstein
condensate in 1995. In a context where the theoretical understanding of these
systems still remains challenging, the growing power of computers offers a unique
and efficient way to tackle such challenges. In our theory group, we particularly
use powerful numerical methods that give exact results, in contrast to other theoretical
approaches based on an a priori assumption, e.g., mean field theory. To
illustrate it, we focus on few typical results that would not be available other
than by using high performance computing. These results have been obtained by
using three numerical methods: quantum Monte Carlo (QMC), Gutzwiller Monte
Carlo (GMC), and the Multiconfigurational Time-dependent Hartree method for
bosons (MCTDHX)
Corallistin A, a second example of a free porphyrin from a living organism : isolation from the demosponge Corallistes sp. of the coral see and inhibition of abnormal cells
It is shown that the demosponge #Corallistes sp. (#Tetractinomorpha, #Lithistida, #Corallistidae) collected in the Coral Sea, contains corallistin A, the second example, of a free porphyrin from a living organism. The compound proved to be active against the Kb cell line. In contrast with the geoporphyrins which do not bear any O-atom corallistin A carries two carboxylic groups. (Résumé d'auteur
Ground state phase diagram of spin-1/2 bosons in a two-dimensional optical lattice
We study a two-species bosonic Hubbard model on a two-dimensional square
lattice by means of quantum Monte Carlo simulations. In addition to the usual
contact repulsive interactions between the particles, the Hamiltonian has an
interconversion term which allows the transformation of two particles from one
species to the other. The phases are characterized by their solid or superfluid
properties and by their polarization, i.e. the difference in the populations.
When inter-species interactions are smaller than the intra-species ones, the
system is unpolarized, whereas in the opposite case the system is unpolarized
in even Mott insulator lobes and polarized in odd Mott lobes and also in the
superfluid phase. We show that in the latter case the transition between the
Mott insulator of total density two and the superfluid can be either of second
or first order depending on the relative values of the interactions, whereas
the transitions are continuous in all other cases.Comment: 10 pages, 17 figure
Spectral Structure and Many-Body Dynamics of Ultracold Bosons in a Double-Well
We examine the spectral structure and many-body dynamics of two and three
repulsively interacting bosons trapped in a one-dimensional double-well, for
variable barrier height, inter-particle interaction strength, and initial
conditions. By exact diagonalization of the many-particle Hamiltonian, we
specifically explore the dynamical behaviour of the particles launched either
at the single particle ground state or saddle point energy, in a
time-independent potential. We complement these results by a characterisation
of the cross-over from diabatic to quasi-adiabatic evolution under finite-time
switching of the potential barrier, via the associated time-evolution of a
single particle's von Neumann entropy. This is achieved with the help of the
multiconfigurational time-dependent Hartree method for indistinguishable
particles (\textsc{Mctdh-x}) -- which also allows us to extrapolate our results
for increasing particle numbers.Comment: 20 pages, 14 figure
Digital mapping of GlobalSoilMap soil properties at a broad scale: a review
Soils are essential for supporting food production and providing ecosystem services but are under pressure due to population growth, higher food demand, and land use competition. Because of the effort to ensure the sustainable use of soil resources, demand for current, updatable soil information capable of supporting decisions across scales is increasing. Digital soil mapping (DSM) addresses the drawbacks of conventional soil mapping and has been increasingly used for delivering soil information in a time- and cost-efficient manner with higher spatial resolution, better map accuracy, and quantified uncertainty estimates. We reviewed 244 articles published between January 2003 and July 2021 and then summarised the progress in broad-scale (spatial extent >10,000 km2) DSM, focusing on the 12 mandatory soil properties for GlobalSoilMap. We observed that DSM publications continued to increase exponentially; however, the majority (74.6%) focused on applications rather than methodology development. China, France, Australia, and the United States were the most active countries, and Africa and South America lacked country-based DSM products. Approximately 78% of articles focused on mapping soil organic matter/carbon content and soil organic carbon stocks because of their significant role in food security and climate regulation. Half the articles focused on soil information in topsoil only (<30 cm), and studies on deep soil (100–200 cm) were less represented (21.7%). Relief, organisms, and climate were the three most frequently used environmental covariates in DSM. Nonlinear models (i.e. machine learning) have been increasingly used in DSM for their capacity to manage complex interactions between soil information and environmental covariates. Soil pH was the best predicted soil property (average R2 of 0.60, 0.63, and 0.56 at 0–30, 30–100, and 100–200 cm). Other relatively well-predicted soil properties were clay, silt, sand, soil organic carbon (SOC), soil organic matter (SOM), SOC stocks, and bulk density, and coarse fragments and soil depth were poorly predicted (R2 < 0.28). In addition, decreasing model performance with deeper depth intervals was found for most soil properties. Further research should pursue rescuing legacy data, sampling new data guided by well-designed sampling schemas, collecting representative environmental covariates, improving the performance and interpretability of advanced spatial predictive models, relating performance indicators such as accuracy and precision to cost-benefit and risk assessment analysis for improving decision support; moving from static DSM to dynamic DSM; and providing high-quality, fine-resolution digital soil maps to address global challenges related to soil resources
A new species of Gymnotus (Gymnotiformes: Gymnotidae) from the Fitzcarrald Arch of southeastern Peru
Characteristics of the Mesophotic Megabenthic Assemblages of the Vercelli Seamount (North Tyrrhenian Sea)
The biodiversity of the megabenthic assemblages of the mesophotic zone of a Tyrrhenian seamount (Vercelli Seamount) is described using Remotely Operated Vehicle (ROV) video imaging from 100 m depth to the top of the mount around 61 m depth. This pinnacle hosts a rich coralligenous community characterized by three different assemblages: (i) the top shows a dense covering of the kelp Laminaria rodriguezii; (ii) the southern side biocoenosis is mainly dominated by the octocorals Paramuricea clavata and Eunicella cavolinii; while (iii) the northern side of the seamount assemblage is colonized by active filter-feeding organisms such as sponges (sometimes covering 100% of the surface) with numerous colonies of the ascidian Diazona violacea, and the polychaete Sabella pavonina. This study highlights, also for a Mediterranean seamount, the potential role of an isolated rocky peak penetrating the euphotic zone, to work as an aggregating structure, hosting abundant benthic communities dominated by suspension feeders, whose distribution may vary in accordance to the geomorphology of the area and the different local hydrodynamic conditions
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