329 research outputs found
Interacting spin-1 bosons in a two-dimensional optical lattice
We study, using quantum Monte Carlo (QMC) simulations, the ground state
properties of spin-1 bosons trapped in a square optical lattice. The phase
diagram is characterized by the mobility of the particles (Mott insulating or
superfluid phase) and by their magnetic properties. For ferromagnetic on-site
interactions, the whole phase diagram is ferromagnetic and the Mott
insulators-superfluid phase transitions are second order. For antiferromagnetic
on-site interactions, spin nematic order is found in the odd Mott lobes and in
the superfluid phase. Furthermore, the superfluid-insulator phase transition is
first or second order depending on whether the density in the Mott is even or
odd. Inside the even Mott lobes, we observe a singlet-to-nematic transition for
certain values of the interactions. This transition appears to be first order
Dirac Point and Edge States in a Microwave Realization of Tight-Binding Graphene-like Structures
We present a microwave realization of finite tight-binding graphene-like
structures. The structures are realized using discs with a high index of
refraction. The discs are placed on a metallic surface while a second surface
is adjusted atop the discs, such that the waves coupling the discs in the air
are evanescent, leading to the tight-binding behavior. In reflection
measurements the Dirac point and a linear increase close to the Dirac point is
observed, if the measurement is performed inside the sample. Resonances due to
edge states are found close to the Dirac point if the measurements are
performed at the zigzag-edge or at the corner in case of a broken benzene ring.Comment: 4 pages, 6 figure
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
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
Sterol composition of the "living fossil" crinoid Gymnocrinus richeri
The compn. of sterol mixt. from the living fossil crinoid G. richeri collected off Noumea (New Caledonia) was investigated. The free 3β-OH sterol mixt. was found to contain 14 components, Δ5 and ring satd. stanols, identified by GC-MS. Cholest-4-en-3-one, cholesta-1,4-dien-3-one, 5α-8α-epidioxy sterols, and 5α-ergosta-7,22-diene-3β,5,6β-triol were also present, their characterization being accomplished by EI-MS and 1H-NMR. The methanol ext. also contained sterol sulfates, which were identified by GC-MS after solvolysis to remove the sulfate group
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
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