2,313 research outputs found
Finite-size, magnetic and chemical-potential effects on first-order phase transitions
We perform a study about effects of an applied magnetic field and a finite
chemical potential on the size-dependent phase structure of a first-order
transition. These effects are introduced by using methods of quantum fields
defined on toroidal spaces, and we study in particular the case of two
compactified dimensions, imaginary time and a spatial one (a heated film). It
is found that for any value of the applied field, there is a minimal size of
the system, independent of the chemical potential, below which the transition
disappears.Comment: 19 pages, 3 figures, version accepted for publication in Phys. Lett.
Canonical transformation for stiff matter models in quantum cosmology
In the present work we consider Friedmann-Robertson-Walker models in the
presence of a stiff matter perfect fluid and a cosmological constant. We write
the superhamiltonian of these models using the Schutz's variational formalism.
We notice that the resulting superhamiltonians have terms that will lead to
factor ordering ambiguities when they are written as operators. In order to
remove these ambiguities, we introduce appropriate coordinate transformations
and prove that these transformations are canonical using the symplectic method.Comment: Revtex4 Class, 3 pages, No Figure
Temporary use of a coastal ecosystem by the fish, Pomadasys corvinaeformis (Perciformes: Haemulidae), at Guaratuba Bay, Brazil
Pomadasys corvinaeformis is Qne ofthe most abundant species found in Guaratuba Bay, an Atlantic coastal, estuarine ecosystem in Paraná, Brazil. We tested whether this species comprises a resident or migratorial population. Abundance, size distribution, and gonad development of individuaIs captured during the year show that pre-adults enter the system when salinities are greater (autumn and early winter). During this time, they are mostly associated with mangroves and adjacent areas. When salinities begin to fall (late spring and summer) these now maturing individuaIs return to the sea where they spawn. Thus, P. corvinaeformis is migratorial and estuarine-dependent and uses this area prior to sexual maturity. During the period in this bay, P. corvinaeformis presents a great plasticity of feeding habits, that include inainly Crustacea, MoIlusca and Thaliacea.Pomadasys corvinaeformis é uma das espécies de maior representatividade na ictiofauna da Baía de Guaratuba, litoral do Estado do Paraná. Este trabalho investigou se a espécie compõe ali uma população permanente, residente no sistema, ou se é migratória, fteqüentando-o apenas em épocas determinadas. Resultados de 12 meses sobre variações de abundância, distribuição de tamanho e aspectos reprodutivos indicam que indivíduos subadultos ingressam no sistema quando a salinidade é maior (outono, começo de inverno). Nesta época, os indivíduos utilizam a área de manguezal e suas adjacências. Quanto a salinidade começa a cair (final de primavera, verão), estes indivíduos agora em maturação gonadal retomam para o mar, onde desovam. P. corvinaeformis é, portanto, uma espécie migratória e estuarino-dependente que utiliza a Baía em período anterior ao de maturidade gonadal. Ali, apresenta uma dieta muito variada, constituída basicamente de Crustacea, MoIlusca e Thaliacea
Comportamento dos fluxos de N2O por dois métodos de quantificação: câmaras estáticas e micrometeorológico.
O objetivo do trabalho consistiu em comparar os fluxos de óxido nitroso (N2O) obtidos pelo método da câmara estática com um método micrometeorológico (gradiente de fluxo) em área consolidada com sistema de integração lavoura-pecuária, fase pastagem.Pôster - pós-graduação
A canonical transformation and the tunneling probability for the birth of an asymptotically DeSitter universe with dust
In the present work, we study the quantum cosmology description of closed
Friedmann-Robertson-Walker models in the presence of a positive cosmological
constant and a generic perfect fluid. We work in the Schutz's variational
formalism. If one uses the scale factor and its canonically conjugated momentum
as the phase space variables that describe the geometrical sector of these
models, one obtains Wheeler-DeWitt equations with operator ordering
ambiguities. In order to avoid those ambiguities and simplify the quantum
treatment of the models, we introduce new phase space variables. We explicitly
demonstrate that the transformation leading from the old set of variables to
the new one is canonical. In order to show that the above canonical
transformations simplify the quantum treatment of those models, we consider a
particular model where the perfect fluid is dust. We solve the Wheeler-DeWitt
equation numerically using the Crank-Nicholson scheme and determine the time
evolution of the initial wave function. Finally, we compare the results for the
present model with the ones for another model where the only difference is the
presence of a radiative perfect fluid, instead of dust.Comment: Revtex4, 18 pages, 2 EPS figure
- …