Quantum Monte Carlo and variational approaches to the Holstein model

Based on the canonical Lang-Firsov transformation of the Hamiltonian we develop a very efficient quantum Monte Carlo algorithm for the Holstein model with one electron. Separation of the fermionic degrees of freedom by a reweighting of the probability distribution leads to a dramatic reduction in computational effort. A principal component representation of the phonon degrees of freedom allows to sample completely uncorrelated phonon configurations. The combination of these elements enables us to perform efficient simulations for a wide range of temperature, phonon frequency and electron-phonon coupling on clusters large enough to avoid finite-size effects. The algorithm is tested in one dimension and the data are compared with exact-diagonalization results and with existing work. Moreover, the ideas presented here can also be applied to the many-electron case. In the one-electron case considered here, the physics of the Holstein model can be described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte

Stonefly (Plecoptera) fauna in a mountainous area of Central Brazil: composition and adult phenology

A survey of the stonefly (Plecoptera) fauna of streams of the Almas River basin, Pirenópolis, Goiás State, Central Brazil, is presented as well as data of some factors that could affeet the temporal distribution of the adults. For checking adult phenology, light sources were used in three stations from June 1993 to Jully 1994. The sampled individuais were identified to species or morphospecies, as possible. In this study, 301 individuais belonging to the perlid genera Anacroneuria Klapálek, 1909, Kempnyia Klapálek, 1916 and Macrogynoplax Enderlein, 1909 were collected. Adults of most species were collected along the studied period, except for those of Kempnyia that were restricted to the warm-rainy season, the same pattern for this genus in southeastern Brazil. Although adults of most species were collected along most of the studied period, the largest number of adults was collected in the months with larger mean temperatures, showing a clear seasonality in abundance

Natural history of three Hydropsychidae (Trichoptera, Insecta) in a "cerrado" stream from northeastern S\ue3o Paulo, Brazil

The faunal composition of the Hydropsychidae (Trichoptcra) of Pedregulho Stream is presented. Feeding habits of larvae and behavioural aspects of both larvae and adults are described