Pedestrian Solution of the Two-Dimensional Ising Model

The partition function of the two-dimensional Ising model with zero magnetic field on a square lattice with m x n sites wrapped on a torus is computed within the transfer matrix formalism in an explicit step-by-step approach inspired by Kaufman's work. However, working with two commuting representations of the complex rotation group SO(2n,C) helps us avoid a number of unnecessary complications. We find all eigenvalues of the transfer matrix and therefore the partition function in a straightforward way.Comment: 10 pages, 2 figures; eqs. (101) and (102) corrected, files for fig. 2 fixed, minor beautification

An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model

We have calculated S(q) and the single particle distribution function for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site lattice with periodic boundary conditions; we justify the use of this lattice in compariosn to those of having the full square symmetry of the bulk. This new cluster has a high density of vec k points along the diagonal of reciprocal space, viz. along k = (k,k). The results clearly demonstrate that when the single hole problem has a ground state with a system momentum of vec k = (pi/2,pi/2), the resulting ground state for N holes involves a shift of the peak of the system's structure factor away from the antiferromagnetic state. This shift effectively increases continuously with N. When the single hole problem has a ground state with a momentum that is not equal to k = (pi/2,pi/2), then the above--mentioned incommensurability for N holes is not found. The results for the incommensurate ground states can be understood in terms of rigid--band filling: the effective occupation of the single hole k = (pi/2,pi/2) states is demonstrated by the evaluation of the single particle momentum distribution function . Unlike many previous studies, we show that for the many hole ground state the occupied momentum states are indeed k = (+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include

Species of Eurydice (Isopoda, Flabellifera) from southern Brazil

This paper reports species of Eurydice (Isopoda, Flabellifera) occurring on the continental shelf of southern Brazil, from lat. 22º00'S. Three species are recorded. E. littoralis (Moore, 1902), is a new occurrence from Brazil. Detailed illustrations and new morphologi cal data are furnished for this species hitherto inadequately described. E. elongata sp. n. and E. emarginata sp. n., are new to science. The flagellar process on the antenna 2, reported in three species of Eurydice, seems to be an exclusively male characteristic, and probably it will also occur in the males of most of the already described species. Genus and species diagnosis are provided, together with a classification key for all species recorded up to date from southern Brazil. All available distributional and ecological infor mation completes the account of each species.No presente trabalho faz-se um levantamento das especies do gênero Eurydice (Isopoda, Flabellifera) ocorrendo na plataforma continental centrosul do Brasil, a partir da Lat. 22º00'S. Tres espécies são assinaladas para a região: E. littoralis (Moore, 1902), E. elongata sp. n. e E. emarginata sp.n. Novos dados morfológicos e detalhadas ilustrações são apresentadas para a espécie E. littoralis. O processo flagelar da antena 2, observado em três espécies de Eurydice, parece constituir uma característica exclusiva dos machos, donde se pressupõe possa também ser encontrado em outros machos de espécies já descritas. São fornecidas diagnoses para o gênero e espécie, assim como urna chave de classificação para todas as espécies tratadas. Tanto quanto possível, observações ecológicas completam a descrição das espécies

Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in \Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low Temp. Phys. in honour of Prof. H. von Loehneyse