1,721,190 research outputs found
Quantum compass model on the square lattice
No full textUsing exact diagonalizations, Green's function Monte Carlo simulations and high-order perturbation theory, we study the low-energy properties of the two-dimensional spin-1/2 compass model on the square lattice defined by the Hamiltonian H=-Sigma(r)(J(x)sigma(r)(x)sigma(r+ex)(x)+J(z)sigma(r)(z)sigma(r+ez)(z)). When J(x)not equal J(z), we show that, on clusters of dimension L x L, the low-energy spectrum consists of 2(L) states which collapse onto each other exponentially fast with L, a conclusion that remains true arbitrarily close to J(x)=J(z). At that point, we show that an even larger number of states collapse exponentially fast with L onto the ground state, and we present numerical evidence that this number is precisely 2 x 2(L). We also extend the symmetry analysis of the model to arbitrary spins and show that the twofold degeneracy of all eigenstates remains true for arbitrary half-integer spins but does not apply to integer spins, in which cases the eigenstates are generically nondegenerate, a result confirmed by exact diagonalizations in the spin-1 case. Implications for Mott insulators and Josephson junction arrays are briefly discussed
Theory of spin-density profile and lattice distortion in the magnetization plateaus of SrCu_2(BO_3)_2
No full textThe two-dimensional spin-gap system SrCu2(BO3)(2) shows unique physical properties due to the low-dimensionality character and the strong quantum fluctuations. Experimentally, 1/8, 1/4, and 1/3 plateaus have been observed in the magnetization curve under magnetic fields up to 70 T, and the 1/2 plateau is expected to be stabilized at higher magnetic fields. We argue that spin-lattice effects are necessary to describe the spin density profile at the plateaus, and we propose a simple microscopic model of spins interacting adiabatically with the lattice to reproduce the main features of the recent experimental results by nuclear magnetic resonance
Peierls-like transition induced by frustration in a two dimensional antiferromagnet
No full textWe show that the introduction of frustration into the spin-1/2 two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via a next-nearest-neighbor exchange interaction can lead to a Peierls-like transition, from a tetragonal to an orthorhombic phase, when the spins are coupled to adiabatic phonons. The two different orthorhombic ground states define an Ising order parameter, which is expected to lead to a finite temperature transition. Implications for Li(2)VOSiO(4) , the first realization of that model, will be discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ising transition driven by frustration in a 2D classical model with continuous symmetry
No full textWe study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J(1)) and next-nearest (J(2)) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J(2)/J(1)>1/2, thermal fluctuations give rise to an effective Z(2) symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T-c-->0 with an infinite slope when J(2)/J(1)-->1/2
Li2VO(Si,Ge)O4 a prototype of a two-dimensional frustrated quantum Heisenberg antiferromagnet
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Finite-temperature properties of frustrated classical spins coupled to the lattice
No full textWe present extensive Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both nearest (J(1)) and next-nearest (J(2)) exchange couplings on the square lattice coupled to the lattice degrees of freedom. The Ising-like phase transition, which appears for J(2)/J(1)>1/2 in the pure spin model, is strengthened by the spin-lattice coupling and is accompanied by a lattice deformation from a tetragonal symmetry to an orthorhombic one. Evidence that the universality class of the transition does not change with the inclusion of the spin-lattice coupling is reported. Implications for Li2VOSiO4, the prototype for a layered J(1)-J(2) model in the collinear regime, are also discussed
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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