1,721,020 research outputs found
Phase separation and three-site hopping in the 2-dimensional t-J model
Full text linkWe study the t-J model with the inclusion of the so-called three-site term which is due to the t/U → 0 expansion of the Hubbard model. We find that this singlet pair hopping term has no qualitative effect on the structure of the pure mean field phase diagram for non-magnetic states. In accordance with experimental data on high-Tc materials and some numerical studies, we also find wide regions of phase coexistence whenever the coupling J is greater than a critical value Jc. We show that Jc varies linearly with the temperature T, going to zero at T = 0
Estimating quasi-long-range order via Rényi entropies
No full textWe show how entanglement entropies allow for the estimation of quasi-long-range order in one-dimensional systems whose low-energy physics is well captured by the Tomonaga-Luttinger liquid universality class. First, we check our procedure in the exactly solvable XXZ spin-1/2 chain in its entire critical region, finding very good agreement with Bethe ansatz results. Then, we show how phase transitions between different dominant orders may be efficiently estimated by considering the superfluid-charge density wave transition in a system of dipolar bosons. Finally, we discuss the application of this method to multispecies systems such as the one-dimensional Hubbard model. Our work represents the first proof of a direct relationship between the Luttinger parameter and Re ́nyi entropies in both bosonic and fermionic lattice models
From the equations of motion to the canonical commutation relations
No full textThe problem of whether or not the equations of motion of a quantum system determine the commutation relations was posed by E.P.Wigner in 1950. A similar problem (known as ”The Inverse Problem in the Calculus of Variations”) was posed in a classical setting as back as in 1887 by H.Helmoltz and has received great attention also in recent times. The aim of this paper is to discuss how these two apparently unrelated problems can actually be discussed in a somewhat unified framework. After review- ing briefly the Inverse Problem and the existence of alternative structures for classical systems, we discuss the geometric structures that are intrinsically present in Quantum Mechanics, starting from finite-level systems and then moving to a more general setting by using the Weyl-Wigner ap- proach, showing how this approach can accomodate in an almost natural way the existence of alternative structures in Quantum Mechanics as well
Path integrals for spinning particles, stationary phase and the Duistermaat-Heckmann theorem
Full text linkWe examine the problem of the evaluation of both the propagator and of the partition function of a spinning particle in an external field at the classical as well as the quantum level, in connection with the asserted exactness of the stationary phase approximation. At the classical level we argue that exactness of this approximation stems from the fact that the dynamics (on the two-sphere S2) of a spinning particle in a magnetic field is the reduction from R4 to S2 of a linear dynamical system on R4. At the quantum level, however, and within the path integral approach, the restriction, inherent to the use of the stationary phase approximation, to regular paths clashes with the fact that no regulators are present in the action that enters the path integral. This is shown to lead to a prefactor for the path integral that is strictly divergent, except in the classical limit. A critical comparison is made with the various approaches that have been presented in the literature. The validity of a formula given in literature for the spin propagator is extended to the case of motion in an arbitrary magnetic field
Effective actions for spin ladders
Full text linkWe derive a path-integral expression for the effective action in the continuum limit of an antiferromagnetic Heisenberg spin ladder with an arbitrary number of legs. The map is onto an O(3) nonlinear sigma model (NL sigma M) with the addition of a topological term that is effective only for odd-leg ladders and half-odd integer spins. We derive the parameters of the effective NL sigma M and the behavior of the spin gap for the case of even-leg ladde
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
Noncommutative Lattices and the Algebras of their Continuous Functions
No full text Recently a new kind of approximation to continuum topological spaces has been introduced, the approximating spaces being partially ordered sets (posets) with a finite or at most a countable number of points. The partial order endows a poset with a nontrivial non-Hausdorff topology. Their ability to reproduce important topological information of the continuum has been the main motivation for their use in quantum physics. Posets are truly noncommutative spaces, or noncommutative lattices, since they can be realized as structure spaces of noncommutative C*-algebras. These noncommutative algebras play the same rôle as the algebra of continuous functions [Formula: see text] on a Hausdorff topological space M and can be thought of as algebras of operator valued functions on posets. In this article, we will review some mathematical results that establish a duality between finite posets and a certain class of C*-algebras. We will see that the algebras in question are all postliminal approximately finite dimensional (AF) algebras. </jats:p
- …
