1,720,963 research outputs found
Phase diagram and multicritical behaviors of mixtures of three-dimensional bosonic gases
Full text linkWe investigate the Bose-Einstein condensation (BEC) patterns, the critical and multicritical behaviors of three-dimensional mixtures of bosonic gases with density-density interactions, characterized by a global U(1) U(1) symmetry [one U(1) transformation for each species]. In particular, we consider the three-dimensional Bose-Hubbard model for two lattice bosonic gases coupled by an on-site interspecies density-density interaction. We study the phase diagram and the critical behaviors along the transition lines of the BEC of one or both species. We present mean-field calculations and finite-size scaling analyses of quantum Monte Carlo data. We also investigate the nature of the multicritical points where the BEC transition lines of the two species meet. The corresponding universality classes are inferred from a renormalization-group analysis of the corresponding multicritical U(1)U(1) Landau-Ginzburg-Wilson Φ4 theory. We find two distinct critical behaviors, associated with bicritical and tetracritical points, respectively, depending on the relative strength of the interspecies and intraspecies interactions
Finite-size scaling at the first-order quantum transitions of quantum Potts chains
No full textWe investigate finite-size effects at first-order quantum transitions. For this purpose we consider the onedimensional
q-state quantum Potts chain, in particular with q = 10, which undergoes a first-order transition,
separating the quantum disordered and ordered phases with a discontinuity in the energy density of the ground
state. In agreement with the general theory, around the transition the low-energy properties show finite-size
scaling with respect to appropriate scaling variables. Their size dependence is particularly sensitive to boundary
conditions, which is a specific feature of first-order quantum transitions. Finally, we also discuss the finite-size
behavior of the q-state Potts model (q 2) at the first-order transitions driven by a parallel magnetic field,
occurring in the ferromagnetic phase
Bose-Einstein condensation and critical behavior of two-component bosonic gases
No full textWe study Bose-Einstein condensation (BEC) in three-dimensional two-component bosonic gases, character-
izing the universal behaviors of the critical modes arising at the BEC transitions. For this purpose, we use
field-theoretical (FT) renormalization-group (RG) methods and perform mean-field and numerical calculations.
The FT RG analysis is based on the Landau-Ginzburg-Wilson ! 4 theory with two complex scalar fields which
has the same symmetry as the bosonic system. In particular, for identical bosons with exchange Z2 symmetry,
coupled by effective density-density interactions, the global symmetry is Z2,e ⊗ U(1) ⊗ U(1). At the BEC
transition, it may break into Z2 ,e ⊗ Z2 ⊗ Z2 when both components condense simultaneously, or to U(1)
⊗ Z2 when only one component condenses. This implies different universality classes for the corresponding critical
behaviors. Numerical simulations of the two-component Bose-Hubbard model in the hard-core limit support
the RG prediction: when both components condense simultaneously, the critical behavior is controlled by a
decoupled XY fixed point, with unusual slowly decaying scaling corrections arising from the onsite interspecies
interaction
Scaling phenomena driven by inhomogeneous conditions at first-order quantum transitions
Full text linkWe investigate the effects of smooth inhomogeneities at first-order quantum transitions (FOQTs), such as those arising in the presence of a space-dependent external field, which smooths out the discontinuities of the low-energy properties at the transition. We argue that a universal scaling behavior emerges in the space transition region close to the point in which the external field takes the value for which the homogeneous system undergoes the FOQT. We verify the general theory in two model systems. We consider the quantum Ising chain in the ferromagnetic phase and the q-state Potts chain for q = 10, investigating the scaling behavior which arises in the presence of an additional inhomogeneous parallel and transverse magnetic field, respectively. Numerical results are in full agreement with the general theory
Finite-Size Scaling at First-Order Quantum Transitions
No full textWe study finite-size effects at first-order quantum transitions (FOQTs). We show that the low-energy properties show a finite-size scaling (FSS) behavior, the relevant scaling variable being the ratio of the energy associated with the perturbation driving the transition and the finite-size energy gap at the FOQT point. The size dependence of the scaling variable is therefore essentially determined by the size dependence of the gap at the transition, which in turn depends on the boundary conditions. Our results have broad validity and, in particular, apply to any FOQT characterized by the degeneracy and crossing of the two lowest-energy states in the infinite-volume limit. In this case, a phenomenological two-level theory provides exact expressions for the scaling functions. Numerical results for the quantum Ising chain in transverse and parallel magnetic fields support the FSS Ansatzes
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
Variations 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
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
Scaling behaviour of quantum systems at thermal and quantum phase transitions
No full textExperimental setups are finite in space and hardly ever in homogeneous
conditions.
This is very different from the ideal settings of the thermodynamic limit
often adopted in condensed matter theories.
Therefore, close to phase transitions, where typically long range correlations
build up, it is important to correctly take into account the way in which
boundaries and inhomogeneities affect the critical behaviour.
This can be achieved by means of the finite-size (FSS) and trap-size (TSS)
scaling theories, which generally apply to continuous phase transitions, where
one can define a diverging length scale.
FSS and TSS are reviewed in the first part of this work, together with some
general properties of systems close to phase transitions.
We then numerically study the TSS properties of the continuous
finite-temperature phase transition of the Bose-Hubbard model (BH) in two and
three dimension.
This quantum model realistically describes experiments with ultra-cold bosonic
gases trapped in optical lattices.
In three dimensions, the BH exhibits a standard normal-to-superfluid
transition.
In two dimensions, the transition becomes of the Berezinski-Kosterlitz-Thouless
type, characterised by logarithmic corrections to scaling.
We perform thorough FSS analyses of quantum Monte Carlo data in homogeneous
conditions to extract the value critical temperature.
In two dimensions, this requires devising a matching method in which the FSS
behaviour of the 2D BH is matched to the classical 2D XY model, whose
transition belongs to the same universality class.
We subsequently verify the validity of the TSS ansatz by simulating the trapped
systems at the critical temperature.
We find that the TSS theory is general and universal once one takes into
account the effective way in which the trapping potential couples to the
critical modes of the system.
In the last part of this Thesis, we extend the FSS and TSS to discontinuous (or
first order) quantum phase trnasitions.
Discontinuous transitions do not develop a diverging length scale in the
thermodynamic limit, but are rather characterised by the coexistence of
domains of different phases at the transition.
The typical size of single-phase domains induce a behaviour that closely
resembles finite size scaling.
We find that the scaling variable that parametrises the scaling behaviour at
discontinuous transitions is the ratio of the perturbation energy driving the
transition to the finite-size energy gap.
We further find that inhomogeneous systems exibiting first order transitions
can be treated heuristically in analogy with the TSS behaviour at continuous
transitions.
These findings are confirmed numerically on the quantum Ising and quantum Potts
chains, which are simulated using density matrix renormalisation group
techniques
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