[Show abstract][Hide abstract] ABSTRACT: The relevance of the various dynamical thermal processes which affect the optical properties of a dye solution under flashlamp illumination is examined by means of the appropriate hydrodynamic equations. It is found that the main contribution to the density fluctuations comes from a local Rayleigh term. After a brief discussion of the mechanisms which convert absorbed pump light into heat, the space and time-dependent thermal distortions and the refractive index profiles are evaluated for the case of Rhodamine 6 G in ethanol in a planar geometry. These results yield a basis for a detailed study of the mode behaviour in planar dye lasers. Such an analysis is carried out in the following contribution.
Journal of Modern Optics 11/2010; November 1976(11-Vol. 23):923-932. DOI:10.1080/713819185 · 1.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A magnetic system is usually described in terms of the exchange coupling between neighboring spins lying on the sites of a given lattice. Our goal here is to account for the unavoidable quantum effects due to the further coupling with the vibrations of the ions constituting the lattice. A Caldeira-Leggett scheme allows one to treat such effects through the analysis of the associated influence action, obtained after tracing-out the phonons. In a physically sound model, it turns out that one must deal with an environmental coupling which is nonlinear in the system's variables. The corresponding path integral can be dealt with by suitably extending the pure-quantum self-consistent harmonic approximation. In this way one can obtain extended phase diagrams for magnetic phase transitions, accounting for the environmental interaction.
[Show abstract][Hide abstract] ABSTRACT: A nonperturbative approach giving the spin-energy coupling effects present in the magnetization fluctuation spectra is developed for the quantum one-dimensional ferromagnetic chain to explain the features observed in CsNiF3 by new experiments employing polarized neutrons. Quantum effects are found to be essential to reproduce the features observed in the data.
[Show abstract][Hide abstract] ABSTRACT: We study the two-spin entanglement distribution along the infinite $S=1/2$ chain described by the XY model in a transverse field; closed analytical expressions are derived for the one-tangle and the concurrences $C_r$, $r$ being the distance between the two possibly entangled spins, for values of the Hamiltonian parameters close to those corresponding to factorized ground states. The total amount of entanglement, the fraction of such entanglement which is stored in pairwise entanglement, and the way such fraction distributes along the chain is discussed, with attention focused on the dependence on the anisotropy of the exchange interaction. Near factorization a characteristic length-scale naturally emerges in the system, which is specifically related with entanglement properties and diverges at the critical point of the fully isotropic model. In general, we find that anisotropy rule a complex behavior of the entanglement properties, which results in the fact that more isotropic models, despite being characterized by a larger amount of total entanglement, present a smaller fraction of pairwise entanglement: the latter, in turn, is more evenly distributed along the chain, to the extent that, in the fully isotropic model at the critical field, the concurrences do not depend on $r$.
Journal of Physics A Mathematical and Theoretical 03/2007; DOI:10.1088/1751-8113/40/32/010 · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An efficient Path Integral Monte Carlo procedure is proposed to simulate the behavior of quantum many-body dissipative systems described within the framework of the influence functional. Thermodynamic observables are obtained by Monte Carlo sampling of the partition function after discretization and Fourier transformation in imaginary time of the dynamical variables. The method is tested extensively for model systems, using realistic dissipative kernels. Results are also compared with the predictions of a recently proposed semiclassical approximation, thus testing the reliability of the latter approach for weak quantum coupling. Our numerical method opens the possibility to quantitatively describe real quantum dissipative systems as, e.g., Josephson junction arrays.
[Show abstract][Hide abstract] ABSTRACT: A variational approach based on the path-integral formulation of the statistical mechanics is applied to calculate the partition function and the related thermodynamic quantities of one-dimensional kink bearing fields. This is done by determining an effective potential which includes in a complete quantum way the linear modes of the field, while treating variationally the nonlinear excitations. The treatment is applied both to integrable systems, like Sine-Gordon and to non integrable ones, like 4 and Double Sine-Gordon. The temperature renormalization can be separately studied both for the vacuum and the one-soliton sector in order to find the low temperature properties in self consistent one-loop approximation. Moreover a low-coupling expansion is given and its range of applicability is found to be much wider than the Wigner expansion. Comparisons with quantum Monte-Carlo results and exact results obtained by Bethe Ansatz are presented.
Physica Scripta 11/2006; 40(4):451. DOI:10.1088/0031-8949/40/4/002 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Entanglement represents a pure quantum effect involving two or more particles. Spin systems are good candidates for studying this effect and its relation with other collective phenomena ruled by quantum mechanics. While the presence of entangled states can be easily verified, the quantitative estimate of this property is still under investigation. One of the most useful tool in this framework is the concurrence whose definition, albeit limited to $S=1/2$ systems, can be related to the correlators. We consider quantum spin systems defined along chains and square lattices, and described by Heisenberg-like Hamiltonians: our goal is to clarify the relation between entanglement and quantum phase transitions, as well as that between the concurrence the and the specific quantum state of the system.
Open Systems & Information Dynamics 06/2006; DOI:10.1007/s11080-006-9014-2 · 0.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the pairwise entanglement close to separable ground states of a class of one dimensional quantum spin models. At T=0 we find that such ground states separate regions, in the space of the Hamiltonian parameters, which are characterized by qualitatively different types of entanglement, namely parallel and antiparallel entanglement; we further demonstrate that the range of the Concurrence diverges while approaching separable ground states, therefore evidencing that such states, with uncorrelated fluctuations, are reached by a long range reshuffling of the entanglement. We generalize our results to the analysis of quantum phase transitions occurring in bosonic and fermionic systems. Finally, the effects of finite temperature are considered: At T>0 we evidence the existence of a region where no pairwise entanglement survives, so that entanglement, if present, is genuinely multipartite. Comment: 6 pages, 4 figures
Physical Review A 02/2006; 74(2). DOI:10.1103/PhysRevA.74.022322 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The first-order superconducting fluctuation corrections to the thermal conductivity of a granular metal are calculated. A suppression of thermal conductivity proportional to Tc∕(T−Tc) is observed in a region not too close to the critical temperature Tc. As T≃Tc, a saturation of the correction is found, and its sign depends on the ratio between the barrier transparency and the critical temperature. In both regimes, the Wiedemann-Franz law is violated.
[Show abstract][Hide abstract] ABSTRACT: We consider a quantum many-body system made of $N$ interacting $S{=}1/2$ spins on a lattice, and develop a formalism which allows to extract, out of conventional magnetic observables, the quantum probabilities for any selected spin pair to be in maximally entangled or factorized two-spin states. This result is used in order to capture the meaning of entanglement properties in terms of magnetic behavior. In particular, we consider the concurrence between two spins and show how its expression extracts information on the presence of bipartite entanglement out of the probability distributions relative to specific sets of two-spin quantum states. We apply the above findings to the antiferromagnetic Heisenberg model in a uniform magnetic field, both on a chain and on a two-leg ladder. Using Quantum Monte Carlo simulations, we obtain the above probability distributions and the associated entanglement, discussing their evolution under application of the field.
The European Physical Journal D 06/2005; 38(3). DOI:10.1140/epjd/e2006-00090-6 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We analyze the interplay of dissipative and quantum effects in the proximity of a quantum phase transition. The prototypical system is a resistively shunted two-dimensional Josephson junction array, studied by means of an advanced Fourier path-integral Monte Carlo algorithm. The reentrant superconducting-to-normal phase transition driven by quantum fluctuations, recently discovered in the limit of infinite shunt resistance, persists for moderate dissipation strength but disappears in the limit of small resistance. For large quantum coupling our numerical results show that, beyond a critical dissipation strength, the superconducting phase is always stabilized at sufficiently low temperature. Our phase diagram explains recent experimental findings.
[Show abstract][Hide abstract] ABSTRACT: Making use of exact results and quantum Monte Carlo data for the entanglement of formation, we show that the ground state of anisotropic two-dimensional S=1/2 antiferromagnets in a uniform field takes the classical-like form of a product state for a particular value and orientation of the field, at which the purely quantum correlations due to entanglement disappear. Analytical expressions for the energy and the form of such states are given, and a novel type of exactly solvable two-dimensional quantum models is therefore singled out. Moreover, we show that the field-induced quantum phase transition present in the models is unambiguously characterized by a cusp minimum in the pairwise-to-global entanglement ratio R, marking the quantum-critical enhancement of multipartite entanglement.
[Show abstract][Hide abstract] ABSTRACT: The weak localization correction to the conductivity of a granular metal is calculated using the diagrammatic technique in the reciprocal grain lattice representation. The properties of this correction are very similar to that one in disordered metal, with the replacement of the electron mean free path $\ell $ by the grain diameter $d$ and the dimensionless conductance $g$ by the tunnelling dimensionless conductance $g_{T}$. In particular, we demonstrate that at zero temperature no conducting phase can exist for dimensions $D\leq 2$. We also analyze the WL correction to magnetoconductivity in the weak field limit. Comment: 4 pages, 3 figures; minor corrections added
[Show abstract][Hide abstract] ABSTRACT: We perform an extensive quantum Monte Carlo investigation of entanglement properties in quantum spin systems close to or at a quantum critical point. Making use of the Stochastic Series Expansion method, we can systematically estimate the bipartite entanglement of the ground-state wavefunction in a large class of anisotropic spin models on unfrustrated lattices and in a uniform magnetic field. The behavior of the entanglement estimators as a function of the field shows remarkable universal features independent of the lattice dimensionality, marking both the occurrence of a field-induced quantum phase transition and of an exactly factorized state.
Journal of Low Temperature Physics 01/2005; 140(3):293-302. DOI:10.1007/s10909-005-6315-8 · 1.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the field dependence of the entanglement of formation in anisotropic S=1/2 antiferromagnetic chains and two-leg ladders displaying a T=0 field-driven quantum phase transition. The analysis is carried out via Quantum Monte Carlo simulations. At zero temperature the entanglement estimators show abrupt changes at and around criticality, vanishing below the critical field, in correspondence with an exactly factorized state, and then immediately recovering a finite value upon passing through the quantum phase transition. At the quantum critical point, a deep minimum in the pairwise-to-global entanglement ratio shows that multi-spin entanglement is strongly enhanced; moreover this signature represents a novel way of detecting the quantum phase transition of the system, relying entirely on entanglement estimators.
[Show abstract][Hide abstract] ABSTRACT: We study the field dependence of the entanglement of formation in anisotropic S=1/2 antiferromagnetic chains displaying a T=0 field-driven quantum phase transition. The analysis is carried out via quantum Monte Carlo simulations. At zero temperature the entanglement estimators show abrupt changes at and around criticality, vanishing below the critical field, in correspondence with an exactly factorized state, and then immediately recovering a finite value upon passing through the quantum phase transition. At the quantum-critical point, a deep minimum in the pairwise-to-global entanglement ratio shows that multispin entanglement is strongly enhanced; moreover this signature represents a novel way of detecting the quantum phase transition of the system, relying entirely on entanglement estimators.
[Show abstract][Hide abstract] ABSTRACT: We study the two-dimensional (2D) S=12 Heisenberg antiferromagnet with weak easy-plane exchange anisotropy (down to 0.1%) by quantum Monte Carlo simulations. We observe a crossover between isotropic and XY behavior at a temperature which is ∼30% above the Kosterlitz–Thouless transition. Evidences of the latter are hard to detect in real layered materials due to the interlayer exchange; however, below the crossover there exists a range where experimental detection of non-critical 2D XY behavior is possible, as shown in the case of Sr2CuO2Cl2.
Journal of Magnetism and Magnetic Materials 05/2004; 272. DOI:10.1016/j.jmmm.2003.12.407 · 1.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the thermodynamics of the spin-S two-dimensional quantum Heisenberg antiferromagnet on the square lattice with nearest (J1) and next-nearest (J2) neighbor couplings in its collinear phase (J(2)/J(1)>0.5), using the pure-quantum self-consistent harmonic approximation. Our results show the persistence of a finite-temperature Ising phase transition for every value of the spin, provided that the ratio J(2)/J(1) is greater than a critical value corresponding to the onset of collinear long-range order at zero temperature. We also calculate the spin and temperature dependence of the collinear susceptibility and correlation length, and we discuss our results in light of the experiments on Li2VOSiO4 and related compounds.
[Show abstract][Hide abstract] ABSTRACT: The quantum XY model shows a Berezinskii-Kosterlitz-Thouless (BKT) transition between a phase with quasi long-range order and a disordered one, like the corresponding classical model. The effect of the quantum fluctuations is to weaken the transition and eventually to destroy it. However, in this respect the mechanism of disappearance of the transition is not yet clear. In this work we address the problem of the quenching of the BKT in the quantum XY model in the region of small temperature and high quantum coupling. In particular, we study the phase diagram of a 2D Josephson junction array, that is one of the best experimental realizations of a quantum XY model. A genuine BKT transition is found up to a threshold value $g^\star$ of the quantum coupling, beyond which no phase coherence is established. Slightly below $g^\star$ the phase stiffness shows a reentrant behavior at lowest temperatures, driven by strong nonlinear quantum fluctuations. Such a reentrance is removed if the dissipation effect of shunt resistors is included.