Lattice bosons in a quasi-disordered environment

In this paper, we study non-interacting bosons in a quasi-disordered one-dimensional optical lattice in a harmonic potential. We consider the case of deterministic quasi-disorder produced by an Aubry–André potential. Using exact diagonalization, we investigate both the zero temperature and the finite temperature properties. We investigate the localization properties by using an entanglement measure. We find that the extreme sensitivity of the localization properties to the number of lattice sites in finite size closed chains disappear in open chains. This feature continues to be present in the presence of a harmonic confining potential. The quasi-disorder is found to strongly reduce the Bose–Einstein condensation temperature and the condensate fraction in open chains. The low temperature thermal depletion rate of the condensate fraction increases considerably with increasing quasi-disorder strength. We also find that the critical quasi-disorder strength required for localization increases with increasing strength of the harmonic potential. Further, we find that the low temperature condensate fraction undergoes a sharp drop to 0.5 in the localization transition region. The temperature dependence of the specific heat is found to be only marginally affected by the quasi-disorder.     

Insulator-metal transition in biased finite polyyne systems

A method for the study of the electronic transport in strongly coupled electron-phonon systems is formalized and applied to a model of polyyne chains biased through metallic Au leads. We derive a stationary non equilibrium polaronic theory in the general framework of a variational formulation. The numerical procedure we propose can be readily applied if the electron-phonon interaction in the device hamiltonian can be approximated as an effective single particle electron hamiltonian. Using this approach, we predict that finite polyyne chains should manifest an insulator-metal transition driven by the non-equilibrium charging which inhibits the Peierls instability characterizing the equilibrium state.     

The smooth structural change in mesoscopic Peierls chains

We investigate the Peierls transition in finite chains by exact (Lanczos) diagonalization and within a seminumerical method based on the factorization of the electron-phonon wave function (Adiabatic Ansatz, AA). AA can be applied for mesoscopic chains up to micrometer sizes and its reliability can be checked self-consistently. Our study demonstrates the important role played for finite systems by the tunneling in the double well potential. The chains are dimerized only if their size N exceeds a critical value N_c which increases with increasing phonon frequency. Quantum phonon fluctuations yield a broad transition region. This smooth Peierls transition contrasts not only to the sharp mean field transition, but also with the sharp RPA soft mode instability, although RPA partially accounts for quantum phonon fluctuations. For weak coupling the dimerization disappears below micrometer sizes; therefore, this effect could be detected experimentally in mesoscopic systems. Received: 3 January 1998 / Revised: 13 March 1998 / Accepted: 3 April 1998     

Multicanonical Monte Carlo simulations on intramolecular micelle formation in copolymers

The formation of intramolecular micelles in copolymers with periodic sequence, where hydrophobic units (stickers) are periodically placed along the chain, is studied by using multicanonical Monte Carlo computer simulations for an off-lattice bead-rod model in three dimensions. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and flower-type micelles are formed via the transition. The number of stickers forming a micelle core is limited by the excluded-volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition, takes place. At this transition point, the two intramolecular micelles merge into one micelle. Furthermore, we extend the multicanonical MC method to study elastic properties of single polymer chains with strong attractive interactions under external force fields, and study how the intramolecular micellization affects the elastic property of single polymer chains.     

QCD Phase Transitions and Bag Constants at Finite Chemical Potential

The global colour model at finite temperature is further extended to study the systems at finite chemical potential. The deconfinement and chiral phase transition at finite chemical potential and at temperature T = 0 K are studied simultaneously. Meanwhile the evolution of the bag constants at finite chemical potential is calculated. The dependences of results on the model parameters are discussed in detail     

Studying quantum spin systems through entanglement estimators

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.     

An effective-field study of Ising metamagnet in an external field

The phase diagrams and magnetization curves of a two-sublattice Ising metamagnet at finite temperature with longitudinal crystal field H are investigated by the use of an effective-field theory (EFT) with correlations. In addition to the second-order transition lines, the first-order transition lines are also presented, since a method to calculate the Gibbs free energy numerically at finite temperature within EFT is found in this work. The results show that there is no fourth-order critical point or reentrant phenomenon in the phase diagrams given by using EFT as found by using mean-field theory (MFT).     

Quark potential in a quark–meson plasma

We investigate the effect of the restoration of chiral symmetry on the quark potential in a quark–meson plasma by considering meson exchanges in the two flavor Nambu–Jona-Lasinio model at finite temperature and density. There are two possible oscillations in the chiral restoration phase; one is the Friedel oscillation due to the sharp quark Fermi surface at high density, and the other is the Yukawa oscillation driven by the complex meson poles at high temperature. The quark–meson plasma is strongly coupled in the temperature region 1≤T/T _c≤3, with T _c being the critical temperature of the chiral phase transition. The maximum coupling in this region is located at the phase transition point.     

Elasticity transition and loop formation in vibrated bead chains: A simulation of polymer chains

By measuring the distribution function of the end-to-end distance, we find that strongly shaken bead chains exhibit many properties, such as the rigid-rod-to-Gaussian chain transition, scaling, fast drop of loop formation probability in the short-chain regime, and enhancement of loop formation probability for kinked chains, of long-chain polymers. Though there is difference in local details between our chains and the worm-like chains, our results are consistent with recent calculations based on the worm-like chain model in many respects.     

Calorimetric investigation of the magnetic transition in quasi-one-dimensional molecule-based ferrimagnets: [Mn(OC14H29)4TPP][TCNE]·MeOH and [MnF4TPP][TCNE]·0.5MeOH

Heat capacity study was performed, for the first time, for [MnF₄TPP][TCNE]·0.5MeOH and [Mn(OC₁₄H₂₉)₄TPP][TCNE]·MeOH complexes in the 1.8-100 K temperature range under the 0-9 T magnetic field and disclosed new aspects inherent in such strongly coupled charge-transfer Mn-porphyrin-TCNE linear chain systems, where TPP=5,10,15,20-tetraphenylporphyrinato, TCNE=tetracyanoethylene and MeOH=methanol. Any heat capacity anomaly due to the onset of the magnetic long-range-order was not detected, whereas the magnetic phase transition has clearly been observed around 20 K by previous magnetic studies. As these materials are well approximated by quasi-one-dimensional ferrimagnetic Heisenberg chains with very large intrachain spin-spin interactions, the most part of the magnetic entropy is retained above the phase transition temperature as the dominant short-range order. This is the reason why no magnetic phase transition was detected by calorimetry. On the other hand, the big effect observed in the magnetic susceptibility is well accounted for if the formation of magnetic domains is assumed in the crystal.     

Phase diagram of an Ising model with competitive interactions on a Husimi tree and its disordered counterpart

We consider an Ising competitive model defined over a triangular Husimi tree where loops, responsible for an explicit frustration, are even allowed. We first analyze the phase diagram of the model with fixed couplings in which a “gas of noninteracting dimers (or spin liquid) — ferro or antiferromagnetic ordered state” zero temperature transition is recognized in the frustrated regions. Then we introduce the disorder for studying the spin glass version of the model: the triangular ±J model. We find out that, for any finite value of the averaged couplings, the model exhibits always a finite temperature phase transition even in the frustrated regions, where the transition turns out to be a glassy transition. The analysis of the random model is done by applying a recently proposed method which allows us to derive the critical surface of a random model through a mapping with a corresponding nonrandom model.     

Chiral phase transition at finite temperature in the linear sigma model

We study the chiral phase transition at finite temperature in the linear sigma model by employing a self-consistent Hartree approximation. This approximation is introduced by imposing self-consistency conditions on the effective meson mass equations which are derived from the finite temperature one-loop effective potential. It is shown that in the limit of vanishing pion mass, namely when the chiral symmetry is exact, the phase transition becomes a weak first order accompanying a gap in the order parameter as a function of temperature. This is caused by the long range fluctuations of meson fields whose effective masses become small in the transition region. It is shown, however, that with an explicit chiral symmetry breaking term in the Lagrangian which generates the realistic finite pion mass the transition is smoothed out irrespective of the choice of coupling strength. Recieved: 19 September 1997 / Revised version: 30 October 1997     

Pion decay constant at finite temperature

The values of the pion decay constant, and of the bilinear fermionic condensate, for temperatures from zero up to the critical temperature for chiral phase transition, are calculated using our composite operator approach to finite temperature QCD. For small temperatures our results agree with those found with other approaches. Near the critical temperature we recover the universal behaviour typical of second order phase transition. We have assumed that effects responsible for deconfinement do not strongly perturb our chiral approach for two and possibly three flavours.     

Vortex arrays in a rotating superfluid Fermi gas

The behavior of a dilute two-component neutral superfluid Fermi gas subjected to rotation is investigated within the context of a weak-coupling BCS theory. The microscopic properties at finite temperature are obtained by iterating the Bogoliubov-de Gennes equations to self-consistency. In the model, alkali atoms are strongly confined in quasi-two-dimensional traps produced by a deep one-dimensional optical lattice. The lattice depth significantly enhances the critical transition temperature and the critical rotation frequency at which the superfluidity ceases. As the rotation frequency increases, the triangular vortex arrays become increasingly irregular, indicating a quantum melting transition.     

Thermal stability of mode-locking steps in charge density waves

We examine the stability of mode-locking steps in incommensurate charge density waves in the presence of finite temperature. The steps turn out to be quite stable by showing a series of definite plateaus in the current-voltage characteristics. We also study the dynamical phase transition from a mode-locked state to an unlocked state by assuming the fact that there exists a similar power-law scaling hypothesis as in a pinning-depinning phase transition. The critical exponents around both the edges of the mode-locking steps are found to have similar values for the depinning transition.     

Phase diagrams of a cylindrical transverse Ising ferrimagnetic nanotube; Effects of surface dilution

Phase diagrams (transition temperature and compensation temperature) of a cylindrical ferrimagnetic nanotube with a negative core–shell interaction, described by the transverse Ising model (TIM), are investigated by the use of the effective-field theory with correlations. The phase diagrams of the system are strongly affected by the surface dilution. The possibility of two compensation points is found in the phase diagram, in contrast to the case of a cylindrical TIM nanowire.     

Influence of tensile deformation on the crystalline organization of ethylene copolymers

The influence of tensile deformation on the crystalline properties of ethylene copolymers (ethylene-vinyl acetate [EVA] copolymers) was investigated by differential scanning calorimetry (DSC). The consequence of drawing on the mobility of the amorphous phase also was investigated through the study of the glass transition temperature. The results indicate that more disorganized crystals, melting at a lower temperature, are present after the tensile deformation, reducing the mobility of the amorphous chains, as shown by an increase of the glass transition temperature. For the lowest molecular weight copolymer. less crystalline changes are observed after the tensile test, probably due to the fact that no stiffening appears during the drawing.