Three-state majority-vote model on square lattice

Here, a non-equilibrium model with two states (−1,+1) and a noise q on simple square lattices proposed for M.J. Oliveira (1992) following the conjecture of up-down symmetry of Grinstein and colleagues (1985) is studied and generalized. This model is well-known, today, as the majority-vote model. They showed, through Monte Carlo simulations, that their obtained results fall into the universality class of the equilibrium Ising model on a square lattice. In this work, we generalize the majority-vote model for a version with three states, now including the zero state, (−1,0,+1) in two dimensions. Using Monte Carlo simulations, we showed that our model falls into the universality class of the spin-1 (−1,0,+1) and spin-1/2 Ising model and also agree with majority-vote model proposed for M.J. Oliveira (1992). The exponent ratio obtained for our model was γ/ν=1.77(3), β/ν=0.121(5), and 1/ν=1.03(5). The critical noise obtained and the fourth-order cumulant were q_c=0.106(5) and U^∗=0.62(3).     

Isotropic majority-vote model on a square lattice

The stationary critical properties of the isotropic majority vote model on a square lattice are calculated by Monte Carlo simulations and finite size analysis. The critical exponents, , and are found to be the same as those of the Ising model and the critical noise parameter is found to beq _c =0.075±0.001.     

Spin-glass-like behavior in a Fe-Zr amorphous alloy

The thermomagnetization curves and the hyteresis loops of the amorphous alloy Fe₉₂Zr₈ at low temperature have been investigated. Spin-glass-like behavior such as a thermomagnetic history and relaxation of remanence has been observed.     

Spin-glass-like magnetic ordering in Zn substituted magnetite magnetic fluids

Electron spin resonance (ESR) spectra of magnetic fluids involving polydispersed Zn(0.5)Fe(0. 5)Fe(2)O(4) (FZ5) and Zn(0.7)Fe(0. 3)Fe(2)O(4) (FZ7) nanomagnetic particles are scanned from 4.2 to 300K. The FZ7 fluid exhibits certain distinct features below 40K which are different from FZ5 fluid. These include (i) an isotropic shift in resonance field in zero-field-cooled ESR study, (ii) deviation of resonance field from sin(2)theta behavior (where theta is the angle between axis of the particle and field) in field cooled (FC) sample and (iii) abrupt increase in anisotropy field for FC sample. The results are analyzed in light of the core-shell model for nanomagnetic particles.     

Metal-insulator transition in the Hubbard model with incommensurate magnetic structures

The metal-insulator transition for the square, simple cubic, and body centered cubic lattices has been studied within the Hubbard model at half-filling taking into account nearest- and next-nearest-neighbor electron hopping. Both staggered antiferromagnetic and incommensurate magnetic states (spin-spiral wave) have been considered. The inclusion of the latter states for the three-dimensional lattices does not change the general pattern of the metal-insulator transition, but opens the fundamentally new possibility of the metal-insulator transition of the first order between the magnetically ordered states for the square lattice.     

Phase transition in the Blume-Capel model with second neighbour interaction

A two dimensional antiferromagnetic spin-1 Ising model with negative next- nearest neighbour interaction (J ₂ <0) and under an external magnetic field is investigated by two methods: The mean-field theory and Finite-Size-Scaling based on transfer matrix (TMFSS) calculations. The ground state diagrams exhibit several new phases including frustrated ones. At finite temperature we obtain by these two methods quite rich phase diagrams, with several multicritical points. While Mean field approximation yields phase diagrams which are sometimes even qualitatively incorrect, accurate results are obtained from transfer matrix finite size scaling calculations. For a certain range of interaction parameters, the model is shown to violate the ordinary universality hypothesis. Received: 3 November 1997 / Revised: 31 March 1998 / Accepted: 7 April 1998     

A one-dimensional model with phase transition

Two repellent particles are bound to occupy two among thek _n +1 adjacent sites 0=x ₀ ⁽ⁿ⁾ <x ₁ ⁽ⁿ⁾ <...<x _kn ⁽ⁿ⁾ =1, sayx _q ⁽ⁿ⁾ ,x _q+1 ⁽ⁿ⁾ . Define the Hamiltonian _q ⁽ⁿ⁾ =–ln(x _q+1 ⁽ⁿ⁾ –x _q ⁽ⁿ⁾ ) and the partition function     

A one-dimensional model with phase transition

A one-dimensional model is studied with nearest neighbor interaction and certain forbidden configurations. In this model it is possible to investigate the phase transition even on the microcanonical level, and it turns out that phases can coexist under certain circumstances.     

Spin-glass-like behaviour and long-range order in dilute YTb alloys

Using neutron diffraction, we have examined the magnetic ordering of two YTb alloys whose magnetic properties are characteristic of spin-glass materials. For YTb_{5 at %} we have determined that a spiral state appears below the susceptibility cusp at 26.6 K, despite differences between field coolled and zero field cooled magnetization. No evidence for long-range order was found for YTb_{3 at %} below its cusp at 15.5 K. We conclude that irreversibility alone can not be used to characterize the spin-glass state.     

First-order transition in the ginzburg-landau model

The d-dimensional complex Ginzburg-Landau (GL) model is solved according to a variational method by separating phase and amplitude. The GL transition becomes first order for high superfluid density because of phase fluctuations. We discuss its origin with various arguments showing that, in particular for d = 3, the validity of our approach lies precisely in the first-order domain.     

Spin-glass-like behavior of uncompensated surface spins in NiO nanoparticulated powder

Nickel oxide nanoparticles successfully synthesized by a polymer precursor method are studied in this work. The analysis of X-ray powder diffraction data provides a mean crystallite size of 22±2 nm which is in a good agreement with the mean size estimated from transmission electron microscopy images. Whereas the magnetization (M) vs. magnetic field (H) curve obtained at 5 K is consistent with a ferromagnetic component which coexists with an antiferromagnetic component, the presence of two peaks in the zero-field-cooled trace suggests the occurrence of two blocking process. The broad maximum at high temperature was associated with the thermal relaxation of uncompensated spins at the particle core and the low temperature peak was assigned to the freeze of surface spins clusters. Static and dynamic magnetic results suggest that the correlations of surface spins clusters show a spin-glass-like behavior below T_g=7.3±0.1 K with critical exponents zν=9.7±0.5 and β=0.7±0.1, which are consistent with typical values reported for spin-glass systems.     

Phase transition in the one-dimensional pair-hopping model with unusual one-electron hopping

By modifying the conventional one-electron hopping behavior, we study effects of an occupation-dependent hopping on the ground state of the half-filled one-dimensional pair-hopping model. At weak coupling, the use of bosonization and renormalization-group analysis techniques helps to derive the phase diagram. Such unusual hopping is shown to drive a spin-gap transition and to introduce a new region where the triplet superconducting instability dominates for positively small pair-hopping interaction.     

Time-dependent Mott transition in the periodic Anderson model with nonlocal hybridization

The time-dependent Mott transition in a periodic Anderson model with off-site,nearest-neighbor hybridization is studied within the framework of nonequilibriumself-energy functional theory. Using the two-site dynamical-impurity approximation, wecompute the real-time dynamics of the optimal variational parameter and of differentobservables initiated by sudden quenches of the Hubbard-U and identify the criticalinteraction. The time-dependent transition is orbital selective, i.e., in the final state,reached in the long-time limit after the quench to the critical interaction, the Mott gapopens in the spectral function of the localized orbitals only. We discuss the dependenceof the critical interaction and of the final-state effective temperature on thehybridization strength and point out the various similarities between the nonequilibriumand the equilibrium Mott transition. It is shown that these can also be smoothly connectedto each other by increasing the duration of a U-ramp from a sudden quench to a quasi-staticprocess. The physics found for the model with off-site hybridization is compared with thedynamical Mott transition in the single-orbital Hubbard model and with the dynamicalcrossover found for the real-time dynamics of the conventional Anderson lattice withon-site hybridization.     

Lack of phase transition in the Dicke model with external fields

We show that the addition of external fields to the Dicke Hamiltonian removes the critical behaviour of the model. We analyse this result in terms of order parameter and conjugated field and compare with recent papers.     

Quantum phase transition in the two-dimensional XY model with single-ion anisotropy

In this paper we study the quantum phase transition and low temperature behavior in a square lattice quantum two-dimensional XY model with single-ion anisotropy and spin S=1. Starting with the Villain representation, a Landau-Ginzburg expression is written. The large D phase is studied using the bond operator formalism.     

Phase transition in the lattice gross-neveu model

The confining properties of the leading logarithm approximation to the effective lagrangian $\text{L}\text{= F}{}^2\text{/2g}{}^2\text{(t)}$ [ with g(t) a running coupling function of t = log(F²/μ⁴)] are seen to disappear when the second and the third approximations of the β-function power series expansion are considered.     

Mott-Hubbard transition in the mass-imbalanced Hubbard model

The mass-imbalanced Hubbard model represents a continuous evolution from the Hubbard to the Falicov-Kimball model. We employ dynamical mean field theory and study the paramagnetic metal-insulator transition, which has a very different nature for the two limiting models. Our results indicate that the metal-insulator transition rather resembles that of the Hubbard model as soon as a tiny hopping between the more localized fermions is switched on. At low temperatures we observe a first-order metal-insulator transition and a three peak structure. The width of the central peak is the same for the more and less mobile fermions when approaching the phase transition, which agrees with our expectation of a common Kondo temperature and phase transition for the two species.