A model ferromagnet with first and second order phase transitions

Akai Kurbanovich Murtazaev, Albert Babaevich Babaev

On the basis of a histogram data analysis, phase transitions (PTs) in a three-dimensional diluted ferromagnetic 3- and 4-state Potts models are investigated. Systems with linear dimensions of L = 20–60 and spin concentrations of p = 1.00, 0.95, and 0.65 are studied. It is shown that the introduction of weak disorder ( p ∼ 0.95) into the system in the three-dimensional Potts model with q = 3 is sufficient to change a first-order phase transition to a second-order one, whereas, in the three-dimensional Potts model with q = 4, the change of a first-order phase transition to a second-order one occurs only in the presence of strong disorder ( p ∼ 0.65).

Masuo Suzuki

O. Idigoras, P. Vavassori, Andreas Berger

Shuntaro Okada, Masayuki Ohzeki, Kazuyuki Tanaka

We analytically investigated the phase diagrams of one-dimensional Ising and XY models. A second-order phase transition occurs in both models only with the transverse magnetic field. In this study, the fully connected ferromagnetic and anti-ferromagnetic XX interactions are applied in addition to the transverse magnetic field. The ferromagnetic XX interaction causes a first-order phase transition in both of the one-dimensional Ising and XY models. In contrast, by applying the anti-ferromagnetic one, the second-order phase transition can be avoided in the one-dimensional Ising model, and the critical amplitude becomes smaller in the one-dimensional XY model even though the second-order phase transition remains and the critical exponent does not change. Because the anti-ferromagnetic XX interaction works to destroy the order of the transverse magnetization, it begins to decrease at a larger strength of the transverse magnetic field. As a result, the susceptibility of the transverse magnetization with respect to the strength of the magnetic field becomes small. It is conjectured that the anti-ferromagnetic XX interaction is generally preferable to the ferromagnetic interaction to improve the performance of quantum annealing.

Ger Vertogen, W.A. Smit

José L. Alonso, J. M. Carmona, Jesús Clemente-Gallardo +4 more

We study the phase diagram of the four dimensional Ising model with first and second neighbour couplings, specially in the antiferromagnetic region, by using Mean Field and Monte Carlo methods. From the later, all the transition lines seem to be first order except that between ferromagnetic and disordered phases in a region including the first-neighbour Ising transition point. 1 Antiferromagnetism has been considered in a great variety of models in order to find properties not present in the purely ferromagnetic (FM) system. Mostly they have been models in two and three dimensions. For instance, in several works on High Temperature Superconductivity the transition from paramagnetic (PM) to non-pure FM ordered phases has been studied [1, 2, 3, 4]. On the other hand, in relation to the finite temperature phase transition in pure gauge SU(3), the d = 3 three state Potts model with negative second neighbour couplings was unsuccessfully considered to find a new critical behaviour [5, 6]. In four dimensions, in diluted systems recently new critical exponents have been