Papers by Keyword: Critical Phenomena

Abstract: The phase transitions and critical phenomena in two-dimensional 3-state antiferromagnetic Potts model with account of next-nearest neighbors are investigated by Monte-Carlo method. The systems with linear sizes L=20-144 are explored. Following parities of exchange interactions are considered. Moreover, we analyze the character of phase transitions and determine the critical temperatures.

Abstract: We study the phase transitions and critical phenomena in 3D site-diluted (with nonmagnetic impurities) Potts model with spin states q=4 by Monte-Carlo method. The systems with linear sizes L=20-32 and spin concentrations p=1.00, 0.90, 0.65 are examined. Using the Binder cumulants method the forth order it is shown that the second-order phase transition is observed in strongly diluted model at spin concentration p=0.65; the pure model (p=1.00) and weakly diluted one (p=0.90) reveals the first-order phase transition. On the basis of finite-size scaling theory the static critical parameters of heat capacity, susceptibility, magnetization, and correlation length exponent are calculated.

Abstract: We have investigated stress and temperature dependences of the structure of the X-phase in Ni2MnGa to understand structural relation between the X-phase and other phases. Position and intensity of satellites of the X-phase are different from those of the intermediate (I-) phase under compressive stress, but they approach those of the I-phase with decreasing stress. That is, the structure change associated with the I → X transformation is discontinuous under a compressive stress, while it is continuous under zero stress. In addition, the transformation from the X-phase to the L21-type parent phase is continuous regardless of applied stress. These results strongly suggest the existence of multi-critical point in Ni2MnGa. On the other hand, the transformation from the X-phase to the martensite phase is discontinuous regardless of applied stress.

Abstract: The effect of quenched nonmagnetic impurities on phase transitions in the three-dimensional Potts model with the number of spin states q=3 is studied using the Wolff single-cluster algorithm of the Monte Carlo method. By the method of fourth-order Binder cumulants, it is demonstrated that the second-order phase transition occurs in the model under study at spin concentrations p=0.9, 0.8, 0.7, and 0.65, while the first-order phase transition is observed in the pure model (p=1.0). The static critical exponents (CEs) α (heat capacity), γ (susceptibility), β (magnetization), and ν (correlation length) are calculated based on the finite-size scaling theory.

Abstract: Acoustic emissions (AE) are ultrasonic waves generated by the rapid release of energy from discontinuities or cracks spreading in materials subject to a stress and strain field. By identifying the complete shape of the signals and taking into account a larger quantity of data, it becomes possible to ascertain the three-dimensional location of damage sources from AE sensor records. In this connection, the authors have fine-tuned an original procedure that uses seismic analysis techniques, such as the moment-tensor solution. The experimental program consisted of tests conducted in situ on masonry walls of historical buildings.

Abstract: The apparent shear strength of rock discontinuities is lower than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities. Numerical algorithms permit to calculate contact forces at any point, and to describe the stick-slip transition. On the other hand, the critical aspects are not captured by classical theories. Multiscale simulations show that the contact domain between rough surfaces is a lacunar set. This explains the size-dependence of the apparent friction coefficient. By applying an increasing tangential force, the regime of partial-slip comes into play. However, the continuous and smooth transition to fullsliding predicted by the Cattaneo-Mindlin theory is not occurring in real situations. We implement a numerical renormalization group technique, taking into account the redistribution of stress consequent to partial-slip. This permits the critical value of the tangential force to be found. The critical force is less than the one predicted by Coulomb’s theory, and depends on the specimen size and on the topology of the interface.