arXiv subjects

M. I. Marques

Publications and source records attributed to M. I. Marques.

6 recordsLinked to original sources

arXiv2000

Dynamic Scaling in Diluted Systems Phase Transitions: Deactivation trough Thermal Dilution

Activated scaling is confirmed to hold in transverse field induced phase transitions of randomly diluted Ising systems. Quantum Monte Carlo calculations have been made not just at the percolation threshold but well bellow and above it including the Griffiths-McCoy phase. A novel deactivation phenomena in the Griffiths-McCoy phase is observed using a thermal (in contrast to random) dilution of the system.

cond-mat.stat-mech↗

arXiv2000

Evolution of the universality class in slightly diluted (1>p>0.8) Ising systems

The crossover of a pure (undiluted) Ising system (spin per site probability p=1) to a diluted Ising system (spin per site probability p<0.8) is studied by means of Monte Carlo calculations with p ranging between 1 and 0.8 at intervals of 0.025. The evolution of the self averaging is analyzed by direct determination of the normalized square widths for magnetization and susceptibility as a function of p. We find a monotonous and smooth evolution from the pure to the randomly diluted universality class. The p-dependent transition is found to be independent of the size (L). This property is very convenient for extrapolation towards the randomly diluted universality class avoiding complications resulting from finite size effects.

cond-mat.stat-mech↗

arXiv2000

Revisiting "swings" in the crossover features of Ising thin films near Tc(D)

"Swing" effects at the onset of crossover towards two dimensional behavior in thin Ising films are investigated close to Tc(D) by means of Monte Carlo calculations. We find that the effect is extremely large for the specific heat effective critical exponent, in comparison with the "swing" already noted by Capehart and Fisher for the susceptibility. These effects change considerably the system's evolution with thickness (D) from two-dimensional to three-dimensional behavior, forcing the effective exponents to pass near characteristic Tri Critical Point (TCP) values.

cond-mat.stat-mech↗

arXiv2000

Universality Class of Thermally Diluted Ising Systems at Criticality

The universality class of thermally diluted Ising systems, in which the realization of the disposition of magnetic atoms and vacancies is taken from the local distribution of spins in the pure original Ising model at criticality, is investigated by finite size scaling techniques using the Monte Carlo method. We find that the critical temperature, the critical exponents and therefore the universality class of these thermally diluted Ising systems depart markedly from the ones of short range correlated disordered systems. Our results agree fairly well with theoretical predictions previously made by Weinrib and Halperin for systems with long range correlated disorder.

cond-mat.stat-mech↗

arXiv2000

Possible evidence for a change in cosmic equation of state at decoupling

Indirect but significative evidence (from fusion research data, and from the known 4He abundance) relevant to primordial nucleosynthesis is examined. It is shown that this evidence supports a change in cosmic equation of state at decoupling, compatible with Einstein's cosmological equations, from ROm T^4 aprox. cte (ROm=ROr) prior to decoupling (t taf), being taf the atom formation time. Implications at the baryon threshold time (temperature) and beyond are discussed.

astro-ph↗

arXiv1999

Self-averaging of random and thermally disordered diluted Ising systems

Self-averaging of singular thermodynamic quantities at criticality for randomly and thermally diluted three dimensional Ising systems has been studied by the Monte Carlo approach. Substantially improved self-averaging is obtained for critically clustered (critically thermally diluted) vacancy distributions in comparison with the observed self-averaging for purely random diluted distributions. Critically thermal dilution, leading to maximum relative self-averaging, corresponds to the case when the characteristic vacancy ordering temperature is made equal to the magnetic critical temperature for the pure 3D Ising systems. For the case of a high ordering temperature, the self-averaging obtained is comparable to that in a randomly diluted system.

cond-mat.stat-mech↗