Nematic Phases and Kosterlitz-Thouless Phase Transitions in the Ising model with Long-range Interactions

BAB, MARISA ALEJANDRA; SARACCO, GUSTAVO PABLO

Buenos Aires

Congreso; StatPhys 27; 2019

IUPAP-UBA

The ferromagnetic bidimensional Ising model with dipolar interactions has been proposed to model ultrathin films with strong out-of-plane anisotropy. The phase diagram at zero external field presents a rich phenomenology that includes low-temperature phases characterized by stripes of width n (h=n) and a high-temperature phase with domains of stripes with mutually perpendicular orientations, named tetragonal liquid (TL). The latter phase can be reached by two possible ways. One of them is the direct transition from h=n to TL, and the other one is an intermediate phase with orientational order but short-range positional disorder, named nematic phase (NM). The regions of the phase diagram where these transitions occur remain as open question. Moreover, the character of these phase transitions has been the object of a long-standing controversy. In order to clarify this topic, intensive Monte Carlo simulations were performed by employing short-time dynamics method as the main tool for studying the phase transition behavior. The dynamic evolution of the orientational order parameter and its second moment were measured for selected values of the ratio between the ferromagnetic exchange and dipolar constants, called δ. The obtained results indicate that the intermediate NM phases are present for δ≥2 in narrow ranges of temperatures. The scaling of nonequilibrium ground-state relaxation curves of the order parameter is compatible with that expected for the Kosterlitz-Thouless phase transition. This allows us to estimate the corresponding transition temperature. Furthermore, the quenching from the high temperature paramagnetic phase of the second moment of the order parameter is sensible to report both phase transitions, i.e. h=n-NM and NM-TL. In fact, the power-law behavior of this observable in the range where the NM phase exists, as well as for low temperatures, suggests that both transitions belong to the Kosterlitz-Thouless transition type. In particular, in the case of h=n-NM transition both initial conditions give the same transition temperature?within the error bars--. It is interesting to mention that this type of topological transition is observed in continuous bidimensional models and have been proposed for discrete ones, as in the case of the present work.