No abstract

Abstract. Phase transitions of first and second order can easily be distinguished in small systems in the microcanonical ensemble. Configurations of phase coexistence, which are suppressed in the conventional canonical formulation, carry important information about the main characteristics of first order phase transitions like the transition temperature, the latent heat, and the interphase surface tension. The characteristic backbending of the microcanonical caloric equation of state T ( E ) (not to be confused with the well known Van der Waals loops in ordinary thermodynamics or mean field approximations) leading to a negative specific heat is intimately linked to the interphase surface entropy.Keywords: Microcanonical thermodynamics; Phase transitions; Surface tension.Microcanonical thermodynamics describes the dependence of the volume RN of the N-body phase-space of an interacting many-body system on the globally conserved quantities like the total energy, momentum, angular momentum, mass, charge etc. This is the fundamental starting point of any statistical definition of thermodynamics. By Laplace transform of &(E) from the extensive quantities like the energy to intensive ones like inverse temperature Cp = l / T ) one obtains the more familiar Gibb's canonical partition function Z @ ) . For systems interacting by short range two-body forces (with hard cores) both formulations are identical in the thermodynamic limit of infinitely many particles at the same particle density [l]. However, the two formulations are different for finite systems and more essentially are different even in their physical content for systems with long range forces like gravity [2] or Coulomb dominated systems see for example [3]. This will be discussed in a forthcoming paper.One of the most interesting phenomena in thermodynamics are phase transitions. Computer simulations of simple models give a deep insight into the mechanism. Naturally these calculations can only be performed for small systems. The main characteristics like the transition temperature TI,, the specific latent heat qIar, and the interphase surface tension aSwrf are extrapolated to the infinite system. In the thermodynamic limit it should not matter whether the calculations for the finite systems are performed canonically or microcanonically. However, we will show for the two-dimensional, q = 10 states, Potts model, which has a clear phase transition of first order from ordered to disordered spins, the extrapolations converge faster when started from the microcanonical-ensemble. Moreover, our investigation will clarify

In investigations of the orientation of the magnetization of thin ferromagnetic films, the single-ion anisotropy coefficients play a crucial role. By applying a thermodynamic perturbation theory we calculate the temperature dependence of the second- and fourth-order single-ion anisotropies for a Heisenberg monolayer with Tyablikov decoupling (the random-phase approximation, RPA) and compare with results obtained from mean-field theory. In order to assess the accuracy of the Tyablikov (RPA) and also the Callen decoupling approximations in the Green's function many-body theory, we calculate the magnetization of a Heisenberg spin pair and of a ferromagnetic monolayer, and compare the results with exact solutions available for the spin pair with arbitrary spins, and a recent quantum Monte Carlo calculation (for spin 1/2) for the monolayer. The RPA decoupling provides a fairly good approximation to the exact results for the magnetization over the whole temperature range of interest. Because of this, we expect the calculated anisotropy coefficients to be approximated well by this method.

We discuss far-from-equilibrium electron transport in quantum waveguide structures at low temperatures.On slowly cooling the devices in the dark, the current-voltage characteristics are found to be similar to those of a quantum point contact. Exposure to light at low temperature alters the characteristics dramatically, with one or more regions of current-controlled negative differential conductance occurring. The characteristics can be returned to their prelight condition by annealing the samples above 120 K, which indicates that the effect is associated with the occupancy of DX centers in the Al Gai As. We argue that the negative differential conductance arises &om hotelectron bistabilities due to puddles of charge trapped in the waveguide by potential inhomogeneities associated with ionized DX centers.

No abstract