11 research outputs found
On a microcanonical relation between continuous and discrete spin models
A relation between a class of stationary points of the energy landscape of
continuous spin models on a lattice and the configurations of a Ising model
defined on the same lattice suggests an approximate expression for the
microcanonical density of states. Based on this approximation we conjecture
that if a O(n) model with ferromagnetic interactions on a lattice has a phase
transition, its critical energy density is equal to that of the n = 1 case,
i.e., a system of Ising spins with the same interactions. The conjecture holds
true in the case of long-range interactions. For nearest-neighbor interactions,
numerical results are consistent with the conjecture for n=2 and n=3 in three
dimensions. For n=2 in two dimensions (XY model) the conjecture yields a
prediction for the critical energy of the Berezinskij-Kosterlitz-Thouless
transition, which would be equal to that of the two-dimensional Ising model. We
discuss available numerical data in this respect.Comment: 5 pages, no figure
Kinetic energy and microcanonical nonanalyticities in finite and infinite systems
In contrast to the canonical case, microcanonical thermodynamic functions can
show nonanalyticities also for finite systems. In this paper we contribute to
the understanding of these nonanalyticities by working out the relation between
nonanalyticities of the microcanonical entropy and its configurational
counterpart. If the configurational microcanonical entropy has
a nonanalyticity at , then the microcanonical entropy
has a nonanalyticity at the same value of
its argument for any finite value of the number of degrees of freedom . The
presence of the kinetic energy weakens the nonanalyticities such that, if the
configurational entropy is times differentiable, the entropy is -times differentiable. In the thermodynamic limit, however, the
behaviour is very different: The nonanalyticities do not longer occur at the
same values of the arguments, but the nonanalyticity of the microcanonical
entropy is shifted to a larger energy. These results give a general explanation
of the peculiar behaviour previously observed for the mean-field spherical
model. With the hypercubic model we provide a further example illustrating our
results.Comment: 14 pages, 2 figures; v2: minor corrections, final versio