Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Thermal and Quantum Phase Transitions in Atom-Field Systems: a
  Microcanonical Analysis   The thermodynamical properties of a generalized Dicke model are calculated
and related with the critical properties of its energy spectrum, namely the
quantum phase transitions (QPT) and excited state quantum phase transitions
(ESQPT). The thermal properties are calculated both in the canonical and the
microcanonical ensembles. The latter deduction allows for an explicit
description of the relation between thermal and energy spectrum properties.
While in an isolated system the subspaces with different pseudo spin are
disconnected, and the whole energy spectrum is accesible, in the thermal
ensamble the situation is radically different. The multiplicity of the lowest
energy states for each pseudo spin completely dominates the thermal behavior,
making the set of degenerate states with the smallest pseudo spin at a given
energy the only ones playing a role in the thermal properties, making the
positive energy states thermally inaccesible. Their quantum phase transitions,
from a normal to a superradiant phase, are closely associated with the thermal
transition. The other critical phenomena, the ESQPTs occurring at excited
energies, have no manifestation in the thermodynamics, although their effects
could be seen in finite sizes corrections. A new superradiant phase is found,
which only exists in the generalized model, and can be relevant in finite size
systems.

23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
24
The categories of this paper are: dg-ga
25
The categories of this paper are: patt-sol
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio