In recent years, the theory of quantum phase transitions has rapidly
developed. These are transitions at a zero temperature which are associated
with a change of the theory parameters like couplings. In contrast, the
classical phase transitions occur ``within'' the same theory (in particular,
with the same couplings) and are associated with a change in temperature.
Within the framework of a simple model of Dirac fields in the Euclidean Rindler
space, we establish an intermediate case when the phase transition occurs at a
finite temperature, but the temperature itself is of a quantum origin (the
Unruh temperature). Moreover, the phase transition point is uniquely associated
with the behavior of individual levels, namely at the Unruh temperature the two
lowest Matsubara modes become singular on the Rindler horizon. This provides a
new manifestation of the duality between the thermodynamic description and the
geometric approach (the behavior of the quantum levels of particles living on a
nontrivial geometric manifold). Although the considered example refers to the
physics of black holes, we note the formal similarity of the Unruh temperature
with the parameter characterizing quantum transitions in the theory of
condensed matter.Comment: 11 pages, 2 figure