  The term "hidden order" refers to an as yet unidentified form of
broken-symmetry order parameter that is presumed to exist in the strongly
correlated electron system URu2Si2 on the basis of the reported similarity of
the heat capacity at its phase transition at To~17 K to that produced by
Bardeen-Cooper-Schrieffer (BCS) mean field theory. Here we show that the phase
boundary in URu2Si2 has the elliptical form expected for an entropy-driven
phase transition, as has been shown to accompany a change in valence. We show
one characteristic feature of such a transition is that the ratio of the
critical magnetic field to the critical temperature is defined solely in terms
of the effective quasiparticle g-factor, which we find to be in quantitative
agreement with prior g-factor measurements. We further find the anomaly in the
heat capacity at To to be significantly sharper than a BCS phase transition,
and, once quasiparticle excitations across the hybridization gap are taken into
consideration, loses its resemblance to a second order phase transition. Our
findings imply that a change in valence dominates the thermodynamics of the
phase boundary in URu2Si2, and eclipses any significant contribution to the
thermodynamics from a hidden order parameter.
