  We consider a classical XY-like Hamiltonian on a body-centered tetragonal
lattice, focusing on the role of interlayer frustration. A three-dimensional
(3D) ordered phase is realized via thermal fluctuations, breaking the
mirror-image reflection symmetry in addition to the XY symmetry. A heuristic
field-theoretical model of the transition has a decoupled fixed point in the 3D
XY universality, and our Monte Carlo simulation suggests that there is such a
temperature region where long-wavelength fluctuations can be described by this
fixed point. However, it is shown using scaling arguments that the decoupled
fixed point is unstable against a fluctuation-induced biquadratic interaction,
indicating that a crossover to nontrivial critical phenomena with different
exponents appears as one approaches the critical point beyond the transient
temperature region. This new scenario clearly contradicts the previous notion
of the 3D XY universality.
