  Lead and tin-based chalcogenide semiconductors like PbTe or SnSe have long
been known to exhibit an unusually low thermal conductivity that makes them
very attractive thermoelectric materials. An apparently unrelated fact is that
the excitonic bandgap in these materials increases with temperature, whereas
for most semiconductors one observes the opposite trend. These two anomalous
features are also seen in a very different class of photovoltaic materials,
namely the halide-perovskites such as CsPbBr3. It has been previously proposed
that emphanisis, a local symmetry-breaking phenomenon, is the one common origin
of these unusual features. Discovered a decade ago, emphanisis is the name
given to the observed displacement of the lead or the tin ions from their cubic
symmetry ground state to a locally distorted phase at high temperature. This
phenomenon has been puzzling because it is unusual for the high-temperature
state to be of a lower symmetry than the degenerate ground state. Motivated by
the celebrated vibration-inversion resonance of the ammonia molecule, we
propose a quantum tunneling-based model for emphanisis where decoherence is
responsible for the local symmetry breaking with increasing temperature. From
the analytic expression of the temperature dependence of the tunnel splitting
(which serves as an order parameter), we provide three-parameter fitting
formulae which capture the observed temperature dependence of the ionic
displacements as well as the anomalous increase of the excitonic bandgap in all
the relevant materials.
