  We report experimental observations of the long-lived rhodium Mossbauer
emissions by the time- and energy-resolved spectroscopy. The extraordinary
observations manifest the open-up of photonic band gap in analogy to the
superconducting gap of remarkable symmetry breakings at transition point. These
observations are of potential importance for detecting gravitational waves and
development of the two-photon gamma laser. Firstly, phase transitions shown by
spectral evolution of characteristic emissions reveal the different aggregate
exciton orderings at room temperature. Six different phases are identified by
spectra profiles emitted from the color centers. Secondly, the cascade
branching of the multipolar nuclear transition is discovered being the
spontaneous cascade down-conversion to generate entangled gammas. The
macroscopic angular distribution of entangled gammas from the polycrystalline
sample manifests a global photon-nucleus-photon bound state across the grain
boundaries. Thirdly, the gamma-energy distributions depending on exciton phases
reveal the photonic band gap typically on the order of several hundred eV.
