By using Bayesian spectroscopy, we studied polarization dependence of photoluminescence (PL) spectra in doubly‐split 1S yellow ortho‐excitons in a Cu2O thin‐crystal recrystallized in a small gap between paired MgO substrates. In these thin‐crystals, biaxial stresses are expected to be involved due to a small lattice mismatch between Cu2O and MgO. Under a planar‐isotropic biaxial stress, the cubic symmetry degrades to the tetragonal one in Cu2O, and subsequently, the ortho‐exciton state splits into two states having different symmetries. Consequently, the resonant PL intensities of the doubly‐split ortho‐excitons are expected to show different polarization dependencies. To elucidate such biaxial stress effect, we measured polarization dependence of the PL spectra at 4.2 K. Although resonant weak PL bands of the doubly‐split exciton states and their intense phonon sidebands co‐exist, we succeeded in decomposing to the respective spectral components by the Bayesian spectroscopy with a replica exchange Monte Carlo algorithm. As a result, it is found that the resonant PL band appearing on higher energy side shows hardly polarization dependence, whereas the resonant PL band appearing on lower energy side is further weak and shows noticeable polarization dependence. These results can be explained by the selection rule and polarization dependences on the transition matrix elements of quadrupole transitions of the doubly‐split ortho‐exciton states, and it clearly shows that the crystal symmetry degrades to D4h by the isotropic biaxial stress involved in the Cu2O thin‐crystals.

Using a hot‐wall technique, BiI3 thin films are deposited on α‐Al2O3 substrates at different evaporation rates and substrate temperatures in order to optimize these conditions. From X‐ray diffraction (XRD) data, it is confirmed that BiI3 layers are regularly stacked. Absorption due to direct excitons in BiI3 is observed in the deposited thin films. To study the translational symmetry and homogeneity of the films, changes in direct exciton transitions are examined. Direct excitons can be split into inner XInner and interface XInter excitons due to a collapse of translational symmetry along the stacking direction for a finite thickness, and the mean value truetrueEˆ and the energy difference ΔE for the transition energies for these excitons are obtained. To evaluate the sample quality, the dependence of truetrueEˆ and ΔE on the deposition conditions is investigated based on a tight‐binding model for flake‐like crystals consisting of a finite number n of BiI3 layers. In this model, a site shift energy δ is introduced for the interfaced BiI3 layers, which represents the difference of the on‐site energy for the excitons. From the magnitude of ΔE, the BiI3 thin films are considered to consist of packed flake‐like crystals, whose translational symmetry will be maintained within four or five (n = 4 or 5) BiI3 layers. From the variations of δ, it is found that an evaporation rate of about 0.8 Å s−1 and a substrate temperature of 75 °C are the best conditions for BiI3 thin film deposition on α‐Al2O3 substrates.