The luminescence spectra of the mixed-mode exciton-polariton
in CuGaS2 are studied at 8.6 K in conjunction with the
absorption spectra.
By measuring emitted photons as a function of propagation direction with
respect to the z axis of the crystal, it is possible to vary the magnitude
of a longitudinal-exciton and extraordinary-photon coupling.
The effects of this coupling on the resonance energy and oscillator strength
of the extraordinary exciton-polariton are established for the emission line
of the Γ4 free exciton which exhibits a doublet structure.
In contrast, the emission spectra due to the ordinary photons show one
weak, sharp line in the same intrinsic exciton region as
the Γ4 line.
The polarization dependence of the emission intensity is greatly reduced
for this sharp line. The behavior is interpreted as arising due to
the optically forbidden exciton having Γ3 symmetry.
The resonant exciton transitions to the lower exciton-polariton branch
and the forbidden state are observed in the extraordinary and
ordinary absorption spectra, respectively.
CuInS2 bulk polycrystals have been prepared from stoichiometric mixtures of the elements in rapid time scales using a domestic microwave oven. The growth procedure involves a direct synthetic reaction and a succeeding melt solidification with the assistance of a CuO microwave heater. This leads to the successful preparation of homogeneous and densified CuInS2 crystals having n-type conductivity. The photoluminescence spectra of these crystals at 9 K exhibit sharp emission lines corresponding to free-exciton, bound-exciton and donor-to-valence-band emissions.
We study the level spacing distribution p(s) in the spectrum of random networks. According to our numerical results, the shape of p(s) in the Erdős-Rényi (E-R) random graph is determined by the average degree k and p(s) undergoes a dramatic change when k is varied around the critical point of the percolation transition, k = 1. When k 1, the p(s) is described by the statistics of the Gaussian orthogonal ensemble (GOE), one of the major statistical ensembles in Random Matrix Theory, whereas at k = 1 it follows the Poisson level spacing distribution. Closely above the critical point, p(s) can be described in terms of an intermediate distribution between Poisson and the GOE, the Brodydistribution. Furthermore, below the critical point p(s) can be given with the help of the regularized Gamma-function. Motivated by these results, we analyse the behaviour of p(s) in real networks such as the internet, a word association network and a protein-protein interaction network as well. When the giant component of these networks is destroyed in a node deletion process simulating the networks subjected to intentional attack, their level spacing distribution undergoes a similar transition to that of the E-R graph.
The photoluminescence spectra of CuGaSe2 are measured on melt-solidified crystals, iodine vapour transported as-grown crystals and annealed crystals in various conditions. From the change in the spectra among them and the influence of annealing on luminescence, it is concluded that the iodine impurity acts as a deep donor which has been unintentionally incorporated into crystals during growth, and that the selenium vacancy acts as a shallow donor. The latter defect can be filled back by annealing in selenium vapour at 600°C. All photoluminescence bands observed in the iodine vapour transported crystals are explained in trems of these two donors (0.38 eV and 80 meV) and one acceptor (40 meV), which is probably due to the copper vacancy.
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