Excitonic spectra and band structure of CdGa2Se4 birefractive crystals
We report on the intersection of spectral dependences of refractive indices n o and n e at the wavelengths 546 nm ( λ 0 ) and 450 nm ( λ 01 ) in CdGa 2 Se 4 single crystals. The value of difference Δ n = n e −n o is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa 2 Se 4 exhibit a transmission band at the wavelength of λ 0 =546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa 2 Se 4 crystals, and other parameters of excitons and bands were determined. In the Г point of Brillouin zone the effective mass of electrons m c is equal to 0.14 m 0 , and the effective masses of holes m v 2 and m v 3 are equal to 0.76 m 0 and 0.94 m 0 , respectively. The hole mass m v 1 depends upon the direction of wave vector k : at polarization E ∥ c , k ∥ a the mass m v 1 =1.15 m 0 , and at polarization E ∥ c , k ∥ b m v 1 =0.84 m 0 . The values of valence bands splitting in the center of Brillouin zone by the crystal field ( Δ cf =49 meV) and spin–orbital interaction ( Δ so =351 meV) were determined. The optical functions n , k , ε 1 and ε 2 in polarizations E ⊥ c and E ∥ c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by Kramers–Kronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa 2 Se 4 crystals.