Fine structure of the reflectivity maximum of CdCr2Se4 single crystals

Abstract

on the occasion of his 75th birthday 1)Wavelength modulation as a derivative method is very sensitive to a weak structure in the spectrum. Wavelength-modulated (WM) specular reflectivity of CdCr2Se4 single crystals has been measured in the spectral region E = 1 . 3 to 2.7 eV. A detailed investigation of this absorption region is important owing to determination of the fundamental electronic transitions. The modulation of the wavelength of the incident light has been accomplished by vibrating a quartz plate placed in front of the exit slit of a C a r l Zeiss SPM 1 monochromator. As light detector, either an EMI 9558 QB photomultiplier o r an EG-d G HW-4000B silicon photovoltaic photodiode have been used in the appropriate spectral region. Measuring with a photomultiplier, the dc component of the anode current has been kept constant by means of a simple stabilizer and by dynode chain voltage regulation. The ac signal detected by a'lock-in amplifier was then proportional to the relative change of reflectivity A R / R and it was directly registered by H P 9603A measurement and control system. The true relative reflectivity change AR/R of the sample has been calculated from the difference between the modulated signal reflected from the sample and that reflected from the aluminium reference m i r r o r . The samples were ground and poliSed from single crystals grown from the melt /l/. Typical results of the WM reflection meas-I I I I I Fig. 1. Wavelength-modulated reflectivity 11 I( 16 16 20 21 24 26 spectra of CdCrZSeq single c r y s t a l s a t 293 K I L (evI -(broken line) and 78K (full line), respectivelyly 1) K e Karlovu 5, 12116 Prague 2, CSSR.