We have investigated the effects of isotopic composition on the band gap of CuCl on a series of samples made out of the stable isotopes 63 Cu, 65 Cu, 35 Cl, and 37 Cl. Besides specimens containing elements with the natural abundances, we have measured samples with monoisotopic sublattices as well as artificial mixtures of isotopes. With nonlinear ͑two-photon absorption, second-harmonic generation͒ and linear ͑luminescence͒ optical spectroscopy we find that the fundamental gap of CuCl increases by 364͑18͒ eV/amu when increasing the Cl mass. However, it decreases by 76͑5͒ eV/amu when increasing the Cu mass. Using a two-oscillator model for the lattice dynamics of CuCl we show that these rates are consistent with the anomalous increase of the band gap with increasing temperature. These effects can be traced back to the strong p-d mixing in the copper halides. From the temperature dependence of the band gap of CuBr we also estimate the changes of its gap with isotopic composition.
The dependence of the E 0 direct gap of Ge, GaAs, and ZnSe on isotopic masses at low temperatures has been investigated. Contributions of the variation of the lattice parameter to the gap shift of the binary compounds have been evaluated by using a volume-dependent lattice dynamics, while local empirical pseudopotential techniques have been employed to calculate gap shifts due to electron-phonon interaction. The dependence of these terms on the lattice-dynamical model and on the q→0 extrapolation of the pseudopotential form factors has been investigated. The contributions of the optical and acoustical modes to the isotopic shift are analyzed. The results are compared to previous experimental data, in the case of germanium, and to lowtemperature reflectance measurements performed as part of this work on GaAs samples with different isotopic gallium composition. Particular attention has been paid to the differences in the effects of changing either the cation or the anion masses. The temperature dependence of the E 0 gap of ZnSe has also been calculated, and reasonable agreement with experiment has been found. ͓S0163-1829͑96͒02032-2͔
Raman spectra of CuCl were measured at T 5 K under hydrostatic pressures up to 3.3 GPa. The anomalous line shape of the transverse-optic (TO) scattering, which consists of a broad structure with several maxima, undergoes a drastic change under pressure. The anomaly disappears completely at 3 GPa. These pressure effects are well reproduced by a model calculation of the anharmonic coupling of the TO mode to acoustic two-phonon states (Fermi resonance). The results demonstrate that the ambient pressure TO anomaly does not arise from local vibrational modes of Cu atoms in off-center positions.[S0031-9007(98)08185-X] PACS numbers: 63.20.Kr, 62.50. + p, Copper chloride is a highly ionic I-VII semiconductor crystallizing in the zinc-blende (ZB) structure. It exhibits several unusual properties, e.g., a large negative thermal expansion at low temperatures, a decrease of the elastic shear constants with increasing pressure, large mean square displacements of Cu atoms even at low temperature, and a high value of the ionic conductivity at high temperatures [1]. Another highly unusual feature, which has attracted recent interest [2][3][4][5], is the anomalous firstorder Raman spectrum [5][6][7][8]. Instead of narrow Raman lines corresponding to the characteristic transverse optical (TO) and longitudinal optical (LO) modes of ZB-type crystals, CuCl shows, in the region of the TO mode frequency, a broad structure consisting of three maxima with TO-like polarization characteristics [6].Two different explanations have been proposed for the anomalous TO structure. The Fermi resonance model (FRM) assumes an anharmonic coupling of the TO phonon to resonant acoustic two-phonon states [6,9]. The anharmonic interactions result in a shift and broadening of the TO mode and a transfer of oscillator strength to the two-phonon scattering. In the off-center model (OCM) a sizable fraction of the Cu ions is assumed to occupy off-center positions in the [111] antibonding directions [10,11], resulting in a contribution of local vibrations to the Raman response. Recent ab initio calculations by Wei et al. [2] predict metastable off-center minima along [111] and therefore support this model. According to the first-principles calculations of Park and Chadi [3] various off-center defects should even be stable, including Cu 4 groups formed by displacing four neighboring Cu ions towards each other. On the other hand, the effects of isotope substitution were recently found to be consistent with the FRM [5]. Thus, the question of FRM versus OCM interpretation of the Raman anomaly in CuCl has remained a matter of ongoing debate [4].Raman spectroscopy at high hydrostatic pressures and sufficiently low temperature is a means to discriminate between the two different explanations for the Raman anomaly. Off-center positions are predicted to become energetically more favorable under pressure [3], a fact which should lead to an intensity gain of local vibrational modes. Within the FRM, a major change is expected in the TO-like Raman response because of a detu...
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