2ND-ORDER RAMAN EFFECTS, INELASTIC NEUTRON-SCATTERING AND LATTICE-DYNAMICS IN 2H-WS2

The resonance Raman spectra of 2H-WS2 single crystals are investigated in detail in the region of low-energy indirect transitions including two direct transitions characterized by A and B exciton peaks. The spectra reveal the activities of first-order resonance Raman signals as well as of second-order Raman (SOR) processes due to coupling phonons with a non-zero momentum. In addition, the longitudinal acoustic (LA) dispersion curves in the (00-zeta), (zeta-00) and (zeta-zeta-0) directions were determined using coherent inelastic neutron scattering (INS) techniques: It is concluded that the more intense SOR band at 352 cm-1 is due to the 2 x LA (K) overtone, while other SOR peaks can be assigned t to combinations of sum or difference bands involving phonons at the K point coupled to the LA (K) mode. Consequently, we have made use of all these data to fix some unknown parameters in complete valence force field lattice dynamics calculations which reproduce satisfactorily the essential features of the Raman and INS results. Also, values of some elastic constants (C11, C33, C44) are estimated and compared with literature data. In agreement with recent band-structure calculations, it is thus definitively concluded that the direct gap nature of the A and B excitons in 2H-WS2 is responsible for the enhanced intensity of first-order GAMMA-point phonons (A1g), whereas the K-point phonons, which contribute to the indirect gap absorption edge, are enhanced in the two-phonon resonance Raman spectra: Distinct and simultaneous first- and second-order resonance Raman scattering mechanisms are thus taking place in this material.