1. Pressure-Raman study of resonant TO(  )-two-phonon decay processes in ZnS: Comparison of three isotope compositions R.E. Tallman a, J. Serrano b, A. Cantarero c, N. Garro c, R. Lauck b, T. M. Ritter d, B. A. Weinstein a, and M. Cardona b a SUNY at Buffalo, Department of Physics, Buffalo, USA; b Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany; c Dept. of Physics and Materials Science, Univ. of Valencia, Spain; d Department of Chemistry and Physics, UNC Pembroke, USA Relevant Citations 1. B.A. Weinstein, Solid State Commun., 20, 999,1976 2. S.Ves, I. Loa, K. Syassen, F. Widulle, and M. Cardona, phys. stat. sol. (b) 223, 241 (2001) 3. C. Ulrich, A. Göbel, K. Syassen, and M. Cardona, Phys. Rev. Lett. 82, 351 (1999) 4. J. Serrano, A.H. Romero, F.J. Manjon, R. Lauck, M. Cardona, and A. Rubio, Phys. Rev. B69, 094306 (2004) 5. J. Serrano, A. Cantarero, M. Cardona, N. Garro, R. Lauck, R.E. Tallman, T.M. Ritter, and B.A. Weinstein, Phys. Rev. B69, 014301 (2004) 6. B.D. Rajput and D.A. Browne, Phys.Rev. B 53, 9052 (1996) 7. N. Vagelatos, D. Wehe, and J.S. King, J. Chem. Phys. 60, 3613 (1974) Conclusions  The TO(  ) Raman line-shapes exhibit qualitatively similar changes under applied pressure in 68 Zn 32 S, 64 Zn 34 S and natural ZnS.  The changes occur at different pressures depending on the isotopic composition.  These effects are due to third order resonant anharmonic interactions of the type: TO(  ) TA+LA (near W).  Line-shape calculations using perturbation theory and a BCM DOS corrected to fit INS data agree qualitatively with the Raman data.  Mass scaling explains the isotopic dependence of P th (i.e., pressure for emergence of “bare” TO(  ) peak), and yields the estimate  LA(W) ≈ 1.2. Acknowledgements Work at UB partially supported by CAPEM research center. T.M. Ritter acknowledges support from a U. North Carolina-Pembroke travel grant. Summary: Raman data & line-shape calculations for 68 Zn 32 S, 64 Zn 34 S & nat. ZnS.  LO(  ) peaks have sharp Lorentzion line-shapes  TO(  ) peaks exhibit unconventional behavior  They are broad and weak at 1 atm  Distinct features shift through the TO(  ) region with increasing pressure  A sharp Lorentzian TO(  ) peak emerges at: P Th ~ 7.3 GPa ± 0.5 in 68 Zn 32 S, 8.5 GPa ± 0.5 in nat. ZnS, 11.5 GPa ± 0.5 in 64 Zn 34 S  Calculations for the three isotope compositions predict emergence of the sharp TO(  ) phonon peak at different P Th values and reproduce other line- shape changes in approx. accord with experiment. Inset  Observed effect of pressure (points) on TO(  ) & LO(  ) line-widths compared to  + 2 [  TO(  ) (P)] (solid curve).  Pressure variations in the TO(  ) FWHM correspond to singularities in the 2-phonon DOS as predicted in Eqs 1& 2  Lower arrows mark significant Van Hove singularities.  Upper arrows indicate the pressure-dependent position of the TO(  ) phonon relative to these singularities.  At 12.5 GPa the TO(  ) frequency crosses (TA 2 + LA) W and moves into the DOS gap. This defines P Th. Inset plots P Th vs. (  TA+LA –  TO(  ) ) for the 3 ZnS isotope compositions  Variation of P Th is due to mass scaling of frequencies  for TO(  ),   for (TA 2 and LA) W summation modes,   Isotope dependence of P Th implies  LA(W)  1.2 FWHM (cm -1 ) Pressure (GPa) 8 6 4 2 0 5.0 10.0 0.0 TO LO 15.0 Measured Effect of Pressure on 68 Zn 32 S TO(  ) Raman Spectrum at T=16K