Lattice Mode Study of La 2 CuO 4-y Implanted with Sr

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LATTICE MODE STUDY OF La 2 CuO4 _y IMPLANTED WITH Sr I. OHANAa G.L. DOLL," A. LUSNIKOV,a,'b S.P. WITHROW,c P.J. PICONE,d M.S. DRESSELHAUS,- G. DRESSELHAUSe H.P. JENSSENd and D.R. GABBEd 'Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 bNow at Department of Physics, U. of Nebraska at Omaha, Omaha, NB 68182-0266 "cSolid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 dCenter for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 eNational Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Dramatic implantation-induced changes in the Raman and infrared spectra of polycrystalline samples of La 2 CuO 4_y are reported for implantation of Sr in the fluence range 0.5 to 1.5 x 1016/ cm 2 . The Raman and ir-reflectance spectra of the Sr implanted material reveal disorder-induced lines corresponding to breakdown of selection rules and to non-zone-center phonon modes. Whereas the main features in the Raman spectra of the implanted sample are broad disorder-induced lines centered at - 560 - 5 cm- 1 , reststrahlen lineshapes are observed above - 800 cm- 1 in the ir-reflectance spectra. After thermal annealing (550°C), the multiphonon lines in the ir spectra diminish, and the disorder-induced lines in the Raman spectra split to form well-resolved, sharper lines at 530 and 580 cm- 1, identified with maxima in the phonon density of states. The sensitivity and complementarity of Raman and infrared spectroscopy to implantation-induced disorder and subsequent partial recovery by thermal annealing suggest that these techniques can be used to characterize sensitively disorder in ion implanted La 2 CuO4 _y high T, materials. INTRODUCTION Ion implantation has been shown to provide a controlled means for suppressing superconductivity in very thin layers or in patterned arrays on the surface of YBa 2Cu 3O7 [1J. Implantation therefore promises to be useful for applications of high T, superconductors. In the present work we show that ion implantation also provides valuable scientific information about the identification of the disorder-induced lines in the Raman spectra and those associated with the maximum in the phonon density of states. The main focus of this paper is the demonstration of the sensitivity and the complementarity of Raman and infrared spectroscopy in characterizing changes in the high T, superconductor La 2 CuO4 _y induced by ion implantation and subsequent annealing. Raman scattering is particularly sensitive for a study of these effects since the penetration depth of the laser radiation is often comparable to the implantation depth. Furthermore, implantation-induced damage causes a breakdown in the selection rules for the zone-center phonons and the consequent appearance of disorder-induced lines in the Raman and ir spectra. In addition, heavy lattice damage gives rise to contributions to the Raman spectra from phonons throughout the Brillouin zone, emphasizing the maximum in the phonon densit