Journal of Low Temperature Physics, VoL 105, Nos. 3/4, 1996 Electronic Raman Scattering in CuO2 Superconductors R. Hackl, M. Opel, P.F. Miiller, G. Krug, B. Stadlober, R. Nemetschek, H. Berger*, and L. Forr6* Walther Meissner Institut, Bayerische Akademie der Wissenschaflen, D-85748 Garching *Ecole Polytechnique Fdd~rale de Lausanne, Ecublens, CH-1015 Lausanne Experimental results for the electronic Raman effect in differently doped cuprate super- conductors will be presented. We show that the B2g-.symmetry data are generally closely related to ordinary transport and are therefore most likely originating from the carriers, while the response at Big symmetry cannot be assigned to a specific type of excitations. In the superconducting state the B2g pair breaking peaks scale with the transition tem- perature over a wide doping range. All results consistently suggest a strong anisotropy of the gap and can be modeled by assuming dx2.y2 symmetry for the order parameter. PACS numbers: 78.30 Er, 74.72 Hs, 74.25 Gz, 1. INTRODUCTION The physical properties of the copper-oxide materials depend in a unique way on the doping level of the CuOz planes. Great theoretical and experimental effort is therefore focused on the study of the copper-oxygen layers as the only structural element in common to all compounds of this material class. In the superconducting state in-plane anisotropies have attracted a lot of interest since most likely unconventional Cooper pairing was found to occur very likely in the cuprates. 1-4 In this context, inelastic scattering of light turned out to be a useful technique which, in contrast to other methods, provides consistent data about most of the popular cuprate families.5'6 On the other hand, the knowledg e about in-plane anisotropies in the normal state and, in particular, its doping dependence is limited and, after all, there is no consensus about the interpretation of the data. Recently, in overdoped T12Ba2CuO6+a measurements of the c-axis transport in a magnetic field aligned parallel to the planes showed a strong variation of the resistivity at low temperatures with ~/2 periodicity when the sample was rotated about the c-axis 7. Earlier, the Hall conductivity was studied. The results were interpreted either in a band picture 8'9 or by assuming spin-charge separation. 1~ As the intensity is lost completely at (re,0) the photo-emission data are consistent with both the LDA band structure or Fermi surface pockets around (Tz/2,Tr/2) which may originate from strong correlation effects, n Once again, one can expect contributions from Raman-scattering experiments and the theory of light scattering from carriers provides a relation between the photon polari- zations eI, es and k space. 5'~2'13Indeed, anisotropies in the normal state have been found experimentally for optimally doped Bi2SrzCaCuzOs (Bi2212) and YBa2Cu307 (Y123) a 733 0022-2291/96/1100-0733509.50/0 9 1996 Plenum Publishing Corporation