We describe the results of electronic Raman-scattering experiments in differently doped single crystals of YBa 2 Cu 3 O 6ϩx and Bi 2 Sr 2 (Ca x Y 1Ϫx )Cu 2 O 8 . The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates ⌫()ϭ1/() and mass-enhancement factors 1ϩ()ϭm*()/m of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In B 2g (xy) symmetry selecting predominantly electrons with momenta along the diagonals of the CuO 2 planes the Raman data compare well with the results obtained from dc and dynamical transport. In B 1g (x 2 Ϫy 2 ) symmetry projecting out momenta along the Cu-O bonds the dc scattering rates of underdoped materials become temperature independent and considerably larger than in B 2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B 2g symmetry in the range between the superconducting transition at T c and a characteristic temperature T* of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in B 2g symmetry. The peak frequencies scale with T c . We do not find a direct relation between the pseudogap and the superconducting gap.

Raman spectra of YBa 2 Cu 3 O 72x and Bi 2 Sr 2 ͑Ca 0.62 Y 0.38 ͒Cu 2 O 81d with T c Х 0.65T max c in the underdoped regime of the phase diagram are studied as a function of temperature and polarization. At B 2g ͑xy͒ symmetry a reduction of spectral weight by 10% for frequencies less than 700 cm 21 or 15k B T c is found below approximately 200 K. Below T c , a superconducting gap opens up which closely resembles that observed at higher doping levels. It is compatible with d x 2 2y 2 pairing and its amplitude 2D 0 can be estimated to be 8k B T c . [S0031-9007(97)

Single-crystalline Nd I &4Ceo 16Cu04 with a superconducting transition temperature of 19.3 K has been studied above and below T, using polarized Raman scattering. Electronic, crystal-field, and vibrational excitations have been detected. In the superconducting state, the formation of an almost isotropic gap 2A is observed which varies between 4.1 and 4.9 in units of kT, . This indicates that Ndl «Ceo &6Cu04 is different from the other cuprate systems. PACS numbers: 74.25.Gz, 74.72.Hs, 78.30.Er Apart from the unexpectedly low superconducting transition temperature, Nd2, Ce, Cu04 (NCCO), along with the La2Cu04 family, seems optimally suited for systematic studies, since the whole phase diagram can be accessed easily by appropriate doping. It is therefore considered as one of the model systems for the high-T, compounds. However, in addition to the low T" there are several other properties which do not fit into the established picture. In contrast to the other cuprates, the charge carriers are probably electrons [1]. The in-plane normal-state resistivity p, b(T) varies almost quadratically [2] and not linearly with temperature. The temperature dependence of the penetration depth [3] and the tunneling conductance below T, [4] are closerto classical superconducting behavior than in any other high-T, material. In addition, the electron-phonon spectral function a2F(to) could be obtained and was found to be in fair agreement with the generalized phonon density of states measured by neutron scattering [4].From the latter results there arise basically two questions: Is the superconducting ground state significantly different from that of other cuprates and, secondly, can we get some idea of how electron-electron coupling works in this material class? In the hole-doped systems the mechanism of superconductivity is not at all clear, but the results from several independent experiments can be interpreted with an energy gap which depends strongly on the momentum k and which may even have a reduced symmetry [5]. It is a challenge to find an answer for NCCO since the measurement of gap anisotropies within the copper-oxygen plane is a nontrivial task using traditional methods. Among other problems the resolution of angle-resolved photoemission spectroscopy (ARPES) is, at the moment, of the same size as the expected gap energy and therefore hinders a measurement similar to that in Bi2Sr2CaCu20s [6].Partly due to the short coherence length, directional electron tunneling into high-T, superconductors does not reveal consistent results to date and quantum interference experiments such as for YBa2Cu307 [7] do not exist for NCCO to our knowledge.We therefore tried to get some more insight into the structure of the energy gap of this system by electronic Raman scattering, since according to recent work a k dependence of the gap is found to produce Raman spectra which vary significantly and characteristically with polarization [8].We also look briefly at other excitations which couple to light, particularly to detect possible anomalies at...

Single crystals of Bi2Sr2CaCu20s+£ with different oxygen concentrations were investigated by Raman scattering. Dependent on the polarizations of the incident and the scattered light, the electronic spectra measured well below T c show either zero or finite intensity at small energy transfers. In oxygenannealed crystals, the superconducting order parameter A is temperature dependent. In contrast, A is almost constant in the essentially two-dimensional argon-annealed samples. PACS numbers: 74.70.Vy, 74.30.Gn, 78.30.Er The energy gap in the quasiparticle excitation spectrum of superconductors A(k, 70 reveals important information about the superconducting ground state. In classical superconductors the gap has been found to be completely developed over the whole Fermi surface. Remaining anisotropics are small and the temperature dependence obeys the prediction of the BCS theory [1]. In the heavy-fermion superconductor UPt3 several properties indirectly suggest that the gap vanishes for certain directions of k space [2]. Metallic superconductors may be completely gapless in a certain impurity concentration range [2,3l. In the Cu-O superconductors, it is not clear whether these materials are fully gapped and which features are actually intrinsic. Contradictory results have been found for the low-temperature limiting value and for the temperature dependence of the energy gap or, more precisely, for the experimentally observable related quantities. In this Letter, results of Raman scattering experiments on Bi2Sr2CaCu208+>=[010], *' = [110], and/-[lT0]. Just before the samples were mounted into the cryostat, a thin surface layer was peeled off with an adhesive tape. In any case spot temperatures will be indicated.The samples were prepared with a self-flux method [7]

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