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...
