High-resolution photoemission is used to study the electronic structure of the cuprate superconductor, Bi(2)Sr(2)CaCu(2)O(8+delta), as a function of hole doping and temperature. A kink observed in the band dispersion in the nodal line in the superconducting state is associated with coupling to a resonant mode observed in neutron scattering. From the measured real part of the self-energy it is possible to extract a coupling constant which is largest in the underdoped regime, then decreasing continuously into the overdoped regime.

New tunneling data are reported in Bi 2 Sr 2 CaCu 2 O 81d which show quasiparticle excitation gaps, D, reaching values as high as 60 meV for underdoped crystals with T c 70 K. These energy gaps are nearly 3 times larger than those of overdoped crystals with similar T c . Despite the large differences in gap magnitude, the tunneling spectra display qualitatively similar characteristics over the entire doping range. Detailed examination of the spectra, including the Josephson I c R n product measured in break junctions, indicates that these energy gaps are predominantly of superconducting origin. , and thus if the gap has a superconducting origin it strongly suggests that the pseudogap state is due to some type of precursor superconductivity [1,4,5]. However, the smooth dependence on doping may also originate from a quasiparticle gap that evolves from superconducting character in the overdoped phase to another type (e.g., charge density wave) in the underdoped phase [6]. In support of this picture are some measurements that suggest a superconducting order parameter (or coherence gap) that scales with T c [7][8][9]. Thus, a critical question is whether the large energy gap found in tunneling originates entirely from superconducting pairing or has a contribution from some other electronic effect. Here we address the nature of the gap measured by tunneling and report new data in more heavily underdoped Bi2212. Energy gaps, D, of 51 6 2, 54 6 2, and 58 6 2 meV are observed for three underdoped crystals with T c 77, 74, and 70 K, respectively, extending the previously reported trend [2] further into the underdoped regime. Detailed examination of the tunneling spectra over a wide doping range, including the Josephson I c R n product, show that these energy gaps are predominantly of superconducting origin.Superconductor-insulator-superconductor (SIS) junctions provide an accurate measure of the gap from the peaks in tunneling conductance (at a bias voltage jeV p j 2D) which are only weakly affected by thermal smearing or quasiparticle scattering [2,10]. However, the large magnitudes of energy gaps observed here lead to such extraordinarily large values of 2D͞kT c (as high as 20) that it is necessary to examine carefully the entire tunneling spectrum to clarify their physical origin. Most theoretical models of HTS stress the importance of electronic correlations which lead to spin density waves [11,12], or charge density waves in the underdoped phase [6,13]. These correlations give rise to gaps (or pseudogaps) in the electronic excitation spectrum, D c ͑k͒, that are distinct from those arising from superconductivity, D s ͑k͒. Since these other correlation gaps are used to explain pseudogap phenomena above T c , our investigation here has a direct bearing on this issue. The experimental goal is to determine whether the single-particle excitation gap as measured in tunneling has contributions from both D s and D c . We argue that if two distinct gaps exist, (i) they should have different magnitudes as well as different dopi...

New break-junction tunneling data are reported in Bi(2)Sr(2)CaCu(2)O(8+delta) over a wide range of hole concentration from underdoped (T(c) = 74 K) to optimal doped (T(c) = 95 K) to overdoped (T(c) = 48 K). The conductances exhibit sharp dips at a voltage, Omega/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Omega scales as 4.9kT(c) over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.

Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.