The electronic structure of single crystals Na0.6CoO2, which are closely related to the superconducting Na0.3CoO2.yH2O (Tc ∼ 5K), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes (Γ-K, Γ-M ). Our ARPES result also indicates that an extended flat band is formed slightly above EF along Γ-K. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the Γ-M direction, which differs from the predicted bilayer splitting.

We report a systematic high-resolution angle-resolved photoemission spectroscopy on high-T(c) superconductors Bi(2)Sr(2)Ca(n-1)Cu(n)O(2n+4) (n=1-3) to study the origin of many-body interactions responsible for superconductivity. For n=2 and 3, a sudden change in the energy dispersion, so called "kink", becomes pronounced on approaching (pi,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. For n=1, the kink shows no significant temperature dependence even across T(c). This could suggest that the coupling of electrons with Q=(pi,pi) magnetic mode is dominant in the superconducting state for multilayered cuprates, while the interactions at the normal state and that of single-layered cuprates have a different origin.

We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-Tc cuprate Bi2Sr2Ca2Cu3O 10+δ . We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below EF as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.PACS numbers: 74.72. Hs, 74.20Fg, 79.60.Bm It is well known that BCS (Bardeen, Cooper, and Schrieffer) theory [1] explains many of fundamental thermodynamic, transport and magnetic properties of superconductors by introducing a simple picture that two electrons with opposite momenta and spins near the Fermi surface form a pair (Cooper pair) in the superconducting state. In the language of Fermi liquids [2], the quasiparticles (QPs) of the Cooper pairs are called Bogoliubov quasiparticles (BQPs) [3] which are defined as excitation of a single electron "dressed" with an attractive interaction between paired electrons. The BQPs play an essential role in characterizing the superconducting state via quantities such as the superconducting gap and its symmetry. Despite the excellent description of the BQPs in BCS theory, there has been no direct experimental observation of the predicted full energy dispersion (electron and hole branches) of BQPs, although the pairing of electrons is evidenced by ac Josephson-junction experiment [4]. For high-T c cuprates, the direct observation of the BQPs is even more significant, since many microscopic ideas of BCS theory have been seriously challenged. Since the BQPs are a natural consequence of the starting Hamiltonian including the two-body attractive interaction assumed by BCS theory [1], the existence of BQPs would prove the validity of the basic BCS description of the superconducting state in the cuprates.In this Letter, we report the direct observation of BQPs in triple-layered high-T c cuprate Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ by angle-resolved photoemission spectroscopy (ARPES). By using ultrahigh resolution in energy and momentum, we have succeeded in directly observing the full energy dispersion (electron and hole branches) of BQPs below and above the Fermi level (E F ). We have determined experimentally the coherence factors as a function of momentum from ARPES intensity and compared the result with the prediction from BCS theory to investigate the basic

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (π, 0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Néel temperature (TN ) while the AF pseudogap is gradually filled up at much higher temperatures, possibly due to the short-range AF correlation.Since the discovery of cuprate high-temperature superconductors (HTSCs), intensive experimental and theoretical studies have been performed to elucidate the origin and mechanism of the anomalously high superconducting (SC) transition temperature. It is now widely accepted that electrons or holes doped into the parent Mott insulator interact antiferromagnetically with each other on the quasi-two dimensional CuO 2 plane. Although it has been suggested that the antiferromagnetic (AF) interaction plays an essential role for pairing of electrons (holes) in the SC state, it is still unclear how the antiferromagnetism interplays with the superconductivity at the microscopic level. This problem is a central issue not only in HTSCs but also in other exotic superconductors such as heavy-fermion and organic-salt superconductors. In the phase diagram of electron-doped cuprates, the AF and SC phases are adjacent to each other or somewhat overlap at the boundary, in contrast to the hole-doped case where the two phases are well separated [1]. The proximity or overlapping between the AF and SC phases in the electron-doped cuprates yields a good opportunity for studying the interplay between the AF interaction and the superconductivity [2,3,4,5]. In fact, a recent elastic neutron-scattering experiment reported a competitive nature between the AF long-range order and the superconductivity [2]. On the other hand, an inelastic neutron scattering experiment observed coexistence of the gapped commensurate spin fluctuation and the superconductivity [3]. In contrast to these intensive studies on the "qresolved" spin dynamics by neutron scattering, a limited number of photoemission studies on the "k-resolved" electronic structure have been reported for electron-doped cuprates [6,7]. The k-dependence of the AF correlation effect on the electronic structure is essential to understand the interplay between the antiferromagnetism and the superconductivity.In this Letter, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) on electron-doped cuprate Nd 1.87 Ce 0.13 CuO 4 (NCCO, x = 0.13) located at the phase boundary between the AF and SC phases. We found the mass-renormalized quasiparticle (QP) state near (π, 0), which gradually evolves into the high-energy gap [6] around the hot spot. The observed continuous evolution of the electronic structure near the Fe...

We have performed systematic angle-resolved photoemission spectroscopy on the high-T c superconducting family of Bi 2 Sr 2 Ca nϪ1 Cu n O 2nϩ4 (nϭ1-3). In addition to the generic features of the large Fermi surface and d-wave superconducting gap, we have found that there exists a scaling of the doping dependence of the energy gap. Moreover, comparison of the nodal dispersion in the superconducting state shows that the ''kink'' ͑the sudden change of band dispersion͒, while occurring at a similar binding energy, becomes more pronounced with increasing layer number n, indicating stronger coupling between electrons and a collective mode.