ORAL SESSION 4A5: ARPES and Fermi Surface of HTSs

Thursday, Feb. 24, 10:00 a.m. – 12:15 p.m., Room 303A (GRB)

Chairs: P.W. Anderson (Princeton), N. Nücker (Karlsruhe)

4A5.2 Photoemission and the Origin of High Temperature Superconductivity

M.R. Norman, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439

Presenting Author: M.R. Norman

I review recent work our group has done on the doping, momentum, frequency, and temperature dependence of the spectral lineshape from angle resolved photoemission. In particular, I will emphasize the implications this has for the electron self energy in the normal, superconducting, and pseudogap phases, and also the connection of these results with other spectroscopies such as optical conductivity and neutron scattering. I will then relate the change in the photoemission lineshape when going below T_c to similar behaviors observed in other probes, and use this to comment on the source of the superconducting condensation energy in the cuprates.

4A5.3 Fermi Surface, Pseudogap and Superconducting Gap in LSCO

A. Fujimori, Deptartment of Physics and Department of Complexity Science and Engineering, University of Tokyo, Tokyo 113-0033, Japan

Presenting Author: A. Fujimori

We have performed ARPES measurements on La_2-xSr_xCuO₄ (LSCO), whose composition covers from the insulating to the heavily overdoped regimes. The Fermi surface appears electron-like in heavily overdoped (x = 0.3) samples while it recovers the hole-like feature for x < 0.2. In underdoped (x < 0.15) samples, the superconducting gap around k = (p , 0) continues to increase with decreasing x, exhibiting a pseudogap behavior. In addition, the quasi-particle peak along the (p , p ) direction is obscured and the Fermi surface around (p /2, p /2) disappears, resulting in the total absence of Fermi-surface crossing. With further decreasing hole concentration, insulator-like features develop near the superconductor-insulator transition x = 0.05. In addition to the the "low-energy pseudogap" of order 10 meV, a "high-energy" pseudogap of order 100 meV develops in angle-integrated photoemission spectra. The superconducting gap is found to be strongly anisotropic and decreases away from (p ,0) much more rapidly than predicted for simple d-wave pairing. We discuss possible relationship between these observations and fluctuating charge stripes, which are particularly pronounced in LSCO.

This work has been done in collaboration with T. Yoshida, A. Ino, C. Kim, Z.-X. Shen, T. Mizokawa, H. Eisaki, S. Uchida and K. Kishio.

4A5.4 On the Topological Features at the Fermi Surface of Bi2212 System Revealed by Angle Scanning Photoemission

N.L. Saini, Unita INFM, Dipartimento di Fisica, Universita di Roma "La Sapienza", 00185 Roma, Italy

Presenting Author: N.L. Saini

*This work is made in collaboration with A. Bianconi, A. Lanzara, J. Avila, M.C. Asensio, S. Tajima, G.D. Gu and N. Koshizuka.

4A5.5 The Fermi surface and its excitations in the high temperature superconductors

Juan-Carlos Campuzano, University of Illinois at Chicago, Dept. of Physics, Chicago, IL 60607

Presenting Author: Juan-Carlos Campuzano

We first define the Fermi surface (FS) in strongly correlated metals, and then study excitations at the FS of the high T_c superconductor Bi2201 and Bi2212 as a function of doping, momentum and temperature with angle-resolved photoemission. We find that the excitations in the normal state of optimally doped materials cannot be described as quasiparticles anywhere along the FS, although the do become so in the superconducting state everywhere along the FS.

4A5.6 Theory of normal-state single particle and transport properties

Julius Ranninger, Centre de recherche sur les tres basses temperatures, CNRS; Boite Postale 38042, Grenoble, France

Presenting Author: J.H. Ranninger

We present a phenomenological model (Boson-Fermion mode consisting of a mixture of localized tightly bound electron-pairs and itinerant electrons, interacting with each other via a charge-exchange term) for HTS which satisfies two major experimentally established facts: i) a superconducting transition at T_c being driven by phase-fluctuations of the order parameter and ii) an opening of a pseudogap in the normal state below T_F*, being controlled by amplitude-fluctuations of it. Focusing on the normal state, we discuss the opening of the pseudogap in terms of the appearance of diffusive Cooper-pair like two-particle excitations which are manifest in a characteristic three-peak structure in the single-particle spectra of ARPES and ARIPES. We show to what extent the thermodynamic and transport properties (optical conductivity, Hall coefficient, etc.) can be attributed to those anomalous single-particle features. Upon decreasing the temperature towards T_c we envisage a region below T_B* (T_F*>T_B*>T_c) in which the two-particle Cooper-pair-like entities acquire itinerant behavior. This shows up in the appearance of a Drude like peak in the optical conductivity already quite T_c.

4A5.7 A re-examination of the electronic structure and Fermi Surface topology of BSCCO

D.S. Dessau, Y.-D. Chuang, and A.D. Gromko, University of Colorado, Boulder, CO 80309

Presenting Author: D.S. Dessau

We present a re-examination of the electronic structure and Fermi Surface (FS) topology of BSCCO as obtained from angle-resolved photoemission experiments. By applying a stricter set of FS crossing criteria as well as by varying the incident photon energy outside the usual range, we have found very different behavior from that previously observed. In particular, the FS observed using 33 eV photons is centered around the (0,0) point of the Brillouin zone and contains electron-like portions, while the standard FS observed is hole-like and centered around the (p,0) points. These results are robust and are not due to matrix-element effects. We will discuss how these results fit into the broader picture of high temperature superconductivity.

4A5.8 Electronic structure of Nd_1.85Ce_0.15CuO₄: evidence for disparity between hole and electron doped cuprate superconductors

N. Peter Armitage, Department of Physics and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford CA 94305

Presenting Author: N. Peter Armitage

The high-temperature superconductors are known to be doped Mott insulators. The symmetry, or the lack thereof, between doping with electrons (n-type) or holes (p-type) has important theoretical implications as most models implicitly assume symmetry. Most of what we know about high-T_c superconductors comes from the p-type materials. The limited experiments that have been done on the n-types suggest that there is a real difference between the two sides of the phase diagram. Despite this, the issue of electron/hole symmetry has not been seriously discussed, perhaps, because the database is rather limited. We report a high-resolution angle-resolved photoemission spectroscopic (ARPES) study of the n-type cuprate superconductor Nd_1.85Ce_0.15CuO₄, the results of which contrast strongly with the p-type. Evidence has been found for two distinct components in low energy electronic structure. While one retains much of the properties of the undoped insulator, the other appears to be a result of electron doping. Moreover, we have not seen the remarkable anisotropic leading edge gap that is the hallmark of d-wave superconductivity in the p-type superconductors. These distinct differences in electronic structure strengthen the case for an asymmetry between electron and hole doping.