J. Fink

Electronic structure and Fermi surface of Bi2Sr2CaCu2O8

Applying angle-resolved photoemission spectroscopy of high angular and energy resolution we have measured the electronic structure of single crystals of Bi2Sr2CaCu2O8 near the Fermi energy. Along the high symmetry direction ΓX a band is observed to disperse upwards and to cross the Fermi level, whose unoccupied part constitutes the hole-like carriers responsible for the superconductivity. From spectra along the Γ M direction we find evidence for an electron pocket around the M point. The measured band widths appear to be drastically reduced compared with band structure calculations indicating strong electronic correlation effects. From the observation of Fermi-Dirac-like cut-offs in the spectra due to band crossings through the Fermi level we can confirm the existence and, in particular, the shape of the Fermi surface as calculated by band structure calculations.

Dielectric function of YBa2Cu3O7-δ between 50 meV and 50 eV

Electronic excitations with polarization parallel to the CuO2-planes in single-crystalline YBa2Cu3O7 and YBa2Cu3O6 have been investigated by optical reflectance and by highenergy electron energy-loss spectroscopy in transmission in the energy ranges from 50 meV to 6 eV and 0.2 eV to 150 eV, respectively. From a combination of these experimental data the dielectric function, the reflectivity and the optical conductivity have been obtained in a wide energy range. The observed spectra are interpreted in terms of optical phonons, free-carrier absorption, transitions across the charge-transfer gap, further interband transitions, low-lying core-level excitations, excitonic transitions and valence conservingd-d transitions within the Cu 3d shell combined with O 2p intraband transitions. The charge carrier plasmon and the 4 eV-excitation in YBa2Cu3O6 show quadratic dispersions in momentum transfer. From the dispersion constant of the plasmon a mean value of the Fermi velocity of the charge carriers parallel to the CuO2 planes has been derived.

On the electronic structure and superconducting gap of Bi2Sr2CaCu2O8

Abstract Photoemission spectra taken at room temperature with high energy and angle resolution on Bi 2 Sr 2 CaCu 2 O 8 single crystals reveal a band near E F with weak dispersion along λX. It crosses E F at about 1/2 λX and the occupied part becomes visible up to the Brillouin zone boundary in angle-resolved inverse photoemission spectra. Starting for T>Tc with a clear Fermi edge, for T F to higher binding energy related to the opening of the superconducting gap of Δ=(30±4) meV. These findings strongly favour Fermi liquid based theories for high-Tc superconductivity in this material.

High-energy spectroscopy studies of high-Tc superconductors

Abstract High-energy spectroscopy studies of cuprate superconductors and their undoped parent compounds are reviewed. In particular, we report on investigations by electron energy-loss, x-ray absorption, and angle-resolved photoemission spectroscopy. Information on the character and the symmetry of states close to the Fermi level has been obtained by core level excitations. Angle-resolved photoemission spectra yield “bands” and the shape of the Fermi surface.

Evidence against hole formation by Ca in YBA2Cu4O8

Abstract We report on electron energy-loss studies of Ca-doped YBa 2 Cu 4 O 8 . O 1s and Ca 2p absorption edges indicate that there is almost no change of the hole concentration upon Ca doping and that a major fraction of the Ca enters the Ba site. This suggests that the increase of T c upon Ca doping is not caused by a formation of additional holes as expected for the replacement of trivalent Y by divalent Ca.

Preparation and electronic structure of phase pure K3C60

Phase pure K3C60 films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.

Surface effects on YBa2Cu3O7−δ by angle-resolved photoemission

Abstract High-resolution angle-resolved photoemission performed on cleaved surfaces of twinned YBa 2 Cu 3 O 7−δ single crystals at 20 K revealed an intrinsic surface-derived electronic state for this high-temperature superconductor. In the spectra it is very pronounced around the X(Y) point of the Brillouin zone where its binding energy is 0.92 eV. It shows slight k ∥ -, no k ⊥-dispersion , and a strong sensitivity to surface contamination. The existence of this state points to a rearrangement of electron charge in the surface layers and consequences for the superconducting behaviour on the surface of 1:2:3 superconductors will be discussed.

Electron energy-loss studies on high-temperature superconductors

Abstract The electronic structure of the high-temperature superconductors La 2−x Sr x CuO 4 , Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8 , and Nd 2−x Ce x CuO 4−δ have been studied by high-energy electron energy-loss spectroscopy in transmission. Core-level spectroscopy, in particular on the O 1s and Cu 2p levels, yields information on the partial unoccupied density of states, i.e., on the character of hole states close to the Fermi level.

Investigations of the electronic structure of cuprate superconductors by high-energy spectroscopy

Abstract Studies of the electronic structure of high-Tc superconductors and related compounds by high-energy spectroscopies are reviewed. In particular, we report on investigations by electron energy-loss, angle-resolved photoemission, and inverse angle-resolved photoemission spectroscopy.

Plasmon dispersion and the dielectric function in YBa2Cu4O8 single crystals

Abstract Electron energy-loss spectroscopy studies have been performed in order to investigate the in-plane loss function between 0 and 50 eV and the plasmon dispersion of YBa 2 Cu 4 O 8 single crystals. Strongly anisotropic dielectric functions and plasmon frequencies have been observed. The plasmon dispersion has been found to be quadratic, with an a-b anisotropy of about two. The Fermi velocity and the effective mass of the charge carriers could be derived.