ARPES measures the photoemission intensity as a function of two variables, the electron binding energy (obtained from the photoelectron kinetic energy) and electron momentum (obtained from the angle of emission from the sample surface). With its ability to directly reveal energy–momentum relationships (dispersion curves) and lifetimes, ARPES provides a unique opportunity to look for energy scales that manifest themselves in dynamical parameters, such as the velocity (slope of energy–momentum curve) and scattering rate. The angular resolution of ±0.1 degrees, which is about an order of magnitude better than in many previous ARPES studies of these materials, made the new results possible.

Raw ARPES data set showing photoemission intensity as a function of energy and reduced momentum k' (momentum at the Fermi surface minus the actual momentum).

The experimenters recorded their spectra at ALS Beamline 10.0.1 (some data were also taken at Stanford Synchrotron Radiation Laboratory Beamline 5.4) at several temperatures and photon energies on single crystals of Bi₂Sr₂CaCu₂O₈ (Bi221), lead-doped Bi₂Sr₂CaCu₂O₈ (Pb-Bi2212), lead-doped Bi₂Sr₂CuO₆ (Pb-Bi2201), and La_2–xSr_xCuO₄ (LSCO). These representative high-temperature superconductors exhibited a range of transition temperatures and energy gap values.

Vibrating Atoms Stage a Superconductivity Comeback

Superconductivity is the resistanceless flow of electricity in certain materials when cooled to a sufficiently low temperature, usually within a few degrees of absolute zero (0 Kelvin). Why it occurs in metals and alloys is well established. The same cannot be said for "high-temperature" superconductivity, which persists in certain ceramic oxide compounds at temperatures above the boiling point of liquid nitrogen (77 Kelvin). So, for the last 15 years, understanding high-temperature superconductivity has remained toward the top of anybody's list of important unsolved problems in solid-state physics.

The preponderance of the wide variety of research conducted to date supports the contention that the mechanism driving high-temperature superconductivity is quite different from the interaction between electrons and vibrating atoms that underlies conventional superconductivity. However, recent experiments at the ALS by a collaboration between researchers from Stanford University, the University of Tokyo, and the ALS suggests that some form of interaction between electrons and lattice vibrations may yet play an important role in resolving the high-temperature superconductivity conundrum. This controversial finding by Lanzara et al. is sure to stimulate a flurry of work, both experiments and theory, designed to explore its consequences.

A typical momentum distribution curve (MDC), obtained by plotting the photoemission intensity as a function of scanning angle at a constant binding energy, shows a peak on a constant background that can be fitted to obtain one point on an energy–momentum curve. The dispersion curves derived from many MDCs for each material clearly showed the energy moving linearly towards the Fermi energy (binding energy = 0) as the momentum decreased. Most important, however, the curves exhibited an obvious kink in the slope near a binding energy of 50-80 meV, independent of the material's superconducting transition temperature and energy gap.

Electron–phonon coupling modifies the electron-momentum dispersion curve near the Fermi energy.

Dispersion curves for three families of high-temperature superconductors show a common kink at an energy (arrow) that matches an oxygen lattice vibration. The parameter d is the doping concentration that determines the transition temperatures in the materials.

The change in slope to a lower value close to the Fermi energy suggests the onset of a many-body effect involving electrons and some other entity to form a heavier, slower quasiparticle. Universality of the kink in the various materials and its uniformity for different directions of momentum in the Brillouin zone lead naturally to the conclusion that a very strong electron–phonon coupling is responsible. Persistence of the kink above the transition temperature further supports this conclusion because phonons would be active over a wide temperature range. Neutron-scattering experiments by another group on La_2–xSr_xCuO₄ show that the energy of an oxygen stretching vibration (longitudinal optical phonon) matches that of the kink, suggesting this phonon mode is involved.

Additional evidence comes from energy distribution curves (EDCs), obtained from the photoemission intensity variation with binding energy at a fixed angle for several Bi₂Sr₂Cu₂O₈ samples with different transition temperatures. The set of EDCs for each material exhibited a common structure showing a quasiparticle peak at energies close to the Fermi energy, a dip occurring approximately at the phonon energy, and a broad feature at higher energy. Similar EDCs are observed for the beryllium surface, whose electrons are known to have a strong coupling to a single phonon mode, and to simulated EDC spectra for the simple case of isotropic coupling to a single phonon mode.

Set of photoemission energy distribution curves (EDCs) at different angles (colors) for a high-temperature superconductor (HTSC). Similar sets measured for three families of HTSCs and the nonsuperconducting beryllium surface and simulated for the simple case of isotropic coupling to a single phonon mode share common features, suggesting electron–phonon coupling is operative in HTSCs.

These findings and others obtained from additional detailed analysis of the ARPES data, bring the electron–phonon interaction back as an important player in the high-temperature superconductivity puzzle.

Research conducted by A. Lanzara (Stanford University, Stanford Synchrotron Radiation Laboratory, and ALS); P.V. Bogdanov, X.J. Zhou, S.A. Kellar, D.L. Feng, H. Eisaki, and Z.-X. Shen (Stanford University and Stanford Synchrotron Radiation Laboratory); E.D. Lu and Z. Hussain (ALS); and T. Yoshida, A. Fujimori, K. Kishio, J.I. Shimoyama, T. Noda, and S. Uchida (University of Tokyo).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); National Science Foundation; Istituto Nazionale Fisica della Materia (INFM); and University of Rome "La Sapienza." Operation of the ALS is supported by BES.

Publication about this research: A. Lanzara, P.V. Bogdanov, X.J. Zhou, S.A. Kellar, D.L. Feng, E.D. Lu, T. Yoshida, H. Eisaki, A. Fujimori, K. Kishio, J.I. Shimoyama, T. Noda, S. Uchida, Z. Hussain, and Z.X. Shen, "Evidence for Ubiquitous Strong Electron–Phonon Coupling in High-Temperature Superconductors," Nature 412, 510 (2001).
 

ALSNews Vol. 186, October 17, 2001