Evidence for ubiquitous strong electron–phonon coupling in high-temperature superconductors

Abstract

Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity 1 . The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high transition temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle resolved photoemission spectroscopy to probe electron dynamicsvelocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. We investigated the electronic quasiparticle dispersions in three different families of hole-doped cuprates, Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) and Pb doped Pb-Bi2212, Pb-doped Bi 2 Sr 2 CuO 6 (Pb-Bi2201) and La 2-x Sr x CuO 4 (LSCO). Except for the Bi2201 (overdoped, T c =7K) data and that in Fig. 3b, recorded at the beam-line 5.4 of the Stanford Synchrotron Radiation Laboratory (SSRL), all the data were recorded at the Advanced Light Source (ALS), as detailed elsewhere 2 . The top panels of figure 1 report the momentum distribution curve (MDC) derived dispersions along the (0, 0)-(π, π) direction for LSCO (panel a) and Bi2212 (panel b) superconducting state and for Pb-Bi2201 normal state (panel c) vs the rescaled momentum, k ' , defined by normalizing to one the momentum k relative to the Fermi momentum k F , (k-k F ), at the binding energy E=170meV. A "kink" in the dispersion around 50-80meV, highlighted by thick arrows in the figure, is the many-body effect of

Phonon anomalies in trilayer high-Tc cuprate superconductors

“…Figure 9 displays frequency dependencies of the ratios E loc bl /E, E loc int /E and E loc ocp /E in the SCS obtained using Eqs. (4)- (12). The phonons have not been taken into account and S bl = 0.…”

“…It is important to establish what particular type of electron-phonon interaction is responsible for these effects (so called phonon anomalies), and whether the anomalous phonons are "active players" in the mechanism of the high-T c superconductivity [11,12,13] or whether they simply respond to changes of the electronic ground state that have an independent cause. Here we focus on the anomalies of c-axis polarized infrared-active phonons.…”

Spectral properties and isotope effect in strongly interacting systems: Mott-Hubbard insulator versus polaronic semiconductor

“…Recent photoemission experiments in the high-Tc cuprates [1,2,3,4], which are generally described in terms of purely electronic models due to the proximity to a Mott insulating phase, have revealed the existence of an important electron-lattice coupling. This has given rise to intense theoretical work focusing on the excitation spectra in the presence of electron-boson interactions, both in models with [5,6,7] and without [8,9,10,11,12,13] electronic correlations.…”

Photoemission as a Probe of the Collective Excitations in Condensed Matter Systems

“…[79] In examining the mass enhancement some groups have focused more closely on the associated "kink" in the dispersion and suggested that its presence at a similar energy in studies of all of the different cuprates is an indication of coupling to a phonon mode. [75] Indeed neutron studies do indicate the presence of phonon modes at similar energies. [80] The authors of these studies have also suggested that an "unconventional isotope effect" is an indication of the role of phonons in that the substitution of O 18 for O 16 results in a change in the velocity of the higher energy electrons as opposed to the lower energy electrons in the vicinity of the Fermi level.…”

“…This observation has potentially important implications for the mechanism driving high T C superconductivity and an obvious question is whether or not it points to a BCS like mechanism whereby the electrons or renormalization and associated "kink" have become central issues in subsequent ARPES work with considerable controversy regarding their source. [73,74,75,76,77] Are they related to the presence of spin excitations or do they reflect an interaction with phonons or indeed any other collective mode? In the case of the cuprates, this is not an easy issue to resolve as the various energy scales are nearly identical.…”