An unusual isotope effect in a high-transition-temperature superconductor

Abstract

In conventional superconductors, the electron pairing that allows superconductivity is caused by exchange of virtual phonons, which are quanta of lattice vibration. For high-transition-temperature (high-T(c)) superconductors, it is far from clear that phonons are involved in the pairing at all. For example, the negligible change in T(c) of optimally doped Bi2Sr2CaCu2O8+delta (Bi2212; ref. 1) upon oxygen isotope substitution (16O --> 18O leads to T(c) decreasing from 92 to 91 K) has often been taken to mean that phonons play an insignificant role in this material. Here we provide a detailed comparison of the electron dynamics of Bi2212 samples containing different oxygen isotopes, using angle-resolved photoemission spectroscopy. Our data show definite and strong isotope effects. Surprisingly, the effects mainly appear in broad high-energy humps, commonly referred to as 'incoherent peaks'. As a function of temperature and electron momentum, the magnitude of the isotope effect closely correlates with the superconducting gap--that is, the pair binding energy. We suggest that these results can be explained in a dynamic spin-Peierls picture, where the singlet pairing of electrons and the electron-lattice coupling mutually enhance each other.

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

“…Particular attention is devoted to the effect of a shift of the boson frequency, as can be achieved through an isotopic substitution if the bosonic mode that couples to the electrons is a lattice vibration, or more generally if it is a collective mode with a sizeable lattice component (in which case a nontrivial IE can still arise, provided that the frequency of the boson is modified by the isotopic substitution). In the present approximation, the IE on the high energy part of the spectrum turns out to be qualitatively similar in the Mott-Hubbard insulator and in the polaronic semiconductor, being mainly determined by the strength of the electron-boson coupling (albeit slightly enhanced by the presence of electronic correlations): in both cases it is strongly temperature and energy dependent, in agreement with the recently measured IE in the high temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ [17,18]. A tentative analysis of the experimental results, performed in section IV, is compatible with the existence of a moderate electron-phonon coupling in the cuprates.…”

“…The value inferred from the photoemission experiments in ref. [17] on the mode at ω 0 = 70meV is comparable with this value, |∆ω 0 |/ω 0 > ∼ 7% , pointing to a strongly phononic character of the bosonic excitation.…”

“…The results of section II can be tentatively compared with the experimental results of references [17,18]. If we associate the variance σ with the observed width ∼ 0.2eV of the active IE region, and take the value ω 0 = 0.07eV for the boson energy, a "polaron" binding energy E P ∼ 0.5eV is obtained.…”

Isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8 due to quadratic electron–phonon coupling

“…Nowadays, with the great improvement of energy resolution, ARPES is able to observe the fine structures due to e-ph interaction 2 and the tiny superconducting gap 3 with an energy resolution less than 1 meV. Recently, the oxygen isotope effect has been studied with ARPES on the high-T c superconductor Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) by two groups 4,5 . Comparing their data, at least one common feature has become clear, the spectra are shifted when 16 O is substituted by its isotope 18 O, whose movement in this crystal is regarded as a lattice vibration, i.e., phonon.…”

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

“…[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. [81] If the phonons play any role in the superconductivity, this observation is again counterintuitive. However we note that subsequent attempts to reproduce this effect have failed.…”