Electron–Phonon Interaction at Interfaces

Abstract

Many-body effects and their interplay are at the heart of some of the most interesting problems in condensed matter physics, and frequently the simultaneous presence of different effects is found in complex materials. The electron-phonon (eÀph) interaction is one such effect that limits the lifetime of excited electrons (or holes) and has long been studied because of its role in many phenomena, from electrical conductivity to electronic heat capacity and BCS-type superconductivity. Several experimental techniques such as tunneling spectroscopy or heat capacity measurements have provided information on the electron-phonon coupling strength averaged over the bulk Fermi surface of metals [1].Experimentally, recent advances in angle-resolved photoemission (ARPES) have opened the opportunity for a study of many-body effects in unprecedented detail. Most importantly, studies are not confined to averages over the Fermi surface but detailed information about the energy and k-dependence of the interaction has come within reach. This permits, for instance, to establish the symmetry of the superconducting gap in novel superconductors [2,3].The eÀph interaction stands out as a fundamental many-body process that can be tested by both experimental and theoretical methods. Much has been learned by studying the eÀph coupling on carefully chosen electronic surface states, for which good arguments can be made for the eÀph interaction to be the only many-body effect giving rise to a bosonic spectroscopic signature. Surface states have also played an important role because they have, as do the states in the cuprates, a merely two-dimensional dispersion, an essential prerequisite for the analysis of ARPES data.In the most simple picture, the eÀph coupling changes the dispersion and the lifetime of the electronic states in a material. This situation is illustrated in Figure 7.1a. Very close to the Fermi level, within a typical phonon energy hv D , the dispersion is renormalized such that it is flatter at the Fermi energy. Consequently, the effective mass of the electrons at the Fermi level and the density of states are

Strong momentum-dependent electron–magnon renormalization of a surface resonance on iron

“…Outside the characteristic energy scale x of the coupled excitations, the electrons resume the non-coupling, steeper band dispersion, leading to kinks in the overall band dispersion. This quasiparticle renormalization is equally induced by interactions with phonons, [14][15][16] electrons, [17][18][19][20] magnons, 8,9 and plasmons. 21 Apart from kinks in the electronic bands, strong electron-boson coupling can also be reflected in replica band or satellite formation, which was, for example, detected for hydrogen-induced surface states on W(110), 14 the Be(0001) surface, 22,23 TiO 2 , 24 as well as high-temperature Fe-based superconductors.…”