Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStrong band kinks in magic-thickness Yb films arising from interfacial electron-phonon coupling
102
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Interfacial electron-phonon coupling in ultrathin films has attracted much interest; it can give rise to novel effects and phenomena, including enhanced superconductivity. Here we report an observation of strong kinks in the energy dispersions of quantum well states in ultrathin Yb films grown on graphite. These kinks, arising from interfacial electron-phonon coupling, are most prominent for films with a preferred (magic) thickness of 4 monolayers, which are strained and hole doped by the substrate. The energy position of the kinks agrees well with the optical phonon energy of graphite, and the extracted electron-phonon coupling strength {lambda} shows a large subband dependence, with a maximum value up to 0.6. The kinks decay rapidly with increasing film thickness, consistent with its interfacial origin. The variation of {lambda} is correlated with evolution of the electronic wave function amplitudes at the interface. A Lifshitz transition occurs just beyond the magic thickness where the heavy Yb 5d bands begin to populate right below the Fermi level.
rate research
Read More
Angle-resolved photoemission spectroscopy reveals pronounced kinks in the dispersion of the sigma band of graphene. Such kinks are usually caused by the combination of a strong electron-boson interaction and the cut-off in the Fermi-Dirac distribution. They are therefore not expected for the $sigma$ band of graphene that has a binding energy of more than 3.5 eV. We argue that the observed kinks are indeed caused by the electron-phonon interaction, but the role of the Fermi-Dirac distribution cutoff is assumed by a cut-off in the density of $sigma$ states. The existence of the effect suggests a very weak coupling of holes in the $sigma$ band not only to the $pi$ electrons of graphene but also to the substrate electronic states. This is confirmed by the presence of such kinks for graphene on several different substrates that all show a strong coupling constant of lambda=1.
First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the $sigma$ band with $lambda$ values of up to 0.9. Near the top of the $sigma$ band, $lambda$ is found to be $approx 0.7$. This value is consistent with the recently observed kinks in the $sigma$ band dispersion by angle-resolved photoemission. While the photoemission intensity from the $sigma$ band is strongly influenced by matrix elements due to sub-lattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of $0.6(1)$, in excellent agreement with the calculations.
Ultrathin FeSe films grown on SrTiO$_{3}$ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. Using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO$_{3}$ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1 unit cell, 3 unit cell, and 60 unit cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00(2) to 5.25(2) THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A$_textrm{1g}$ phonon. The dominant mechanism for the phonon softening in 1 unit cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.
The title compound is investigated by specific heat measurements in the normal and superconducting state supplemented by upper critical field transport, susceptibility and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities and partial densities of states the presence of both strong electron-phonon interactions and considerable pair-breaking has been revealed. The specific heat and the upper critical field data can be described to first approximation by an effective single band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated s-wave superconductivity somewhat suppressed by sizeable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a van Hove type singularity in the electronic density of states near the Fermi energy is suggested.
Linear response methods are applied to identify the increase in electron-phonon coupling in elemental yttrium that is responsible for its high superconducting critical temperature Tc, which reaches nearly 20 K at 115 GPa. While the evolution of the band structure and density of states is smooth and seemingly modest, there is strong increase in the 4d content of the occupied conduction states under pressure. We find that the transverse mode near the L point of the fcc Brillouin zone, already soft at ambient pressure, becomes unstable (in harmonic approximation) at a relative volume V/Vo=0.60 (P ~ 42 GPa). The coupling to transverse branches is relatively strong at all high symmetry zone boundary points X, K, and L. Coupling to the longitudinal branches is not as strong, but extends over more regions of the Brillouin zone and involves higher frequencies. Evaluation of the electron-phonon spectral function $alpha^2F(omega)$ shows a very strong increase with pressure of coupling in the 2-7 meV range, with a steady increase also in the 7-20 meV range. These results demonstrates strong electron-phonon coupling in this system that can account for the observed range of Tc.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
