Atom scattering is becoming recognized as a sensitive probe of the electron-phonon interaction parameter $λ$ at metal and metal-overlayer surfaces. Here, the theory is developed linking $λ$ to the thermal attenuation of atom scattering spectra (in particular, the Debye-Waller factor), to conducting materials of different dimensions, from quasi-one dimensional systems such as W(110):H(1$\times$1) and Bi(114), to quasi-two dimensional layered chalcogenides and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of $λ$ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition the number of layers contributing to the electron-phonon interaction that is measured in an atom surface collision.
We have investigated coupling constants in elementary electron-phonon scattering processes on a graphite surface by the combined use of high-resolution electron-energy-loss spectroscopy (HREELS) and very low-energy electron diffraction (VLEED). HREELS is used to measure the modulations of electron transition probabilities from incoming electrons in vacuum to outgoing electrons in vacuum where the transition includes one-phonon scattering processes inside a solid. Determining the electronic band structures of graphite with VLEED, we defined electronic states of the solid surface that electrons …
We study the electron and phonon spectral functions as well as superconductivity for a two dimensional electron gas couped to dipolar phonons via small-momentum transfer scattering. The results reported here are obtained through sign-problem-free quantum Monte-Carlo simulation. Hence aside from modeling there is no further approximations. Our results can be relevant to the high-temperature superconductor at the interface between a unit-cell-thick FeSe and SrTiO3.
Soft phonons and diffuse scattering in insulating La$_{2-x}$Sr$_{x}$CuO$_4$ ($x=0.05$) have been studied by the neutron scattering technique. The X-point phonon softens from high temperature towards the structural transition temperature $T_{s}=410$ K, and the Z-point phonon softens again below 200 K. The Z-point phonon softening persists to low temperature, in contrast to the behavior observed in the superconducting $x=0.15$ compound, in which the Z-point phonon hardens below $T_c$. The diffuse scattering associated with the structural phase transition at 410 K appears at commensurate position…
We report the first systematic measurements of the Raman scattering by electrons in elemental metals of Al, Mo, Nb, Os, Pb, Re, Ta, Ti, V, W and metallic compound La$B_6$. Experimental spectra are modelled on the base of the band structures, calculated within the density functional theory, taking properly into account the effects of electron-phonon scattering. The agreement between our measured and calculated spectra is excellent for the variety of metals, thus providing estimates for the electron-phonon coupling constants and temperature-dependent relaxation rates. The method can be applied f…
Tunneling and point contact spectroscopy show clear phonon features and together with optic measurements give strong support that the electron-phonon interaction (EPI) is large in HTS oxides. Strong correlations in HTS oxides renormalize the EPI (and interaction with impurities) so that the forward scattering peak (FSP) develops for small hole doping δ<<1. The FSP mechanism explains important properties of the normal and superconducting state.
We investigate the dispersion and temperature dependence of a number of phonons in the recently discovered superconductor CaC6 utilizing inelastic x-ray scattering. Four [00L] and two ab-plane phonon modes are observed, and measured at temperatures both above and below T_c. In general, our measurements of phonon dispersions are in good agreement with existing theoretical calculations of the phonon dispersion. This is significant in light of several discrepancies between experimental measurements of phonon-derived quantities and theoretical calculations. The present work suggests that the origi…
The discovery of an enhanced superconducting transition temperature $T_c$ in monolayers of FeSe grown on several oxide substrates has opened a new route to high-$T_c$ superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge in…
Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single crystal sample of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon w…
The paper presents detailed Raman scattering study of the unusually broad E2g phonon mode in MgB2 crystal. For the first time, it is shown by the polarized Raman scattering on few-micron-size crystallites with natural faces that the observed broad Raman feature really does obey the selection rules of an E2g mode. Raman spectra on high quality polycrystalline superconducting MgB2 wires reveal a very symmetric E2g phonon line near 615 1/cm with the room temperature linewidth of 260 1/cm only. Additional scattering of different polarization dependence, observed in certain crystallites is interpre…
The momentum dependent lineshape of the out-of-phase oxygen vibration as measured in recent neutron scattering measurements is investigated. Starting from a microscopic coupling of the phonon vibration to a local crystal field, the phonon lineshift and broadening is calculated as a function of transfered momentum in the superconducting state of YBa$_{2}$Cu$_{3}$O$_{7}$. It is shown that the anisotropy of the density of states, superconducting energy gap, and the electron-phonon coupling are all crucial in order to explain these experiments.
We argue that Raman scattering in $B_{1g}$ symmetry allows one to distinguish between phonon-mediated and magnetically-mediated $d-$wave superconductivity. In spin mediated superconductors, $B_{1g}$ Raman intensity develops a resonance at a frequency $ω_{res} < 2Δ_{max}$, whose origin is similar to a neutron resonance. In phonon-mediated $d-$wave superconductors, such a resonance does not develop. Several extensions of the argument are presented.
Inelastic neutron-scattering experiments have been carried out on polycrystalline samples of the FeSe$_{1-x}$ superconductors. We report the phonon density of states for FeSe$_{1-x}$ with Tc$\approx$8 K. The phonon cutoff frequency is observed around 40 meV. No significant change is observed across the superconducting transition. The measurements support the published first-principles calculations [A. Subedi et al., Phys. Rev. B \textbf{78}, 134514 (2008)].
We examine the effect on the DC resistivity of small-q electron-phonon scattering, in a system with the electronic topology of the high-T_c oxides. Despite the fact that the scattering is dominantly forward, its contribution to the transport can be significant due to ``ondulations'' of the bands in the flat region and to the umpklapp process. When the extended van-Hove singularities are sufficiently close to $E_F$ the acoustic branch of the phonons contribute significantly to the transport. In that case one can obtain linear $T$ dependent resistivity down to temperatures as low as 10 K, even i…
Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB$_2$ are presented. The $E_{2g}$ phonons, which involve in-plane B displacements, couple strongly to the $p_{x,y}$ electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting $λ$ in the clean limit is calculated to be significantly larger. The $E_{2g}$ phonons are strongly anharmonic, and the non-linear contribution to the coupling between the $E_{2g}$ modes and the p$_{x,y}$ bands is signi…
He atom scattering has been shown to be a sensitive probe of electron-phonon interaction properties at surfaces. Here it is shown that measurements of the thermal attenuation of the specular He atom diffraction peak (the Debye-Waller effect) can determine the electron-phonon coupling constant $λ$ for ultrathin films of metal overlayers on various close-packed metal substrates. Values of $λ$ obtained for single and multiple monolayers of alkali metals, and for Pb layers on Cu(111), extrapolated to large thicknesses, agree favorably with known bulk values. This demonstrates that He atom scatteri…
The sharp forward electron-phonon $(FEP)$ and impurity $(FIS)$ scattering change the normal and superconducting properties significantly. The pseudo-gap like features are present in the density of states for $ω<Ω$, where $Ω$ is the phonon frequency. The superconducting critical temperature $T_c$, due to the $FEP$ pairing, is linear with respect to the electron-phonon coupling constant. The $FIS$ impurities are pair weakening for $s-$ and $d-wave$ pairing.
Two-phonon contributions to meV-resolved inelastic x-ray scattering spectra of MgB2 at 300K are identified, in good agreement, in both intensity and energy, with a harmonic calculation using the force constant matrix from ab-inito LDA calculations. This contribution impacts the determination of the linewidth of the E2g phonon mode that is so important for the high Tc of this material. To the best of our knowledge, this is the first observation of peaks in measurements of phonon dispersion (q>0) due to 2-phonon scattering in a non-rare-gas solid.
The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured down to 50 mK. Using the spin-flop transition to switch on and off the acoustic Nd magnons, we can reliably separate the magnon and phonon contributions to heat transport. We find that magnons travel ballistically below 0.5 K, with a thermal conductivity growing as $T^3$, from which we extract their velocity. We show that the rate of scattering of acoustic magnons by phonons grows as $T^3$, and the scattering of phonons by magnons peaks at twice the average Nd magnon frequency.
We propose a new Rashba-free mechanism to realize topological superconductivity with electron-phonon interaction. In the presence of a magnetic field, electron-phonon interaction with small momentum transfer is found to favor spin-triplet Cooper pairing. This process facilitates the formation of chiral topological superconductivity even when Rashba spin-orbital coupling is absent. As a proof of concept, we propose an experimentally feasible heterostructure to systematically study the entangled relationship among forward-phonon scatterings, Rashba spin-orbital couplings, pairing symmetries, and…
