Search

Highlights • Decay properties of quantum-well states of a Pb overlayer on Si(111) are calculated. • Phonon-induced linewidths depend sensitively on binding energy. • Unoccupied Pb-derived states in the Si bulk band gap exhibit moderate linewidths. • Linewidths of occupied Pb states are enhanced by additional decay into Si bands. Graphical abstract Abstract We report a study of the electron–phonon contribution to the linewidth of quantum-well states in a surface superstructure formed by a 4/3-monolayer of Pb on Si(111), a dense phase with a 3 × 3 unit cell. Ab initio calculations based on the density-functional theory were carried out using a linear response approach in the mixed-basis pseudopotential representation. The phonon-induced contribution to the lifetime broadening is analyzed for both excited electrons and holes. The phonon contribution is found to be generally very sensitive to the energy position of the excited electron (hole) except for unoccupied Pb electronic states inside the Si band gap where the phonon-induced linewidth varies smoothly around ∼ 13 meV irrespective of electron energy. This differs for occupied Pb electronic states, which exhibit larger linewidths, or shorter lifetimes, because numerous phonon-mediated transitions to Si electronic bands substantially increase the electron–phonon coupling. [ABSTRACT FROM AUTHOR]

The ferropnictidesuperconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in ≈40 Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to ≈0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons. [ABSTRACT FROM AUTHOR]

The electron-phonon interaction is a major factor influencing the competition between collective instabilities in correlated-electron materials, but its role in driving high-temperature superconductivity in the cuprates remains poorly understood. We have used high-resolution inelastic X-ray scattering to monitor low-energy phonons in YBa2Cu3O6.6 (superconducting transition temperature Tc = 61 K), which is close to a charge-density-wave (CDW) instability. Phonons in a narrow range of momentum space around the CDW ordering vector exhibit extremely large superconductivity-induced line-shape renormalizations. These results imply that the electron-phonon interaction has sufficient strength to generate various anomalies in electronic spectra, but does not contribute significantly to Cooper pairing. In addition, a quasi-elastic 'central peak' due to CDW nanodomains is observed in a wide temperature range above and below Tc, suggesting that the gradual onset of a spatially inhomogeneous CDW domain state with decreasing temperature is a generic feature of the underdoped cuprates. [ABSTRACT FROM AUTHOR]

We have studied the lattice dynamical properties of the tetragonal double-layer manganite system La2-2xSr1+2xMn207 (x = 0.4) within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual-crystal approximation for modeling the alloy. Performing phonon dispersion calculations for both ferromagnetic (FM) and nonmagnetic (NM) ground states in the plane qz = 0 revealed a strong influence of the magnetic order on high-frequency modes which involve oxygen vibrations with Mn-0 bond-stretching modulations. They can be traced back to changes in the dynamical coupling of medium-ranged bonds like Mn-Mn or O-O, while the nearest-neighbor Mn-0 bonds remain practically unchanged. Electronically they originate from a replacement of the predominant Mn d{t2g) orbital polarization in the NM phase by a d(eg)-type polarization in the FM phase. Good agreement is found between theoretical predictions for phonon frequencies and intensities in the [110] direction and a previous neutron scattering experiment performed in the FM phase. Finally, calculations of the electron-phonon interaction revealed that dips in the dispersion of the high-frequency bond-stretching branches are connected to an enhanced electron-phonon coupling, but on average only a moderate coupling constant of &lgr; = 0.65 is obtained. [ABSTRACT FROM AUTHOR]

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron{phonon coupling. YN2B2C was chosen as a test example because it is a conventional superconductor with a fairly high Tc (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron{phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth. [ABSTRACT FROM AUTHOR]

We have studied the lattice dynamical properties of the magnetic intermetallic Ni 1 − x Cu x alloy within the framework of density functional perturbation theory using a mixed-basis pseudopotential method and the self-consistent virtual crystal approximation for modeling the alloy. A transition from a ferromagnetic (FM) to a non-magnetic (NM) state is found at a critical concentration x c =0.45. Phonon spectra as a function of x for both FM and NM states obtained for the entire range of Cu content show good agreement with available experimental data. The magnetic order for x < x c has only a subtle influence on the phonon modes, but induces important changes in the dynamical coupling of first- and second-nearest neighbor bonds. While variations in the first-nearest neighbor coupling are linked to structural, chemical and magnetic changes along the alloy series, the second-nearest neighbor is almost entirely sensitive to the magnetic order only. [ABSTRACT FROM AUTHOR]

We present an investigation of the lattice dynamics of the charge density wave (CDW) compound 2H-NbSe2. We analyze the precise nature of the wave vector-dependent electron-phonon coupling (EPC) and derive the bare dispersion of the CDW soft phonon mode using inelastic x-ray scattering combined with ab initio calculations. Experimentally, phonon modes along the Γ - M line, i.e., q = (h,0,0), with 0 ⩽ h ⩽ 0.5 and the same longitudinal symmetry (Σ1) as the CDW soft mode, were investigated up to 32 meV. In agreement with our calculations, we observe significant EPC in the optic modes at h ⩽ 0.2. We analyze the EPC in the optic, as well as acoustic, mode and show that the q dependences stem from scattering processes between two bands at the Fermi surface that both have a Nb 4d character. Finally, we demonstrate that the soft mode dispersion at T = 33 K (=TCDW) can be well described on the basis of a strongly q-dependent EPC matrix element and an acousticlike bare phonon dispersion in agreement with observations near room temperature. [ABSTRACT FROM AUTHOR]

Highlights • Surface phonon dispersion curves of ultrathin 3–6 ML Pb films on Si(111) and Ge(111) measured with Helium atom scattering (HAS). • Density functional theory accounts for shallow Kohn anomalies observed in surface acoustic phonon branches. • The electron-phonon mass enhancement factor of ultrathin Pb films is measured from specular HAS intensity during growth. Abstract The surface phonon dispersion curves of ultrathin (3–6 ML) Pb layers grown on Si(111) and Ge(111) substrates, measured with inelastic He atom scattering, reveal shallow Kohn anomalies in the surface acoustic branches. Ab-initio calculations based on density functional perturbation theory for the 3–6 monolayers of Pb on a rigid substrate, besides reproducing well the observed dispersion curves, allow for the assignment of the anomalies to well defined flat segments of the Fermi contours. Unlike Pb films grown on Cu(111), where both the optical surface and the interface phonon branches above the bulk frequency maximum are observed, for Si(111) and Ge (111) only the surface-localized branch is observed, indicating the comparatively weaker film-substrate interaction. Consistently with the observation of superconductivity in ultrathin Pb films on Si(111) down to one monolayer (wetting layer), the mass-enhancement factor derived from the specular scattering intensity dependence on the film thickness is found for both substrates in the range of unity, however with slightly larger values for the Ge(111) substrate. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67 under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along the a axis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials. [ABSTRACT FROM AUTHOR]

Motivated by model calculations, for the heavy fermion superconductor CePt3Si predicting phonon anomalies because of antisymmetric spin-orbit coupling, we performed a detailed experimental study of the lattice dynamical properties of CePt3Si. In particular, we investigated the dispersion of transverse acoustic and low energy optic phonon branches along the [110] direction using inelastic neutron scattering. In these branches we found deviations from our ab initio lattice dynamical calculations, which overall give a good description of the phonon dispersion in CePt3Si. However, the agreement for the [110] transverse modes can be improved if we neglect the Ce 4 f states, done in an additional calculation. We conclude that the lattice dynamics of CePt3Si are conventional and that the observed deviations are not related to effects of antisymmetric spin-orbit coupling. More likely, ab initio calculations overestimate the exchange between different phonon branches, particularly in the presence of 4 f electron states. Our results imply that the ASOC plays less a role in noncentrosymmetric superconductors than commonly believed. [ABSTRACT FROM AUTHOR]

We present a combined density functional perturbation theory and inelastic neutron scattering study of the lattice dynamical properties of YNi2B2C. In general, very good agreement was found between theory and experiment for both phonon energies and line widths. Our analysis reveals that the strong coupling of certain low energy modes is linked to the presence of large displacements of the light atoms, i.e., B and C, which is unusual in view of the rather low phonon energies. Specific modes exhibiting a strong coupling to the electronic quasiparticles were investigated as a function of temperature. Their energies and line widths showed marked changes on cooling from room temperature to just above the superconducting transition at Tc = 15.2 K. Calculations simulating the effects of temperature allow us to model the observed temperature dependence qualitatively. [ABSTRACT FROM AUTHOR]

We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi2B2C. Using the sweep mode, a newly developed time-of-flight technique involving the continuous rotation of a single crystal specimen, allowed us to measure a four-dimensional volume in (Q, E) space and, thus, determine the dispersion surface and linewidths of the Au (≈ 102 meV) and Au (≈ 159 meV) type phonon modes over the whole Brillouin zone. Despite of having linewidths of r = 10 meV, A1g modes do not strongly contribute to the total electron-phonon coupling constant A. However, experimental linewidths show a remarkable agreement with ab initio calculations over the complete phonon energy range, demonstrating the accuracy of such calculations in a rare comparison to a comprehensive experimental data set. [ABSTRACT FROM AUTHOR]

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e., q = (0.5, 0, 0.5), at T ≈ TCDW. Detailed ab initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the soft phonon mode. The observed broad range of renormalized phonon frequencies, (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5), is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature. [ABSTRACT FROM AUTHOR]

Motivated by predictions of a substantial contribution of the "buckling" vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below ~105 K, but no corresponding anomaly at q = (0.3, 0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity. [ABSTRACT FROM AUTHOR]

Abstract: Inelastic neutron scattering was used to search for an influence of superconductivity on the phonons in optimally doped and in slightly overdoped Ba(Fe1− x Co x )2As2, x =0.06 and x =0.10. The study focused on phonons with energies close to the superconducting gap energy 2Δ because it is well known that such phonons will respond most strongly to the opening of the gap. We were able to obtain high quality data but nevertheless, we could not detect any influence of superconductivity on the phonons, neither on the linewidths nor on the frequencies. Our results imply that any coupling of low energy phonons to the electrons has to be very small, much smaller than observed in conventional superconductors with a high T c. Our results are in line with the low coupling strength predicted by density functional theory for the investigated phonon branches. [Copyright &y& Elsevier]