C. Marini, C. Mirri, G. Profeta, S. Lupi, D. Di Castro, R. Sopracase, P. Postorino, P. Calvani, A. Perucchi, S. Massidda, G. M. Tropeano, M. Putti, A. Martinell… 展开作者
C. Marini, C. Mirri, G. Profeta, S. Lupi, D. Di Castro, R. Sopracase, P. Postorino, P. Calvani, A. Perucchi, S. Massidda, G. M. Tropeano, M. Putti, A. Martinelli, A. Palenzona, P. Dore
We measured the Raman and the Infrared phonon spectrum of SmFeAsO polycrystalline samples. We also performed Density Functional Theory calculations within the pseudopotential approximation to obtain the structural and dynamical lattice properties of both the SmFeAsO and the prototype LaFeAsO compounds. The measured Raman and Infrared phonon frequencies are well predicted by the optical phonon frequencies computed at the Gamma point, showing the capability of the employed ab-initio methods to describe the dynamical properties of these materials. A comparison among the phonon frequencies of different oxypnictides suggests a possible role of the high frequency phonons in the pairing mechanism leading to superconductivity in these materials.
The interplay between spin dynamics and lattice vibration has been suggested as an important part of the puzzle of high-temperature superconductivity. Here we report the strong interaction between spin fluctuation and phonon in SmFeAsO, a parent compound of the iron arsenide family of superconductors, revealed by low-temperature Raman spectroscopy. Anomalous zone-boundary-phonon Raman scattering from spin superstructure was observed at temperatures below the antiferromagnetic ordering point, which offers compelling evidence on spin dependent electron-phonon coupling in pnictides.
We report measurements of the phonon density of states as measured with inelastic x-ray scattering in SmFeAsO$_{1-x}$F$_y$ powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO$_{1-x}$F$_y$ single crystals allow us to identify the 21 meV A$_{1g}$ in-phase (Sm,As) and the 26 meV B$_{1g}$ (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.
Hydrostatic pressure Raman measurements have been carried out on the SmFeAsO series of oxypnictides with varying amount of doping (F substitution for O and Co for Fe) and transition temperature (T_{c}), in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with increasing F concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 …
We report inelastic x-ray scattering experiments on the lattice dynamics in SmFeAsO and superconducting SmFeAsO$_{0.6}$F$_{0.35}$ single crystals. Particular attention was paid to the dispersions along the [100] direction of three optical modes close to 23 meV, polarized out of the FeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron-pnictide…
A comparative study of thermal properties of SmFeAsO, SmFeAs(O0.93F0.07) and SmFeAs(O0.85F0.15) samples is presented. Specific heat and thermal conductivity show clear evidences of the spin density wave (SDW) ordering below TSDW 135 K in undoped SmFeAsO. At low level of F-doping, SmFeAs(O0.93F0.07), SDW ordering is suppressed and superconducting features are not yet optimally developed in both specific heat and thermal conductivity. At optimal level of F-doping SmFeAs(O0.85F0.15) anomalies related to the superconducting transition are well noticeable. By a compared analysis of doped and undope…
We measured dispersive spin excitations in $\mathrm{SmFeAsO}$, parent compound of $\mathrm{SmFeAsO_{\text{1-x}}F_{\text{x}}}$ one of the highest temperature superconductors of Fe pnictides (T$_{\text{C}}\approx$55~K). We determine the magnetic excitations to disperse with a bandwidth energy of ca 170 meV at (0.47, 0) and (0.34, 0.34), which merges into the elastic line approaching the $Γ$ point. Comparing our results with other parent Fe pnictides, we show the importance of structural parameters for the magnetic excitation spectrum, with small modifications of the tetrahedron angles and As hei…
Single crystals of LaFeAsO, NdFeAsO, and SmFeAsO have been prepared by means of a NaAs flux growth technique and studied by optical spectroscopy measurements. We show that the spectral features corresponding to the partial energy gaps in the spin-density-wave (SDW) state are present below the structural phase transition. This indicates that the electronic state below the structural phase transition is already very close to that in the SDW state. We also show that in-plane infrared phonon modes display systematic shifts towards high frequency upon rare-earth element substitutions for La, sugges…
Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states for superconducting MgCNi_3, T_c=8 K, give evidence for a soft-mode behavior of low-frequency Ni phonon modes. Results are compared with ab initio density functional calculations which suggest an incipient lattice instability of the stoichiometric compound with respect to Ni vibrations orthogonal to the Ni-C bond direction.
The ferropnictide superconductors 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 develo…
The anomalous features of the Raman spectroscopy measurement in MgB$_2$ represent a still unresolved puzzle. In particular highly debated are the origin of the huge $E_{2g}$ phonon linewidth, the nature of the low energy ($ω< ω_{E_{2g}}$) background and the evolution of the Raman spectra with Al doping. In this paper we compute the self-energy of the $E_{2g}$ phonon mode in a fully self-consistent way taking into account electron-phonon effects on the electronic properties. We show that all the anomalous features can be naturally understood in a framework where the whole electron-phonon spectr…
We study superconductivity in doped solid picene (C22H14) with linear response calculations of the phonon spectrum and electron-phonon (ep) interaction. We show that the coupling of the high-energy C bond-stretching phonons to the π molecular orbitals for a doping of ~3 electrons per picene molecule is sufficiently strong to reproduce the experimental Tc of 18 K within Migdal-Eliashberg theory. For hole doping, we predict a similar coupling leading to a maximum Tc of 6 K. However, we argue that, due to its molecular nature, picene may belong to the same class of strongly correlated ep supercon…
Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed …
There is an ongoing debate about the nature of the bosonic excitations responsible for the quasiparticle self energy in high temperature superconductors -- are they phonons or spin fluctuations? We present a careful analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV in angle resolved photoemission data using Eliashberg theory for a d-wave superconductor. Starting from the assumption that nodal quasiparticles are not coupled to the $(π,π)$ magnetic resonance, the sharp structure at $70 $meV can be assigned to phonons. We find that not only can we account for the shi…
We study the zero-temperature ($T = 0$) quantum rotor model with on-site disorder in the charging energy. Such a model may serve as an idealized Hamiltonian for an array of Josephson-coupled small superconducting grains, or superfluid $^4$He in a disordered environment. In the approximation of small-amplitude phase fluctuations, the Hamiltonian maps onto a system of coupled harmonic oscillators with on-site disorder. We study the effects of disorder in this harmonic regime, using the coherent potential approximation (CPA), obtaining the density of states and the lifetimes of the spin-wave-like…
We analyse a model where the anomalies of the bond-stretching LO phonon mode are caused by the coupling to electron dynamic response in the form of a damped oscillator and explore the possibility to reconstruct the spectrum of the latter from the phonon measurements. Preliminary estimates point to its location in the mid infrared region and we show how the required additional information can be extracted from the oxygen isotope effect on the phonon spectrum. The model predicts a significant measurable deviation from the "standard value" of the isotope effect even if the phonon frequency is far…
We present results of ab-initio calculations of the electronic band structure, lattice dynamical properties, and elastic constants of LaOFeAs, the parent compound of the recently discovered superconducting iron-oxypnictide (LaO1-xFxFeAs). The total and partial electronic density of states (EDOS) of the undoped LaOFeAs (in the insulating or metallic state) are extracted from the electronic band structure. The phonon dispersion and the phonon density of states (PDOS) are also studied. Possible implications of the band structure, EDOS, and PDOS of LaOFeAs on the eventual appearance of high-Tc sup…
We have performed Raman-scattering measurements on a high-quality single crystal of the recently discovered Fe-based superconductor K$_{0.8}$Fe$_{1.6}$Se$_2$ ($T_c$ = 32 K). At least thirteen phonon modes were observed in the wave number range 10$-$300 cm$^{-1}$. The spectra possess a four-fold symmetry indicative of bulk vacancy order in the Fe-deficient planes. We perform a vibration analysis based on first-principles calculations, which both confirms the ordered structure and allows a complete mode assignment. We observe an anomaly at $T_c$ in the 180 cm$^{-1}$ $A_g$ mode, which indicates a…
We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C$_{22}$H$_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the phonon peaks experimentally observed . Through a detailed analysis of the phonon eigenvectors, We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these p…
The cuprates exhibit a prominent charge-density-wave (CDW) instability with wavevector along [100], i.e., the Cu-O bond direction. Whereas CDW order is most prominent at moderate doping and low temperature, there exists increasing evidence for dynamic charge correlations throughout a large portion of the temperature-doping phase diagram. In particular, signatures of incipient charge order have been observed as phonon softening and/or broadening near the CDW wavevector approximately half-way through the Brillouin zone. Most of this work has focused on moderately-doped cuprates, for which the CD…
We reinterpret neutron scattering and Raman spectroscopy experiments in La(2-x)Sr(x)CuO(4), showing modulation of spin and lattice degrees of freedom both in the low-temperature orthorhombic and tetragonal phases, in terms of a magneto-elastic coupling between noncollinear configurations for the copper spins and the oxygen octahedra. The magneto-elastic coupling furthermore explains the recently discovered magnetic-field induced untwinning in La(2-x)Sr(x)CuO(4), for x<2%, and allows us to understand why in La(2-x-y)Sr(x)Nd(y)CuO(4) with y=0.4 and x=1/8, for example, when (1,0) and/or (0,1) sta…
We study influence of the phonon modes on a subgap spectrum and Andreev conductance for the double quantum dot vertically coupled between a metallic and superconducting lead. For the monochromatic phonon reservoir we obtain replicas of the interferometric Fano-type structures appearing simultaneously in the particle and hole channels. We furthermore confront the induced on-dot pairing with the electron correlations and investigate how the phonon modes affect the zero-bias signature of the Kondo effect in Andreev conductance.
Features of the phonon spectrum of a chiral crystal are examined within the micropolar elasticity theory. This formalism accounts for not only translational micromotions of a medium but also rotational ones. It is found that there appears the phonon band splitting depending on the left/right-circular polarization in a purely phonon sector without invoking any outside subsystem. The phonon spectrum reveals parity breaking while preserving time-reversal symmetry, i.e. it possesses true chirality. We find that hybridization of the micro-rotational and translational modes gives rise to the acousti…
We study an interacting spinless Bose-Einstein condensate to clarify theoretically whether the spectra of its quasiparticles (one-particle excitations) and collective modes (two-particle excitations) are identical, as concluded by Gavoret and Nozières [Ann. Phys. 28, 349 (1964)]. We derive analytic expressions for their first and second moments so as to extend the Bijl-Feynman formula for the peak of the collective-mode spectrum to its width (inverse lifetime) and also to the one-particle channel. The obtained formulas indicate that the width of the collective-mode spectrum manifestly vanishes…
