W. Kress

Lattice vibrations of Y1-xPrxBa2Cu3O7 : theory and experiment

Abstract We report measurements of the far-infrared reflectance of Y 1− x Pr x Ba 2 Cu 3 O 7 thin films. Detailed understanding of the experimental lineshapes is achieved with the proper treatment of the SrTiO 3 substrate, the finite film thicknesses, the background contribution of free carriers and the mid-infrared band, and the phonon modes. Shell-model calculations predict the frequencies of the infrared-active phonons to be very similar for YBa 2 Cu 3 O 7 and PrBa 2 Cu 3 O 7 with the exception of rare-earth-related vibrations. The experimental frequencies of the in-plane as well as z -polarized infrared-active phonons of PrBa 2 Cu 3 O 7 agree satisfactorily with the calculated ones. An in-plane mode at 270 cm -1 , assigned to vibrations confined to the Y/Pr and neighboring oxygen ions, is found to be sensitive to the rare-earth-ion substitution. We report also the frequencies of the Raman-active B 2g and B 3g phonons of PrBa 2 Cu 3 O 7 , thus giving a nearly complete picture of all k =0 eigenmode frequencies in the RBa 2 Cu 3 O 7 crystal structure.

Lattice dynamics of the high-Tc superconductor La2−xMxCuO4

Abstract The phonon dispersion curves, the one-phonon density of states and the specific heat of the high-T c superconductor La 2− x M x CuO 4 have been calculated in the framework of a shell model. The zone center frequencies are classified according to their vibrational character and compared with the available IR and Raman data. A slight adjustment of the parameters obtained from the known interaction potentials of oxides with perovskite and rock salt structure yields reasonable agreement with the IR, Raman, specific heat, and inelastic neutron scattering data of the polycrystalline material.

Phonons in Nd2−xCexCuO4

We have measured far infrared reflection and Raman spectra on high quality single crystals and ceramic samples of the Nd2−xCexCuO4 system. The TO and LO frequencies of all infrared-active phonons were determined from a Kramers-Kronig analysis of the infrared reflection spectra. The frequencies of the detected Raman-active phonons are 228 cm−1 (A1g Nd), 328 cm−1 (B1g O2) and 480 cm−1 (Eg 02). A resonantly enhanced peak at 580 cm−1 is observed for Ce-doped samples only with polarization properties like an A1g phonon. We have also performed lattice-dynamical calculations with parameters extracted from those of perovskites and metallic oxides. The frequencies obtained agree well with our experimental findings. We present the eigenvectors of all zone center phonons. Finally, we also detected a feature at 165 cm−1 in the ab-polarized infrared spectra in Nd2CuO4 which might be of magnetic origin.

Effect of isotopic substitution of oxygen on Tc and the phonon frequencies of high Tc superconductors

Abstract The Raman and infrared spectra of YBa2Cu318Oβ16O7−β and TmBa2Cu318Oβ16O7−β with various amounts β of 16O substituted by 18O have been measured. The observed phonon frequency shifts of the ir and Raman active modes are compared with predictions from a recently proposed model for the lattice dynamics of these compounds. The application of this model allows a detailed study of the frequency shifts induced by istope substitution of various inequivalent oxygen positions (OI, OII, III, OIV). The resistive transitions of these samples have also been measured. No isotope effect is ound if only one out of seven 16O is replaced by 18O. For 85% substition, a shift of ΔTc = 0.3±0.1 K is observed in agreement with previous determinations.

Effect of pressure on Raman modes in MBa2Cu3O7-type materials

Abstract The effect of pressure on Raman modes of ceramic superconducting samples of MBa 2 Cu 3 O 7−x (M = Y, Eu) has been investigated up to 16 GPa at 300 K. Linear pressure coefficients are reported for the strongest modes at around 145, 300–340, 440, and 500 cm −1 . All these modes exhibit an increase in frequency, with average mode Gruneisen pameters falling in the range from 1.2 to 2.2. The pressure dependence of the mode frequencies is calculated within the framework of a shell model, which has recently been used to aid the mode assignment. Calculated mode Gruneisen parameters are found to be in satisfactory agreement with experiment if the proper volume dependence of the polarizability is taken into account.

Phonon anomalies in transition metal nitrides: δ-NbN

Abstract The phonon dispersion curves in δ-NbN have been measured at room temperature in the high symmetry directions Δ, Σ, and ʌ by coherent inelastic neutron scattering. Anomalies in the dispersion of the acoustic branches have been detected which are quite different from those which have already been reported for superconducting transition metal compounds with 9 valence electrons. In the 10 valence electron compound δ-NbN the soft mode region has moved from the middle of the Σ-direction to the zone boundary. The experimental results are well described by a double shell model which has also been used to calculate the density of states. Using the formalism of Varma and Weber the measured dispersion curves have been reproduced. This shows that the change in the anomalies when going from 9 to 10 valence electrons, is due to a shift in Fermi energy in an essentially rigid band scheme.

Lattice dynamics of high-Tc superconductors: Pb2Sr2YCu3O8

Abstract Raman and infrared active modes of Pb2Sr2YCu3O8 have been calculated in the framework of a shell model which uses short range overlap and long range Coulomb potentials. Eigenfrequencies and eigenvectors for the zone center modes are presented and compared with available Raman scattering data. The agreement is excellent.

Relaxation and dynamics of the (001) surfaces of KZnF3and KMnF3

Abstract We report calculations of the static and dynamical properties of the K-F and Zn-F 2 (Mn-F 2 ) (001) surfaces of the fluoridic perovskites KZnF 3 and KMnF 3 . Several surfacelocalized modes have been identified in both relaxed and unrelaxed geometries. In particular a soft mode at M was obtained for the K-F surface of KMnF 3 . The surface reconstruction associated with this soft mode is discussed: it can be considered as a precursor of the bulk transition.

Phonon dispersion curves in dilute KTN crystals

The phonon dispersion curves of the transverse acoustic (TA) and the lowest transverse optic (TO) branches in the (001) direction of two (KTN) crystals with niobium concentrations of 0.8 and 1.2 at.% have been measured by inelastic neutron scattering at various temperatures. A pronounced temperature dependence has been observed for both the lowest TO phonon at the centre of the Brillouin zone and the TA phonons in the (001) direction for reduced wave vectors larger than 0.04. The temperature dependence of the TA phonons at large wavevectors shows a significant minimum. From the observation of this minimum it is conjectured that both crystals undergo phase transitions. The measured data have been analysed in the framework of a shell model with an anisotropic, nonlinear core - shell interaction. It is shown that the anomalous behaviour of the TA branch is due to an exchange of vibrational character between the phonons of the lowest TO branch and those of the TA branch. An estimate of the size of dipolar clusters around the niobium impurities is derived from the analysis of the dispersion of the lowest TO modes in the long-wavelength range.

Mode softening in quantum ferroelectric KTa1-xNbXO3

Abstract The temperature dependence of the lowest frequency TO and of the corresponding TA phonon branches in the (00ζ) direction has been investigated by means of coherent inelastic neutron scattering in KTa1-xNbxO3 for two different Nb concentrations x1 = 0. 008 and X2 = 0.012.