Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper-oxide superconductors

The attempt to understand cuprate superconductors is complicated by the presence of multiple strong interactions. While many believe that antiferromagnetism is important for the superconductivity, there has been revived interest in the role of electron-lattice coupling. The recently studied conventional superconductor MgB2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon), that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bond-stretching phonon in the cuprate superconductors La2-xSrxCuO4 (with x=0.07, 0.15). This is in contrast to conventional theory, which does not predict such behavior. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order. It occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron-phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron-phonon coupling may be important to our understanding of superconductivity, although its contribution to the mechanism is likely to be indirect.Comment: to appear in Nature, 16 pages, 4 figures, very minor changes in text and figures from previous versio

A study of the condensed phases and solid-solid phase transition in toluene: A Monte Carlo investigation

A Monte Carlo study of the orthorhombic(beta), monoclinic(alpha), and liquid phases of toluene in the isobaric isothermal ensemble employing variable shape simulation cell is reported here. The intermolecular potential of Williams and Starr is seen to reproduce the lattice parameters and other known properties reasonably well for the alpha-phase. The beta-phase is not reproduced as well. The structure has been characterized in terms of the radial distribution functions and orientational correlation functions. The transition from the orthorhombic low temperature beta-phase to the high temperature monoclinic alpha-phase has been successfully simulated. The transition is first order and lies between 140 and 145 K in agreement with experiment. The reverse transition from the alpha- to the beta-phase does not take place in agreement with experiment. The liquid phase density and the heat of vaporization are reproduced well. The potential employed predicts an interaction energy which is about 5% in excess of the experimental value. The orientational correlation function and the radial distribution functions are sensitive to the potential and suggest where improvements are possible

Dynamics of propane in Na-Y zeolite

The dynamics of propane confined in Na-Y zeolite was studied using the quasi-elastic neutron scattering technique at different temperatures. The data were analysed using a model in which the propane molecules undergo random-walk diffusion characterised by a Gaussian distribution of jump lengths inside zeolite cages. The diffusion constant and root-mean-square jump length were determined in the temperature range of 300-350 K

Diffusion of 1,3-butadiene adsorbed in Na–Y zeolite: Neutron scattering study

Here we report the quasi elastic neutron scattering (QENS) study of the dynamics of 1,3-butadiene molecules adsorbed in Na–Y zeolite at saturation loading. Data analysis showed that the adsorbed 1,3-butadiene molecules undergo motion, which could be described by jump diffusion model. Within this, various plausible theoretical models have been considered to describe the experimentally observed data. Residence time and the jump length are evaluated for all the models considered. The diffusivity values obtained for the three models are found to be consistent within themselves and also while compared with those obtained for other guest molecules studied earlier

Phonons and oxygen diffusion in Bi2O3 and (Bi0.7Y0.3)2O3

We report investigation of phonons and oxygen diffusion in Bi2O3 and (Bi0.7Y0.3)2O3. The phonon spectra have been measured in Bi2O3 at high temperatures up to 1083 K using inelastic neutron scattering. Ab-initio calculations have been used to compute the individual contributions of the constituent atoms in Bi2O3 and (Bi0.7Y0.3)2O3 to the total phonon density of states. Our computed results indicate that as temperature is increased, there is a complete loss of sharp peak structure in the vibrational density of states. Ab-initio molecular dynamics simulations show that even at 1000 K in δ-phase Bi2O3, Bi-Bi correlations remain ordered in the crystalline lattice while the correlations between O-O show liquid like disordered behavior. In the case of (Bi0.7Y0.3)2O3, the O-O correlations broadened at around 500 K indicating that oxygen conductivity is possible at such low temperatures in (Bi0.7Y0.3)2O3 although the conductivity is much less than that observed in the undoped high temperature δ-phase of Bi2O3. This result is consistent with the calculated diffusion coefficients of oxygen and observation by QENS experiments. Our ab-initio molecular dynamics calculations predict that macroscopic diffusion is attainable in (Bi0.7Y0.3)2O3 at much lower temperatures, which is more suited for technological applications. Our studies elucidate the easy directions of diffusion in δ-Bi2O3 and (Bi0.7Y0.3)2O3