Singular Effect of Disorder on Electronic Transport in Strong Coupling Electron-Phonon Systems

We solve the disordered Holstein model in three dimensions considering the phonon variables to be classical. After mapping out the phases of the `clean' strong coupling problem, we focus on the effect of disorder at strong electron-phonon (EP) coupling. The presence of even weak disorder (i) enormously enhances the resistivity (\rho) at T=0, simultaneously suppressing the density of states at the Fermi level, (ii) suppresses the temperature dependent increase of \rho, and (iii) leads to a regime with d\rho/dT <0. We locate the origin of these anomalies in the disorder induced tendency towards polaron formation, and the associated suppression in effective carrier density and mobility. These results, explicitly at `metallic' density, are of direct relevance to disordered EP materials like covalent semiconductors, the manganites, and to anomalous transport in the A-15 compounds.Comment: Final versio

Structural Basis for the Anomalously Low Spontaneous Polarisation Values of the Polar Phase of Sr1-xCaxTiO3 (x=0.02, 0.04): Evidence for a Ferrielectric Ordering

Full pattern Le-Bail refinement using x-ray powder diffraction profiles of Sr1-xCaxTiO3 for x=0.02, 0.04 in the temperature range 12 to 300 K reveals anomalies in the unit cell parameters at 170, 225 K due to an antiferrodistortive (cubic to tetragonal I4/mcm) phase transition and at ~32, ~34 K due to a transition to a polar phase (tetragonal I4/mcm to orthorhombic Ic2m), respectively. The lower transition temperatures obtained by us are in excellent agreement with those reported on the basis of the dielectric studies by Bednorz and Muller, [10] who attributed these to ferroelectric transition. Rietveld analysis of the diffraction profiles of the polar phase reveals off-centre displacements of both Sr2+/Ca2+ and Ti4+ ions in the X-Y plane along pseudocubic directions, in agreement with the experimentally reported direction of easy polarization by Bednorz and Muller, but the resulting dipole moments are shown to be ferrielectrically coupled in the neighbouring (001) planes along the [001] direction leading to anomalously low values of the spontaneous polarization at 12K.Comment: 5 pages, 4 figures and 1 tabl

Electroweak Measurements of Neutron Densities in CREX and PREX at JLab, USA

Measurement of the parity-violating electron scattering asymmetry is an established technique at Jefferson Lab and provides a new opportunity to measure the weak charge distribution and hence pin down the neutron radius in nuclei in a relatively clean and model-independent way. This is because the Z boson of the weak interaction couples primarily to neutrons. We will describe the PREX and CREX experiments on ${}^{208}$Pb and ${}^{48}$Ca respectively; these are both doubly-magic nuclei whose first excited state can be discriminated by the high resolution spectrometers at JLab. The heavier lead nucleus, with a neutron excess, provides an interpretation of the neutron skin thickness in terms of properties of bulk neutron matter. For the lighter ${}^{48}$Ca nucleus, which is also rich in neutrons, microscopic nuclear theory calculations are feasible and are sensitive to poorly constrained 3-neutron forces.Comment: A contribution to the upcoming EPJA Special Volume on Nuclear Symmetry Energ