Low-Temperature Permittivity of Insulating Perovskite Manganites

Measurements of the low-frequency (f<=100 kHz) permittivity and conductivity at T<= 150 K are reported for La(1-x)Ca(x)MnO(3) (0<=x<=1) and Ca(1-y)Sr(y)MnO(3) (0<=y<=0.75) having antiferromagnetic, insulating ground states covering a broad range of Mn valencies from Mn(3+) to Mn(4+). Static dielectric constants are determined from the low-T limiting behavior. With increasing T, relaxation peaks associated with charge-carrier hopping are observed in the real part of the permittivities and analyzed to determine dopant binding energies. The data are consistent with a simple model of hydrogenic impurity levels and imply effective masses m*/m_e~3 for the Mn(4+) compounds. Particularly interesting is a large dielectric constant (~100) associated with the C-type antiferromagnetic state near the composition La(0.2)Ca(0.8)MnO(3).Comment: 6 pages, 8 figures, PRB in pres

Heat Conduction and Magnetic Phase Behavior in Electron-Doped Ca_{1-x} La_x MnO_3(0 <= x <= 0.2)

Measurements of thermal conductivity (kappa) vs temperature are reported for a series of Ca_{1-x} La_x MnO_3(0 <= x <= 0.2) specimens. For the undoped (x=0), G-type antiferromagnetic compound a large enhancement of kappa below the Neel temperature (T_N ~ 125 K) indicates a strong coupling of heat-carrying phonons to the spin system. This enhancement exhibits a nonmonotonic behavior with increasing x and correlates remarkably well with the small ferromagnetic component of the magnetization reported previously [Neumeier and Cohn, Phys. Rev. B 61 14319 (2000).] Magnetoelastic polaron formation appears to underly the behavior of kappa and the magnetization at x <= 0.02.Comment: submitted to PRB; 4 pp., 4 Fig.'s, RevTex

Atomistic modeling and experimental studies of radiation damage in monazite-type LaPO4 ceramics

We simulated the threshold displacement energies (Ed), the related displacement and defect formation probabilities, and the energy barriers in LaPO4 monazite-type ceramics. The obtained Ed values for La, P, O primary knock-on atoms (PKA) are 56eV, 75eV and 8eV, respectively. We found that these energies can be correlated with the energy barriers that separate the defect from the initial states. The Ed values are about twice the values of energy barriers, which is explained through an efficient dissipation of the PKA kinetic energy in the considered system. The computed Ed were used in simulations of the extent of radiation damage in La0.2Gd0.8PO4 solid solution, investigated experimentally. We found that this lanthanide phosphate fully amorphises in the ion beam experiments for fluences higher than ~1013 ions/cm2

Sediment resuspension and nepheloid layers induced by long internal solitary waves shoaling orthogonally on uniform slopes

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 72 (2014): 21-33, doi:10.1016/j.csr.2013.10.019.Two-dimensional, nonlinear and nonhydrostatic field-scale numerical simulations are used to examine the resuspension, dispersal and transport of mud-like sediment caused by the shoaling and breaking of long internal solitary waves on uniform slopes. The patterns of erosion and transport are both examined, in a series of test cases with varying conditions. Shoreward sediment movement is mainly within boluses, while seaward movement is within intermediate nepheloid layers. Several relationships between properties of the suspended sediment and control parameters are determined such as the horizontal extent of the nehpeloid layers, the total mass of resuspended sediment and the point of maximum bed erosion. The numerical results provide a plausible explanation for acoustic backscatter patterns observed during and after the shoaling of internal solitary wavetrains in a natural coastal environment. The results may further help interpret sedimentary structures that may have been shaped by internal waves and add an another e ective mechanism for o shore dispersal of muddy sediments.This research was funded by the Natural Sciences and Engineering Research Council of Canada (D. Bourgault) and by the Spanish Research Project CGL2009-13254 (M. Morsilli)

Negative thermal expansion of MgB2_{2} in the superconducting state and anomalous behavior of the bulk Gr\"uneisen function

The thermal expansion coefficient $\alpha$ of MgB$_2$ is revealed to change from positive to negative on cooling through the superconducting transition temperature $T_c$. The Gr\"uneisen function also becomes negative at $T_c$ followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.Comment: 5 figures. submitted to Phys. Rev. Let

Inhomogeneous magnetism in La-doped CaMnO3. (I) Nanometric-scale spin clusters and long-range spin canting

Neutron measurements on Ca{1-x}La{x}MnO3 (0.00 <= x <= 0.20) reveal the development of a liquid-like spatial distribution of magnetic droplets of average size ~10 Angstroms, the concentration of which is proportional to x (one cluster per ~60 doped electrons). In addition, a long-range ordered ferromagnetic component is observed for ~0.05 < x < ~0.14. This component is perpendicularly coupled to the simple G-type antiferromagnetic (G-AFM) structure of the undoped compound, which is a signature of a G-AFM + FM spin-canted state. The possible relationship between cluster formation and the stabilization of a long-range spin-canting for intermediate doping is discussed.Comment: Submitted to Physical Review