X-ray scattering from surfaces: discrete and continuous components of roughness

Incoherent surface scattering yields a statistical description of the surface, due to the ensemble averaging over many independently sampled volumes. Depending on the state of the surface and direction of the scattering vector relative to the surface normal, the height distribution is discrete, continuous, or a combination of the two. We present a treatment for the influence of multimodal surface height distributions on Crystal Truncation Rod scattering. The effects of a multimodal height distribution are especially evident during in situ monitoring of layer-by-layer thin-film growth via Pulsed Laser Deposition. We model the total height distribution as a convolution of discrete and continuous components, resulting in a broadly applicable parameterization of surface roughness which can be applied to other scattering probes, such as electrons and neutrons. Convolution of such distributions could potentially be applied to interface or chemical scattering. Here we find that this analysis describes accurately our experimental studies of SrTiO3 annealing and homoepitaxial growth.Comment: 15 pages, 7 figure

Double exchange-driven spin pairing at the (001) surface of manganites

The (001) surface of La_{1-x}Ca_xMnO_3 system in various magnetic orderings is studied by first principle calculations. A general occurrence is that z^2 dangling bond charge -- which is ``invisible'' in the formal valence picture -- is promoted to the bulk gap/Fermi level region. This drives a double-exchange-like process that serves to align the surface Mn spin with its subsurface neighbor, regardless of the bulk magnetic order. For heavy doping, the locally ``ferromagnetic'' coupling is very strong and the moment enhanced by as much as 30% over the bulk value.Comment: 6 pages, 4 figure

Spin-Charge Coupling in lightly doped Nd2−x_{2-x}Cex_{x}CuO4_4

We use neutron scattering to study the influence of a magnetic field on spin structures of Nd$_2$CuO$_4$. On cooling from room temperature, Nd$_2$CuO$_4$ goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. While a c-axis aligned magnetic field does not alter the basic zero-field noncollinear spin structures, a field parallel to the CuO$_2$ plane can transform the noncollinear structure to a collinear one ("spin-flop" transition), induce magnetic disorder along the c-axis, and cause hysteresis in the AF phase transitions. By comparing these results directly to the magnetoresistance (MR) measurements of Nd$_{1.975}$Ce$_{0.025}$CuO$_4$, which has essentially the same AF structures as Nd$_2$CuO$_4$, we find that a magnetic-field-induced spin-flop transition, AF phase hysteresis, and spin c-axis disorder all affect the transport properties of the material. Our results thus provide direct evidence for the existence of a strong spin-charge coupling in electron-doped copper oxides.Comment: 12 pages, 12 figure

Structure of MnO nanoparticles embedded into channel-type matrices

X-ray diffraction experiments were performed on MnO confined in mesoporous silica SBA-15 and MCM-41 matrices with different channel diameters. The measured patterns were analyzed by profile analysis and compared to numerical simulations of the diffraction from confined nanoparticles. From the lineshape and the specific shift of the diffraction reflections it was shown that the embedded objects form ribbon-like structures in the SBA-15 matrices with channels diameters of 47-87 {\AA}, and nanowire-like structures in the MCM-41 matrices with channels diameters of 24-35 {\AA}. In the latter case the confined nanoparticles appear to be narrower than the channel diameters. The physical reasons for the two different shapes of the confined nanoparticles are discussed.Comment: 8 pages, including 9 postscript figures, uses revtex4.cl

Magnetic reconstruction at (001) CaMnO3_3 surface

The Mn-terminated (001) surface of the stable anti-ferromagnetic insulating phase of cubic perovskite CaMnO$_3$ is found to undergo a magnetic reconstruction consisting on a spin-flip process at surface: each Mn spin at the surface flips to pair with that of Mn in the subsurface layer. In spite of very little Mn-O charge transfer at surface, the surface behavior is driven by the $e_g$ states due to $d_{xy}$ $\to$ $d_{z^2}$ charge redistribution. These results, based on local spin density theory, give a double exchange like coupling that is driven by $e_g$ character, not additional charge, and may have relevance to CMR materials.Comment: 4 pages, 5 figures reference added Fig. 3 modified. Caption of Fig. 5 modifie

Can Reproductive Health Voucher Programs Improve Quality of Postnatal Care? A Quasi-Experimental Evaluation of Kenya’s Safe Motherhood Voucher Scheme

This study tests the group-level causal relationship between the expansion of Kenya’s Safe Motherhood voucher program and changes in quality of postnatal care (PNC) provided at voucher-contracted facilities. We compare facilities accredited since program inception in 2006 (phase I) and facilities accredited since 2010-2011 (phase II) relative to comparable non-voucher facilities. PNC quality is assessed using observed clinical content processes, as well as client-reported outcome measures. Two-tailed unpaired t-tests are used to identify differences in mean process quality scores and client-reported outcome measures, comparing changes between intervention and comparison groups at the 2010 and 2012 data collection periods. Difference-in-differences analysis is used to estimate the reproductive health (RH) voucher program’s causal effect on quality of care by exploiting group-level differences between voucher-accredited and non-accredited facilities in 2010 and 2012. Participation in the voucher scheme since 2006 significantly improves overall quality of postnatal care by 39% (p=0.02), where quality is defined as the observable processes or components of service provision that occur during a PNC consultation. Program participation since phase I is estimated to improve the quality of observed maternal postnatal care by 86% (p=0.02), with the largest quality improvements in counselling on family planning methods (IRR 5.0; p=0.01) and return to fertility (IRR 2.6; p=0.01). Despite improvements in maternal aspects of PNC, we find a high proportion of mothers who seek PNC are not being checked by any provider after delivery. Additional strategies will be necessary to standardize provision of packaged postnatal interventions to both mother and new-born. This study addresses an important gap in the existing RH literature by using a strong evaluation design to assess RH voucher program effectiveness on quality improvement

Poisson-Boltzmann Theory of Charged Colloids: Limits of the Cell Model for Salty Suspensions

Thermodynamic properties of charge-stabilised colloidal suspensions are commonly modeled by implementing the mean-field Poisson-Boltzmann (PB) theory within a cell model. This approach models a bulk system by a single macroion, together with counterions and salt ions, confined to a symmetrically shaped, electroneutral cell. While easing solution of the nonlinear PB equation, the cell model neglects microion-induced correlations between macroions, precluding modeling of macroion ordering phenomena. An alternative approach, avoiding artificial constraints of cell geometry, maps a macroion-microion mixture onto a one-component model of pseudo-macroions governed by effective interactions. In practice, effective-interaction models are usually based on linear screening approximations, which can accurately describe nonlinear screening only by incorporating an effective (renormalized) macroion charge. Combining charge renormalization and linearized PB theories, in both the cell model and an effective-interaction (cell-free) model, we compute osmotic pressures of highly charged colloids and monovalent microions over a range of concentrations. By comparing predictions with primitive model simulation data for salt-free suspensions, and with predictions of nonlinear PB theory for salty suspensions, we chart the limits of both the cell model and linear-screening approximations in modeling bulk thermodynamic properties. Up to moderately strong electrostatic couplings, the cell model proves accurate in predicting osmotic pressures of deionized suspensions. With increasing salt concentration, however, the relative contribution of macroion interactions grows, leading predictions of the cell and effective-interaction models to deviate. No evidence is found for a liquid-vapour phase instability driven by monovalent microions. These results may guide applications of PB theory to soft materials.Comment: 27 pages, 5 figures, special issue of Journal of Physics: Condensed Matter on "Classical density functional theory methods in soft and hard matter

Charge Renormalization, Effective Interactions, and Thermodynamics of Deionized Colloidal Suspensions

Thermodynamic properties of charge-stabilised colloidal suspensions depend sensitively on the effective charge of the macroions, which can be substantially lower than the bare charge in the case of strong counterion-macroion association. A theory of charge renormalization is proposed, combining an effective one-component model of charged colloids with a thermal criterion for distinguishing between free and associated counterions. The theory predicts, with minimal computational effort, osmotic pressures of deionized suspensions of highly charged colloids in close agreement with large-scale simulations of the primitive model.Comment: 15 pages, 7 figure

BRYNTRN: A baryon transport model

The development of an interaction data base and a numerical solution to the transport of baryons through an arbitrary shield material based on a straight ahead approximation of the Boltzmann equation are described. The code is most accurate for continuous energy boundary values, but gives reasonable results for discrete spectra at the boundary using even a relatively coarse energy grid (30 points) and large spatial increments (1 cm in H2O). The resulting computer code is self-contained, efficient and ready to use. The code requires only a very small fraction of the computer resources required for Monte Carlo codes

Renormalization group approach to multiscale modelling in materials science

Dendritic growth, and the formation of material microstructure in general, necessarily involves a wide range of length scales from the atomic up to sample dimensions. The phase field approach of Langer, enhanced by optimal asymptotic methods and adaptive mesh refinement, copes with this range of scales, and provides an effective way to move phase boundaries. However, it fails to preserve memory of the underlying crystallographic anisotropy, and thus is ill-suited for problems involving defects or elasticity. The phase field crystal (PFC) equation-- a conserving analogue of the Hohenberg-Swift equation --is a phase field equation with periodic solutions that represent the atomic density. It can natively model elasticity, the formation of solid phases, and accurately reproduces the nonequilibrium dynamics of phase transitions in real materials. However, the PFC models matter at the atomic scale, rendering it unsuitable for coping with the range of length scales in problems of serious interest. Here, we show that a computationally-efficient multiscale approach to the PFC can be developed systematically by using the renormalization group or equivalent techniques to derive appropriate coarse-grained coupled phase and amplitude equations, which are suitable for solution by adaptive mesh refinement algorithms