Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Implementations of Nosé-Hoover and Nosé-Poincaré thermostats in mesoscopic dynamic simulations with the united-residue model of a polypeptide chain

Molecular dynamics (MD) simulations generate a canonical ensemble only when integration of the equations of motion is coupled to a thermostat. Three extended phase space thermostats, one version of Nose-Hoover and two versions of Nose-Poincare, are compared with each other and with the Berendsen thermostat and Langevin stochastic dynamics. Implementation of extended phase space thermostats was first tested on a model Lennard-Jones fluid system; subsequently, they were implemented with our physics-based protein united-residue (UNRES) force field MD. The thermostats were also implemented and tested for the multiple-time-step reversible reference system propagator (RESPA). The velocity and temperature distributions were analyzed to confirm that the proper canonical distribution is generated by each simulation. The value of the artificial mass constant, Q, of the thermostat has a large influence on the distribution of the temperatures sampled during UNRES simulations (the velocity distributions were affected only slightly). The numerical stabilities of all three algorithms were compared with each other and with that of microcanonical MD. Both Nose-Poincare thermostats, which are symplectic, were not very stable for both the Lennard-Jones fluid and UNRES MD simulations started from nonequilibrated structures which implies major changes of the potential energy throughout a trajectory. Even though the Nose-Hoover thermostat does not have a canonical symplectic structure, it is the most stable algorithm for UNRES MD simulations. For UNRES with RESPA, the "extended system inside-reference system propagator algorithm" of the RESPA implementation of the Nose-Hoover thermostat was the only stable algorithm, and enabled us to increase the integration time step.     

Simplified categorization of outdoor activities for male and female U.S. indoor workers-a feasibility study to improve assessment of ultraviolet radiation exposures in epidemiologic study questionnaires

Skin cancer studies depend on questionnaires to estimate exposure to ultraviolet light and subsequent risk but are limited by recall bias. We investigate the feasibility of developing a short checklist of categories comprising outdoor activities that can improve sun exposure questionnaires for use in epidemiologic studies. We recruited 124 working and retired U.S. radiologic technologists (52% women). Each subject was instructed to complete a daily activity diary, listing main indoor and outdoor activities between 9:00 A.M. and 5:00 P.M. during a 7 day period. A total of 4697 entries were associated with 1408 h (21.1%) of the total 6944 h spent outdoors. We were able to classify the activities into seven main activity categories: driving, yard work, home-maintenance, walking or performing errands, water activities, other recreational or sports activities and leisure activities or relaxing outside. These activities accounted for more than 94% of time spent outdoors both for working and retired men and women. Our data document the feasibility and guidance for developing a short checklist of outdoor activities for use in epidemiologic questionnaires for estimating sunlight exposures of working and retired indoor workers.     

Agreement between diary records of time spent outdoors and personal ultraviolet radiation dose measurements

Little is known about the validity of self-recorded sun exposure and time spent outdoors for epidemiological research. The aims of the current study were to assess how well participants' self-recorded time outdoors compared to objective measurements of personal UVR doses. We enrolled 124 volunteers aged 40 and above who were identified from targeted subgroups of US radiologic technologists. Each volunteer was instructed to wear a polysulfone (PS) dosimeter to measure UVR on their left shoulder and to complete a daily activity diary, listing all activities undertaken in each 30 min interval between 9:00 A.M. and 5:00 P.M. during a 7 day period. In a linear regression model, self-recorded daily time spent outdoors was associated with an increase of 8.2% (95% CI: 7.3-9.2%) in the personal UVR exposure with every hour spent outdoors. The amount of self-recorded total daily time spent outdoors was better correlated with the personal daily UVR dose for activities conducted near noon time compared to activities conducted in the morning or late afternoon, and for activities often performed in the sun (e.g. gardening or recreation activities) compared to other outdoor activities (e.g. driving) in which the participant is usually shaded from the sun. Our results demonstrated a significant correlation between diary records of time spent outdoors with objective personal UVR dose measurements.     

Mössbauer study of structural inhomogeneities formed upon the FCC–L10 transformation in equiatomic FePd alloy from different initial states

A comparative Mössbauer study of structural inhomogeneities that arise in the course of А1→ L1₀ phase transformation in nondeformed (as cast and quenched from 950°C and melt-spun from the ingot) and severely deformed samples of equiatomic FePd alloy upon ordering annealing at Т?=?450°С has been performed. According to the known experimental works, the chosen temperature of annealing is optimal for achieving the highest coercive force H_c in both quenched and deformed samples. It is shown that in the high-coercivity state both quenched and deformed samples FePd possess an inhomogeneous tetragonal structure, which is preserved even after quite a prolonged (40–100?h) annealing. All the samples contain, along with the configurations of the nearest neighbourhood that are assigned to the ordered L1₀ phase, significant volume fractions of configurations typical of nonequiatomic compositions. This conclusion is inconsistent with the commonly accepted concept on single-phase L1₀ type alloys with maximal values of H_c. An inference is made that the structural inhomogeneities detected in the samples under study result from the mismatch of the position of the point of congruent А1→ L1₀ transformation (≈58 at.% Pd) in the phase diagram of the FePd system to equiatomic composition.     

Structure of La(Fe0.88Si x Al0.12 − x )13 compounds in the paramagnetic state

X-ray diffraction patterns and nuclear gamma resonance spectra of La(Fe_0.88Si _x Al_{0.12 ? x} )₁₃ compounds in the paramagnetic state at room temperature have been investigated. It has been found that all samples have a cubic structure of the NaZn₁₃ type, in which Si and Al atoms disorderly substitute for iron in the crystallographic position 96i. An analysis of the Mössbauer spectra using the fitting with doublets with different quadrupole splittings has revealed that the distributions of the aluminum and silicon impurity atoms substituting for iron differ significantly. Aluminum is statistically distributed over nine positions of the 96i type in the generalized coordination sphere of the Fe2 atom, whereas silicon predominantly substitutes for only six of the nine positions.     

Cr(VI) Oxidation of Cholesterol—A Kinetic Study Using N-Cetylpicolinium Dichromates,A Class of Novel Phase Transfer Oxidants

Kinetics and Catalysis - Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichromates (CPDC), a class of phase transfer oxidants, in acetic acid medium...     

Investigating aspects of the formation of structure in FePd alloy upon ordering

The Mössbauer effect is used to study changes in the structure of Fe₅₀Pd₅₀ alloy in the course of annealing for ordering at T = 450°C from different initial states: cast and quenched from 950°C, then subjected to severe plastic deformation by shear under pressure, and that obtained by fast quenching from the melt. Differences in the kinetics of phase transformations are observed depending on the initial state of the material.     

Intermolecular interactions and spectral shifts in energy transfer dye lasers

Short-range dye-dye intermolecular interactions in energy transfer dye lasers can produced pronounced blue shifts in the superradiant wavelength of the sensitized dye. The magnitude of the shift, which in some cases can exceed 20 nm, depends on the nature and concentration of the lasing dye and the donor:acceptor ratio.     

Elastic properties of the Ta–V system: bcc Ta0.33V0.67 and C15 TaV2

Resonant ultrasound spectroscopy was used to measure the elastic constants of bcc Ta_0.33V_0.67 over the temperature range 3.5–300?K; the results were compared to earlier measurements on C15 TaV₂. The temperature dependence of the polycrystalline shear modulus is completely different in the two phases; that of the bcc phase decreases with temperature whereas that of the C15 phases increases in an anomalous fashion. This difference is consistent with a model involving doubly-degenerate levels at the X point of the Brillouin zone in the C15 phase with the Fermi level lying near the doubly degenerate level. This model accounted for the unusual behaviour of the C15 phase. Debye temperatures were determined from the ultrasonic measurements: 295?K for the C15 phase and 315?K for the bcc phase.