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.     

On the kinematic approximation to specular reflectivity data

The kinematic approximation to specular reflectivity is very useful for understanding neutron and X-ray scattering observations, because it provides a simple analytical formula that relates the scattering-length density profile of a layered surface or interface, β(z), to the data, R (q_z ). Although its derivation from the wave equation, or the optical transfer model, for large q_z , is straightforward, the origin of the q_z -factor in the denominator is less obvious when the problem is viewed in the limit of Fraunhofer diffraction from the outset. This paper, which is principally educational, aims to bring clarity to the issue.     

Vibrational analysis of the inelastic neutron scattering spectrum of s-triazine and trichloro-s-triazine

The inelastics neutron scattering (INS) spectra of s-triazine and trichloro-s-triazine at 5 K provide the first observation of the IR and Raman inactive modes, v₄ and v₅. A full assingment is made by fitting a harmonic force field to the INS spectra profile. In the case of trichloro-s-triazine we demonstrate that this approach can be used successfully for a molecule without H atoms.     

Neutron brillouin scattering in a metallic glass

The dispersion of collective modes in a metallic glass (Mg₇₀Zn₃₀) measured previously at the thermal neutron time-of-flight spectrometer, IN4, of the HFR of the ILL has been extended towards lower momentum transfers, down to the first pseudo-Brillouin zone, for the first time. This extension to momentum transfers not accessible up to now was possible using the high-resolution time-of-flight spectrometer, HET, at the new spallation source, ISIS. In the region of overlap, the two parts of the dispersion determined with different samples of the same metallic glass on different instruments agree very well. Also, the earlier discrepancies with the dispersion determined for this metallic glass from a computer simulation have been almost completely eliminated due to a more recent and more complete investigation.     

Muon level crossing resonance in aluminium

Cross relaxation between implanted positive muons and²⁷Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 3/2 and 3/2 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante ²qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.     

Surface ordering in dilute dihexadecyl dimethyl ammonium bromide solutions at the air-water interface

At elevated temperatures and in dilute solution, we have observed lamellar surface ordering at the air-water interface of dihexadecyl dimethylammonium bromide, DHDAB, in the presence of electrolyte. With increasing temperature, the onset in ordering is observed between 35 and 40 degrees C. At 40 degrees C, there is an abrupt change in the lamellar spacing, from approximately 33 to approximately 40 A. Furthermore, in the presence of the cosurfactant benzyl alcohol, the ordering occurs at a lower temperature, between 20 and 25 degrees C. The change in lamellar spacing with temperature is attributed to a surface-induced transition, similar to the Lbeta to Lalpha phase transition observed in bulk lamellar dispersions.     

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...     

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.