Brillouin Scattering Studies of La_{0.77}Ca_{0.23}MnO_3 Across Metal-Insulator Transition

Temperature-dependent Brillouin scattering studies have been carried out on La_{0.77}Ca_{0.23}MnO_3 across the paramagnetic insulator - ferromagnetic metal (I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves (BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts, where as the frequencies of the BAW decrease below the I-M transition temperature (T_C) of 230 K. These results can be understood based on the temperature dependence of the elastic constants. We also observe a central peak whose width is maximum at T_C.Comment: 7 pages, 8 figure

Magnetic and electron transport properties of the rare-earth cobaltates, La0.7-xLnxCa0.3CoO3 (Ln = Pr, Nd, Gd and Dy) : A case of phase separation

Magnetic and electrical properties of four series of rare earth cobaltates of the formula La0.7-xLnxCa0.3CoO3 with Ln = Pr, Nd, Gd and Dy have been investigated. Compositions close to x = 0.0 contain large ferromagnetic clusters or domains, and show Brillouin-like behaviour of the field-cooled DC magnetization data with fairly high ferromagnetic Tc values, besides low electrical resistivities with near-zero temperature coefficients. The zero-field-cooled data generally show a non-monotonic behaviour with a peak at a temperatures slightly lower than Tc. The near x = 0.0 compositions show a prominent peak corresponding to the Tc in the AC-susceptibility data. The ferromagnetic Tc varies linearly with x or the average radius of the A-site cations, (rA). With increase in x or decrease in (rA), the magnetization value at any given temperature decreases markedly and the AC-susceptibility measurements show a prominent transition arising from small magnetic clusters with some characteristics of a spin-glass. Electrical resistivity increases with increase in x, showed a significant increase around a critical value of x or (rA), at which composition the small clusters also begin to dominate. These properties can be understood in terms of a phase separation scenario wherein large magnetic clusters give way to smaller ones with increase in x, with both types of clusters being present in certain compositions. The changes in magnetic and electrical properties occur parallely since the large ferromagnetic clusters are hole-rich and the small clusters are hole-poor. Variable-range hopping seems to occur at low temperatures in these cobaltates.Comment: 23 pages including figure

Glassy behavior in the ferromagnetic and the non-magnetic insulating states of the rare earth manganates, Ln0.7Ba0.3MnO3 (Ln = Nd or Gd)

While La0.7Ba0.3MnO3 is a ferromagnetic metal (TC = 340 K) with longrange ordering, Nd0.7Ba0.3MnO3 shows a transition around 150 K with a small increase in magnetization, but remains an insulator at all temperatures. Gd0.7Ba0.3MnO3 is non-magnetic and insulating at all temperatures. Low field dc magnetization and ac susceptibility measurements reveal the presence of a transition at around 150 K in Nd0.7Ba0.3MnO3, and a complex behavior with different ordering/freezing transitions at 62, 46 and 36 K in the case of Gd0.7Ba0.3MnO3, the last one being more prominent. The nature of the field dependence of the magnetization, combined with the slow magnetic relaxation, ageing and memory effects, suggests that Nd0.7Ba0.3MnO3 is a cluster glass below 150 K, a situation similar to that found for La_{1-x}SrxCoO3. Gd0.7Ba0.3MnO3, however, shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The difference in nature of the glassy behavior between Gd0.7Ba0.3MnO3 and Nd0.7Ba0.3MnO3 probably arises because of the larger disorder arising from the mismatch between the sizes of the A-site cations in the former. Our results on Nd0.7Ba0.3MnO3 and Gd0.7Ba0.3MnO3 suggest that the magnetic insulating states often reported for rare earth manganates of the type Ln1-xAxMnO3 (Ln = rare earth, A = alkaline earth) are likely to be associated with glassy magnetic behavior.Comment: 20 pages including figure

Non-polio enterovirus association with persistent diarrhea in children as revealed by a follow-up study of an Indian cohort during the first two years of life

Background: We recently reported significant association of non-polio enteroviruses (NPEVs) with acute diarrhea in children. Persistent diarrhea (PD) remains a major cause of morbidity and mortality in infants below two years of age in developing countries. Understanding age-dependent frequency and duration of NPEV infections is important to determine their association with persistent diarrhea and disease burden. Objectives: A cohort of 140 infants was followed for 6 months to 2 years of age to determine the frequency, duration, and association with PD of NPEV infections in comparison with rotavirus and other agents. Study design: Stool samples were collected every 14 days, and diarrheal episodes and their duration were recorded. Enteroviruses were characterized by RT-PCR and VP1 gene sequence analysis, rotavirus by electropherotyping, and other agents by PCR. Results: Of 4545 samples, negative for oral polio vaccine strains, 3907 (85.96%) and 638 (14.04%) were NPEV-negative and NPEV-positive, respectively, representing 403 (8.87%) infection episodes. About 68% of NPEV infections occurred during the first year with every child having at least one episode lasting between four days and four months. Approximately 38% and 22% of total diarrheal episodes were positive for NPEV and RV, respectively. While about 18% of NPEV infection episodes were associated with diarrhea, 6% being persistent, 13% of total diarrheal episodes were persistent involving infections by monotype NPEV strains or sequential infections by multiple strains and other agents. Conclusions: This is the first report revealing NPEVs as the single most frequently and persistently detected viral pathogen in every PD episode. (C) 2014 Elsevier B.V. All rights reserved