Ashok Rao

Bio: Ashok Rao is an academic researcher from Manipal University. The author has contributed to research in topics: Seebeck coefficient & Electrical resistivity and conductivity. The author has an hindex of 15, co-authored 99 publications receiving 888 citations. Previous affiliations of Ashok Rao include Manipal Institute of Technology.

Structural, electrical, magnetic, and thermal studies of Cr-doped La0.7Ca0.3Mn1−xCrxO3 (0≤x≤1) manganites

Abstract: We report detailed structural, electrical, magnetic, and specific heat studies on La0.7Ca0.3Mn1−xCrxO3 manganites. Rietveld analysis of fitted and observed x-ray diffraction patterns exhibited the single-phase nature of all the studied materials, which crystallize in Pbnm space group. Successive substitution of Cr at Mn-site in La0.7Ca0.3Mn1−xCrxO3 manganites increases the electrical resistivity and decreases the characteristic insulator-metal transition temperature (TIM) of the parent compound along with a humplike feature for higher Cr-content (x>0.06) samples. The hump structure basically signifies the onset of antiferromagnetic (AFM) interactions as inferred by both the magnetic and infrared (IR) spectroscopy studies. The systematic suppression of FM state results in a spin glass (SG)-like behavior. IR studies revealed that the vibration mode at 413 cm−1 being associated with internal bending of MnO6 octahedra, becomes softer, indicating an increase in distortion and hence the possible SG behavior. The critical exponents (α, β, and γ) are calculated from the heat capacity (CP) data near the TIM/TFM. The same exhibited variations of their values with doping. In particular, the value of β increases from 0.37(x=0.0) to 0.43(x=0.04), clearly indicating the coexistence of both long and short range magnetic orders, i.e., tendency toward SG state for Cr-doped samples. On the basis of present results, it is suggested that Cr dilutes double-exchange based FM and rather promotes the AFM based superexchange interactions via Cr3+/Mn4+ ions. Substitution of Cr systematically destroys both the metallic state and long range FM order.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Abstract: Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

Intermediate-spin state and properties of LaCoO_3

Abstract: The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

Discovery of High-Performance Thermoelectric Chalcogenides through Reliable High-Throughput Material Screening.

Abstract: High-throughput (HTP) material design is an emerging field and has been proved to be powerful in the prediction of novel functional materials. In this work, an HTP effort has been carried out for thermoelectric chalcogenides with diamond-like structures on the newly established Materials Informatics Platform (MIP). Specifically, the relaxation time is evaluated by a reliable yet efficient method, which greatly improves the accuracy of HTP electrical transport calculations. The results show that all the compounds may have power factors over 10 μW/cm·K2 if fully optimized. A new series of diamond-like chalcogenides with an atomic ratio of 1:2:4 possess relatively higher electrical transport properties among all the compounds investigated. One particular compound, CdIn2Te4, and its variations have been verified experimentally with a peak ZT over 1.0. Further analysis reveals the existence of general conductive networks and the similar Pisarenko relations under the same anion sublattice, and the transport dis...

Electronic Structure of Semiconductor Nanocrystals

Abstract: This paper reviews our recent development of the use of the large-scale pseudopotential method to calculate the electronic structure of semiconductor nanocrystals,such as quantum dots and wires,which often contain tens of thousands of atoms.The calculated size-dependent exciton energies and absorption spectra of quantum dots and wires are in good agreement with experiments.We show that the electronic structure of a nanocrystal can be tuned not only by its size,but also by its shape.Finally,we show that defect properties in quantum dots can be significantly different from those in bulk semiconductors.