Parasharam M. Shirage

Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba0.6K0.4Fe2As2 Probed by 57Fe-NMR

We present 57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0.6 K 0.4 Fe 2 As 2 ( T c = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1/ T 1 ) demonstrate the development of wave-number ( q )-dependent spin fluctuations, except at q = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1/ T 1 results are totally consistent with a s ± -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1/ T 1 data for Ba 0.6 K 0.4 Fe 2 As 2 and LaFeAsO 0.7 , which seemingly follow a T 5 - and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap.

Genuine Phase Diagram of Homogeneously Doped CuO2 Plane in High-Tc Cuprate Superconductors

We report a genuine phase diagram for a disorder-free CuO 2 plane based on the precise evaluation of the local hole density ( N h ) by site-selective Cu-NMR studies on five-layered high- T c cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N h ≈0.17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N h ≤0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM (paramagnetism) phases may be present at N h ≈0.15 and T ≈75 K, (4) T c is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high- T c SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T c of cuprate superconductors is so high.

Absence of an appreciable iron isotope effect on the transition temperature of the optimally doped SmFeAsO(1-y) Superconductor.

We report the iron (Fe) isotope effect on the transition temperature (T(c)) in oxygen-deficient SmFeAsO(1-y), a 50-K-class, Fe-based superconductor. For the optimally doped samples with T(c) = 54  K, a change of the average atomic mass of Fe (M(Fe)) causes a negligibly small shift in T(c), with the Fe isotope coefficient (α(Fe)) as small as -0.024 ± 0.015 (where α(Fe)=-d lnT(c)/dlnM(Fe)). This result contrasts with the finite, inverse isotope shift observed in optimally doped (Ba,K)Fe2As2, indicating that the contribution of the electron-phonon interaction markedly differs between these two Fe-based high-T(c) superconductors.

Effect of growth temperature on the optical properties of ZnO nanostructures grown by simple hydrothermal method

Here we report an easy and rapid synthesis technique of wurtzite ZnO nanostructures in the form of flowers, nano-rods and nano-tubes that are achieved by a facile hydrothermal method. A growth mechanism is proposed based on a series of temperature dependent experiments keeping other parameters during the synthesis in the aqueous medium at optimized levels. Pure ZnO results in nano-rods while Sr doped ZnO material forms flower and tube like structures. The XRD and TEM investigations show that ZnO nanostructures possess good crystalline structures with a growth direction along the c-axis of the crystal plane. Raman spectra confirm five phonon vibration modes for ZnO nanostructures at 99, 333, 382, 438 and 582 cm−1 and one more defect induced low intensity peak at 663 cm−1 for Sr doped ZnO. Ultraviolet-visible (UV-vis) spectroscopy shows the band gap energy of ZnO nanostructures decreases from 3.24 to 3.22 eV with the substitution of Sr into the ZnO lattice. Photoluminescence spectra reveal the existence of several defect states in all of the samples. Defect intensity seems negligibly affected by the variation of growth temperature, whereas, Sr doping plays a major role in controlling oxygen and Zn related defects. I–V characteristics of the ZnO and Sr doped ZnO show rectification behaviour of the Schottky diodes.

Sr- and Ni-doping in ZnO nanorods synthesized by a simple wet chemical method as excellent materials for CO and CO2 gas sensing

In this study, the effect of Sr- and Ni-doping on the microstructural, morphological and sensing properties of ZnO nanorods has been investigated. Nanorods with different Sr and Ni loadings were prepared using a simple wet chemical method and characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) analysis. XRD data confirmed that Sr- and Ni-doped samples maintain the wurtzite hexagonal structure of pure ZnO. However, unlike Sr, Ni doping modifies the nanorod morphology, increases the surface area (SA) and decreases the ratio of the IUV/Igreen photoluminescence peak to a greater extent. Sensing tests were performed on thick film resistive planar devices for monitoring CO and CO2, as indicators of indoor air quality. The effect of the operating temperature, nature and loading of the dopant on the sensibility and selectivity of the fabricated sensors towards these two harmful gases was investigated. The gas sensing characteristics of Ni- and Sr-doped ZnO based sensors showed a remarkable enhancement (i.e. the response increased and shifted towards a lower temperature for both gases) compared to the ZnO-based one, demonstrating that these ZnO nanostructures are promising for the fabrication of sensor devices for monitoring indoor air quality.

Controlling of ZnO nanostructures by solute concentration and its effect on growth, structural and optical properties

ZnO nanostructured films were prepared by a chemical bath deposition method on glass substrates without any assistance of either microwave or high pressure autoclaves. The effect of solute concentration on the pure wurtzite ZnO nanostructure morphologies is studied. The control of the solute concentration helps to control the nanostructure to form nano-needles, and -rods. X-ray diffraction (XRD) studies revealed highly c-axis oriented thin films. Scanning electron microscopy (SEM) confirms the modification of the nanostructure dependent on the concentration. Transmission electron microscopy (TEM) results show the single crystalline electron diffraction pattern, indicating high quality nano-material. UV–vis results show the variation in the band gap from 3.20 eV to 3.14 eV with increasing concentration as the nanostructures change from needle- to rod-like. Photoluminescence (PL) data indicate the existence of defects in the nanomaterials emitting light in the yellow–green region, with broad UV and visible spectra. A sharp and strong peak is observed at ~438 cm−1 by Raman spectroscopy, assigned to the optical mode of ZnO, the characteristic peak for the highly-crystalline wurtzite hexagonal phase. The solute concentration significantly affects the formation of defect states in the nanostructured films, and as a result, it alters the structural and optical properties. Current–voltage characteristics alter with the measurement environment, indicating potential sensor applications.

Search for Origin of Room Temperature Ferromagnetism Properties in Ni-Doped ZnO Nanostructure

The origin of room temperature (RT) ferromagnetism (FM) in Zn1-xNixO (0< x < 0.125) samples are systematically investigated through physical, optical, and magnetic properties of nanostructure, prepared by simple low-temperature wet chemical method. Reitveld refinement of X-ray diffraction pattern displays an increase in lattice parameters with strain relaxation and contraction in Zn/O occupancy ratio by means of Ni-doping. Similarly, scanning electron microscope demonstrates modification in the morphology from nanorods to nanoflakes with Ni doping, suggests incorporation of Ni ions in ZnO. More interestingly, XANES (X-ray absorption near edge spectroscopy) measurements confirm that Ni is being incorporated in ZnO as Ni2+. EXAFS (extended X-ray absorption fine structure) analysis reveals that structural disorders near the Zn sites in the ZnO samples upsurges with increasing Ni concentration. Raman spectroscopy exhibits additional defect driven vibrational mode (at 275 cm-1), appeared only in Ni-doped samples and the shift with broadening in 580 cm-1 peak, which manifests the presence of the oxygen vacancy (VO) related defects. Moreover, in photoluminescence (PL) spectra, we have observed a peak at 524 nm, indicating the presence of singly ionized VO+, which may be activating bound magnetic polarons (BMPs) in dilute magnetic semiconductors (DMSs). Magnetization measurements indicate weak ferromagnetism at RT, which rises with increasing Ni concentration. It is therefore proposed that the effect of the Ni ions as well as the inherent exchange interactions arising from VO+ assist to produce BMPs, which are accountable for the RT-FM in Zn1-xNixO (0< x < 0.125) system.

Studies on the control of ZnO nanostructures by wet chemical method and plausible mechanism

Most of the applications of the nano structures dependent on the morphology which affects the opto electronics properties. This research article provides a pathway of guiding optical properties like band-gap and fluorescence properties by controlled growth of nano-rods, -flowers, -needles or- tubes without external chemical doping, by simple hydro thermal method by controlling over synthesis parameter, temperature.This research article provides a pathway of controlled growth of ZnO nano-rods, -flowers, -needles or -tubes without external chemical catalysis, via a simple wet chemical method by control of synthesis temperature. Morphological effects on structural and optical properties are studied by Ultraviolet-visible (UV-vis) spectroscopy shows slight enhancement in the band gap, with increasing synthesis temperature. Photoluminescence (PL) data indicates the existence of defect in the nanomaterials, which is more elaborately explained by schematic band diagram. A sharp and strong peak in Raman spectroscopy is observed at ∼438cm−1 is assigned to the E2high optical mode of the ZnO, indicating the wurtzite hexagonal phase with high crystallinity.

Origin of Tc Enhancement Induced by Doping Yttrium and Hydrogen into LaFeAsO-Based Superconductors: 57Fe-, 75As-, 139La-, and 1H-NMR Studies

We report our extensive 57 Fe-, 75 As-, 139 La-, and 1 H-NMR studies of La 0.8 Y 0.2 FeAsO 1- y (La 0.8 Y 0.2 1111) and LaFeAsO 1- y H x (La1111H), where doping yttrium (Y) and hydrogen (H) into optimally doped LaFeAsO 1- y [La1111(OPT)] increases T c =28 to 34 and 32 K, respectively. In the superconducting (SC) state, the measurements of nuclear-spin lattice-relaxation rate 1/ T 1 have revealed in terms of a multiple fully gapped s ± -wave model that the SC gap and T c in La 0.8 Y 0.2 1111 become larger than those in La1111(OPT) without any change in doping level. In La1111H, the SC gap and T c also increase slightly even though a decrease in carrier density and some disorders are significantly introduced. As a consequence, we suggest that the optimization of both the structural parameters and the carrier doping level to fill up the bands is crucial for increasing T c among these La1111-based compounds through the optimization of the Fermi surface topology.

Shape-controlled CoFe2O4 nanoparticles as an excellent material for humidity sensing

The humidity sensing performance of cobalt ferrite nanoparticles (CoFe2O4 NPs) with controlled morphology obtained via a solution route is reported in this work. The humidity sensing properties of the presented CoFe2O4 NPs ferrite sensor were investigated by exposing it to a broad humidity range of 8–97% at room temperature. CoFe2O4 NPs with spherical, cubic, and hexagonal shapes have been successfully achieved by tuning the growth conditions like reaction time and amount of solvent. These CoFe2O4 NPs exhibit morphology-dependent chemi-resistive humidity sensing behaviors. The highest humidity sensitivity value of ∼590 along with response/recovery value of 25/2.6 s at room temperature was obtained for CoFe2O4 hexagonal (CF-H) shape as compared to CoFe2O4 spherical (CF-S) and CoFe2O4 cubic (CF-C) shapes. Freundlich adsorption isotherm model was well fitted with the experimental results which turned up in support of a plausible humidity sensing mechanism. The morphology-dependent CoFe2O4 nanostructures exhibit promising sensing capabilities which ensure them as a potential candidate for magnetic recording media and next-generation humidity sensors.