Somaditya Sen

Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3.

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E(g)(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb(2)Se(3) crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.

Structure and other physical properties of magnesium vanadate glasses

Structure and other physical properties of (100-x)MgO-xV 2 O 5 glasses of different compositions are reported in this paper. Different studies like X-ray diffractograms and oxygen molar volume show that homogeneous glasses are obtained in the composition domain x=60-90 mol%. It has been observed that the network structure for all glass compositions is built up of VO 4 polyhedra. The glass transition temperatures are observed to decrease with an increase in V 2 O 5 content in the compositions. An increase in the concentration of the reduced V 4+ ions with an increase in V 2 O 5 content is observed from the magnetic susceptibility studies.

Multiphonon assisted hopping in strontium vanadate semiconducting glasses

The temperature and compositional dependence of the electrical conductivity of semiconducting strontium vanadate glasses have been reported in the temperature range 80-500 K. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, which considers the strong interaction of electrons with both the optical and acoustical phonons, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. Motts optical phonon assisted hopping model at high temperatures yields smaller values of the localization length. However, Motts variable range hopping model can predict the low temperature data. Schnakenbergs model provided higher values of the hopping energy than the activation energy obtained at the highest temperature range.

A study of intermetallic compound formation in a copper–tin bimetallic couple

The formation of intermetallics in copper–tin bimetallic couples has been studied from room temperature to 183 °C by measuring the evolution of contact resistance and composite electrical resistance with time and temperature in order to assess the kinetic behavior of the system. X‐ray diffractogram (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies have also been performed on the samples. As regards bulk diffusion, copper diffuses interstitially into tin rapidly at room temperature with the formation of η’‐Cu6Sn5 intermetallic compound. Further diffusion through this phase as evaluated by composite electrical resistivity measurements is given by 0.40 eV, assuming a model of defect‐assisted diffusion into the grains. The grain‐boundary diffusion is found to occur with an activation energy of 0.78 eV as estimated from contact resistivity measurements. SEM confirms the presence of grain‐boundary diffusion of tin in copper, whereas XRD and TEM measurements indicate t...

Ac conductivity of strontium vanadate semiconducting glasses

We have studied the ac conductivity of several compositions of strontium vanadate semiconducting glasses in the frequency range 10 Hz-2 MHz and in the temperature range 80-450 K. We have analysed the experimental results in the framework of the quantum tunnelling and classical hopping models. We observe that tunnelling of the overlapping large polarons is the most suitable mechanism for the ac conductivity of the strontium vanadate glasses.

Semiconducting properties of magnesium vanadate glasses

The electrical conductivity of semiconducting magnesium vanadate glasses has been reported for a wide composition range in the temperature range of 80–500 K. The experimental results have been analyzed in the framework of different hopping models. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by Emin, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is consistent with the low temperature data. Schnakenberg’s model yields higher values of the hopping and the disorder energies than the activation energy obtained at the highest and the...

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.

Electrical properties of semiconducting barium vanadate glasses

The preparation of vanadate glasses containing barium oxide and their electrical properties in the temperature range of 80–500 K have been reported in this article. Analysis of the electrical properties has been made in the light of different hopping models. The multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by D. Emin [Phys. Rev. Lett. 32, 303 (1974)] have been observed to describe the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with the glass composition. J. Schnakenberg’s model [Phys. Status Solidi 28, 623 (1968)] is also consistent with the temperature dependence of the conductivity data. Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is valid at low temperatures.