C. Bansal

Surface charge induced variation in the electrical conductivity of nanoporous gold

The electrical conductivity of nanoporous gold was measured in situ during the charging and decharging of the surface of the metal. The nanoporous gold samples were prepared by the process of selective dealloying of Ag from Au–Ag alloy. Charge was induced on the surface by making the sample a working electrode in an electrochemical cell. The conductivity was observed to vary reversibly with the induced surface charge.

A Mössbauer effect investigation of Fe hyperfine fields in Fe3−xMnxSi alloys

Abstract The dependence of average hyperfine fields on Mn concentration at the two inequivalent Fe(B) and Fe(A, C) sites in the pseudo-binary alloy system Fe 3− x Mn x Si (0 ⩽ x ⩽ 0.9) has been investigated by 57 Fe Mossbauer spectroscopy. The observed behaviour suggests a preferential choice of neighbours trying to build up the Fe 3 Si structure around Fe when the central atom is an Fe atom.

Dependence of average hyperfine fields and order-disorder kinetics on near neighbour environments in Fe1-xMnxSi alloys

Abstract The average hyperfine field at the inequivalent Fe(A,C) and Fe(B) sites in the pseudo-binary system Fe3-xMnxSix are presented in the entire composition range 0 ⩽ x ⩽ 1.8, in which single phase alloys are formed. The observed behaviour is understood in terms of the distinct near neighbour environments of these sites. The kinetics of ordering process at these sites in Fe3Si has also been found to be different and is likely to be governed by near neighbour effects.

Preparation, characterization, emission (Eu3+), and electron spin resonance (Gd3+) studies of Y2−xLnxTi2O7 (Ln=Eu and Gd, x=0.0,0.05)

Bulk and nanopyrochlore materials of composition Y2−xLnxTi2O7 (Ln=Eu and Gd, x=0.0,0.05) have been prepared by sol-gel method via peroxo titanium complex precursor. All the samples were characterized by powder x-ray diffraction (XRD), Raman spectra, and transmission electron microscopy (TEM). The unit cell parameters were obtained from Rietveld analysis of XRD patterns. The influence of particle size on the emission of Eu3+ (in Y2−xEuxTi2O7), the electron spin resonance (ESR) of Gd3+ (in Y2−xGdxTi2O7) and optical absorption spectra have been studied. The band gap energy of Y1.95Ln0.05Ti2O7 (Ln=Eu and Gd) was determined and compared with parent Y2Ti2O7. The variation in the emission intensity of Eu3+ is accounted with the particle size and short range order. The spin-Hamiltonian parameters of Gd3+ were obtained from the powder ESR spectra. The single-ion anisotropy was taken in to account in the analysis of powder ESR spectra. The crystallite size was obtained from powder XRD and TEM images.

Electric field induced reversible tuning of resistance of thin gold films

The change in resistance of nanostructured metals with respect to an applied field is believed to be due to a change in carrier concentration and hence a linear variation of resistance with the surface charge is expected. In this article, we propose a different approach to explain the resistance variation based on a change in the effective thickness of the film due to a shift of the electron density profile resulting from the applied surface charge. The change in effective thickness together with its effect on surface scattering of electrons account for the majority of the observed variation in resistance. The thin film geometry with different thicknesses and hence a controlled variation of the surface-to-volume ratio allows a deep quantitative understanding and interpretation of the observed phenomena. The model presented in this work shows that a nominal nonlinear response of the resistance of a metal on electrochemically applied surface charge does not necessarily indicate an onset of a redox reaction.

Optical absorption edge spectra of amorphous ambipolar semiconductors (PbS)x(GeS)0.7−x(GeS2)0.3

Abstract Study of the optical absorption edge spectra of amorphous ambipolar semiconductors Ge 42 S 58 , (PbS) x (GeS) 0.7− x (GeS 2 ) 0.3 ( x = 0.23, 0.46) using photo-acoustic spectroscopic technique, has been undertaken for the first time. Photoacoustic spectra of the samples reveal the three regions of the fundamental absorption edge spectra: (a) Interband transition (b) Urbachs tail and (c) sub-band gap tail. Optical energy gap, slope of Urbachs tail and density of defect states responsible for sub-band gap tail have been estimated. Results are discussed in the light of microscopic structure of these glasses.

Thermal expansion study of Fe–Mn–Si alloys

Abstract Thermal expansion measurements have been performed on Fe3−xMnxSi alloys in the concentration range 1.2⩽x⩽1.8 to study the effect of Mn-substitution on their thermal and magnetic properties. A theoretical model has been used to determine the vibrational and the magnetic contributions to thermal expansion, specific heat and entropy and to obtain the various Gruneisen parameters.

Anomalous thermal expansion behavior of the YBaCuO superconductor. Indirect evidence of polaron formation

Abstract A simple-minded semiclassical method is used to study the thermal expansion data of YBa2Cu3O7−δ (δ = 0.15) both in the normal and superconducting phases. It is shown that there is an anomaly in the temperature variation of the thermal expansion which begins to show up at a temperature somewhat higher than Tc. We argue that this anomaly can be attributed to the phenomenon of polaron formation. This might give an indirect support to the bipolaronic mechanism of high-temperature superconductivity.

Thermopower behaviour of incommensurate charge density wave systems — (NbSe4)103 I and (TaSe4)2I

The temperature dependence of thermoelectric power has been measured in the quasi-one dimensional charge density-wave (CDW) systems (NbSe4)103 I and (TaSe4)2 I. In the (NbSe4)103 I system, the thermopower remains positive throughout the temperature range investigated (120–350 K) whereas for (TaSe4)2 I it changes sign from negative values above the Peierls transition temperature (Tp = 263 K to positive values below Tp. Both these results are in agreement with Hall effect measurements. In addition we deduce from the temperature dependence of thermopower below Tp that there exists larger electron-hole asymmetry (σe/σh) in (NbSe4)103 I as compared to (TaSe4)2 I. The unusual behaviour of thermopower below a temperature (Tm) in these and other incommensurate systems is attributed to CDW drag effects of Umklapp type.

Efros-Shklovskii variable range hopping transport in nanocluster metallic films

It is shown that a film composed of nanoclusters of metal alloys of magnetic materials with a small oxide shell exhibits a Coulomb gap behaviour and the electronic transport in these nanostructured systems is governed by a variable range hopping mechanism as given by the Efros-Shklovskii model. This interesting observation is likely to have far-reaching consequences in several transport-related behaviour in micro-electronics of nano-clusters and in ultra-high density data storage devices and is expected to have a great impact on the new generation magnetic recording media and magnetic sensors made out from these clusters.