Publications

This chapter discusses a model that was proposed 30 years ago deceptive in its simplicity and laid the foundations for much of the science underlying the metal–nonmetal transition. The apparent simplicity of the problem—to transform a metal into a nonmetal or insulator, and vice versa—belies the subtlety and complexity of this, the most fundamental of electronic phase transitions in condensed matter. Both experimental and theoretical has moved toward the actual transition between metal and nonmetal, the greater the degree of sophistication of the theoretical constructions. At the level of advanced theoretical treatments, the problem of the disordered and interacting electron gas at finite temperature has emerged as one of the central concerns of modern-day condensed matter science. The chapter highlights the continued effectiveness of simple physical concepts and criteria for identifying and describing key aspects of this quantum phase transition. It discusses that successes can be recorded for the Goldhammer–Herzfeld prescription for metallization. Despite the fact that this criterion is obtained from simple classical discussions, it yields predictions for the metal–nonmetal transition that often agree closely with experiment as well as theoretical estimates from elaborate quantum–mechanical calculations. Finally, it concludes that the range and number of experimental systems traversing the metal–nonmetal transition are continually increasing.

We report Electron Paramagnetic Resonance measurements on single crystalline and powder samples of Nd_0.5Ca_0.5MnO_3 across the charge-ordering transition at T_co=240 K down to the antiferromagnetic ordering transition at T_N = 140 K. The changes in the linewidth, g-factor and intensity as functions of temperature are studied to understand the nature of spin-dynamics in the system. We explain the observed large decrease in the linewidth from T_N to T_co in terms of motional narrowing caused by the hopping of the Jahn-Teller polarons yielding an activation energy of E_a = 0.13 eV. Similar analysis of data on Pr_0.6Ca_0.4MnO_3 published earlier gives E_a=0.2 eV. Below T_co, the g-value increases continuously suggesting a gradual strengthening of the orbital ordering. We give a qualitative explanation of the temperature dependence and of the maximum in the asymmetry ratio A/B observed at T_co in single crystal spectra which also supports the themodel of motional narrowing.

Nonlinear transmission of 80 and 140fs pulsed light with 0.79μm wavelength through single walled carbon nanotubes suspended in water containing sodium dodecyl sulfate is studied. Pulse-width independent saturation absorption and negative cubic nonlinearity are observed, respectively, in open and closed aperture z-scan experiments. The theoretical expressions derived to analyze the z-dependent transmission in the saturable limit require two photon absorption coefficient β0∼1.4cm∕MW and a nonlinear index γ∼−5.5×10−11cm2∕W to fit the data.

No abstract is provided for this article.

Metal Auger intensity ratios of the type I(CVV)/I(CC′V) and I(CVV)/I(CC′C″), where C, C′ and C″ denote core levels and V stands for a valence level, are shown to increase progressively with the number of valence electrons in the metal in the case of second-row transition metals and their oxides. Metal Auger intensity ratios in chalcogenides of transition metals can be correlated by taking the effective atomic charge on the metal into consideration. The possible use of metal Auger intensity ratios in the study of surface oxidation of second-row transition metals is illustrated in the case of zirconium.

The only reported synthesis of a [<italic>n</italic>]-azacubane, 1,4-diazacubane, on re-examination revealed a disordered piperazine that constitutes the previously reported Ni²⁺ Kagome metal organic framework (MOF).

Catalytic activities of rare earth cobaltites and related compounds are shown to be related to the spin and valence states of cobalt.

Electron-doped rare-earth manganates of the type Ca1-xLnxMnO3 (Ln = La, Nd, Gd or Y) with x = 0.2 and 0.3 show charge ordering in the 150 - 270 K range, but the charge-ordering transition temperature, Tco, generally decreases with the decrease in the size of the A-site cations, a trend exactly opposite to that for hole-doped manganates. On the other hand, Tco increases with x or the electron concentration. These trends are also seen for Ca1-xLnxMnO3 compounds (Tco = 300K for x = 0.3) which show transitions to a more distorted orthorhombic structure below Tco. In Ca1-xLnxMnO3, Cr doping does not melt the charge-ordered state, unlike the case for the hole-doped systems. CaMnO2.82, for which electron doping is affected by anion vacancies, appears to show charge ordering at around 200 K.