Showing papers by "Debnarayan Jana published in 2009"

Synthesis and characterization of single-phase Mn-doped ZnO

Abstract: Different samples of Zn1−xMnxO series have been prepared using conventional solid-state sintering method. We identified up to what extent doping will enable us to synthesize single-phase polycrystalline Mn-doped ZnO sample, which is one of the prerequisites for dilute magnetic semiconductor, and we have analyzed its some other physical aspects. In synthesizing the samples, proportion of Mn varies from 1 to 5 at%. However, the milling time varied (6, 12, 24, 48 and 96 h) only for 2 at% Mn-doped samples while for other samples (1, 3, 4 and 5 at% Mn doped) the milling time has been fixed to 96 h. Room-temperature X-ray diffraction (XRD) data reveal that all of the prepared samples up to 3 at% of Mn doping exhibit wurtzite-type structure, and no segregation of Mn and/or its oxides has been found. The 4 at% Mn-doped samples show a weak peak of ZnMn2O4 apart from the other usual peaks of ZnO and the intensity of this impurity peak has been further increased for 5 at% of Mn doping. So beyond 3 at% doping, single-phase behavior is destroyed. Band gap for all the 2 at% Mn-doped samples has been estimated to be between 3.21 and 3.19 eV and the reason for this low band gap values has been explained through the grain boundary trapping model. The room-temperature resistivity measurement shows an increase of resistivity up to 48 h of milling and with further milling it saturates. The defect state of these samples has been investigated using the positron annihilation lifetime (PAL) spectroscopy technique. Here all the relevant lifetime parameters of positron i.e. free annihilation (τ1) at defect site (τ2) and average (τav) increases with milling time.

First principles calculations of the optical properties of CxNy single walled nanotubes.

Abstract: The optical properties of (8, 0) single walled carbon nanotubes alloyed with nitrogen (N) have been examined using relaxed carbon-carbon (C-C) bond length ab initio density functional theory (DFT) calculations in the long wavelength limit. The maximum value of the absorption coefficient is shown to depend strongly on the concentration of N in a non-linear way as well as on the direction of polarization. The reflectivity at normal incidence vanishes at some unique concentration of N. It is also observed that the peak of the loss function (in parallel polarization and unpolarized cases) shifts to a higher frequency indicating the enhanced metallic character. The observed variation of the plasma resonance frequencies with N concentration indicates the existence of a unique maximum for parallel polarization and a step function like behavior for the unpolarized situation with concentration.

Non-universal finite size scaling of rough surfaces

Abstract: We demonstrate the non-universal behavior of finite size scaling in (1+1) dimension of a nonlinear discrete growth model involving extended particles in generalized point of view. In particular, we show the violation of the universal nature of the scaling function corresponding to the height fluctuation in (1+1) dimension. The 2nd order moment of the height fluctuation shows three distinct crossover regions separated by two crossover time scales namely, tx1 and tx2. Each regime has different scaling property. The overall scaling behavior is postulated with a new scaling relation represented as the linear sum of two scaling functions valid for each scaling regime. Besides, we notice the dependence of the roughness exponents on the finite size of the system. The roughness exponents corresponding to the rough surface is compared with the growth rate or the velocity of the surface.

Non-universal finite-size scaling of rough surfaces

Abstract: We demonstrate the non-universal behavior of finite-size scaling in (1+1) dimension of a nonlinear discrete growth model involving extended particles in a generalized point of view. In particular, we show the violation of the universal nature of the scaling function corresponding to the height fluctuation in (1+1) dimension. The second-order moment of the height fluctuation shows three distinct crossover regions separated by two crossover timescales namely, t×1 and t×2. Each regime has different scaling properties. The overall scaling behavior is postulated with a new scaling relation represented as the linear sum of two scaling functions valid for each scaling regime. Besides, we note the dependence of the roughness exponents on the finite size of the system. The roughness exponents corresponding to the rough surface is compared with the growth rate or the velocity of the surface.

Quantum Degeneracy in Two Dimensional Systems

Abstract: Degeneracy is an important concept in physics and chemistry. Degeneracy and symmetry are closely connected. We distinguish between two types of degeneracy - symmetric or systematic and accidental one. In this paper, we illustrate the concept of degeneracy through some two dimensional quantum mechanical problems. We have also indicated the breaking of the degeneracy by suitable application of perturbing potentials.