Ratchet effect, observed in many systems starting from living organism to artificially designed device, is a manifestation of motion in asymmetric potential. Here we report results of a conductivity study of Polypyrrole nanowires, which have been prepared by a simple method to generate a variation of doping concentration along the length. This variation gives rise to an asymmetric potential profile that hinders the symmetry of the hopping process of charges and hence the value of measured resistance of these nanowires become sensitive to the direction of current flow. The asymmetry in resistance was found to increase with decreasing nanowire diameter and increasing temperature. The observed phenomena could be explained with the assumption that the spatial extension of localized state involved in hopping process reduces as the doping concentration reduces along the length of the nanowires. Flow of particles becomes direction sensitive in presence of a ratchet potential whose principal feature is loss of inversion symmetry [1,2,3,4,5,6,7,8,9,10,11]. Depending upon the origin of asymmetric potential and fluctuating force several kinds of ratchets are observed ranging from molecular motors [3] in which proteins move in a deterministic way along filaments, to electron pumps [5] engineered in semiconductor channels. Ratchet effect has been utilized in diverse fields like in the process of particle separation [8] and in optical tweezing [9]. We report here an evidence of ratchet potential formation in nanowires of a conducting polymer. The resistance of these nanowires can differ by several kilo-Ohms as the direction of current flow along the length is reversed. This effect become more pronounced with decrease of diameter of the nanowires and increase in temperature.
The structural, electronic and optical properties of nearly ferromagnetic compound HfZn 2 have been studied using ab-initio technique. We have carried out the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented within the CASTEP code. The calculated structural parameters show a good agreement with the experimental results. In our work we have used GGA-PBE to calculate the electronic properties and it is found that the results exhibit similar band structure qualitatively with the results calculated using LDA. The electronic band structure reveals metallic conductivity and the major contribution comes from Hf-5d states. Our results for structural and electronic properties are compared with the experimental and other theoretical results wherever these are available. For the first time we have investigated the optical properties of HfZn 2 since no other experimental and theoretical studies on optical properties and dielectric functions of HfZn 2 has been reported yet. The reflectivity spectrum shows that the reflectivity is high in the visibleultraviolet region up to 16 eV indicating promise as a good coating material to avoid solar heating.
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