Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

Material Physics 145Fig.5.1.1.3. We have observed [Rahman and Sanyal, Adv. Mater. 19, 3956 (2007);Phys. Rev. B 76, 045110 (2007)] a novel switching transition to highly conducting statein conducting polymer (Polypyrrole) nanowires - the threshold voltage (current) of theobserved transition can be easily tuned by changing diameter of nanowires and/or temperature.Negative differential resistance, enhancement of noise above the threshold andseveral other similarities with sliding motion of charge density wave (CDW) observedin structurally ordered systems indicate formation of Wigner crystals in these wiresThe switching mechanism observed in this structurally disordered material is found to be similarto that of charge-density-wave systems observed in nanowires of crystalline substances.Atikur Rahman, Milan K SanyalSP5.1.1.4 Ferromagnetism in Zinc sulfide nanocrystals: Dependence on manganese concentrationFerromagnetic ordering in nanocrystallites of dilute magnetic seminconductors (DMS) arises dueto fascinating interplay of carrier concentration, randomness of magnetic impurity sites and sizeinduced quantum confinement. We have studied magnetization behavior of ZnS nanoparticles( 2.5nm) by varying doping concentration of substitutional Mn ions that occupy cationic sites.For doping above 1.5%, these nanoparticles show intrinsic ferromagnetic ordering marked by giantZeeman splitting below 30K. The change in coercive field HC exhibited T temperature dependenceexpected for non-interacting nanoparticles. The values of blocking temperature TB and HC are