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Physical properties of quasi-one-dimensional (1D) and quasi-two-dimensional (2D) systems formed in chemicallygrown nano-wires and nano-films attract much interest as they provide a unique test bed for studying the theoreticallypredicted novel characteristics of low-dimensional physics. Here we shall discuss 1D electronic transport properties ofpolymer nanowires and 2D magnetic ordering in Langmuir-Blodgett films.
Conducting polymer nanowires constitute a rich quasi-1D model system to study the effects of disorder andinteractions. We shall discuss here our recent results on low temperature electronic transport properties of nearlymonodispersed, chemically synthesized, low doped polypyrrole nanowires and nanowires of polypyrrole-goldcomposite that exhibit several interesting features such as the existence of a 'gap' at low bias, a switching transitionto a highly conducting state at relatively large bias, power law behaviour in between these two bias regimes, negativedifferential resistance (NDR) in the switched state, noise enhancement, the presence of negative capacitance etc.All of these experimental observations could be explained by assuming the formation of 1D Wigner crystal in thesenanowires.
We shall also discuss the results of a polarized neutron reflectivity (PNR) study of Gd-based Langmuir-Blodgettfilms in the temperature range of 55 mK to 15 K, representing two-dimensional magnets. A model based on in-planespin vortices can explain the low-field as well as high-field PNR data. The branching of zero-field-cooled and fieldcooledmagnetization curves are explained by reconfiguration of the vortex structure. The low-field magnetization datais found to follow a power-law behavior as predicted by the Berezinski--Kosterlitz-Thouless (BKT) transition for a finitesizesystem. We have also observed that a field of 15 kOe annihilates vortex-antivortex pairs completely to produce ahomogeneous phase with saturation moment of 7 Bohr Magnetron per Gd ion below the BKT transition temperature ofaround 600 mK.
References
1. Bias dependent crossover from variable range hopping to power law characteristics in the resistivity of polymernanowires, Rahman A. and Sanyal, M.K., J. Phys.: Condens. Matter 22 175301 (2010).2. Negative capacitance in Wigner crystal forming polymer nanowires, Rahman A. and Sanyal, M.K., Appl. Phys. Lett.94, 242102 (2009).3. Novel switching transition of resistance observed in conducting polymer nanowires, Rahman, A. and Sanyal, M.K.,Adv. Mater. 19, 3956 (2007).4. Observation of charge density wave characteristics in conducting polymer nanowires: possibility of Wignercrystallization, Rahman, A., Sanyal, M. K., Phys. Rev. B 76, 045110 (2007).5. Polarized neutron reflectivity study of spin vortices formed in Gd-based Langmuir-Blodgett films, Gayen, S. Sanyal,M.K., Sharma, A., Wolff, M., Zhernenkov, K. and Zabel, H., Phys. Rev. B 82, 174429 (2010).
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