Electrodynamic response properties of low-dimensional conductors

Two classes of low-dimensional materials are examined to expand current knowledge on their potentially useful electrical and/or optical properties. First, complex AC conductance measurements from 0.01 to 50 GHz, across temperatures of 4.2 to 300 K and magnetic fields up to 2.0 T were made on textile sheets of highly aligned multi-wall carbon nanotubes drawn from 329, 420 and 520 microm-high forests. The AC conductance of sheets with strands oriented parallel and perpendicular to the electric field polarization is roughly modeled by a shunt capacitance in parallel with a frequency-independent conductance, with no inductive contribution. This is consistent with diffusive Drude AC conduction up to 50 GHz. Further, AC conductance is found to be essentially independent of temperature and magnetic field. The absence of temperature dependence implies elastic defect and impurity scattering is dominant in these materials, while a lack of magnetoconductance suggests uncompensated single band conduction with no coherent weak localization backscattering. Second, the effect of Cr doping on properties of Cr(x)V(1-x)O2 thin films across the metal-insulator transition (MIT) has been studied. Resistance, Hall effect and infrared reflectance show Cr doping systematically increases the transition temperature Tc from 59 C at x=0 to 70 C at x=0.11, but the effect appears to saturate. This is in contrast to a prior study of bulk ceramic samples where the transition temperature increased without saturation for chromium doping fractions up to x=0.20. Results also show conductance changes across the MIT for the Cr(x)V(1-x)O2 thin films to be largely due to increases in carrier density rather than mobility, consistent with theoretical expectations.