Transport properties of correlated electron systems in the nanoscale
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The goal of this dissertation is to understand the microscopic origin of metal-insulator transition (MIT) in strongly correlated electron systems. Two such systems, VO 2 and V 2 O 5 in their nanoscale form are investigated. Results from experimental studies of electrical transport properties and of Raman spectroscopic measurements from single nanowires of these systems across the MIT are presented. In the VO 2 system, the doping and confinement effects of MIT on the individual W -doped VO 2 nanowires are studied. The hysteretic and abrupt transitions in the temperature or in the electrical driven MIT are observed and studied. While increasing the W content, the pronouncedly decreasing rate (- (48-56) K/at. % W ) of the MIT transition temperature (T c ) and low activation energy (E a <100 meV) suggest a more complex phase domain nucleation in this quasi-1D system. Also, while driving MIT by applying voltage across sample, the temperature dependence of threshold voltages (V TH ) suggests the charge ordering and Joule heating in these nanowires, respectively. To understand the role of structural phase during the MITs, the simultaneous electrical transport and Raman spectroscopic measurements across temperature- and voltage-driven phase transitions are performed. Our results indicate no intermediate structural phases are required to mediate the structural phase transition. Moreover, both MITs show the coexistence of monoclinic (M 1 ) and rutile (R) during the transitions. The implications of these results are discussed. In the V 2 O 5 system, two single-crystal vanadium oxide bronzes in their nanowire form, β'-Cu x V 2 O 5 and δ-K x V 2 O 5 , were studied. The hysteretic temperature driven MIT ranging up to almost 5 orders of magnitude in both nanowires has not been previously seen in their bulk form. Furthermore, the above room temperature transition (T c =360-390 K) is also in strong contrast with the typical sub 200 K values in bulk bronzes, indicating the importance of matrix confinement and of ion intercalation in the vanadium oxide bronzes. Our results from the studies of temperature- and of electrical-driven MIT suggest that the intrinsic electronic instability in single nanowire are the result of approaching single domain limits.