Noise spectroscopy near phase transitions in nanoscale systems
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Strongly correlated electron systems manifest themselves in various phases, due to a delicate balance between many types of competing interactions, such as electron-electron interaction, electron-phonon interaction, and disorder induced interaction. Application of certain external stimuli, such as temperature, electric field, stress to such systems often breaks this balance, leading to phase transitions. Furthermore, in systems with reduced dimensions, confinement effects often play important roles in driving these phase transitions. Among the many experimental tools that have been used to study phase transitions in strongly correlated electron systems, resistance noise spectroscopy, assisted with conventional transport measurements, provides an unique perspective in exploring the microscopic dynamics near well-studied phase transitions in superconductors, magnetic materials, semiconductors, 2D electron systems etc.. In this thesis, using noise spectroscopy and transport measurements, two classes of strongly correlated electron systems in the nanoribbon form were studied: charge density wave systems (NbSe 3 and o-TaS 3 ), and tungsten (W) doped vanadium dioxide (VO 2 ) system. Due to the size of the samples, finite size effects were found to be important in the transport and noise measurements of both NbSe 3 and o-TaS 3 . A model that treats the pinning by bulk, surface and contacts separately was proposed to explain an anomaly, which was observed in the differential resistance vs. electric field at temperatures below the Peierls transition in NbSe 3 . This model, combined with slow motions of the CDW due to the fast freezing of the thermal excitations over the Peierls gap, were suggested to account for discrete peaks in the Ohmic regime of o-TaS 3at temperatures below 100 K. In addition, a nonmonotonic behaivor in the electric field dependence of noise magnitude was seen in NbSe 3 in certain temperature ranges, and can be explained as signatures of thermally activated phase slip events. A non-thermal behavior was observed in the rest of the temperature range, possibly related to mobility fluctuations of the quasiparticles. However, the nonmonotonic behavior in the electric field dependence of noise magnitude was not observed in o-TaS 3and the underlying physics is still under study. Results from our noise measurements in W-doped VO 2 suggest an onset of avalanche dynamics and a deviation from conventional percolation theory near the temperature-driven insulator-metal transition. Implications of these results for understanding the insulator-metal transition in VO 2 are presented.