Metal insulator transition in vanadium dioxide nanobeams and magnetic-field asymmetry of nonlinear transport in carbon nanotubes

This dissertation includes two research projects. The first project of vanadium dioxide nanobeams (VO2) shows that there are many advantages to be gained from working with strongly correlated materials in nanoscale crystalline form. Small VO2 single crystals display abundant new properties. It is shown that metallic and insulating phases can coexist in VO2 due to the growth introduced strain. From the simple physical system of length-confined suspended VO2 naonobeam, it is observed that metallic phase supercools and the resistivity of insulating phase remains constant when metal and insulator coexist. Most importantly, the constant resistivity indicates that the MIT in VO2 is driven by strong electron-electron interactions. Also based on the unique mechanical buckling behavior of suspended VO2 nanobeam, we developed a new way of measuring the intrinsic transition temperature Tc, which is independent of hysteresis. Beyond these findings, we proposed a new phase diagram including a stable M2 phase, which can more accurately describe and explain all the variety of behavior of VO2 nanocrystals. However, many basic question remain about the behavior of VO2 such as the detailed form of the phase diagram including the precise phase boundaries between all three phases; effects of surface energy and finite geometry, surface doping, and other parameters on stability in small crystals; and kinetics of the transition. We may hope that once the fundamental behavior of the system has been better established theoretical progress will finally be possible in understanding the basic nature of the transition. In second research project, we have carried out the first experimental study of a new transport coefficient in nanoscale devices, namely, the magnitude of the V2B term in the I-V characteristics. This coefficient provides a way to quantify the electron-electron interaction strength, which is of particular interest in our chosen system of single-walled carbon nanotubes. We also find unexplained magnetoresistance in disordered metallic nanotubes at high and low temperatures that act as a further indication that basic aspects of these 1D conductors remain to be addressed.