Phase transitions on the nano-scale

 We study how the properties of phase transition in condensed matter change with the dimensionality and size of the system under investigation. When the size of the system is shorter (in one or more dimensions) than an important length scale (for example, the mean free path), we would expect the physical properties concerned with this length scale to also change. In addition, at these short length scales, being physically close to other materials with different properties could also affect the system of interest – the properties of the other system ‘leaking’ to the system in question. This is generally called a proximity effect.

We identified avalanche-like transitions of the phase transition in Vanadium oxide when measuring nano-size devices of this material. A statistical power-law dependence of the avalanche magnitude points toward criticality of the transition. Many other aspects of the properties are currently under investigation.

Transport Measurement Dependence on Phase Seperation

Vanadium dioxide is a transition-metal-oxide which exhibits an insulator to metal (IMT) transition near room temperature at ~63C, showing changes of over 3 orders in the resistance. The transition can be driven by a variety of external excitations, such as temperature, light and electric field. Interestingly, in thin films the temperature driven phase transition occurs via spatial phase separation: upon heating, metallic islands nucleate in the insulating phase growing and propagating till the transition is completed. Previous measurements of the carrier density in VO2, via the Hall Effect, showed large density changes, but no evidence of phase separation. Our research goal is to explain how the transport measurements are affected by the phase separation. In our lab we grow epitaxial-high-quality thin films of VO2 on different orientations of sapphire substrate, using our Magnetron RF sputtering.

Ramp Reversal Memory

Ramp Reversal Memory is a unique type of non-volotile memory effect found in systems with temperature driven Metal-Insulator transitions. It is manifested by a local shift in transition temperature at designated temperatures. By reversing temperature ramping mid-transition (minor loop) one causes a shift in the transition temperature at that specific temperature. A model has been suggested in the form of local strain stabilization in the form of scars created by phase boundaries between metallic and insulating domains. These scars are local strains held in place by the substrate and act as barriers for the growing metallic domains. In addition, the scars can be “healed” by heating past the temperature they were created at. Read more about the Ramp Reversal Effect in Advanced Materials Volume 29, Issue 21