Final PhD Oral Examination (Thesis Title: “Nonequilibrium Transport in Electron Solids”)

Speaker: 
OLEKSANDR ROSSOKHATY
Event Date and Time: 
Thu, 2016-12-01 09:00 - 11:00
Location: 
Room 200, Graduate Student Centre
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
Electron-electron interactions inside of two dimensional electron gases (2DEG) in out-of-plane magnetic field and at very low temperatures under certain conditions can lead to electron localization in Wigner crystals or even more complex periodic
structures, demonstrating anisotropic transport. These states are usually referred to as electron solid phases and result in Reentrant Integer Quantum Hall Effect (RIQHE) in transport measurements. However, their microscopic description remains unclear, as
insulating phases with different microscopic structure demonstrate indistinguishable macroscopic transport properties.

Here the transport of the electron solids is investigated under non-equilibrium conditions. This approach allows us to break an insulating state by application of significant current bias to the 2DEG. As bias current increases, longitudinal and Hall resistivities measured for these states show multiple sharp breakdown transitions. Whereas the high bias breakdown of fractional quantum Hall states is consistent with simple heating, the nature of RIQH breakdown remains to be a subject of a considerable debate.

A comparison of RIQH breakdown characteristics at multiple voltage probes indicates that these signatures can be ascribed to a phase boundary between broken-down and unbroken regions, spreading chirally from source and drain contacts as a function of bias current and passing voltage probes one by one. It is shown that the chiral sense of the spreading is not set by the chirality of the edge state itself, instead depending on electronor hole-like character of the RIQH state.

Although at high current bias the electron temperature is unmeasurable with standard techniques, the data shows that electron solid
states appear to stay temperature sensitive even after the RIQHE is destroyed. A comparison of temperature dependence and the spatial distribution of the Hall potential along the edge provides an evidence, that the bulk 2DEG remains insulating up to surprisingly high biases, indicating strong imhomogeneity of the sample during the breakdown.

In the last part of this work a metastable stripe phase around =9/2 is investigated under non-equilibrium conditions at electron density close to the stripe reorientation critical point. Despite the metastability at zero bias, the anisotropy of nonequilibrium stripe phase shows a strong dependence of the natural orientation of stripes on exact filling factor around half-filling of the last Landau level.

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