Access Type

Open Access Dissertation

Date of Award

January 2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Jian Huang

Abstract

The integer and the fractional quantum Hall effects are essential to the exploration of quantum matters characterized by topological phases. A quantum Hall system hosts one-dimensional (1D) chiral edge channels that manifest zero magnetoresistance, dissipationless due to the broken time reversal symmetry, and quantized Hall resistance v h e^2 with v being the topological invariant (or Chern number). The 1-1 correspondence between the conducting gapless edge channels to the gapped incompressible bulk states is a defining character of a topological insulator (TI). Understanding this correspondence in real systems, especially the origin of its robustness (in terms of the limit of breakdown), is important both fundamentally and practically (i.e. in relation to spintronics). However, the breakdown mechanism, especially in light of the edge-bulk correlation, is still an open question.

We adopt GaAs two-dimensional (2D) high-mobility hole systems confined in a 20 nm wide (100)-GaAs quantum wells and have perform transport measurement for a range of charge densities between 4 and 5 x 10^10 cm^-2 with a carrier mobility of 2 – 4 x 10^6cm^2/Vs down to millikelvin temperatures. Systematic characterization of the 2D systems through Shubnikov-de Haas (SdH) oscillations yields an effective mass between 0.30 and 0.50m_e, in good agreement with the cyclotron resonance results. We then modify a regular Hall bar system into a unique anti-Hall bar geometry that provides an extra set of independent chiral edge channels without altering the topological invariant. We perform systematic measurement of quantum oscillations via chiral edges while simultaneously probing the bulk dynamics, through measuring across independent edges, in respond to the edge excitations. The edge-bulk correspondence reveals a non-equilibrium dynamical development of the incompressible bulk states that leads to a novel asymmetrical 1-0 Hall potential distribution. Moreover, probing the breakdown via inner and outer edges reveals a breakdown in discontinuous steps characterized by exactly the Landau level spacing. These results are the first-time evidence for a resonant quantum tunneling mechanism realized through aligning the edge and bulk energy levels.

We also explored the fundamental physics of graphene-based devices with an eye focused on possible applications as ultra-high gain and quantum efficiency hybrid graphene-quantum dots phototransistors.

We have confirmed that natural graphene presents better transport properties as mobility (10-fold higher) and delta point closer to zero. Also we have been able to observe the anomalous quantum Hall effect, unique to graphene, symbol of the high quality of the device.

More experimental work is needed to gain more insights on the real efficiency of the devices and a more efficient fabrication.

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