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Studies on Electron Dynamics in Deformed Graphene

Zhai, Dawei
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1540985604827894

Abstract Details

2018, Doctor of Philosophy (PhD), Ohio University, Physics and Astronomy (Arts and Sciences).
Graphene, a monolayer of carbon atoms arranged in a honeycomb lattice structure, has been the focus of intense research efforts in the past decade due to its unusual electronic, mechanical, thermal, and optical properties, which might lead the next generation of electronic devices. The possibility of countless potential applications is not the only aspect that makes graphene attractive. The low energy electron dynamics in graphene is governed by the massless Dirac equation with an energy dispersion composed of two inequivalent conical structures, known as K and K’ valleys. The corresponding spinor wave function, usually called pseudo-spin, has two components that label the occupation of the two inequivalent triangular sublattices that constitute the honeycomb lattice. This relativistic nature makes graphene an accessible platform to explore many of the quantum electrodynamics phenomena, among which the anomalous integer quantum Hall effect is one of the most prominent examples. In this dissertation, we investigate some of the quantum Hall effect related physics without external magnetic fields. This is made possible by the intimate relation between graphene's electronic and mechanical properties. Mechanical deformations introduce strain into graphene, the effect of which can be incorporated into the Dirac description as a pseudo-magnetic field and a scalar potential. The pseudo-magnetic field is originated from hopping energy modifications that are produced by strain induced changes in the C-C distance. In contrast to a real magnetic field, it exhibits opposites signs in the two valleys, thus respects time-reversal symmetry. The existence of pseudo-magnetic fields suggests that graphene deformations might be designed for various applications, as discussed in this dissertation. First, we examine the pseudo-magnetic field as a tool to spatially separate electrons from the two valleys, a property known as valley polarization/filtering. This is a prerequisite for valleytronics applications, where the valley index is employed to carry information similar to the role of spin in spintronics. We study two typical deformation geometries-- local bubble-like and extended fold-like-- and found that the latter is more efficient for valley filtering. Second, we explore the pseudo-magnetic barrier-assisted confinement effect in graphene folds. The motion of electrons are found to be restricted in the direction across the fold, while they are allowed to propagate along its axis. Our results demonstrate that graphene folds can function as either a confining potential or a waveguide depending on the direction of incident current. Next, we study the development of pseudo-spin polarization in the above mentioned two types of deformations. Such a phenomenon is similar to the Zeeman effect, but fundamentally different: It is only achievable with a field that respects time-reversal symmetry, serving as a fingerprint to differentiate strain-induced pseudo- and real magnetic fields. Physically, pseudo-spin polarization means that there exists a charge density redistribution between the two sublattices, which can be detected experimentally. Finally, we will show that strain-induced charge redistributions can be exploited to explore strong correlation regimes that are challenging to access in pristine samples. We use the Kondo effect as an example, and show that the Kondo temperature can be enhanced by an order of magnitude when a magnetic impurity is placed on a graphene deformation with negligible strain. All these findings demonstrate that graphene deformations have great potential as versatile building blocks for electronic devices or platforms to explore exotic phenomena.
Nancy Sandler (Advisor)
Katherine Cimatu (Committee Member)
Martin Kordesch (Committee Member)
Horacio Castillo (Committee Member)
186 p.
Condensed Matter Physics; Materials Science; Physics; Theoretical Physics
Graphene; spinor wave function; quantum Hall effect; pseudo-spin polarization

Recommended Citations

Citations

  • Zhai, D. (2018). Studies on Electron Dynamics in Deformed Graphene [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1540985604827894

    APA Style (7th edition)

  • Zhai, Dawei. Studies on Electron Dynamics in Deformed Graphene. 2018. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1540985604827894.

    MLA Style (8th edition)

  • Zhai, Dawei. "Studies on Electron Dynamics in Deformed Graphene." Doctoral dissertation, Ohio University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1540985604827894

    Chicago Manual of Style (17th edition)

ohiou1540985604827894
345
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This open access ETD is published by Ohio University and OhioLINK.