Electronic properties of strained graphene and supercritical charge centers
28 September 2016
U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Physics
François Peeters & Massoud Ramezani Masir
PhD defence Dean Moldovan - Faculty of Science, Department of Physics
Graphene has many superior properties compared to classical semiconductors, however the lack of a band gap makes electron confinement a challenge. Good conductivity does not matter if the current cannot be turned off as needed. Traditional electric barriers are ineffective in graphene, thus an alternative approach is needed. To that end, this thesis explores ways of controlling the electronic properties of graphene using mechanical strain as well as supercritical electric fields.
Strain has long been used to enhance the electrical properties of semiconductors, but the very high strain tolerance of graphene makes it especially well-suited for strain engineering. The effect of mechanical deformations on the electrons in graphene can be described in terms of the so-called pseudo-magnetic field. This field mimics many of the properties of a real magnetic field, including the ability to confine electrons in graphene. Several models of the strain-induced field are investigated, including both in-plane and out-of-plane strain geometries.
Supercritical charge centers are a long-standing prediction of quantum electrodynamics. The same effect can be replicated in graphene thanks to the relativistic behavior of its low-energy electrons. While regular electric potentials only have a marginal effect in graphene, the situation changes in the presence of strong charge centers which can tap into the supercritical regime. It is shown that quasi-bound states form around supercritical charges hosted by external impurities, vacancies and the tip of a scanning tunneling microscope. The theoretical results are compared to experimental observations and good agreement is found.
These phenomena were investigated mainly using the tight-binding approach. In order to aid the numerical work in this thesis, a code framework was developed and made available as an open source project. The framework greatly simplifies the process of constructing models while still being generic and applicable to any tight-binding system.