Photonic density of states enhancement in finite graphene multilayers

We consider the optical properties of finite systems composed of a series of graphene sheets separated by thin dielectric layers. Because these systems respond as conductors to electric fields in the plane of the graphene sheets and as insulators to perpendicular electric fields, they can be expected to have properties similar to those of hyperbolic metamaterials. We show that under typical experimental conditions graphene/dielectric multilayers have enhanced Purcell factors, and enhanced photonic densities-of-states in both the THz and mid-IR frequency range. These behaviors can be traced to the coupled plasmon modes of the multi-layer graphene system. We show that these results can be obtained with just a few layers of graphene.Comment: 8 figure

Origin of band gaps in graphene on hexagonal boron nitride

Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. In this paper we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene's carbon atoms when placed on a boron nitride substrate and then for the influence of the substrate on low-energy $\pi$-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions.Comment: 16 pages 15 figures. This version corrects minor numerical error

Mechanism for current saturation and energy dissipation in graphene transistors

From a combination of careful and detailed theoretical and experimental studies, we demonstrate that the Boltzmann theory including all scattering mechanisms gives an excellent account, with no adjustable parameters, of high electric field transport in single as well as double-oxide graphene transistors. We further show unambiguously that scattering from the substrate and superstrate surface optical (SO) phonons governs the high field transport and heat dissipation over a wide range of experimentally relevant parameters. Models that neglect SO phonons altogether or treat them in a simple phenomenological manner are inadequate. We outline possible strategies for achieving higher current and complete saturation in graphene devices.Comment: revtex, 5 pages, 3 figures, to appear in Phys. Rev. Lett

The stability of the fractional quantum Hall effect in topological insulators

With the recent observation of graphene-like Landau levels at the surface of topological insulators, the possibility of fractional quantum Hall effect, which is a fundamental signature of strong correlations, has become of interest. Some experiments have reported intra-Landau level structure that is suggestive of fractional quantum Hall effect. This paper discusses the feasibility of fractional quantum Hall effect from a theoretical perspective, and argues that while this effect should occur, ideally, in the $n=0$ and $|n|=1$ Landau levels, it is ruled out in higher $|n|$ Landau levels. Unlike graphene, the fractional quantum Hall effect in topological insulators is predicted to show an interesting asymmetry between $n=1$ and $n=-1$ Landau levels due to spin-orbit coupling.Comment: 8 pages, 2 figure

Observation of the superconducting proximity effect and possible evidence for Pearl vortices in a candidate topological insulator

We report the observation of the superconducting proximity effect in nanoribbons of a candidate topological insulator (Bi2Se3) which is interfaced with superconducting (tungsten) contacts. We observe a supercurrent and multiple Andreev reflections for channel lengths that are much longer than the inelastic and diffusive thermal lengths deduced from normal state transport. This suggests that the proximity effect couples preferentially to a ballistic surface transport channel, even in the presence of a coexisting diffusive bulk channel. When a magnetic field is applied perpendicular to the plane of the nanoribbon, we observe magnetoresistance oscillations that are periodic in magnetic field. Quantitative comparison with a model of vortex blockade relates the occurrence of these oscillations to the formation of Pearl vortices in the region of proximity induced superconductivity.Comment: Submitted to Physical Review B (regular article

Navigating research: Expectations and guidelines for supervisors and graduate students

A healthy work environment is crucial for graduate students to succeed in scientific research. As the head of the research team, the supervisor shapes the team culture and has the responsibility to relay expectations to students and uphold these to provide an inclusive work and learning environment (Maher et al., 2020). Without these expectations, destructive behaviours, such as abuse of power by senior graduate students, toxic rivalry for space, equipment, and/or attention, and continual exposure to microaggressions can lead to a lack of intra-lab mentorship for junior members and an isolating, hostile learning environment, all of which can slow graduate student progress and may lead to withdrawal from the program (Reithmeier & Williams, 2020), particularly for students from equity-deserving groups. Programs must set clear expectations for behaviour and provide training on handling breaches to help supervisors maintain a safe and collaborative research environment, ultimately reducing conflicts and the time spent resolving them. Adverse outcomes from a lack of behavioural expectations are not limited to thesis-based programs but can also impact course-based graduate science programs, particularly those with laboratory courses/rotations. Foundational guideline frameworks exist (Council of Ontario Universities, 2023), but the extent of implementation of such expectations is varied in Canadian graduate science programs. Using the literature as a starting point, presenters and participants will explore the impact of setting guidelines for science supervisors and graduate students, and how to better train/support faculty in conveying and upholding them. Participants are encouraged to bring their own devices

Moiré band model and band gaps of graphene on hexagonal boron nitride

Nearly aligned graphene on hexagonal boron nitride (G/BN) can be accurately modeled by a Dirac Hamiltonian perturbed by smoothly varying moir\'e pattern pseudospin fields. Here, we present the moir\'e-band model of G/BN for arbitrary small twist angles under a framework that combines symmetry considerations with input from ab-initio calculations. Our analysis of the band gaps at the primary and secondary Dirac points highlights the role of inversion symmetry breaking contributions of the moir\'e patterns, leading to primary Dirac point gaps when the moir\'e strains give rise to a finite average mass, and to secondary gaps when the moir\'e pseudospin components are mixed appropriately. The pseudomagnetic strain fields which can reach values of up to $\sim$40 Tesla near symmetry points in the moir\'e cell stem almost entirely from virtual hopping and dominate over the contributions arising from bond length distortions due to the moir\'e strains.Comment: 14 pages, 8 figures, 3 table