High temperature quantum transport in graphene/hexagonal-boron nitride heterostructures

The past decade has seen a new paradigm in solid state physics, where a new class of layered crystals can be thinned down to a monolayer and exhibit drastic changes in their electronic and optical properties in comparison to their bulk counterpart. Graphene was the first, and certainly most outstanding, of this set of so called two-dimensional (2D) materials. Aside from its obvious appeal which earnt its discovery the 2010 Nobel Prize, the electronic properties of graphene are truly unique. Perhaps the most familiar is its linear electron dispersion which hosts quasi-particles that obey the Dirac equation. This has enabled the study of a plethora of transport phenomena, as well as the realisation of novel device architectures that will be used in the next generation electronics. In general, experimental signatures of electron transport are most prominent at liquid helium temperatures when lattice vibrations are weak, for example in quantum hall physics. In this Thesis, we explore the regime of intermediate temperatures where the physics of interest is strongest between 100 and 300 K. Equipped with the state of the art high quality graphene samples, we demonstrate novel electron transport unique to graphene. The experimental work consists of two themes. In the first work, we study hydrodynamic electron flow in graphene encapsulated with hexagonal boron nitride devices. At elevated temperatures, electron-electron collisions become significant, and the electron viscosity starts to influence the steady state current distribution in a variety of surprising ways. In the first work, we perform transport experiments on standard graphene hall bars in a unique measurement geometry which allows the detection of negative non-local voltages intrinsic to viscous flow. In another experiment, we study viscous electron flow through graphene nano-constrictions/classical point contacts. Here, we observed anomalous temperature dependence in the conductance measured across the constriction. Specifically, the conductance increases with increasing temperature and even exceeded the semi-classical limit which is expected for single-particle ballistic transport. The underlying mechanism originates from electron-electron collisions, which, counter-intuitively, act to enhance current flow. In the second work, we slightly change our experimental system by studying magneto transport in a graphene/hexagonal boron nitride superlattice. Owed to the large periodicity of the superlattice unit cell, these devices have allowed experimental observation of the long sought Hofstadter butterfly, which addresses the electronic dispersion of electrons in a periodic potential and magnetic field. Here, we again go to elevated temperatures, where all the spectral gaps related to Hofstadter butterflies are completely smeared, and instead find a new type of quantum oscillation. These new oscillations are periodic in 1/B with a frequency corresponding to one flux quantum piercing the superlattice unit cell. Whilst these oscillations are related to Hofstadter physics, they are in fact more primal in origin. The most fascinating feature is their robustness with respect to increasing temperature. The oscillations are easily observable at room temperature in fields as low as 3 T and still remained prominent at 373 K, the boiling point of water