High temperature quantum transport in graphene/hexagonal-boron nitride
      heterostructures
Creators: name: Krishna Kumar, Roshan
Publisher: name: Lancaster University
Year: 2017

description: 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