Search

In graphite crystals, layers of graphene reside in three equivalent, but distinct, stacking positions typically referred to as A, B, and C projections. The order in which the layers are stacked defines the electronic structure of the crystal, providing an exciting degree of freedom which can be exploited for designing graphitic materials with unusual properties including predicted higherature superconductivity and ferromagnetism. However, the lack of control of the stacking sequence limits most research to the stable ABA form of graphite. Here, we demonstrate a strategy to control the stacking order using van der Waals technology. To this end, we first visualize the distribution of stacking domains in graphite films and then perform directional encapsulation of ABC-rich graphite crystallites with hexagonal boron nitride (hBN). We found that hBN encapsulation, which is introduced parallel to the graphite zigzag edges, preserves ABC stacking, while encapsulation along the armchair edges transforms the stacking to ABA. The technique presented here should facilitate new research on the important properties of ABC graphite.

A fundamental issue to transport studies of topological insulator crystals has been the unavoidable albeit limitable parallel thermally activated bulk conduction. These crystals require careful crystal growth under laboratory conditions to unleash their topological properties, unlike rhombohedral graphite films which are exfoliated from high quality crystals of naturally occurring graphite.The bulk band gap of rhombohedral graphite films, in the thicknesses of the films studied (9 to 50 layers) with tight binding predicted bulk gap sizes ranging from 370 to 71 meV is comparable to bulk gap sizes of ordinary topological insulators. During experiments, one would expect a smaller transport gap size to manifest itself as it has been the case in other topological insulators, where one would expect, the conduction at low temperature at least, to be dominated by the surface states of topological origins.

Despite the low energy topological surface bands attracting main interest gapped bulk bands of rhombohedral graphite are also peculiar. Here we report on three-dimensional transport properties bulk electrons in high quality thin film devices of rhombohedral graphite, a nodal line system in bulk limit. We show that bulk electrons show a thickness dependent cross-over from three dimensional weak anti-localization to weak localization as well as a quadratic negative magnetoresistance resembling chiral anomaly.

In samples of rhombohedral graphite ranging from 3 to 4.5 nm thickness, corresponding to 9–12 layers of graphene, at low temperatures, a rise in resistance was found. This chapter is dedicated to the interaction induced gap at the charge neutrality point of rhombohedral graphite films, which is responsible for the rise in zero doping low temperature resistance.

This chapter is dedicated to experimental observation of predicted displacement field induced gap opening in pristine rhombohedral graphite films of up to 50 layers thickness. Supported by theoretical calculations, we show that the gap opening is a signature of absence of stacking faults.

In this chapter a general introduction and overview of electronic transport concepts such as effective mass and mobility will be given where Ohms law will be derived through both from a classical picture as well as a semi-classical picture. Fundamental electron transport phenomena of Shubnikov-de Haas oscillations as well as quantum (anomalous) Hall effect will be introduced. Lastly, quantum transport phenomena of weak (anti)localization will be explained in a pedagogical way.

We have seen that at low temperatures, the transport properties of rhombohedral graphite films are dominated by two-dimensional low energy surface states. One of the most powerful techniques to characterise electronic structure of high-quality two-dimensional systems is to study their magnetic field dependence, including the study of quantum Hall effect and carrying out of Landau level spectroscopy experiments. Hence, this chapter will be dedicated to Landau level spectroscopy of the low energy surface states, with the overall goal of gaining a complete understanding of dispersion properties of low energy surface states of rhombohedral graphite films.

In this introductory chapter we will summarise the existing theoretical and experimental work on rhombohedral graphite, using the simplest tight binding model of graphene as the starting point.

Of the two stable forms of graphite, hexagonal (HG) and rhombohedral (RG), the former is more common and has been studied extensively. RG is less stable, which so far precluded its detailed investigation, despite many theoretical predictions about the abundance of exotic interaction-induced physics. Advances in van der Waals heterostructure technology have now allowed us to make high-quality RG films up to 50 graphene layers thick and study their transport properties. We find that the bulk electronic states in such RG are gapped and, at low temperatures, electron transport is dominated by surface states. Because of topological protection, the surface states are robust and of high quality, allowing the observation of the quantum Hall effect, where RG exhibits phase transitions between gapless semimetallic phase and gapped quantum spin Hall phase with giant Berry curvature. An energy gap can also be opened in the surface states by breaking their inversion symmetry via applying a perpendicular electric field. Moreover, in RG films thinner than 4 nm, a gap is present even without an external electric field. This spontaneous gap opening shows pronounced hysteresis and other signatures characteristic of electronic phase separation, which we attribute to emergence of strongly-correlated electronic surface states.

van der Waals technology, since its creation in 2010, enabled the development of many materials, which never existed before, and led to the observation of numerous exciting physical phenomena. Here we present an overview of recent developments in the technology of two-dimensional (2D) crystals and their heterostructures and discuss novel physical phenomena observed in these artificial materials. We discuss the paradigm of tailor-made materials with properties on demand and overview novel phenomena in 2D nanomechanics, valleytronics, superconductivity, ferromagnetism, and novel topological states.

The quantum Hall effect (QHE) originates from discrete Landau levels forming in a two-dimensional (2D) electron system in a magnetic field. In three dimensions (3D), the QHE is forbidden because the third dimension spreads Landau levels into multiple overlapping bands, destroying the quantisation. Here we report the QHE in graphite crystals that are up to hundreds of atomic layers thick - thickness at which graphite was believed to behave as a 3D bulk semimetal. We attribute the observation to a dimensional reduction of electron dynamics in high magnetic fields, such that the electron spectrum remains continuous only in the direction of the magnetic field, and only the last two quasi-one-dimensional (1D) Landau bands cross the Fermi level. In sufficiently thin graphite films, the formation of standing waves breaks these 1D bands into a discrete spectrum, giving rise to a multitude of quantum Hall plateaux. Despite a large number of layers, we observe a profound difference between films with even and odd numbers of graphene layers. For odd numbers, the absence of inversion symmetry causes valley polarisation of the standing-wave states within 1D Landau bands. This reduces QHE gaps, as compared to films of similar thicknesses but with even layer numbers because the latter retain the inversion symmetry characteristic of bilayer graphene. High-quality graphite films present a novel QHE system with a parity-controlled valley polarisation and intricate interplay between orbital, spin and valley states, and clear signatures of electron-electron interactions including the fractional QHE below 0.5 K.

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.

The excellent electronic and mechanical properties of graphene allow it to sustain very large currents, enabling its incandescence through Joule heating in suspended devices. Although interesting scientifically and promising technologically, this process is unattainable in ambient environment, because graphene quickly oxidises at high temperatures. Here, we take the performance of graphene-based incandescent devices to the next level by encapsulating graphene with hexagonal boron nitride (hBN). Remarkably, we found that the hBN encapsulation provides an excellent protection for hot graphene filaments even at temperatures well above 2000 K. Unrivalled oxidation resistance of hBN combined with atomically clean graphene/hBN interface allows for a stable light emission from our devices in atmosphere for many hours of continuous operation. Furthermore, when confined in a simple photonic cavity, the thermal emission spectrum is modified by a cavity mode, shifting the emission to the visible range spectrum. We believe our results demonstrate that hBN/graphene heterostructures can be used to conveniently explore the technologically important high-temperature regime and to pave the way for future optoelectronic applications of graphene-based systems.

Resistance drift of the amorphous states of multilevel phase change memory (PCM) cells is currently a great challenge for the commercial implementation of a reliable multiple-bit-per-cell memory technology. This paper reports observation of a stable intermediate state for a multilevel PCM cell that is achieved through nonuniform heating with a square current injection top electrode. Drift coefficient of the intermediate state is an order of magnitude lower than reset and has weaker temperature dependence. Using finite-element simulations and an analytical model for the subthreshold current–voltage characteristics, based on thermally activated hopping of charge carriers across Coulombic donor-like traps, we conclude that the defect density is two orders of magnitude larger in the intermediate state. We attribute the low drift coefficient of the intermediate state to a large number of stable interfacial defects which dominate the electron transport. Current findings give way to a more stable ultrahigh-density PCM device.