ETD

• Graphene
• Quantum Hall
• Quantum Technology

In this thesis, we present the nanofabrication and magneto-transport characterization of dual-gated 30° devices based on twisted bilayer graphene grown by chemical vapor deposition (CVD).
The primary objective of this work is to demonstrate that CVD-based graphene devices represent a competitive platform for investigating new quantum phenomena in high magnetic fields. Unlike exfoliated graphene, which has dominated the field of two-dimensional materials research, CVD growth offers a scalable fabrication process that is potentially compatible with industrial applications. In addition for this study, CVD synthesis enables precise control over structural alignment between graphene layers, a critical requirement for achieving the twist angle regime that enables the physics explored in this work.
Establishing CVD graphene as a viable alternative to exfoliated heterostructures would open new pathways for both fundamental research into quantum many-body physics and future technological applications based on two-dimensional materials.
Building on techniques and approaches previously developed at NEST laboratory, during this project we optimized comprehensive fabrication protocols specifically tailored for CVD-grown twisted bilayer graphene. Our protocols incorporate several key innovations aimed at maximizing device quality while maintaining the structural integrity of the twisted bilayer system
The fabrication process integrates sensitive characterization methods, particularly atomic force microscopy (AFM) and Raman spectroscopy, to precisely identify and select optimal device areas before committing to the complete fabrication sequence. AFM allows us to assess surface quality, identify contamination, and verify the uniformity of the bilayer structure, while Raman spectroscopy provides crucial information about the electronic properties and presence of defects in the graphene. This careful pre-selection step significantly improves device yield and quality. Additionally, we developed new strategies to minimize and avoid fabrication-induced artifacts that can degrade device performance, such as damage from etching steps and increase the efficiency of the contacts.
The success of these optimized protocols is demonstrated by the exceptional electronic properties of the resulting devices. Our fabricated samples exhibit carrier mobilities 800000 cm2/(Vs) at low temperature, a value that approaches the highest mobilities reported for exfoliated graphene heterostructures and far exceeds typical values for CVD graphene devices reported in literature produced from outside of our laboratory. This high mobility indicates minimal disorder and impurity scattering in our devices, confirming that CVD-grown twisted bilayer graphene can achieve electronic quality comparable to the best exfoliated heterostructures. The achievement is particularly significant because it demonstrates that the scalability advantages of CVD growth do not necessarily come at the cost of reduced electronic quality, challenging the conventional assumption that exfoliated materials are inherently superior for fundamental physics studies.
The high electronic quality of our devices proved essential for observing strongly correlated phenomena that emerge in the quantum Hall regime. When subjected to strong perpendicular magnetic fields at low temperatures, our devices exhibit well-developed quantum Hall states, including fractional quantum Hall ones. The fractional quantum Hall effect, which arises from strong electron-electron interactions in two-dimensional systems under high magnetic fields, represents one of the most remarkable manifestations of many-body quantum physics. The observation of these fractional states in our CVD-grown devices provides direct evidence that our fabrication protocols are capable of producing samples with sufficiently low disorder that interaction-driven quantum phases can emerge with remarkable clarity. The quality of these quantum Hall features again rivals that observed in exfoliation-based devices.
A unique and crucial feature of the 30° twist angle configuration is that the bilayer graphene effectively decouples into two independent monolayer systems. Unlike small twist angles where interlayer coupling leads to a wholly different band structures and phenomena, such as those observedat the so-called magic-angle, the 30° configuration results in minimal interlayer hybridization.
The two graphene layers therefore behave as two independent two-dimensional electron systems that are spatially separated by the interlayer distance of approximately 30°. This unique configuration, combined with our dual-gate architecture, creates unprecedented experimental opportunities for controlling and manipulating quantum Hall states.
The dual-gate architecture implemented in our devices provides independent control over the carrier density in each graphene layer. By applying different voltages to the top and bottom gates, we can tune the carrier type and density in each layer separately, allowing us to create configurations where one layer is electron-doped while the other is hole-doped, or where both layers have the same carrier type but different densities. This independent control is essential for investigating novel transport phenomena in the quantum Hall regime. In the quantum Hall effect, charge transport occurs along one-dimensional edge channels that propagate along the sample boundaries, with their direction of propagation determined by the sign of the charge carriers and the direction of the magnetic field. By creating opposite charge polarizations in the two decoupled layers, we can engineer a situation where edge states propagate in opposite directions along the same physical edge, i.e. counter-propagating edge states residing on separate layers. Under specific conditions, these states present spin-moment locking, meaning that the spin polarization is locked to the direction of propagation. In this case, the edge channels are helical and reproduce the physics of the quantum spin Hall effect in 2D topological insulators.
Our magneto-transport measurements not only reveal signatures of this phenomena in the integer regime, but show hints of fractional helical transport arising from these counter-propagating edge states, a phenomena which has not been observed yet in graphene.
The signatures we observe include resistance features and non-local voltage signals that are consistent with theoretical predictions for helical transport in systems with fractional quantum Hall edge states. While definitive confirmation of fractional helical transport requires additional experiments and analysis, our observations represent promising evidence for this exotic phenomenon and demonstrate the potential of our platform for investigating correlated edge state physics.
If these results will be confirmed, our material will represent a new platform to study completely unexplored physics that could be leveraged for future applications in the realm of quantum technology.