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The defence will be in English.
Abstract: The zeroth Landau level (zLL) in high-mobility graphene, with a spin-polarized ground state at filling factor ν = 0, hosts a quantum spin Hall phase. This phase is characterized by an insulating bulk and spin-polarized, counterpropagating helical edge states at the charge neutral point (CNP). Coupling this helical phase to an s-wave superconductor is a promising route toward realizing topological superconductivity and studying Majorana physics. Experimentally, however, the small g-factor in graphene and prevalent Coulomb interactions have proven to be a roadblock, favoring a trivial insulating valley-polarized ground state at low fields. Recent experiments in our group claimed to tune the ground state at ν = 0 in the zLL to the helical phase at low fields by screening the long-range part of the Coulomb interactions, using a high dielectric constant substrate, SrTiO3. However, the low mobilities observed in samples on SrTiO3, coupled with its non-linear and hysteretic dielectric constant and the presence of ferroelectric domain walls, make it less than ideal for stable and reproducible high-mobility devices, which are crucial for future applications.
In this thesis, we introduce a more stable material platform for screening Coulomb interactions in hexagonal boron nitride (hBN) encapsulated graphene (hBN-Gr-hBN) heterostructures. We utilize a 2D metallic bismuth selenide (Bi2Se3) electrode as both the screening substrate and gate electrode, separated from the encapsulated graphene plane by an ultrathin hBN layer. In the resulting heterostructures, we consistently measure ultra-high Hall mobility, allowing us to observe the quantum Hall effect and helical edge transport at CNP at fields as low as 200 mT.
In the resulting devices, we first investigate the significant impact of contact doping and size on the quantization of helical edge resistance. Notably, we observe non-additivity of 2-terminal helical edge resistance in devices with narrow contacts, which we attribute to imperfect equilibration of helical edge states in the contacts. We present numerical simulations on the effect of doping and contact width on the equilibration of helical edge states and demonstrate that wider contacts promote better quantization.
In subsequently fabricated devices featuring wider contacts, we systematically measure near ideal quantization at both 4 K and 75 mK. We further characterize helical edge transport in these devices by selectively transmitting and backscattering helical edge states through a topgate electrode that acts as a quantum Hall barrier. We characterize the transport of helical edge states in this regime using a model based on the Landauer-Büttiker formalism, which also accounts for the equilibration of these edge states with the states in the topgate region. Our calculations align well with the experimental results. This comprehensive set of data, obtained from several samples, provides convincing evidence for a reproducible quantum spin Hall phase in charge-neutral graphene on Bi2Se3 and prompts further efforts to incorporate superconductivity to look for topological superconductivity.
