Raman Spectroscopy and Electronic Transport in Exfoliated α-RuCl3 and Graphene/α- RuCl3 Heterostructures

The layered antiferromagnetic Mott insulator α-RuCl3, having a honeycomb lattice of spin-1/2 moments, has been identified as a potential candidate material to realize the Kitaev quantum spin liquid. In particular, bulk RuCl3 crystals have been studied and found to be on the cusp of manifesting QSL behavior. Since the QSL is primarily a two-dimensional phenomenon, it may be possible to stabilize the QSL phase in exfoliated single- or few-layer α-RuCl3. In this work, we study the exfoliated α-RuCl3 via Raman spectroscopy and electronic transport. The α-RuCl3 bulk crystals were mechanically exfoliated onto Si/SiO2 substrates using the scotch tape technique. Raman spectroscopy was performed on monolayer, bilayer and multilayer α-RuCl3 samples on Si/SiO2 over a range of temperatures. The Raman spectra are consistent with the overall structure remaining same as the bulk, however in mono- and bilayers the appearance of a symmetry-forbidden mode in crossed polarization suggests the development of significant in- plane distortions away from the perfect honeycomb lattice in the thinnest samples. Additionally, the anomalous temperature dependence of a mode at 164 cm-1 that is known to couple to a con- tinuum of magnetic excitations in the mono- and bilayer may suggest an enhancement of magnetic fluctuations. The Raman spectra demonstrate the possibility of tuning the Kitaev paramagnetic state in α-RuCl3 via exfoliation. The electronic transport in exfoliated α-RuCl3 is then studied. We performed DC transport measurement on α-RuCl3 flakes of various thicknesses and found the resistance to freeze out well above the Neel temperature. To access the low temperature regime with transport, we fabricated van der Waals heterostructures containing thin α-RuCl3 flakes in contact with monolayer graphene Hall bars. We find an anomalously large conductivity implying the RuCl3 has become conducting by proximity to graphene. Additionally, the temperature derivative of the resistivity at low temperature shows clear signals associated with magnetic phase transitions at temperatures 2-3 times higher than the native antiferromagnetic transition in α-RuCl3. The collaborative work on the infrared cyclotron resonance of high mobility encapsulated monolayer graphene is also presented. Interband Landau level transitions are studied. The transition energies show a pronounced non-monotonic dependence on the Landau level filling factor, ν ~ n/B, providing direct evidence that electron-electron interactions contribute to the Landau level transition energies in graphene, beyond the single-particle picture. Additionally, a splitting occurs in transitions to or from the lowest Landau level, which is interpreted as a Dirac mass arising from coupling of the graphene and boron nitride lattices. Sample fabrication and measurement details are described. The experimental setup is shown as well as the data processing procedures