QDev Seminar: Eoin O'Farrell

Centre for Advanced 2D Materials, National University of Singapore, Singapore. 

Strongly interacting van der Waals heterostrustures: Unconventional superconductivity, Rashba interactions and magnetism in graphene and chalcogenide based heterostructures

Van der Waals interfaces offer the opportunity to combine materials with distinct ground states into heterostructures, under relatively relaxed lattice matching constraints. In this talk I will discuss a number of material systems that realize highly correlated and tunable electronic states based on van der Waals materials.

First, I will discuss semimetallic 1T-TiSe₂ [1], which demonstrates a rich interplay between charge density wave formation (CDW) and superconductivity. By using 1T-TiSe₂ single crystals with thicknesses less than 10 nanometres, encapsulated in two-dimensional layers of hexagonal boron nitride, we demonstrate the use of electric field to suppress the charge density wave transition, and study the phase transition to superconductivity. The observation of periodic oscillations of magnetoresistance in the superconducting state, due to the Little–Parks effect, shows that the superconducting order parameter and phase has a spatial modulation corresponding to a two-dimensional matrix. I will show these spatially modulated electronic states are likely to originate in fluctuations of the CDW order that stabilize superconductivity.

Second, I will discuss spin orbit coupling (SOC) that can be induced in graphene by hybridization with heavy metals. Here we demonstrate the mechanical intercalation of Au on a dielectric substrate by using a heterostructure of graphene and hBN [2]. Magnetotransport observes spin-splitting due to a Rashba interaction with approximate magnitude 25 meV. Additionally we observe giant negative magnetoresistance, up to 75%, that indicates Au ions form local magnetic moments at the interface. In the region of strongest Rashba coupling an anomalous Hall effect and kinks in the magnetoresistance suggest the proximity to a collective magnetic phase that leads to a strong enhancement of the electron g-factor.

Finally, I will briefly discuss tunneling spectroscopy measurements of an antiferromagnetic Mott insulator. Surprisingly, in this system we observe Coulomb oscillations due to the charging of a quantum dot. While this quantum dot appears to be due to a defect state at the interface, we observe evidence it couples to intrinsic Coulomb interactions of the magnetic system and may thereby allow us a window through which to observe its interactions.

Works in collaboration with: L. Li, J.Y. Tan, B. Özyilmaz, A.H. Castro-Neto (National University of Singapore) K. Watanabe, T. Taniguchi (N.I.M.S., Japan) T. Higo, S. Nakatsuji (University of Tokyo, Japan).

[1] L. Li, E. O’Farrell et al. Nature 529, 185 (2016).

[2] E. O’Farrell et al., arXiv:1607.06893, Phys. Rev. Lett. in press.