Spin–orbit-driven band inversion in bilayer graphene by the van der Waals proximity effect

Abstract

Spin orbit coupling (SOC) is the key to realizing time-reversal invariant topological phases of matter [1, 2]. Famously, SOC was predicted by Kane and Mele[3] to stabilize a quantum spin Hall insulator; however, the weak intrinsic SOC in monolayer graphene [4][5][6][7] has precluded experimental observation. Here, we exploit a layer-selective proximity effect-achieved via van der Waals contact to a semiconducting transition metal dichalcogenide[8-21]-to engineer Kane-Mele SOC in ultra-clean bilayer graphene. Using high-resolution capacitance measurements to probe the bulk electronic compressibility, we find that SOC leads to the formation of a distinct incompressible, gapped phase at charge neutrality. The experimental data agrees quantitatively with a simple theoretical model in which the new phase results from SOC-driven band inversion. In contrast to Kane-Mele SOC in monolayer graphene, the inverted phase is not expected to be a time reversal invariant topological insulator, despite being separated from conventional band insulators by electric field tuned phase transitions where crystal symmetry mandates that the bulk gap must close [22]. Electrical transport measurements, conspicuously, reveal that the inverted phase has a conductivity ∼ e 2 /h, which is suppressed by exceptionally small in-plane magnetic fields. The high conductivity and anomalous magnetoresistance are consistent with theoretical models that predict helical edge states within the inversted phase, that are protected from backscattering by an emergent spin symmetry that remains robust even for large Rashba SOC. Our results pave the way for proximity engineering of strong topological insulators as well as correlated quantum phases in the strong spin-orbit regime in graphene heterostructures. arXiv:1901.01332v2 [cond-mat.mes-hall]

Superconductivity in metallic twisted bilayer graphene stabilized by WSe2

“…4). Previous works established that WSe 2 can induce large SOI of both Ising and Rashba type into monolayer and bilayer graphene 15,16 , and it is therefore reasonable to assume that the SOI is similarly generated in the upper (proximitized) layer of TBG in our devices. Continuum-model calculations taking into account this effect show that the SOI lifts the degeneracy of both flat and dispersive bands, thereby breaking four-fold spin-valley symmetry.…”

“…This mismatch implies that the moiré pattern formed between TBG and WSe 2 has a maximum lattice constant ∼ 1 nm, in other words, much smaller than that formed in small-angle TBG (> 10 nm). Second, it is well-established that WSe 2 can induce a spin-orbit interaction (SOI) in graphene via van der Waals proximity 15,16 . And finally, due to hybridization effects, WSe 2 may also change both the Fermi velocity of the proximitized graphene sheet and the system's phonon spectrum.…”

Large spin-relaxation anisotropy in bilayer-graphene/ WS2 heterostructures

“…At the Gr/TMD interface, the spatial inversion symmetry is broken, and the graphene sublattices having K(K') valleys experience different crystal potentials and spin-orbit coupling magnitudes from the underlying TMD. The electron-spin degree of freedom and its interaction with other properties such as valley pseudospins in the presence of SOC provide access to spintronic phenomena such as spin-valley coupling [4][5][6][7]9], spin-Hall effect [10,11], (inverse) Rashba-Edelstein effect [12][13][14][15][16] and even topologically protected spin-states [17][18][19][20][21] which are not possible to realize in pristine graphene. The mentioned effects are sought after for realizing enhancement and electric field control of SOC [1,4,8,[22][23][24][25], efficient charge-current to spincurrent conversion and vice versa [10,[27][28][29], which will be the building blocks for developing novel spintronic applications [13,30].…”

Bilayer Graphene: From Stacking Order to Growth Mechanisms

“…For the non‐AB‐stacked bilayer graphene, the top layer is rotated with respect to the bottom layer in a twisted angle (Figure b) and the electronics structures are dependent on the twisted angles. Different twist angles (stacking order) can form different bilayer graphene with different physical properties . MacDonald and Bistritzer suggested that the electronic properties of twisted bilayer graphene (TBG) are very sensitive to the relative orientation of its layers.…”