Combined Minivalley and Layer Control in Twisted Double Bilayer Graphene

Abstract

Control over minivalley polarization and interlayer coupling is demonstrated in double bilayer graphene twisted with an angle of 2.37°. This intermediate angle is small enough for the minibands to form and large enough such that the charge carrier gases in the layers can be tuned independently. Using a dual-gated geometry we identify and control all possible combinations of minivalley polarization via the population of the two bilayers. An applied displacement field opens a band gap in either of the two bilayers, allowing us to even obtain full minivalley polarization. In addition, the carriers, formerly separated by their minivalley character, are mixed by tuning through a Lifshitz transition, where the Fermi surface topology changes. The high degree of control over the minivalley character of the bulk charge transport in twisted double bilayer graphene offers new opportunities for realizing valleytronics devices such as valley valves, filters, and logic gates.

Evidence of Orbital Ferromagnetism in Twisted Bilayer Graphene Aligned to Hexagonal Boron Nitride

“…In such weakly dispersing bands, the kinetic energy is reduced, allowing the electron–electron interactions to favor correlated states. Evidence of correlated behavior has been observed, for example, as the appearance of resistive states at the fractional filling of bands in tBLG with an interlayer twist of ∼1.1°, , ABC-trilayer graphene/hBN moiré, , twisted bibilayer, − monolayer–bilayer graphene, − and twisted WSe 2 . One recently observed consequence of these correlations is ferromagnetism in tBLG in a narrow range of carrier densities around 3/4 filling of the flat conduction band, , where full filling corresponds to four electrons per moiré unit cell accounting for spin and valley degeneracies .…”

Electric-field-tunable electronic nematic order in twisted double-bilayer graphene

“…While this phenomenon has been seen in experiments on tBG [62], experimental investigation of the resistivity anisotropy in tDBG remain to be performed, to the best of our knowledge. As opposed to STM setups, transport measurements [99][100][101] can usually be performed in the presence of top and back gates, which allow one to control both the displacement field and the electronic density independently [10][11][12][13]. This provides more flexibility in stabilizing and tuning nematic order.…”

“…Focusing on the case of moiré nematicity, as supported by the STM data, we microscopically compute the coupling constant for tDBG and show that the director can rotate over a substantial range of about 10 • for experimentally accessible values of the displacement field. We further discuss the predicted experimental manifestations of the nematic director's rotation in STM [63,88,89] and transport [99][100][101] measurements, disentangling it from effects caused by the changes in the band structure prompted by a changing displacement field. These results demonstrate the unique capabilities of moiré systems for electrostatic control of electronic nematicity.…”

Strong interminivalley scattering in twisted bilayer graphene revealed by high-temperature magnetooscillations

“…A notable feature of SA-TBG is that, by employing a single-or dual-gated device architecture, one can selectively populate the minivalleys by appropriately tuning the top and bottom gate voltages 16,17 (V tg and V bg respectively). This combination defines the relative displacement field between graphene layers, D = (C bg V bg − C tg V tg )/2, and total carrier density, n = (C bg V bg + C tg V tg )/e.…”

“…Intuitively, as the minivalleys are predominantly formed from the energy bands of different graphene sheets, an applied electric field dopes the lay- ers unequally resulting in such an imbalance. [16][17][18] In the presence of a perpendicular magnetic field, B, the gateinduced imbalance also determines the relative offset of the Landau levels (LLs) hosted by each minivalley, 19,20 a property that brings us a reliable method to explore the effects of interminivalley electron scattering in SA-TBG, as we now proceed to show.…”