Tunable bandwidths and gaps in twisted double bilayer graphene on the verge of correlations

Abstract

We use temperature-dependent resistivity in small-angle twisted double bilayer graphene to measure bandwidths and gaps of the bands. This electron-hole asymmetric system has one set of non-dispersing bands that splits into two flat bands with the electric field -distinct from the twisted bilayer system. With electric field, the gap between two emergent flat bands increases monotonically and bandwidth is tuned from 1 meV to 15 meV. These two flat bands with gap result in a series of thermally induced insulator to metal transitions -we use a model, at charge neutrality, to measure the bandwidth using only transport measurements. Having two flat bands with tunable gap and bandwidth offers an opportunity to probe the emergence of correlations. arXiv:2001.09916v1 [cond-mat.mes-hall]

Bulk valley transport and Berry curvature spreading at the edge of flat bands

“…The perpendicular electric field has a profound effect on the band structure of TDBG [7][8][9][10][11] . As depicted in Fig.…”

“…2b, we plot the measured nonlocal resistance which is large only at the gaps. Apart from the resistance peak at the gaps, there are other high-resistance regions in the local resistance, characteristics to small-angle TDBG [7][8][9][10][11] . Such examples are the cross-like feature originating at D = 0 in the hole side and the ring-like regions in the electron side for |D|/ϵ 0~0 .3 V nm −1 .…”

“…This is because the bandwidth of the flat bands increases with the electric field, resulting in increase of v F (ref. 9 ).…”

“…T he advancement in twistronics has opened up new avenues to study electron correlations physics, such as Mott insulator states [1][2][3] , superconductivity 3,4 , and orbital ferromagnetism 5,6 in twisted bilayer graphene (TBG). Recent experiments in twisted double bilayer graphene (TDBG) [7][8][9][10][11] and trilayer graphene aligned to hexagonal boron nitride (hBN) [12][13][14] also reveal correlation effects. While low-energy flat bands enhance electronic correlations [15][16][17] , such moiré systems support topological bands with nonzero valley Chern number 5,6,14,18,19 .…”

Developing Graphene‐Based Moiré Heterostructures for Twistronics

“…Graphene‐based moiré heterostructures consist of BLG, multilayer graphene or graphene flakes, and other 2D materials with weak vdW forces between the layers, such as tBLG, [ 36 ] TLG, [ 37 ] tDBG, [ 38 ] and graphene/X [ 39 , 40 ] (X = h ‐BN, TMDCs). In the field of artificial graphene‐based moiré heterostructures, the most important process is the effective preparation of moiré superlattices with specific alignment.…”