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

Abstract

Graphene-based moiré systems have attracted considerable interest in recent years as they display a remarkable variety of correlated phenomena. Besides insulating and superconducting phases in the vicinity of integer fillings of the moiré unit cell, there is growing evidence for electronic nematic order both in twisted bilayer graphene and twisted double-bilayer graphene (tDBG), as signaled by the spontaneous breaking of the threefold rotational symmetry of the moiré superlattices. Here, we combine symmetry-based analysis with a microscopic continuum model to investigate the structure of the nematic phase of tDBG and its experimental manifestations. First, we perform a detailed comparison between the theoretically calculated local density of states and recent scanning tunneling microscopy data [arXiv:2009.11645] to resolve the internal structure of the nematic order parameter in terms of the layer, sublattice, spin, and valley degrees of freedom. We find strong evidence that the dominant contribution to the nematic order parameter comes from states at the moiré scale rather than at the microscopic scale of the individual graphene layers, which demonstrates the key role played by the moiré degrees of freedom and confirms the correlated nature of the nematic phase in tDBG. Secondly, our analysis reveals an unprecedented tunability of the orientation of the nematic director in tDBG by an externally applied electric field, allowing the director to rotate away from high-symmetry crystalline directions. We compute the expected fingerprints of this rotation in both STM and transport experiments, providing feasible ways to probe it. Rooted in the strong sensitivity of the flat bands of tDBG to the displacement field, this effect opens an interesting route to the electrostatic control of electronic nematicity in moiré systems.

Unconventional Superconductivity in Magic-Angle Twisted Trilayer Graphene

“…-Since the discovery of superconductivity and correlated insulating states in magic-angle twisted bilayer graphene (MATBG) [1,2], twisted van der Waals materials have become indispensable for the design of novel quantum materials at will [3]. In the quickly developing field of twistronics [4], tremendous theoretical [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] and experimental [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] efforts have been undertaken to unravel the nature of strong correlations [45,46] and to access new moiré engineered structures with twisted double-bilayer graphene [47][48][49][50], twisted trilayer grap...…”

Unusual magnetotransport in twisted bilayer graphene

“…Third, does the apparent anisotropic effective Hamiltonian emerge from electron interactions? Some previous theoretical [48][49][50][51][52] and experimental [53][54][55][56] results suggest nematic order at a variety of filling factors within the lowest energy moire miniband manifold, both in TBG relatively close to the magic angle of 1.1°, and in twisted double bilayer graphene. Emergence of nematic order likely heralds an anisotropic effective Hamiltonian, plausibly explaining the correspondence between our device's behavior and that of our simple model in which anisotropy was built in.…”