Probing Electronic States in Twisted Bilayer Graphene with Electric Fields: A study of Superconductivity, Non-Local Resistance, Magnetoresistance Hysteresis, and Thermoelectric Effects

Abstract

Introducing a `twist' degree of freedom to modify the low energy band structures and engineer novel electronic phases from the constituent van der Waals 2D materials, was proposed theoretically more than a decade earlier and was first realized experimentally through the discovery of unconventional superconductivity and correlated Mott insulator in magic-angle twisted bilayer graphene (MAtBLG – where the twist angle is close to 1°) in 2018. The results presented in this thesis inquire into this emerging field of `twistronics’, focusing on the effects of external electric fields on twisted bilayer graphene (tBLG). One of the most studied phases of tBLG has been the appearance of a robust superconductivity at integer moiré filling factors (v) at the magic-angle. Superconductivity (SC) in tBLG has been explored by varying carrier concentrations, twist angles, and screening strength, with the aim of uncovering its origin and possible connections to strong electronic correlations in narrow bands and various resulting broken symmetries. In the first work, we investigate an electric field tunable superconducting phase in a near magic-angle (θ~0.95°) tBLG and reveal remarkable competition between SC and other broken symmetries. At higher D, we find the suppression of SC coincides with broken isospin symmetry near half-filling with a lifted degeneracy of g_d~2. Our experimental findings, along with recent theoretical study on SC in tBLG, underscore the competition between SC and other orders promoted by broken symmetries. In the second work, we move onto marginally twisted bilayer graphene (mtBLG) with a twist angle of (θ~0.30°). The measured R_xx at the Dirac point for such a tBLG increases with increasing D due to opening of a gap at the charge neutrality with applied |D|. On the contrary, an anomalous behaviour is measured in the non-local resistance (R_NL) at v ~0, where R_NL decreases with indreasing D. Also, R_NL peak is found to be splitted around v ~0. We propose a theoretical framework to explain the transport in the 2D network of 1D chiral Domain Wall (DW) modes in mtBLG to qualitatively capture the observed anomalous behaviour in R_NL. In the third work, we focus on the observation of hysteresis in R_xx in form of butterfly magnetoresistance (BMR) in near magic-angle tBLG in presence of an in-plane field. The emergent magnetoresistance hysteresis phase bounds the superconducting region with no sign of hysteresis deep into the SC phase at zero D, and evolves into an extended hysteresis phase when superconductivity is suppressed at finite displacement fields. Through measurements with different magnetic field ramp rates and ‘time series’ experiment we look into unveiling the origin (intrinsic versus extrinsic effect) of BMR in near magic-angle tBLG. In the last work, we explore the thermoelectric effects in the near magic-angle tBLG device. Along with conventional sign changes in thermopower at the charge neutrality (v=0) and the superlattice band edges (v±4) of the near magic-angle tBLG, measured thermoelectric voltage (V_2ω) exhibits additional sign changes around v_cs~ |0.3|-|0.5| demonstrating a near robust temperature dependence in the range of T~5 K -50 K at zero D. Finite thermoelectric voltage at the SC region at zero D and a peak in V_2ω at v ~+2 at finite D is also observed in the thermoelectric voltage. Additionally, v_cs is also incommensurate with the appearance of van Hove singularity (vHs) at v_vHs in the flat band probed through Hall density measurements. Such observations indicate a presence of strong correlation effects in twisted graphene platforms.