Investigation of twisted bilayer graphene using electrical and thermoelectric transport

Abstract

The ability to tune the twist angle between di erent layers of two-dimensional materials has opened up a new dimension to band engineering in van der Waals (vdW) heterostructures. By taking advantage of the formation of a moir e superlattice arising from a small lattice mismatch or twist angle between two adjacent atomic layers, one can create materials with tailored electronic, optical, mechanical, thermal and optoelectronic properties. In particular, twisted bilayer graphene (tBLG) has attracted considerable scienti c interest owing to exceptional band tunability. The coupling between the two graphene layers depends strongly on the twist angle, leading to angledependent electronic and phononic hybridization. In addition, when the relative rotation is close to the magic angle ( m = 1:1 ), the low-energy electronic bands are nearly at, leading to a multitude of interaction-driven phases. This includes correlated insulators, superconductivity, magnetism, non-trivial band topology, nematicity and signatures of non-Fermi liquid (NFL) excitations with linear-in-temperature resistivity that persists down to temperatures well below the Bloch{Gr uneisen temperature. Although signi cant progress has been made in understanding the vast phase diagram of tBLG, there is no consensus on the origin of the superconductivity, metallic states and the role of electron-phonon coupling at very low temperatures ( 1 K). In this thesis, we study the in-plane and cross-plane electrical and thermoelectric properties of tBLG with varying twist angles to understand the nature of metallic states and the role of layer breathing phonon modes at low twist angles. The cross-plane thermoelectric transport in large angle tBLG is driven by the scattering of electrons and inter-layer breathing phonon modes. However, the relevance of layer hybridized phonons in thermoelectric transport remains unclear when the electronic hybridization of the two layers becomes strong at low . In the rst part of the thesis, we show out-of-plane thermoelectric measurements across the vdW gap in tBLG, which exhibits an interplay of twistdependent interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is entirely driven by a novel phonon-drag e ect at the subnanometer scale. In contrast, the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to < 6 . Our experiment shows that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion. Although the T-linear resistivity in tBLG at low temperatures has been attributed to the absence of a well-de ned quasiparticle spectrum, experimentally, the manifestation of NFL e ects in transport properties of twisted bilayer graphene remains ambiguous. In the next part of the thesis, we have performed simultaneous measurements of electrical resistivity ( ) and thermoelectric power (S) in tBLG for several twist angles between 1:0 ð€€€ 1:7 . We observe an emergent violation of the semiclassical Mott relation (MR) in the form of excess S close to half- lling for 1:6 that vanishes for & 2 . In addition, for a device with 1:24 , excess S is observed at fractional band lling. The combination of non-trivial electrical transport and violation of Mott relation provides strong evidence of NFL physics intrinsic to tBLG. Next, we study the electrical and thermoelectric transport in marginally tBLG ( 0:5 ), where the electronic band structure at a low twist angle is expected to become qualitatively di erent as compared to magic-angle because of large moir e unit cell and strain-accompanied lattice reconstruction. We observe a strong metallic behaviour accompanied by a T-linear and an emergent violation of the semi-classical Mott relation in the vicinity of van Hove singularities (vHSs). Our experiments show that the thermopower is exceptionally sensitive to the band dispersion in small-angle tBLG even at high temperatures, and the low-T transport is governed by a network of topological channels formed at domain boundaries between AB and BA regions. Finally, we have demonstrated the working of a thermoelectric generator consisting of dual-junction tBLG. We show that the thermopower in cross-plane tBLG can be enhanced by using a dual-junction device. Further, using an external resistor enables us to measure the current-voltage characteristics of the device and estimate the power generated in the system.