Thermopower probing the effect of interaction and strong correlation in twisted multilayer graphene moir´e

Abstract

Two-dimensional quantum materials, especially graphene-based van der Waals heterostructures, have been at the forefront of a new era in scientific research and technological breakthroughs. From field-effect transistors to next-generation optoelectronic devices, these systems have redefined how we explore quantum phenomena, transforming materials into tunable landscapes where geometry and interactions shape the fundamental behavior of electrons. Among them, twist-engineered graphene multilayers, such as twisted bilayer graphene (TBLG), offer an exceptional capability to engineer the electronic band structure through a simple twist. At the so-called magic angle (~1.1°), TBLG exhibits nearly dispersionless flat electronic bands, where interaction and correlation effects dominate. These flat bands serve as fertile ground for many emergent phenomena, including correlated insulators, unconventional superconductivity, orbital magnetism, topological Chern insulators, and more. To understand these phases, it is essential to probe the unique band structure of TBLG and the role of interactions. In this context, thermopower measurements play an indispensable role, providing valuable insights into features of the electronic structure such as the density of states (DOS), particle-hole asymmetry, presence of van Hove singularities (vHs), and the rigidity or malleability of the band structure with changing carrier concentration. In this thesis, we leverage thermopower measurements to explore and elucidate the complex interplay between electronic band structure and correlation in twisted graphene systems. By systematically investigating magic-angle TBLG (MATBLG), as well as other graphene moirés like twisted double bilayer graphene (TDBLG) and related TBLG devices as a function of temperature, carrier concentration, and magnetic field, we uncover the underlying mechanisms driving their rich phase diagrams.

Our investigation begins with thermopower measurements on MATBLG. In the low-temperature range (0.1 K to 10 K), the measured thermopower in the low-energy flat band violates the Mott formula while remaining consistent with expectations for the higher-energy dispersive bands. Most notably, the thermopower of the flat band exhibits pronounced anomalies, with large positive peaks (even exceeding 100 μV/K at 1 K) around the positive integer fillings of the conduction band. This anomaly, supported by theoretical analysis, highlights the role of strong interactions in inducing a highly particle–hole-asymmetric DOS and reconstruction of the non-rigid band structure in MATBLG near the commensurate filling of the band. Additionally, enhanced thermopower observed near the superconducting dome on the hole-doped side is attributed to superconducting phase fluctuations. In the following study done at a higher temperature range (10 K to 60 K), the measured thermopower of the MATBLG flatband shows a unique feature with the three crossing points (sign change in thermopower), which remain independent of the said temperature range. The behaviour of the crossing points with temperature, magnetic field and varying twist angle suggests that a non-interacting band picture is insufficient, and instead supports a theoretical framework involving strong correlation effects. At even higher temperatures (>100 K), as temperature scale goes beyond the correlation energy scale, the thermopower of MATBLG converges toward that of monolayer graphene. We further extend our thermopower measurements to twisted double bilayer graphene (TDBLG), for which the pronounced enhancement of thermopower and large magnetoresistance with increasing magnetic field is observed around the charge neutrality point (CNP). This observation remains consistent with the theory invoking compensated semimetallic state at CNP of the TDBLG, characterized by coexisting electron and hole pockets. Lastly, we explore the interaction effects in the quantum Hall of graphene utilizing thermopower measurement. For the integer quantum Hall, thermopower captures the interplay between Landau level broadening and thermal broadening. For the fractional quantum Hall, thermopower shows how composite fermion theory explains the crossing in the thermopower signal.

Taken together, this thesis establishes thermopower’s success as a broadly applicable and sensitive tool for uncovering the entropic and correlation-driven landscape of moiré and quantum Hall systems.