Title: Proximity engineering of correlated states in graphene heterostructures
Name of the Student: Mr. Saisab Bhowmik (Integrated Ph.D.)
Venue: S V Narsaiah Auditorium, IAP Department.
Date: 06/05/2025, Tuesday
Time: 3 PM
Abstract : Strong electronic correlations are central to many phenomena in condensed matter physics, including unconventional superconductivity, the fractional quantum Hall effect, nematicity, and non-Fermi liquid behavior. In correlated systems, the motion of an electron is strongly influenced by the presence of other electrons, making single-particle band structure approaches insufficient. Although many correlated phases have been discovered in materials such as, Kondo lattices, heavy-Fermion compounds, and high Tc cuprate superconductors, their phase diagrams are often not fully understood. This is due to both the limited number of experimental tuning parameters and the complexity of theoretical models needed to account for strong Coulomb interactions. As we work toward better understanding existing systems, it is equally important to explore new platforms where correlation effects can be more effectively tuned and probed.
One such platform is magic angle twisted bilayer graphene (TBG), where a twist angle of 1.08o between two graphene layers produces flat bands with a large density of states at low energy. These bands enable access to a range of correlated phases, including insulating states, superconductivity, magnetism, and nematic order. Despite significant theoretical and experimental efforts, the origin of some of these phases remains under debate. TBG offers a wide parameter space to explore the correlated phase diagram, including carrier density, electric/magnetic fields, and temperature. In this thesis, we use two different approaches to modify the electronic structure of TBG and Bernal-stacked bilayer graphene (BLG), both of which exhibit flat-band physics. In the first part, we study TBG devices placed on tungsten diselenide (WSe2), which induces spin-orbit coupling (SOC) in graphene layers. We observe correlated states near half-integer band fillings (𝜈 = 0.5, ±3.5) in the zero magnetic field limit. Magnetotransport measurements reveal a sequence of Lifshitz transitions and reset of charge carriers as the density is varied. These phase transitions are attributed to changes in the Fermi surface topology driven by van Hove singularities. In addition, thermoelectric measurements enable observation of 𝜈 = 3.5 without a magnetic field. At 𝜈 ≈ −0.5, a Chern insulating state appears at high magnetic field, consistent with symmetry breaking in an enlarged moiré supercell. These results suggest a spin- or charge density wave ground state at zero field. We also find an anomalous Hall effect and reversal of magnetization near half filling, indicating orbital magnetism. A non-quantized Hall resistance suggests a partial valley polarization. Surprisingly, a valley-polarized ground state is energetically unstable at half filling in TBG without SOC. The addition of SOC can stabilize the valley-polarized ground state at half filling, which is accompanied by a delicate sequence of Fermi surface reconstructions that are tunable with magnetic field and temperature. At finite fields, the Hall resistance becomes quantized, consistent with Chern insulators with complete valley polarization. Our findings demonstrate that SOC can stabilize ordered phases at partial band fillings, which are typically absent in TBG without WSe2. Magnetoresistance measurements further reveal complex behavior with changing density, pointing to rich underlying magnetic and resistive states.
In TBG systems, challenges such as twist angle inhomogeneity, strain, and disorder can degrade device quality, making the observations of strongly correlated phases challenging. Alternative approaches, such as periodic dielectric patterning and the proximity of another moiré lattice near the graphene layer, have been explored in recent years to address these limitations. In the second part, we explore band structure modification in BLG using a patterned graphite gate with a 50 nm period. Applying a superlattice gate voltage leads to new insulating states that are absent in regular BLG. By studying their temperature and magnetic field dependence, we extract energy gaps and probe the Fermi surface topology. These results show that a superlattice potential can significantly alter the BLG band structure and form low-energy minibands in Bernal-BLG.
In summary, this thesis explores how proximity effects, through SOC and superlattice potential modulation, can be used to manipulate the electronic structures of TBG and BLG. These approaches provide useful tools to study and control correlated phases in flat-band graphene systems.