Research Projects

Our research focuses on the understanding of fundamentals of emergent quantum many-body physics. Strong correlations in solid-state systems often make the regular electrons behave differently, and sometimes the resultant quantum states host quasi-particles that are rather immune to local environmental disturbance. These quasi-particles are fundamentally different from electrons or any other fundamental particles. Being fragile, they are elusive and experiments to detect them are much more challenging; yet their understanding may change the way we presently look at advanced technology.

Interlayer correlated states in graphene

(Dr. Ning Ma)

Bilayer quantum Hall system is believed as an ideal platform for the investigation of exotic phenomena which are attributed to the interlayer Coulomb interaction and tunneling effect. In terms of graphene systems, two single carbon layers are usually spaced by an atomically thin insulating material, i.e., hBN, which together with the external magnetic field, decide the strength of the inter- and intra-layer coulomb interaction, respectively. Recently, large angle twisted bilayer (or double bilayer) graphene offers a new pathway to study the interlayer correlation physics. The large relative angle results in a mismatch of the Dirac cones in two layers, which suppresses the interlayer tunneling and hybridization. Besides, the ratio of interlayer and intralayer Coulomb interaction can be larger by over one order of magnitude because of the absence of the insulating spacer, which makes interlayer Coulomb interaction dominant. By fabricating high quality large angle twisted double bilayer graphene (TDBG) devices, we are planning to investigate the correlated states induced the strong interlayer Coulomb interaction, i.e., interlayer excitons, and fractional quantum Hall states.

Exotic states in rhombohedral graphene

(Kilian Krötzsch)

The fabrication of graphene-based 2D material heterostructures can lead to the formation of flat bands in the electronic band structure of graphene, significantly enhancing electron-electron correlations that are normally neglected in the single-particle picture used in condensed matter physics. We perform (magneto-)transport measurements on these systems to study a variety of quantum mechanical phenomena: Phases that belong to the large family of quantum Hall effects, which can arise even in the absence of an external magnetic field. This is exemplified by the quantum anomalous Hall effect and in fractional Chern insulators, which result from spontaneous time reversal symmetry breaking. Furthermore, magnetic ordering, spin-valley polarized phases, and superconductivity have been observed in these systems. The stacking order of few-layer graphene plays a crucial role in the formation of these phases, and its properties can be tuned even further. For example by introducing an additional moiré potential or by adding spin-orbit coupling and superconductivity by proximity effects. Consequently, it is a versatile platform for gaining deeper insight into the physics governing strongly correlated systems at cryogenic temperatures.

Superconductivity in twisted graphene systems

(Dr. Zekang Zhou, Nanyu Yao)

Superconductivity has been widely observed in a variety of twisted graphene systems, with substantial evidence indicating that it is unconventional in origin. Despite significant progress, a complete understanding of the superconducting phases in these materials is not established. Fundamental questions such as the relationship between correlated states and superconductivity, the connection between inter-valley coherence (IVC) order and superconductivity, and the nature of the superconducting order parameter are still unresolved. To address these open issues, we aim to fabricate high-quality twisted graphene devices with diverse geometries and employ a combination of experimental techniques, including transport and noise measurements, to gain deeper insight into the mechanisms underlying superconductivity in these systems.

Interferometry in graphene based heterostructures

(Mario Di Luca)

Electrons confined in two-dimensions (2D) under a perpendicular magnetic field at low temperatures give rise to a collective quantum fluid. One phase of such a fluid is known as quantum Hall effect (QHE). Theoretical models followed by experimental evidence have already shown that strong correlations in fractional quantum Hall effect (FQHE) lead to fractionally charged quasiparticles known as anyons. Anyons do not respect fermionic or bosonic exchange statistics, rather exchanging anyons changes the wave function by a phase other than 0 or pi, which differ from state to state. Anyons can be mainly classified into two groups, abelian and non-abelian. The non-abelian anyons are particularly of interest, given their possible application in topological quantum bits. Electron interferometry is an excellent tool to test the intrinsic properties of anyons. The most common interferometer is the Fabry-Perot interferometer (FPI). There are quite few limitations that make observing electron interference in GaAs an arduous task, hence a new platform is required. Graphene, a one-atom-thick layer of carbon atoms, serves as a natural 2D electron gas and it is a perfect substitute platform. It has a unique electronic band structure in which charge carriers behave as massless Dirac fermions with high mobility. In bilayer graphene (BG) , contrary to monolayer, a gap can be opened in the band structure by applying a displacement field, providing a further degree of freedom to control the system.

Unconventional superconductivity and correlated insulating states in twisted TMDs

(Punam Barman)

The discovery of superconductivity in twisted bilayer and trilayer graphene demonstrated the power of moirĂ© superlattices to generate flat bands with strong correlations, sparking enormous interest in two-dimensional quantum materials. Similar physics has since been realized in transition metal dichalcogenide (TMD) moirĂ© systems, which combine lattice twist and interlayer coupling to produce narrow electronic bands described by simple Hubbard- or Kondo-type models, but with unique material attributes absent in graphene, including a native band gap, strong spin–orbit coupling, spin–valley locking, and intrinsic magnetism. In homo-bilayer WSe₂, superconductivity has been observed adjacent to an antiferromagnetically ordered metallic state, suggesting spin-fluctuation-mediated pairing. At other twist angles, WSe₂/WSe₂ exhibits an antiferromagnetic Mott insulator at one carrier per moirĂ© unit cell, strange metallic behavior at higher temperatures, and superconducting domes upon doping, with the highest Tc​. In contrast, hetero-bilayer MoTe₂/WSe₂ hosts synthetic Kondo lattices, where localized spins in MoTe₂ couple to itinerant carriers in WSe₂, giving rise to gate-tunable heavy fermion behavior, magnetic-field–induced quantum phase transitions, and even a Chern metal phase near Kondo breakdown, characterized by nearly quantized Hall resistance with finite longitudinal resistance and chiral edge transport. Together, these moirĂ© platforms provide a unified and highly tunable setting for exploring the interplay of strong correlations, magnetism, topology, and superconductivity in two dimensions, opening new pathways toward solving the high-Tc​ problem and engineering exotic quantum phases.

Probing properties of fractional quantum Hall states through entropy measurements

(Emily Hajigeorgiou)

This project is dedicated to exploring the fractional statistics of quasiparticles within the context of the Fractional Quantum Hall Effect, using GaAs quantum dots as the experimental platform. Common methodologies for studying anyonic braiding involve electronic interferometers and anyonic colliders. However, these methods use conductance as the experimental probe, which is not always enough to determine the quantum mechanical ground state of the system.  Our approach focuses on measuring entropy, with specific emphasis on quantifying the fractional entropy, which is a signature of nonlocal degrees of freedom such as Majorana zero modes or more exotic non-Abelian anyons.