Tunneling spectroscopy of Fractional Quantum Hall states

A new phase of matter termed Fractional quantum Hall (FQH, 1998 Nobel Prize in Physics) emerge at high magnetic field quantum limit. In FQH, the electrons show collective behavior as if they split into fractionalized charge, termed anyon. These are exciting quasiparticles as some of them, non-abelian anyons, are promising for fault-tolerant quantum computing.

Using scanning tunneling spectroscopy (STS), we performed the high resolution atomic-scale study of the FQHs for the first time, and revealed many microscopic details of the FQHs that were overlooked in previous macroscopic studies. In particular, we reveal the large energy gaps of the non-abelian candidates in graphene, which is about 5 times larger then its counterpart in GaAs. 

Check out in Nature Physics.

Strongly interacting Hofstadter states in MATBG

Moiré quantum materials, composed of rotated 2d atomic layers, are emerging flat band platforms to study the physics of correlation and topology. Unlike the flat Landau level, the moiré lattice potential, modulating on an length scale comparable to the magnetic length (13 nm at 4 T) , plays an significant role and leads to the famous fractal energy spectrum, known as the Hofstadter butterfly. The non-interacting Hofstadter spectrum and its associated topological states are widely explored, yet little is known of the emergent charge density waves (CDW) and fractionalized states in its strong interacting limit. Magic-angle twisted bilayer graphene (MATBG) is a benchmark system with flat electronic bands at zero magnetic field, thus allows access to the strongly interacting Hofstadter states at a comparatively small magnetic fields. 

Using quantum transport measurements, I explore the interacting Hofstadter states in the state-of-art MATBG sample. I uncover a remarkable cascade sequence of symmetry-broken Chern insulator (SBCI) states (the CDWs), which is also captured by our self-consistent Hartree-Fock calculations. I also observe a series of Jain-sequence FQH states, however with a unconventional phase transition towards a dissipative Fermi liquid at a higher magnetic field. We propose that the FQH states in MATBG can be better understood as in-field fractional Chern insulators (in-field FCI), due to its nontrivial quantum geometry properties.

Check out in Nature Physics.

Electron correlation and topology in twisted graphene multilayers

The flat bands are ubiquitous feature of moiré quantum materials beyond MATBG. I also study a variety of correlated insulating states, orbital magnetic states, and interacting Hofstadter states in the twisted graphene multilayers using quantum transport measurements, including those in:

Twisted monolayer-bilayer graphene (tMBG, 1+2) ;

Twisted double bilayer graphene (tDBG, 2+2). 

The phase diagrams of these systems are notably different from that of MATBG, primarily due to the broken C2T symmetry thus the existence of isolated flat Chern bands, sometimes with high Chern number. The flavor symmetry breaking at integer fillings are different and they can be more tunably using displacement field. These are promising platforms to design and engineer correlated and topological states at zero magnetic field.

tDBG (Nature Physics, Nano Letters); tMBG (Nature Physics, Nature communications).

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