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Research Highlights

Illustration of the trilayer graphene/boron nitride moiré superlattice with electronic and ferromagnetic properties.

Topological transport in 2D materials

Topological states of quantum materials offer an exciting platform to test some profound ideas in mathematics and physics. In 2D materials, we achieved to observed topological phases by electrical transport measurement. The first example is a topological valley current in bilayer graphene when the inversion symmetry is broken by an electric field.[1] A second example is topological high order Chern insulator and ferromagnetism in ABC-stacked trilayer graphene on hBN moiré superlattice. By using a electric field, we achieve to tune the topology of the correlated flat band in the moiré superlattice. A high order Chern insulator is observed from the quantized anomalous Hall effect.[2][3]

1. Nature Physics 11 (12), 1027-1031. (2015)

2. Nature, 579, 56–61. (2020)

3. Physical Review Letters, 122, 016401. (2019)

Schematic of graphene/boron nitride Mott insulator.

Tunable correlated states and superconductivity in graphene

Tunable Mott insulator and superconductivity in ABC-stacked trilayer graphene on hBN moiré superlattice. For the first time, we experimentally create an ABC-stacked trilayer graphene on hBN moiré superlattice, and acheive the moiré flat band. The bandwidth and correlated strength can be conviniently tuned by the vertical electric field in this system. In the strong correlated regime, we observe the Mott insulating states[4] and signatures of superconductivity[5].

4. Nature, 572, 215–219 . (2019)

5. Nature Physics, 15, 237–241. (2019)

Engineering graphene's electronic properties by moiré superlattice

Graphene and hBN are both honeycomb lattices with tiny lattice constant mismatch. A long-periodic moiré pattern forms when graphene is put on hBN with a small twisted angle. The moiré pattern can dramatically modify the electronic properties of graphene, such as a band gap opening, secondary and tertary Dirac points from band folding, and Hofstadter butterfly physics.

6. Nano Letters, 17, 3576-3581 . (2017) 7. Nature Materials, 12, 792-797. (2013) 8. Nature Physics, 12, 1111–1115. (2016) 9. Physical Review Letters, 116 (12), 126101. (2016) 10. Nano Letters, 16 (4), 2387-2392. (2016)

Experimental Techniques

Van der Waals heterostructures

By developing different techniques, including mechanical transferring and growth methods, we assemble different 2D materials to form van der Waals (VdW) heterostructures in our home-built setup. By selecting proper 2D componets, we build our own 2D blocks and explore novel quantum phenomena. The shown hBN/BP/hBN/Gr/SiO2/Si is an example of the VdW heterostructures in which the quality of BP electronics is dramatically improved and leads to the observation of quantum Hall effect.

Nano-fabrications

Cleanroom based nano-fabrications are performed for a final functional device after the VdW heterostructures assembling, For the study of quantum Hall related electronic study, the device is usually etched into a standard Hall bar geometry. The optical image above is a dual-gate Hall bar device of hBN/trilayer graphene/hBN.

Low temperature and high magnetic field transport

Electrical transport is our major measurement method. To measure the intrinsic electronic properties of the quantum ground states, we need to overcome the thermal excitations (~kT) and cool down the sample below liquid helium temperature, or even to ~ 0.01 Kelvin range by using dilution refrigerator. Sometimes we go to NHMFL in Tallahassee for very high magnetic field measurement up to 45 Tesla.