Publications
Highlights since 2019
Large quantum anomalous Hall effect in spin-orbit proximitized rhombohedral graphene
Rhombohedral graphene has been identified as a promising platform for investigating correlated and topological phenomena without the need for moiré superlattices. In previous studies, various symmetry-broken states and superconductivities were observed in rhombohedral multilayer graphenes. However, realizing the quantum anomalous Hall effect (QAHE) in these systems, particularly without magnetic elements or moiré effects, has been challenging. Proximity-induced spin-orbit coupling (SOC) from transition-metal dichalcogenides (TMDs) has been suggested as a method to enhance electron correlations in graphene-based systems, potentially enabling QAHE.
In this study, we reported the observation of QAHE in pentalayer rhombohedral graphene proximitized by monolayer TMD. This heterostructure exhibits QAHE at charge neutrality with a quantized Hall resistance corresponding to Chern numbers C = ±5, which is the largest observed in QAHE systems to date. The effect persists up to 1.5 K with clear magnetic hysteresis. The large Chern number arises from the combination of electron correlation in the flat bands of pentalayer graphene, gate-tuning effects, and Ising SOC induced by TMD.
See also T. Han et al., Large quantum anomalous Hall effect in spin-orbit proximitized rhombohedral graphene, Science 384.6696 (2024): 647-651.
Fractional quantum anomalous Hall effect in multilayer graphene
The fractional quantum anomalous Hall effect (FQAHE) is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking, analogous to the fractional quantum Hall effect but at zero magnetic field. Demonstrating FQAHE could lead to the formation of non-Abelian anyons, which are essential for topological quantum computation. Graphene-based moiré superlattices, known for their superior material quality and higher electron mobility, have been proposed as potential hosts for FQAHE.
For the first time, we observed fractional quantum anomalous Hall effects in a rhombohedral pentalayer graphene–hBN moiré superlattice. At zero magnetic field, we detected quantized Hall resistance plateaus at multiple fractional filling factors, accompanied by clear dips in longitudinal resistance. By adjusting the gate-displacement field and moiré filling factor, we observed phase transitions from composite Fermi liquid and FQAH states to various correlated electron states. This system provides a promising platform for exploring charge fractionalization and non-Abelian anyonic braiding, especially with the potential to create lateral junctions between FQAHE and superconducting regions in the same device.
See also Z. Lu et al., Fractional quantum anomalous Hall effect in multilayer graphene, Nature 626 (2024): 759–764.
Orbital multiferroicity in pentalayer rhombohedral graphene
Two-dimensional (2D) materials with honeycomb lattices, such as graphene, offer new opportunities for studying ferroic orders driven purely by orbital degrees of freedom. These materials enable the engineering of unconventional multiferroicity, allowing strong valley-magnetic couplings and significant responses to external fields, paving the way for advanced device applications.
We discovered orbital multiferroicity in pentalayer rhombohedral graphene through low-temperature magneto-transport measurements. We observed anomalous Hall signals with a large Hall angle and orbital magnetic hysteresis at hole doping. Four distinct states with different valley polarizations and orbital magnetizations form a valley-magnetic quartet. The butterfly-shaped hysteresis in the Hall resistance indicates a ferro-valleytronic order coupled to a composite field E · B (where E is the electric field, and B is the magnetic field). This reveals a new type of multiferroicity, suggesting potential for electrically tunable, ultralow-power valleytronic and magnetic devices.
See also T. Han et al., Orbital multiferroicity in pentalayer rhombohedral graphene, Nature 623 (2023): 41-47.
Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice
The ABC-stacked trilayer graphene/hexagonal boron nitride (TLG/hBN) moiré superlattice has become a significant platform for exploring correlated electron physics. Previous studies have primarily used electron transport measurements to investigate this system, revealing phenomena such as superconductivity, correlated insulating states, and so on. To gain deeper insights into the electronic properties of these materials, advanced spectroscopy techniques that can probe the buried heterostructures are essential.
Using Fourier transform infrared (FTIR) photocurrent spectroscopy, we measured dual-gated TLG/hBN and observed a strong optical transition between moiré minibands that narrows continuously as a bandgap is opened by gating, indicating a reduction of the single-particle bandwidth. At half-filling of the valence flat band, a broad absorption peak emerges at ~18 meV, suggesting direct optical excitation across a Mott gap. Similar spectra are found in other correlated insulating states at quarter- and half-filling of the first conduction band. These findings provide crucial parameters for the Hubbard model to understand electron correlations in TLG/hBN.
See also J. Yang et al., Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice, Science 375.6586 (2022): 1295-1299.
Highlights before MIT
Tunable Excitons in Bilayer Graphene
Bilayer graphene has a unique bandstructure where a electrically tunable bandgap exists at corners of the first Brillouin Zone. By opening the gap with external gates, excitons--as the fundamental optical excitation of semiconductors--are expected to host pseudospin-related properties that are distinct from other semiconductors.
For the first time, we observed such excitons in bilayer graphene that have unusual selections rules, significant oscillator strength, extremely narrow linewidth and they can be tuned from mid-infrared to Terahertz range. More interestingly, these excitons exhibit a large valley g-factor of 20 in magnetic fields as determined by the outstanding Berry curvature effects.
Topological Valley Transport at Graphene Domain Walls
Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. Gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators.
We employ near-field infrared nanoscopy to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. These devices feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin.
See also, L. Ju et al., Topological Valley Transport at Bilayer Graphene Domain Walls, Nature 520, 650(2015)
Complete List
L. Ju, A. H. MacDonald, K. F. Mak, J. Shan, X. Xu, "The fractional quantum anomalous Hall effect", Nature Reviews Materials. (2024)
T. Han*, Z. Lu*, Y. Yao*, J. Yang, J. Seo, C. Yoon, K. Watanabe, T. Taniguchi, L. Fu, F. Zhang, L. Ju, "Large Quantum Anomalous Hall Effect in Spin-Orbit Proximitized Rhombohedral Graphene", Science (2024), (arXiv, MRL news, MIT news)
Z. Lu*, T. Han*, Y. Yao*, A. Reddy, J. Yang, J. Seo, K. Watanabe, T. Taniguchi, L. Fu, L. Ju, "Fractional Quantum Anomalous Hall Effect in Multilayer Graphene", Nature (2024), (arXiv, Nature News, CMP Journal Club, CMP Journal Club 2, MIT news, Nature research briefing)
T. Han*, Z. Lu*, G. Scuri, J. Sung, J. Wang, T. Han, K. Watanabe, T. Taniguchi, L. Fu, H. Park, L. Ju, "Orbital Multiferroicity in Pentalayer Rhombohedral Graphene", Nature (2023), (arXiv, MIT news)
T. Han*, Z. Lu*, G. Scuri, J. Sung, J. Wang, T. Han, K. Watanabe, T. Taniguchi, H. Park, L. Ju, "Correlated Insulator and Chern Insulators in Pentalayer Rhombohedral Stacked Graphene", Nature Nanotechnology (2023), (arXiv, Nature News and Views, MIT news, cover story)
J. Yang*, G. Chen*, T. Han*, Q. Zhang, Y-H. Zhang, L. Jiang, B. Lyu, H. Li, K. Watanabe, T. Taniguchi, Z. Shi, T. Senthil, Y. Zhang, F. Wang, L. Ju, "Spectroscopy Signatures of Electron Correlations in a Trilayer Graphene/hBN Moiré Superlattice", Science (2022) (https://arxiv.org/abs/2202.12330)
Z. Lu, P. Hollister, M. Ozerov, S. Moon, E. D. Bauer, F. Ronning, D. Smirnov, L. Ju, B. J. Ramshaw, "Weyl fermion magneto-electrodynamics and ultralow field quantum limit in TaAs", Science Advances. (2022)
T. Han, J. Yang, Q. Zhang, L. Wang, K. Watanabe, T. Taniguchi, P. L. McEuen, L. Ju "Accurate measurement of the gap of graphene/hBN moiré superlattice through photocurrent spectroscopy", Physical Review Letters. (2021)
L. Ju*, L. Wang*, X. Li, S. Moon, M. Ozerov, Z. Lu, T. Taniguchi, K. Watanabe, E. Mueller, F. Zhang, D. Smirnov, F. Rana & P. L. McEuen, "Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene", Nature Communications. (2020)
L. Ju*, L. Wang*, T. Cao, K. Watanabe, T. Taniguchi, S. G. Louie, F. Rana, J. Park, J. Hone, F. Wang & P. L. McEuen, “Tunable Excitons in Bilayer Graphene”, Science. (2017)
J. Velasco Jr*, L. Ju*, D. Wong, S. Kahn, J. Lee, H. Tsai, C. Germany, S. Wickenburg, J. Lu, T. Taniguchi, K. Watanabe, A. Zettl, F. Wang, M. F. Crommie, “Nanoscale Control of Rewriteable Doping Patterns in Pristine Graphene/Boron Nitride Heterostructures”, Nano Letters. (2016)
D. Wong*, J. Velasco Jr*, L. Ju*, J. Lee, S. Kahn, H. Tsai, C. Germany, T. Taniguchi, K. Watanabe, A. Zettl, F. Wang & M. F. Crommie, “Characterization and Manipulation of Defects in Hexagonal Boron Nitride Using Scanning Tunneling Microscopy”, Nature Nanotechnology. (2015)
L. Ju*, Z. Shi*, N. Nair, Y. Lv, C. Jin, J. Velasco Jr., C. Ojeda-Aristizabal, H. A. Bechtel, M. C. Martin, A. Zettl, J. Analytis & F. Wang, “Topological Valley Transport at Bilayer Graphene Domain Walls”, Nature. (2015)
L. Ju*, J. Velasco Jr*, E. Huang, S. Kahn, C. Nosiglia, H. Tsai, W. Yang, T. Taniguchi, K. Watanabe, Y. Zhang, G. Zhang, M. Crommie, A. Zettl & F. Wang, “Photoinduced Doping in Heterostructures of Graphene and Boron Nitride”, Nature Nanotechnology. (2014)
L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H. A. Bechtel, X. Liang, A. Zettl, Y. R. Shen & F. Wang, “Graphene Plasmonics for Tunable Terahertz Metamaterials”, Nature Nanotechnology. (2011)
L. Jiang, Z. Shi, B. Zeng, S. Wang, J. Kang, T. Joshi, C. Jin, L. Ju, J. Kim, T. Lyu, Y. Shen, M. Crommie, H. Gao & F. Wang, “Soliton-dependent Plasmon Reflection at Bilayer Graphene Domain Walls”, Nature Materials. (2016)
Y. Bie, J. Horng, Z. Shi, L. Ju, Q. Zhou, A. Zettl, D. Yu, and F. Wang, “Vibrational Spectroscopy at Electrolyte/Electrode Interfaces with Graphene Gratings”, Nature Communications. (2015)
Z. Shi*, C. Jin*, W. Yang, L. Ju, J. Horng, X. Lu, H. A. Bechtel, M. C. Martin, D. Fu, J. Wu, K. Watanabe, T. Taniguchi, Y. Zhang, X. Bai, E. Wang, G. Zhang & F. Wang, “Gate-dependent Pseudospin Mixing in Graphene/Boron Nitride Moiré Superlattices”, Nature Physics. (2014)
J. Lee, W. Bao, L. Ju, P. J. Schuck, F. Wang, & A. W. Bargioni, “Switching Individual Quantum Dot Emission through Electrically Controlling Resonant Energy Transfer to Graphene”, Nano Letters. (2014)
Z. Yan, Y. Liu, L. Ju, Z. Peng, J. Lin, G. Wang, H. Zhou, C. Xiang, E. L. G. Samuel, C. Kittrell, V. I. Artyukhov, F. Wang, B. I. Yakobson & J. M. Tour, “Large Hexagonal Bi- and Trilayer Graphene Single Crystals with Varied Interlayer Rotation”, Angewandte Chemie. (2014)
S. F. Shi, T.-T. Tang, B. Zeng, L. Ju, Q. Zhou, A. Zettl, & F. Wang, Controlling Graphene “Ultrafast Hot Carrier Response from Metal-like to Semiconductor-like by Electrostatic Gating”, Nano Letters. (2014)
B. Chapler, S. Mack, L. Ju, T. W. Elson, B. W. Boudouris, E. Namdas, J. D. Yuen, A. J. Heeger, N. Samarth, M. Di Ventra, R. A. Segalman, D. D. Awschalom, F. Wang & D. N. Basov, “Infrared Conductivity of Hole Accumulation and Depletion Layers in (Ga, Mn) As-and (Ga, Be) As-based Electric Field-Effect Devices”, Phys. Rev. B. (2012)
K. F. Mak, L. Ju, F. Wang & T. F. Heinz, “Optical Spectroscopy of Graphene: From the Far Infrared to the Ultraviolet”, Solid State Communications. (2012)
M. Liu *, X. Yin *, E. Ulin-Avila, B. Geng, T. Zentgraf, L. Ju, F. Wang & X. Zhang, “A Graphene-based Broadband Optical Modulator”, Nature. (2011)
X. Xie*, L. Ju*, X. Feng, Y. Sun, R. Zhou, K. Liu, S. Fan, Q.Li & K. Jiang, “Controlled Fabrication of High-Quality Carbon Nanoscrolls from Monolayer Graphene”, Nano Letters. (2009)