As a condensed matter theorist, I explore exotic properties of quantum materials through the organizing principles of symmetry and topology. By developing models for topological insulators, topological semimetals, graphene, transition-metal dichalcogenides (TMDs), high Tc cuprates, and their composites formed by angular twists, I explore the novel geometric, topological and correlated phenomena relevant to real materials under investigations by my experimental colleagues. My latest interests have been revolving about (i) the study of correlation and topology, as well as the intriguing interplay between the two, in 2D moiré quantum matter, e.g., twisted graphene/TMDs and twisted unconventional superconductors such as high Tc cuprates, rhombohedral graphene and zerconium nitrides; (ii) quantum geometry in the recently developed 'stacking ferroelectrics' formed in van der Waals-stacked quantum materials.

Quantum-geometric Origin of Out-of-plane Stacking Ferroelectricity

Benjamin T. Zhou†, Vedangi Pathak, Marcel Franz

Accepted in Physical Review Letters (preprint available at arXiv:2309.00728)

Stacking ferroelectricity (SFE) has been discovered recently in a wide variety of bilayer van der Waals materials whose monolayer constituents are otherwise non-polar. In this Letter, we show that the microscopic origin of out-of-plane stacking ferroelectric polarization can be generally understood as a nontrivial Berry phase borne out of an effective Su-Schrieffer-Heeger model description with broken sublattice symmetry, thus elucidating the quantum-geometric origin of polarization in the extremely non-periodic bilayer limit. Our explanatory theory based on the quantum-geometric perspective establishes quantitative understanding of out-of-plane SFE materials beyond symmetry principles.

Non-Abelian topological superconductivity in maximally twisted double-layer spin-triplet valley-singlet superconductors

Benjamin T. Zhou†, Shannon Egan, Dhruv Kush, Marcel Franz†

Communications Physics 6 (1), 47 (2023). Selected as Editor's highlights. 

(See Invited symposium at APS March Meeting 2023)

Motivated by recent advances in twistronics, we propose that an intrinsic chiral f + if'-wave topological superconductor(TSC) hosting non-Abelian Majorana zero modes (MZMs) is one 'maximal twist' away from two layers of unconventional, yet topologically trivial, spin-triplet superconductors found in recent studies on rhombohedral graphene and zerconium nitrides. Different from the magic arising from small twists in magic-angle graphene, the nontrivial chiral TSC emerges at a large twist angle around 30 degrees due to an emergent 12-fold quasi-crystalline symmetry. 

Spontaneous-polarization-induced photovoltaic effect in rhombohedrally stacked MoS2

Dongyang Yang, Jingda Wu, Benjamin T. Zhou, Jing Liang, Toshiya Ideue, Teri Siu, Kashif Masud Awan, Kenji Watanabe, Takashi Taniguchi, Yoshihiro Iwasa, Marcel Franz, Ziliang Ye 

Nature Photonics 16 (6), 469-474 (2022).

In this collaborative work with Ziliang Ye's group at UBC, we provide a theoretical explanation of the Berry phase origin behind the emergence of ferroelectric polarization in rhombohedrally stacked bilayer MoS2, which establishes the (quantum) microscopic mechanism that underpins the working principles of an atomically thin photovoltaic device.  

Moiré flat Chern bands and correlated quantum anomalous Hall states generated by spin-orbit couplings in twisted homobilayer MoS2

Benjamin T. Zhou†, Shannon Egan, Marcel Franz† 

Physical Review Research (Letter) 4, L012032 (2022).

In this Letter, we propose that the unique combination of small angular twist and local symmetry breaking leads to topological moiré flat Chern bands (per each valley) with unusual skyrmion number S=±2 that stem from the conduction band states in homobilayer MoS2. We further discuss how electron correlations lift the valley degeneracy at half-filling and lead to a valley-polarized quantum anomalous Hall state.

Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe2 

Ying-Ming Xie*, Benjamin T. Zhou*, K. T. Law

Physical Review Letters 125, 107001 (2020).

In this Letter, we propose a new form of superconductivity - referred to as spin-orbit-parity-coupled (SOPC) superconductivity - which can withstand in-plane upper critical fields surpassing the conventional Pauli limit. Such exotic pairing states can occur in a general centrosymmetric topological material, which stands in stark contrast with Ising superconductors in which the inversion symmetry breaking is crucial for the strong enhancement of in-plane critical fields.

Our prediction of SOPC superconductivity has been verified in a recent experiment in 2M-WS2 [Nature Physics 19, 106-113 (2023)] by Prof. Faxian Xiu's group at Fudan University. See also "A highly anisotropic polymorph" by Joseph Falson in News and Views in Nature Physics.

Proximity-induced surface superconductivity in Dirac semimetal Cd3As2 

Ce Huang*, Benjamin T. Zhou*, Huiqin Zhang, Bingjia Yang, Ran Liu, Hanwen Wang, Yimin Wan, Ke Huang, Zhiming Liao, Enze Zhang, Shanshan Liu, Qingsong Deng, Yanhui Chen, Xiaodong Han, Jin Zou, Xi Lin, Zheng Han, Yihua Wang, Kam Tuen Law, Faxian Xiu

Nature Communications 10, 2217 (2019). Selected as Editor's highlights on Condensed Matter Physics.

In this collaborative work with Prof. Faxian Xiu's group at Fudan University, we explained the pronounced superconducting proximity effects in the Fermi arcs of a three-dimensional Dirac semimetal(DSM) Cd3As2. Through realistic simulations of the spectroscopic and transport properties of the DSM/superconductor junction, we revealed that the strong proximity-induced pairing in the Fermi arcs manifests itself as a flat conductance plateau in the differential conductance spectra. The presence of non-dissipative surface currents was confirmed by the spatial image of supercurrents resolved from the Fraunhofer pattern formed under superconducting quantum interference.

Spin-orbit-coupling induced valley Hall effects in transition-metal dichalcogenides 

Benjamin T. Zhou*, Katsuhisa Taguchi*, Yuki Kawaguchi, Yukio Tanaka, K. T. Law

Communications Physics 2, 26 (2019). Selected as Editor's highlights.

In collaboration with Prof. Yukio Tanaka's group at Nagoya University, we proposed that the unique combination of intrinsic Ising spin-orbit couplings (SOCs) and Rashba SOCs (induced by external gating or built-in structural asymmetry) leads to novel spin-type valley-contrasting Berry curvatures in TMDs. Different from the conventional Berry curvatures originating from the orbital degrees of freedom, the spin-type Berry curvature can be easily tuned by vertical electric fields, which provides a new scheme for electrical control of valleys in TMD materials. 

Intrinsic valley Hall transport in atomically thin MoS2 

Zefei Wu*, Benjamin T. Zhou*, Xiangbin Cai*, Gui-Bin Liu, Jiangxiazi Lin, Tianyi Han, Liheng An, Yuanwei Wang, Shuigang Xu, Gen Long, Chun Cheng, Kam Tuen Law, Fan Zhang, Ning Wang 

Nature Communications 10, 611 (2019).

In this collaborative work with Prof. Ning Wang's group at HKUST, we explained the observation of non-local charge transport over micron scales mediated by intrinsic valley Hall effects in atomically thin MoS2. This discovery marked an important step toward long-range electrical control of valley degrees of freedom in TMDs. 

Magnetic field driven nodal topological superconductivity in monolayer transition metal dichalcogenides 

Wen-Yu He, Benjamin T. Zhou, James J He, Noah FQ Yuan, Ting Zhang, K. T. Law

Communications Physics 1, 40 (2018). Selected as Editor's highlights.

We proposed that an in-plane magnetic field can drive a monolayer Ising superconductor such as NbSe2 and TaS2 into a nodal topological superconductor in which Dirac nodes appear in the bulk quasi-particle spectrum where the Fermi surface intersects with mirror-invariant lines. The nontrivial bulk topology manifests itself in the presence of a large number of non-dispersive Majorana zero modes, called Majorana flat bands, along the edges of the system which connect Dirac nodes with opposite chiralities in the bulk spectrum. This work provided arguably the first material candidate for nodal topological superconductivity.

Ising superconductivity and Majorana fermions in transition-metal dichalcogenides 

Benjamin T. Zhou, Noah FQ Yuan, Hong-Liang Jiang, K. T. Law

Physical Review B 93, 180501(R)  (2016). Rapid communication. Selected as Editor's suggestion.

Since 2015, Ising superconductivity that withstands exceedingly high in-plane magnetic fields has been discovered experimentally in a family of atomically thin transition-metal dichalcogenides (TMDs), such as MoS2, NbSe2 and TaS2. In this work, we made the first theoretical proposal that the unconventional Ising superconductor possesses equal-spin Cooper pairs with their spin magnetic moments aligned in in-plane directions, which reconciles the competition between spin magnetism and electron pairing that is commonly found in conventional superconductors.  We discussed how these unconventional equal-spin Cooper pairs manifest themselves through spin-anisotropic Andreev reflections, and how they provide a viable route to Majorana fermions.

*: First co-author     †: Corresponding author