Nonlinear light-matter interaction plays a key role in the understanding, probing, and ultimately controlling light and matter. Novel materials and nanostructures with strong nonlinear optical (NLO) responses are highly desirable for many scientific disciplines and technologically important applications, e.g. ultrafast nonlinear optics, nonlinear biosensing and imaging, efficient generation of entangled photon pairs for quantum computing and quantum sensing, and all-optical transistor and computer. Recent discoveries of giant second and third harmonic generation (SHG, THG) in two distinct classes of materials – 2D crystals and topological materials – raise fundamental questions on our knowledge of NLO materials: What are the intrinsic mechanisms in the extraordinary nonlinear light-matter interactions observed in the 2D materials and topological materials? Are they different from conventional materials? Does a fundamental upper limit exist in second and higher order NLO responses?
Supported by NSF CAREER Award, our group is developing first-principles electronic structure theory of NLO responses and carrying out theoretical studies of 2D materials and topological materials to address the above questions. Specifically, the proposed research will 1) formulate first-principles approaches and develop computational modules for efficient and accurate calculation of general second and third order NLO properties; 2) apply the above approaches combined with group theory to understand intrinsic and extrinsic factors of the extraordinary NLO responses in 2D materials and generalize NLO materials design principles; 3) elucidate the role of symmetry, spin-orbit coupling, and topological phase as well as surface in the giant NLO responses of topological materials; and 4) integrate materials theory and simulation into undergraduate and graduate curriculum, outreach activities to K-12 students, underrepresented groups and secondary school teachers, and undergraduate and graduate research.
Since 2017, we have investigated several types of NLO responses in a number of distinct classes of materials as well as the associated microscopic mechanisms. For example, we have studied SHG, shift photocurrent, and circular photocurrent in the 2D semiconducting/ferroelectric materials. Distinct from conventional linear responses, they exhibit ferroelectricity-driven nonlinear photocurrent switching (for example in group IV monochalcogenides GeSe, GeS, SnS, SnSe, SnTe), that is, nonreciprocal behavior whose current flow direction can be controlled by manipulating ferroelectric polarization. Recently, we proposed a theory of ferroelectric nonlinear anomalous Hall effect (FNAHE) in semimetals and topological materials, and predicted an even-odd layer oscillation of FNAHE in few-layer topological semimetals (bilayer, trilayer, and four layer WTe2). Through a close collaboration with Dr. Aaron Lindenberg's group at Stanford University and Dr. Xiang Zhang's group at UC Berkeley, our theoretically-predicted FNAHE and corresponding intriguing low-energy ferroelectric transition pathway in few-layer WTe2 were experimentally demonstrated.
In our theory of FNAHE, Berry curvature dipole and shift dipole are not only treated on an equal footing to account for intraband and interband contributions to nonlinear anomalous Hall effect, but also established as new order parameters for noncentrosymmetric materials. This suggests that ferroelectric metals and Weyl semimetals may be suitable for the development of nonlinear quantum electronics. Moreover, FNAHE provides a facile approach for direct readout of ferroelectric states, which, combined with vertical ferroelectric writing, may realize nonlinear multiferroic memory such as Berry curvature memory. In addition, the distinct ferroelectric transformation pathway may provide potential routes to achieving non-abelian reciprocal braiding of Weyl nodes. These new findings therefore reveal an underexplored realm beyond classical linear Hall effect and conventional ferroelectrics with exciting new opportunities for FNAHE-based nonlinear quantum electronics using ferroelectric metals and Weyl semimetals.
We believe these microscopic theory and insights of nonlinear optical phenomena from the proposed research (obtained so far with more forthcoming) together with their symmetry principles will offer stupendous opportunities for the discovery and design of nonlinear optical materials and enable novel devices such as nonlinear quantum electronics, spintronics, magnetoelectronics, and dynamic quantum materials which may foster the second quantum evolution with unprecedented impact.
Publications:
Nannan Mao, Yue Luo, Ming-Hui Chiu, Chuqiao Shi, Xiang Ji, Tymofii S. Pieshkov, Yuxuan Lin, Hao-Lin Tang, Austin J. Akey, Jules A. Gardener, Ji-Hoon Park, Vincent Tung, Xi Ling, Xiaofeng Qian, William L. Wilson, Yimo Han, William A. Tisdale, and Jing Kong. Giant Nonlinear Optical Response via Coherent Stacking of In-Plane Ferroelectric Layers. Advanced Materials 35, 2210894 (2023)
Hua Wang, Xiuyu Tang, Haowei Xu, Ju Li, and Xiaofeng Qian. Generalized Wilson Loop Method for Nonlinear Light-Matter Interaction. npj Quantum Materials 7, 61 (2022)
Alex Strasser, Hua Wang, and Xiaofeng Qian. Nonlinear Optical and Photocurrent Responses in Janus MoSSe Monolayer and MoS2–MoSSe van der Waals Heterostructure. Nano Letters 22, 4145–4152 (2022)
Baiyu Zhang and Xiaofeng Qian. Competing Superior Electronic Structure and Complex Defect Chemistry in Quasi-One-Dimensional Antimony Chalcogenide Photovoltaic Absorbers. ACS Applied Energy Materials 5, 492-502 (2022)
Joseph V. Handy, Justin L. Andrews, Baiyu Zhang, Doyun Kim, Nattamai Bhuvanesh, Qing Tu, Xiaofeng Qian, and Sarbajit Banerjee. Topochemical stabilization and single-crystal transformations of a metastable 2D γ’-V2O5 intercalation cathode. Cell Reports Physical Science 3, 100712 (2022)
Zhuoliang Ni, Amanda V. Haglund, Hua Wang, Bing Xu, Christian Bernhard, David G. Mandrus, Xiaofeng Qian, Eugene J. Mele, Charles L. Kane, and Liang Wu. Imaging the Néel vector switching in the monolayer antiferromagnet MnPSe3 with strain-controlled Ising order. Nature Nanotechnology 16, 782–787 (2021)
Wenbin Li, Xiaofeng Qian, and Ju Li. Phase transitions in 2D materials. Nature Reviews Materials 6, 829–846 (2021)
Hua Wang and Xiaofeng Qian. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials. npj Computational Materials 6, 199 (2020)
Daniel A. Rhodes, Apoorv Jindal, Noah F. Q. Yuan, Younghun Jung, Abhinandan Antony, Hua Wang, Bumho Kim, Yu-che Chiu, Takashi Taniguchi, Kenji Watanabe, Katayun Barmak, Luis Balicas, Cory R. Dean, Xiaofeng Qian, Liang Fu, Abhay N. Pasupathy, and James Hone. Enhanced Superconductivity in Monolayer Td-MoTe2. Nano Letters 21, 2505–2511 (2021)
Shuyuan Huyan, Yanfeng Lyu, Hua Wang, Liangzi Deng, Zheng Wu, Bing Lv, Kui Zhao, Fei Tian, Guanhui Gao, Rui-Zhe Liu, Xiaojing Ma, Zhongjia Tang, Melissa Gooch, Shuo Chen, Zhifeng Ren, Xiaofeng Qian, and Ching-Wu Chu. Interfacial Superconductivity Achieved in Parent AEFe2As2 (AE = Ca, Sr, Ba) by a Simple and Realistic Annealing Route. Nano Letters 21, 2191-2198 (2021)
Aikaterini Flessa Savvidou, Judith K. Clark, Hua Wang, Kaya Wei, Eun Sang Choi, Shirin Mozaffari, Xiaofeng Qian, Michael Shatruk, and Luis Balicas. Complex Dirac-like Electronic Structure in Atomic Site-Ordered Rh3In3.4Ge3.6. Chemistry of Materials 33, 1218–1227 (2021)
Jia Liang, Qiyi Fang, Hua Wang, Rui Xu, Shuai Jia, Yuxuan Guan, Qing Ai, Guanhui Gao, Hua Guo, Kaijun Shen, Xiewen Wen, Tanguy Terlier, Gary P. Wiederrecht, Xiaofeng Qian, Hanyu Zhu, and Jun Lou. Perovskite‐Derivative Valleytronics. Advanced Materials 32, 2004111 (2020)
Kunyan Zhang, Yunfan Guo, Qingqing Ji, Ang-Yu Lu, Cong Su, Hua Wang, Alexander A. Puretzky, David B. Geohegan, Xiaofeng Qian, Shiang Fang, Efthimios Kaxiras, Jing Kong, and Shengxi Huang. Enhancement of van der Waals Interlayer Coupling through Polar Janus MoSSe. Journal of the American Chemical Society 142, 17499–17507 (2020)
Jun Xiao, Ying Wang, Hua Wang, C. D. Pemmaraju, Siqi Wang, Philipp Muscher, Edbert J. Sie, Clara M. Nyby, Thomas P. Devereaux, Xiaofeng Qian, Xiang Zhang, and Aaron M. Lindenberg. Berry curvature memory through electrically driven stacking transitions. Nature Physics 16, 1028-1034 (2020)
Abhishek Pandey, Ping Miao, M. Klemm, H. He, H. Wang, Xiaofeng Qian, J. W. Lynn, and M. C. Aronson. Correlations and incipient antiferromagnetic order within the linear Mn chains of metallic Ti4MnBi2. Physical Review B 102, 014406 (2020)
Hua Wang and Xiaofeng Qian. Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2. npj Computational Materials 5, 119 (2019)
Hua Wang and Xiaofeng Qian. Ferroicity-driven nonlinear photocurrent switching in time-reversal invariant ferroic materials. Science Advances 5, eaav9743 (2019)
Hua Wang and Xiaofeng Qian. Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics. Nano Letters 17, 5027-5034 (2017)
Daniel Rossi, Hua Wang, Yitong Dong, Tian Qiao, Xiaofeng Qian, and Dong Hee Son. Light-Induced Activation of Forbidden Exciton Transition in Strongly Confined Perovskite Quantum Dots. ACS Nano 12, 12436–12443 (2018)
Mohammad Taghinejad, Zihao Xu, Hua Wang, Hossein Taghinejad, Kyu-Tae Lee, Sean P. Rodrigues, Ali Adibi, Xiaofeng Qian, Tianquan Lian, and Wenshan Cai. Photocarrier-Induced Active Control of Second-Order Optical Nonlinearity in Monolayer MoS2. Small 16, 1906347 (2020)
Advances in high-performance computing have led to the rapidly growing interests in computation-aided materials discovery and device design. However, the complexity in both experiment and theory poses great challenges to the development of more reliable and accurate simulation methods across different lengths and time scales. These challenges are particularly important for many energy applications including understanding fundamental mechanisms of energy, charge, spin, and mass transport. In the past, Qian has developed/co-developed several related computational approaches, including first-principles tight-binding method, time-dependent density functional theory with PAW method and Ultrasoft pseudopotentials, automatic basin filling method for exploring potential energy surface, accelerated many-body perturbation theory within the GW approximation, and nonequilibrium quantum transport with first-principles tight-binding Hamiltonian.
Recently our group has been working on AI for Science to further advance computational materials science:
Active Machine Learning Approach for Accelerated and Convergent Model Generation (Supported by NSF CSSI)
Bridging Quantum Mechanics to Classical Molecular Dynamics via Machine Learning Force Field
Exploring First-Principles Potential Energy Surface under External PerturbationsDeveloping Efficient and Accurate Machine Learning Models for Large Systems
Publications:
Keqiang Yan, Alexandra Saxton, Xiaofeng Qian, Xiaoning Qian, and Shuiwang Ji. A Space Group Symmetry Informed Network for O(3) Equivariant Crystal Tensor Prediction. The Forty-first International Conference on Machine Learning (ICML), accepted (2024)
Keqiang Yan, Cong Fu, Xiaofeng Qian, Xiaoning Qian, and Shuiwang Ji. Complete and Efficient Graph Transformers for Crystal Material Property Prediction. International Conference on Learning Representations (ICLR), accepted (2024)
Xuan Zhang, Limei Wang, Jacob Helwig, Youzhi Luo, Cong Fu, Yaochen Xie, Meng Liu, Yuchao Lin, Zhao Xu, Keqiang Yan, Keir Adams, Maurice Weiler, Xiner Li, Tianfan Fu, Yucheng Wang, Haiyang Yu, YuQing Xie, Xiang Fu, Alex Strasser, Shenglong Xu, Yi Liu, Yuanqi Du, Alexandra Saxton, Hongyi Ling, Hannah Lawrence, Hannes Stärk, Shurui Gui, Carl Edwards, Nicholas Gao, Adriana Ladera, Tailin Wu, Elyssa F. Hofgard, Aria Mansouri Tehrani, Rui Wang, Ameya Daigavane, Montgomery Bohde, Jerry Kurtin, Qian Huang, Tuong Phung, Minkai Xu, Chaitanya K. Joshi, Simon V. Mathis, Kamyar Azizzadenesheli, Ada Fang, Alán Aspuru-Guzik, Erik Bekkers, Michael Bronstein, Marinka Zitnik, Anima Anandkumar, Stefano Ermon, Pietro Liò, Rose Yu, Stephan Günnemann, Jure Leskovec, Heng Ji, Jimeng Sun, Regina Barzilay, Tommi Jaakkola, Connor W. Coley, Xiaoning Qian, Xiaofeng Qian, Tess Smidt, and Shuiwang Ji. Artificial Intelligence for Science in Quantum, Atomistic, and Continuum Systems. arXiv Preprint:2307.08423 (2023)
Haiyang Yu, Meng Liu, Youzhi Luo, Alex Strasser, Xiaofeng Qian, Xiaoning Qian, and Shuiwang Ji. QH9: A Quantum Hamiltonian Prediction Benchmark for QM9 Molecules. Conference on Neural Information Processing Systems (NeurIPS), Track on Datasets and Benchmarks, accepted (2023). arXiv Preprint:2306.09549 (2023)
Haiyang Yu, Zhao Xu, Xiaofeng Qian, Xiaoning Qian, and Shuiwang Ji. Efficient and Equivariant Graph Networks for Predicting Quantum Hamiltonian. Proceedings of the 40th International Conference on Machine Learning, PMLR 202, 40412-40424 (2023). arXiv Preprint:2306.04922 (2023)
Daniel Willhelm, Nathan Wilson, Raymundo Arroyave, Xiaoning Qian, Tahir Cagin, Ruth Pachter, and Xiaofeng Qian. Predicting Van der Waals Heterostructures by a Combined Machine Learning and Density Functional Theory Approach. ACS Applied Materials & Interfaces 14, 25907–25919 (2022)
Nathan Wilson, Daniel Willhelm, Xiaoning Qian, Raymundo Arróyave, and Xiaofeng Qian. Batch active learning for accelerating the development of interatomic potentials. Computational Materials Science 208, 111330 (2022)
Kamal Choudhary, Kevin F. Garrity, Andrew C. E. Reid, Brian DeCost, Adam J. Biacchi, Angela R. Hight Walker, Zachary Trautt, Jason Hattrick-Simpers, A. Gilad Kusne, Andrea Centrone, Albert Davydov, Jie Jiang, Ruth Pachter, Gowoon Cheon, Evan Reed, Ankit Agrawal, Xiaofeng Qian, Vinit Sharma, Houlong Zhuang, Sergei V. Kalinin, Bobby G. Sumpter, Ghanshyam Pilania, Pinar Acar, Subhasish Mandal, Kristjan Haule, David Vanderbilt, Karin Rabe, and Francesca Tavazza. The joint automated repository for various integrated simulations (JARVIS) for data-driven materials design. npj Computational Materials 6, 173 (2020)
Diana Al Husseini, Junchao Zhou, Daniel Willhelm, Trevor Hastings, Gregory S. Day, Hong-Cai Zhou, Gerard L. Cote, Xiaofeng Qian, Ricardo Gutierrez-Osuna, Pao Tai Lin, and Svetlana A. Sukhishvili. All-nanoparticle layer-by-layer coatings for Mid-IR on-chip gas sensing. Chemical Communications 56, 14283-14286 (2020)
Hexin Bai, Peng Chu, Jeng-Yuan Tsai, Nathan Wilson, Xiaofeng Qian, Qimin Yan, and Haibin Ling. Graph Neural Network for Hamiltonian-Based Material Property Prediction. arXiv Preprint, arXiv:2005.13352 (2020)
Yong-Jie Hu, Ge Zhao, Baiyu Zhang, Chaoming Yang, Mingfei Zhang, Zi-Kui Liu, Xiaofeng Qian, and Liang Qi. Local electronic descriptors for solute-defect interactions in bcc refractory metals. Nature Communications 10, 4484 (2019)
Xiaofeng Qian, Paolo Umari, and Nicola Marzari. First-principles investigation of organic photovoltaic materials C60, C70, [C60]PCBM, and bis-[C60]PCBM using a many-body G0W0-Lanczos approach. Physical Review B 91, 245105 (2015)
Paolo Umari, Xiaofeng Qian, Nicola Marzari, Geoffrey Stenuit, Luigi Giacomazzi, and Stefano Baroni. Accelerating GW calculations with optimal polarizability basis. Physica Status Solidi B 248, 527-536 (2011)
Xiaofeng Qian, Paolo Umari, and Nicola Marzari. Photoelectron properties of DNA and RNA bases from many-body perturbation theory. Physical Review B 84, 075103 (2011)
Xiaofeng Qian, Ju Li, and Sidney Yip. Calculating phase-coherent quantum transport in nanoelectronics with ab initio quasiatomic orbital basis set. Physical Review B 82, 195442 (2010)
Xiaofeng Qian, Ju Li, Liang Qi, Cai-Zhuang Wang, Tzu-Liang Chan, Yong-Xin Yao, Kai-Ming Ho, and Sidney Yip. Quasiatomic orbitals for ab initio tight-binding analysis. Physical Review B 78, 245112 (2008)
Xiaofeng Qian, Ju Li, Xi Lin, and Sidney Yip. Time-dependent density functional theory with ultrasoft pseudopotentials: Real-time electron propagation across a molecular junction. Physical Review B 73, 035408 (2006)
Ju Li, Akihiro Kushima, Jacob Eapen, Xi Lin, Xiaofeng Qian, John C. Mauro, Phong Diep, and Sidney Yip. Computing the viscosity of supercooled liquids: Markov network model. PLoS ONE 6, e17909 (2011)
Akihiro Kushima, Xi Lin, Ju Li, Xiaofeng Qian, Jacob Eapen, John C. Mauro, Phong Diep, and Sidney Yip. Computing the viscosity of supercooled liquids. Ii. Silica and strong-fragile crossover behavior. Journal of Chemical Physics 131, 164505 (2009)
Akihiro Kushima, Xi Lin, Ju Li, Jacob Eapen, John C. Mauro, Xiaofeng Qian, Phong Diep, and Sidney Yip. Computing the viscosity of supercooled liquids. Journal of Chemical Physics 130, 224504 (2009)
Ultrafast sensing and control of the state of matter at nanoscale are highly attractive for advanced applications such as energy harvesting and conversion, high-performance high-density information storage, and quantum computing and quantum simulation. Nanoscale multiferroics is an ideal materials platform which allows for direct manipulation and cross-control of charge, spin, and lattice with potentially superior performance (e.g. ultrafast switching and sensing, low-power consumption) owing to reduced dimensionality. However, due to stringent symmetry and coupling constraints, room-temperature multiferroics with strong ferroic coupling has not been demonstrated. In addition, depolarization-induced instability (i.e. ferroic order vanishing below a few nanometers) poses another challenge to nanoscale multiferroics.
Recent breakthroughs in 2D ferroics and multiferroics (including several works from our group) open up a very exciting yet largely-underexploited realm within ultimate thickness of ~1nm. 2D multiferroics are fundamentally different from their bulk counterpart with (a) significantly reduced dielectric screening, (b) increased joint density of states, (c) distinct symmetry, (d) large Rashba spin splitting, and (e) strong many-body interaction (e.g. excitonic photoabsorption and photoluminescence).
Since 2017, we have showed that it is possible to achieve 2D ferroicity/multiferroicity. For example, monolayer group IV monochalcogenides MX (M=Ge, Sn; X=S, Se) possess room-temperature 2D multiferroicity with strongly-coupled large spontaneous in-plane polarization and lattice strain. Encouragingly, 2D in-plane ferroelectricity was recently observed in their cousin atomic thick tin telluride (SnTe) and few odd-layer tin sulfide (SnS). Moreover, we discovered intrinsic coupling between multiferroicity and nonlinear optical response, predicted a new class of 2D ferromagnetic semiconductors in CrSBr and CrSeBr with Curie temperature around ~150K, and predicted the first class of 2D ferroelectric-ferromagnetic multiferroics in monolayer transition metal phosphorus chalcogenides (TMPCs)-CuMP2X6 (M = Cr, V; X = S, Se), suggesting great opportunity in 2D multiferroics.
Motivated by the recent experiment discovery of ferroelectric 2D metal in bilayer and trilayer WTe2, we theoretically studied ferroelectricity in another interesting material - few-layer WTe2. While individual monolayer 1T'-WTe2 is centrosymmetric without spontaneous polarization, in-plane stacking breaks inversion symmetry and induces out-of-plane polarization while maintaining in-plane metallicity. Subsequently, in-plane interlayer gliding leads to low-energy ferroelectric polarization switching in both even-layer and odd-layer WTe2. Ferroelectric transformation, however, is fundamentally different in even-layer and odd-layer WTe2, i.e. inversion in odd-layer WTe2 and mirror plus glide in even-layer WTe2, which leads to Berry curvature and shift vector switching in the case of odd-layer WTe2 only. Hence, Berry curvature dipole and shift dipole exhibit a striking even-odd layer oscillation upon electric polarization switching, giving rise to ferroelectric nonlinear anomalous Hall effect (FNAHE) fundamentally rooted in nonlinear optics. As a result, Berry curvature dipole and shift dipole are formally established as new order parameters for noncentrosymmetric materials. Our theory and results provide a generalized picture of electric polarization and current based on the zero-th order, linear-order, and higher order dipole. It also provides an important practical guidance for developing new electric and optical tools to characterize materials in greater detail and developing novel devices based on the generalized higher-order polarization/current. Through a close collaboration with Dr. Aaron Lindenberg's group at Stanford University and Dr. Xiang Zhang's group at UC Berkeley, our theoretically-predicted intriguing low-energy ferroelectric transition pathway and ferroelectric nonlinear anomalous Hall effect in few-layer WTe2 were experimentally demonstrated.
Publications:
Chuqiao Shi, Nannan Mao, Kena Zhang, Tianyi Zhang, Ming-Hui Chiu, Kenna Ashen, Bo Wang, Xiuyu Tang, Galio Guo, Shiming Lei, Longqing Chen, Ye Cao, Xiaofeng Qian, Jing Kong, and Yimo Han. Domain-dependent strain and stacking in two-dimensional van der Waals ferroelectrics. Nature Communications 14, 7168 (2023)
Duan Luo, Baiyu Zhang, Edbert J. Sie, Clara M. Nyby, Qingyuan Fan, Xiaozhe Shen, Alexander H. Reid, Matthias C. Hoffmann, Stephen Weathersby, Jianguo Wen, Xiaofeng Qian, Xijie Wang, and Aaron M. Lindenberg. Ultrafast Optomechanical Strain in Layered GeS. Nano Letters 23, 2287–2294 (2023)
Jia Liang, Qiyi Fang, Hua Wang, Rui Xu, Shuai Jia, Yuxuan Guan, Qing Ai, Guanhui Gao, Hua Guo, Kaijun Shen, Xiewen Wen, Tanguy Terlier, Gary P. Wiederrecht, Xiaofeng Qian, Hanyu Zhu, and Jun Lou. Perovskite‐ Derivative Valleytronics. Advanced Materials 32, 2004111 (2020)
Kunyan Zhang, Yunfan Guo, Qingqing Ji, Ang-Yu Lu, Cong Su, Hua Wang, Alexander A. Puretzky, David B. Geohegan, Xiaofeng Qian, Shiang Fang, Efthimios Kaxiras, Jing Kong, and Shengxi Huang. Enhancement of van der Waals Interlayer Coupling through Polar Janus MoSSe. Journal of the American Chemical Society 142, 17499–17507 (2020)
Hua Wang, Jingshan Qi, and Xiaofeng Qian. Electrically tunable high Curie temperature two-dimensional ferromagnetism in van der Waals layered crystals. Applied Physics Letters 117, 083102 (2020)
Ziye Zhu, Baiyu Zhang, Xiaofang Chen, Xiaofeng Qian, and Jingshan Qi. Electric Field Control of Molecular Magnetic State by Two Dimensional Ferroelectric Heterostructure Engineering. Applied Physics Letters 117, 082902 (2020)
Jingshan Qi, Hua Wang, Xiaofang Chen, and Xiaofeng Qian. Two-dimensional multiferroic semiconductors with coexisting ferroelectricity and ferromagnetism. Applied Physics Letters 113, 043102 (2018)
Hua Wang and Xiaofeng Qian. Two-dimensional multiferroics in monolayer group IV monochalcogenides. 2D Materials 4, 015042 (2017)
Hua Wang and Xiaofeng Qian. Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics. Nano Letters 17, 5027-5034 (2017)
Hua Wang and Xiaofeng Qian. Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2. npj Computational Materials 5, 119 (2019)
Hua Wang and Xiaofeng Qian. Ferroicity-driven nonlinear photocurrent switching in time-reversal invariant ferroic materials. Science Advances 5, eaav9743 (2019)
The seminal discovery of quantum spin Hall (QSH) effect engendered a new chapter of topological materials research in condensed matter physics and materials science, followed by the discoveries of three-dimensional topological insulator, quantum anomalous Hall insulator, topological crystalline materials, Weyl semimetals etc. These exotic materials share a general aspect, that is, the presence of special surface/edge states that are topologically protected against weak perturbations, hence inelastic scattering induced heat dissipation is minimized. In contrast, conventional electronics suffers from severe local heating as any structural defect or chemical impurity could cause additional scattering and reduce carrier transmission. These topological phases, if materialized and integrated at the device level, could be advantageous for many novel low-power and low-dissipation electronic applications. Novel materials with nontrivial electronic and photonic band topology are, therefore, highly desired for utilizing their topological nature and realizing novel devices with low power consumption and heat dissipation and quantum computing free of decoherence. Furthermore, the ability of controlling topological invariants is also highly desirable for developing configurable topological electronics/photonics.
Our group dedicates special effort to the discovery and design of topological materials using first-principles theoretical approaches. We develop first-principles effective Hamiltonian from density funcional theory or many-body perturbation theory calculations and compute the corresponding topological invariants and surface/edge states in the presence/absence of external electric/stress field using Green's function method. Along with materials discoveries with individual topological phases, our group is also interested in understanding topological phase transition, for examples, from nontrivial to trivial topology and from one nontrivial phase to another through chemical doping, elastic strain engineering, electric and magnetic field etc.
Since 2014, we have theoretically predicted a number of new topological materials, including quantum spin Hall materials in 1T'-transition metal chalcogenides (TMDC, 1T'-MX2 with M=W, Mo & X=Te, Se, and S), topological crystalline insulators in monolayer IV–VI semiconductors, quantum spin Hall phase and Weyl semimetallic phase in ternary transition metal chalcogenides. We also discovered topological phase transition, including (a) electric field induced Z2 nontrivial-to-trivial topological phase transition in 1T'-MX2 TMDC, (b) electrically controlled band gap and topological phase transition in two-dimensional multilayer germanane, and (c) vdW interlayer spacing induced topological phase transition from quantum spin Hall insulator to Weyl Semimetal owing to the symmetry-breaking upon stacking and hence the creation and annihilation of Weyl fermions.
Our theoretical predictions of 1T'-WTe2, 1T'-MoTe2, as well as ternary TMDC topological materials have been experimentally demonstrated using angle-resolved photoemission spectroscopy (ARPES), four-probe conductance measurement, scanning tunneling microscope (STM), etc. The quantum spin Hall effect from the conductance measurement was observed up to 100 kelvin in monolayer 1T'-WTe2, and more strikingly it was demonstrated to be the first 2D materials that performs as both topological insulator and superconductor, showing great promise for realizing Majorana modes towards topological quantum computing.
Publications:
Jian Tang, Thomas Siyuan Ding, Hongyu Chen, Anyuan Gao, Tiema Qian, Zumeng Huang, Zhe Sun, Xin Han, Alex Strasser, Jiangxu Li, Michael Geiwitz, Mohamed Shehabeldin, Vsevolod Belosevich, Zihan Wang, Yiping Wang, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Ziqiang Wang, Liang Fu, Yang Zhang, Xiaofeng Qian, Kenneth S. Burch, Youguo Shi, Ni Ni, Guoqing Chang, Su-Yang Xu, and Qiong Ma. Dual quantum spin Hall insulator by density-tuned correlations in TaIrTe4. Nature 628, 515–521 (2024)
Daniel A. Rhodes, Apoorv Jindal, Noah F. Q. Yuan, Younghun Jung, Abhinandan Antony, Hua Wang, Bumho Kim, Yu-che Chiu, Takashi Taniguchi, Kenji Watanabe, Katayun Barmak, Luis Balicas, Cory R. Dean, Xiaofeng Qian, Liang Fu, Abhay N. Pasupathy, and James Hone. Enhanced Superconductivity in Monolayer Td-MoTe2. Nano Letters 21, 2505–2511 (2021). arXiv Preprint:2102.01793
Aikaterini Flessa Savvidou, Judith K. Clark, Hua Wang, Kaya Wei, Eun Sang Choi, Shirin Mozaffari, Xiaofeng Qian, Michael Shatruk, and Luis Balicas. Complex Dirac-like Electronic Structure in Atomic Site-Ordered Rh3In3.4Ge3.6. Chemistry of Materials 33, 1218–1227 (2021). arXiv Preprint:2102.01793
Hua Wang and Xiaofeng Qian. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials. npj Computational Materials 6, 199 (2020). arXiv Preprint:2006.13573
Junwei Liu, Hua Wang, Chen Fang, Liang Fu, and Xiaofeng Qian. van der Waals Stacking-Induced Topological Phase Transition in Layered Ternary Transition Metal Chalcogenides. Nano Letters 17, 467-475 (2017)
Qihang Zhang, Zhongkai Liu, Yan Sun, Haifeng Yang, Juan Jiang, Sung‐Kwan Mo, Zahid Hussain, Xiaofeng Qian, Liang Fu, Shuhua Yao, Minghui Lu, Claudia Felser, Binghai Yan, Yulin Chen, and Lexian Yang. Lifshitz Transitions Induced by Temperature and Surface Doping in Type‐II Weyl Semimetal Candidate Td‐WTe2. Physica Status Solidi (RRL)-Rapid Research Letters 11, 1700209 (2017)
Electrically controlled band gap and topological phase transition in two-dimensional multilayer germanane. Applied Physics Letters 108, 253107 (2016)
Junwei Liu, Xiaofeng Qian, and Liang Fu. Crystal Field Effect Induced Topological Crystalline Insulators In Monolayer IV–VI Semiconductors. Nano Letters 15, 2657–2661 (2015)
Xiaofeng Qian, Liang Fu, and Ju Li. Topological Crystalline Insulator Nanomembrane with Strain-Tunable Band Gap. Nano Research 8, 967-979 (2015)
Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 346, 1344-1347 (2014)
Over the past few years, our group has been studying electron and ion dynamics in battery materials, nonvolatile memories, electro/photocatalysts, and neuromorphic materials.
Recently, neuromorphic computing emerges as one of the two potential disruptive technologies (together with quantum computing) which may transform the paradigm of computing and information processing. The current data storage, processing, and transmission in cloud computing, edge computing, or Internet of Things (IoT) using inefficient and energy-hungry legacy hardware face the von Neumann bottleneck of the classical computing architecture. Materials with complex nonlinear dynamical behavior such as nonlinear conductance switch and self-oscillation could directly emulate neural elements. Circuits built with those materials could achieve much higher energy efficiency and outperform systems built from thousands of transistors.
The development of neuromorphic computing is in its early stage at present. While initial neural circuits have been realized based on conventional complementary metal-oxide-semiconductor (CMOS) processors, such architecture is still not energy efficient for neuromorphic computing. Hence, there is a surging demand for an integrated investigation from the discovery and design of novel neuromorphic materials, to synthesis and characterization, to the realization of neural elements, and to novel computing and circuit design. Supported by DOE EFRC REMIND Center, we are collaborating with several experiment and theory groups to explore and develop new materials systems for energy efficient neuromorphic computing, such as strongly-correlated oxides, 2D materials, and molecular materials.
In lithium-ion batteries, the movement of charge carriers induces the local atomic lattice distortion, forming new quasiparticles called electron-polaron or hole-polaron. Energy barrier of polaron migration largely determines electronic conductivity, which is one of the rate-limiting factors during charge and discharge. Microscopic mechanism of electron, ion, and/or coupled electron-ion dynamics at nanoscale is therefore crucial for further improvement and the discovery of new battery materials. Coupled electronic-ionic motion also exists in nonvolatile random access memories. In nonvolatile memories, the applied voltage drives charge carriers to overcome the potential energy barrier, which strongly couples the electronic and ionic degrees of freedom, leading to either metallic filament formation or local structural distortion associated with negative U-centers. Microscopic mechanisms of resistance switching highly depends on materials systems, and some of them are still not very clear.
Publications:
Parker Schofield, Erick J. Braham, Baiyu Zhang, Justin L. Andrews, Hayley K. Drozdick, Dexin Zhao, Wasif Zaheer, Rebeca M. Gurrola, Kelvin Xie, Patrick J. Shamberger, Xiaofeng Qian, and Sarbajit Banerjee. Decoupling the metal–insulator transition temperature and hysteresis of VO2 using Ge alloying and oxygen vacancies. Chemical Communications 58, 6586-6589 (2022).
Joseph V. Handy, Justin L. Andrews, Baiyu Zhang, Doyun Kim, Nattamai Bhuvanesh, Qing Tu, Xiaofeng Qian, and Sarbajit Banerjee. Topochemical stabilization and single-crystal transformations of a metastable 2D γ’-V2O5 intercalation cathode. Cell Reports Physical Science 3, 100712 (2022).
Diane G. Sellers, Erick J. Braham, Ruben Villarreal, Baiyu Zhang, Abhishek Parija, Timothy D. Brown, Theodore E. G. Alivio, Heidi Clarke, Luis R. De Jesus, Lucia Zuin, David Prendergast, Xiaofeng Qian, Raymundo Arróyave, Patrick J. Shamberger, and Sarbajit Banerjee. Atomic Hourglass and Thermometer Based on Diffusion of a Mobile Dopant in VO2. Journal of the American Chemical Society 142, 15513–15526 (2020). Texas A&M News: Chameleon-like material spiked with boron comes closer to mimicking brain cells
Hyun Deog Yoo, Yanliang Liang, Hui Dong, Junhao Lin, Hua Wang, Yisheng Liu, Lu Ma, Tianpin Wu, Yifei Li, Qiang Ru, Yan Jing, Qinyou An, Wu Zhou, Jinghua Guo, Jun Lu, Sokrates T. Pantelides, Xiaofeng Qian, and Yan Yao. Fast kinetics of magnesium monochloride cations in interlayer-expanded titanium disulfide for magnesium rechargeable batteries. Nature Communications 8, 339 (2017)
Fenglei Shi, Jing He, Baiyu Zhang, Jiaheng Peng, Yanling Ma, Wenlong Chen, Fan Li, Yong Qin, Yang Liu, Wen Shang, Peng Tao, Chengyi Song, Tao Deng, Xiaofeng Qian, Jian Ye, and Jianbo Wu. Plasmonic- Enhanced Oxygen Reduction Reaction of Silver/Graphene Electrocatalysts. Nano Letters 19, 1371-1378 (2019)
The last two decades have witnessed the increasing demand for renewable and green energy for sustainable society as conventional non-renewable energy resources such as fossil fuels will be depleted by 2060. Photovoltaics, converting solar energy to electric power, provides one of the most efficient approaches to harvest renewable, affordable and environment friendly energy. A variety of solar cells have been developed in the past using different absorbers, including silicon, CdTe, CZTS, CIGS, organic/polymer, and perovskite solar cells. Among them, silicon-, CdTe- and CIGS-based solar cells are dominating the current commercial photovoltaics productions with certified power conversion efficiency (PCE) over 22%. However, the current photovoltaic technologies still face some issues. For example, despite large predominance in photovoltaic market, crystalline silicon (c-Si) solar cells suffer from high cost. The toxicity of Cd and the scarcity of Te are two notable issues of CdTe solar cell. Besides, Indium and gallium in CIGS are not earth abundant. The complexity of defect control hinders the further PCE improvement in CZTS solar cell. Perovskites have attracted tremendous attention in the last decade, and their PCE is approaching 25%, making them very promising for commercialization. Nevertheless, several challenges need to be addressed, such as stability and toxicity of Pb-based perovskites.
Since 2017, we have been collaborating with experimentalists to develop and improve low-dimensional antimony chalcogenides as solar absorbers for thin-film photovoltaic applications. We have investigated the electronic and optical properties of antimony chalcogenides with unique quasi-one-dimensional structure for carrier transport. We have collaborated with Dr. Feng Yan's group at Arizona State University and successfully fabricated Sb2Se3 and Sb2S3 based thin-film solar cells using close space sublimation which is fully compatible with the current thin-film manufacturing process. The power conversion efficiency (PCE) of Sb2Se3 with graphite as electrodes has been improved from 4% to 7% via interfacial engineering. Furthermore, we found the low open-circuit voltage (Voc) limits the further improvement of PCE. We will further study defect physics in antimony chalcogenides using first-principles density functional theory and understand its role in the mid-trap states and the low Voc. We will seek for suitable extrinsic dopants guided by the defect diagram calculations and experimental measurements to provide an effective way to suppress potential defect-induced trap states. In addition to improving the device performance, our group will investigate the role of grain boundary in antimony chalcogenides as well as extrinsic doping.
Besides the development of thin-film photovoltaics, we have collaborated with Dr. Xiaolin Zheng's group at Stanford University and demonstrated the exciton funneling effect in inhomogeneously strained MoS2 which my colleague and I theoretically proposed in 2012. Recently, the concept of inhomogeneously strain engineered "artificial atom" was also explored for enhancing catalytic activities and as single-photon emission center to provide single-photon source for quantum information science applications.
Publications:
Jia Liang, Qing Ai, Xiewen Wen, Xiuyu Tang, Tianshu Zhai, Rui Xu, Xiang Zhang, Qiyi Fang, Christine Nguyen, Yifeng Liu, Hanyu Zhu, Tanguy Terlier, Gary P. Wiederrecht, Pulickel M. Ajayan, Xiaofeng Qian, and Jun Lou. Strong interlayer coupling and long-lived interlayer excitons in two-dimensional perovskite derivatives and transition metal dichalcogenides van der Waals heterostructure. Materials Today, in press (2024)
Liping Guo, Baiyu Zhang, Smriti Ranjit, Jacob Wall, Swapnil Saurav, Adam J. Hauser, Guozhong Xing, Lin Li, Xiaofeng Qian, and Feng Yan. Interface Engineering via Sputtered Oxygenated CdS:O Window Layer for Highly Efficient Sb2Se3 Thin Film Solar Cells with Efficiency above 7%. Solar RRL 3, 1900225 (2019)
Liping Guo, Baiyu Zhang, Shan Li, Lin Li, Guozhong Xing, Qian Zhang, Xiaofeng Qian, and Feng Yan. Interfacial Engineering of Oxygenated Chemical Bath Deposited CdS Window Layer for Highly Efficient Sb2Se3 Thin Film Solar Cells. Materials Today Physics 10, 100125 (2019)
Liping Guo, Corey Grice, Baiyu Zhang, Scott Xing, Lin Li, Xiaofeng Qian, and Feng Yan. Improved stability and efficiency of CdSe/Sb2Se3 thin-film solar cells. Solar Energy 188, 586-592 (2019)
Liping Guo, Baiyu Zhang, Shan Li, Qian Zhang, Michael Buettner, Lin Li, Xiaofeng Qian, and Feng Yan. Scalable and efficient Sb2S3 thin-film solar cells fabricated by close space sublimation. APL Materials 7, 041105 (2019)
Liping Guo, Baiyu Zhang, Ying Qin, Dawen Li, Lin Li, Xiaofeng Qian, and Feng Yan. Tunable Quasi‐One‐Dimensional Ribbon Enhanced Light Absorption in Sb2Se3 Thin‐film Solar Cells Grown by Close‐Space Sublimation. Solar RRL 2, 1800128 (2018)
Hong Li, Alex W. Contryman, Xiaofeng Qian, Sina Moeini Ardakani, Yongji Gong, Xingli Wang, Jeffery M. Weisse, Chi Hwan Lee, Jiheng Zhao, Pulickel M. Ajayan, Ju Li, Hari C. Manoharan, and Xiaolin Zheng. Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide. Nature Communications 6, 7381 (2015)
Ji Feng, Xiaofeng Qian, Cheng-Wei Huang, and Ju Li. Strain-engineered artificial atom as a broad-spectrum solar energy funnel. Nature Photonics 6, 866-872 (2012) Highlighted by the News and Views of Nature Photonics 6, 804 (2012)