Condensed matter physics is well known by its diversity and breadth, ranging from studies to understand complicated phenomena observed in liquids and solids. It roots much of the modern technologies, such as the innovation of read head in hard disk from the study of GMR. And it surely promotes future technologies.

Research in condensed matter physics at our center are active in topological materials, novel superconductors and strongly correlated electronic systems both experimental and theory.

Zengwei Zhu investigates the properties of novel electronic states in semimetals, quantum materials. Currently, he is interested in exotic phases beyond quantum limit under ultrastrong magnetic fields. His group grows various novel quantum materials, as well as customize measurements to access the electrical, magnetic, and thermal properties of those materials especially with electrical and thermal transport methods under high fields and low temperature.

Gang Xu mainly focus on the condensed matter theory and Materials Computation, including the topological materials and topological states such as Chern semimetal, quantum anomalous Hall (QAH) effect and topological superconductivity (TSC); the electronic structures and physical properties of the iron-based superconductors; as well as novel 2D materials. More than 30 papers published on the high level international journals, including 8 PRL, 2 Nat. Nanotech, 1 Nat. Phys, 1 Nat. Commun and 2 Nano letters. Total citations are more than 3500. H-index=20. The detailed publications are at https://scholar.google.com/citations?user=BTtzMdMAAAAJ.

Main results

Topological materials and topological states:1) We have generalized the topological classification from the insulator to semimetal, and predicted that a novel quantum state with topologically unavoidable band-crossing at Fermi level can be realized in ferromagnetic compound HgCr2Se4 [1]. This state is a condensed-matter realization of the Weyl fermions in (3+1) D, and should exhibits remarkable features, such as magnetic monopoles, Fermi arcs, as well as QAH effect. This work is published on PRL, and highlighted with a Synopsis on the Physics website. The citation is more than 500 now. 2) We have predicted that crystalline A3Bi (A = Na, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry [14]. They possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries. This proposal was realized recently, and the citation is more than 580 now. 3) Based on DFT calculations, we first give a new proposal to realize the QAH effect by combining two ferromagnetic insulators, Cr-doped Bi2Te3 and GdI2 together, which can effectively enhance the Chern insulating band gap and Curie temperature [3]. This work points out a new direction of the application of QAH effect and the design of new electronic devices. 4) Based on the ab initio calculations, we propose the realization of the intrinsic QAH effect in the single layer Cs2Mn3F12 Kagome lattice, where the band gap is around 20 meV [2]. A simplified tight binding model based on the in-plane ddσ antibonding states is constructed to understand the topological band structures of the system in the paper, which is published on PRL 115, 186802 (2015). 5) Based on ab initio calculations and the theory analysis, we demonstrate that the three dimensional extended s-wave superconductor Fe1+ySe0.5Te0.5 has a metallic topologically nontrivial band structure and exhibits a normal-topological-normal superconductivity phase transition on the (001) surface. In the TSC phase, a Majorana zero mode can be observed at the ends of a magnetic vortex line in the superconducting Fe1+ySe0.5Te0.5. Our results pave an effective way to realize topological superconductivity and Majorana fermions in a large class of superconductors [7] (see PRL 117, 047001 (2016)).

Iron-based superconductors:1) We have calculated the magnetic diagram of LaOMAs (M=V-Cu), and explained their magnetic ground states from the electronic structure aspect [5]. This paper is selected as ‘Top hundred outstanding scientific and technological papers of china’ in 2008, which is cited more than 260 now. 2) We firstly predicted the stripe anti-ferromagnetic ground state in LaOFeAs, and claim it as the Fermi surfaces nesting induced spin-density-wave state [9]. This work is the important paper for Natural Science Prize on iron-based superconductors, which is selected as ‘Top hundred outstanding scientific and technological papers of china’ in 2008 and cited almost 700. 3) From first principles calculations, we show that the magnetic properties at hole-doped side are different from its electron-doped counterparts in BaFe2As2, and identified that it is mainly originated from their asymmetric electronic structure [6]. Their influence on the superconductivity for both doping side is also studied [11]. 4) By treating the multi-orbital fluctuations with LDA+Gutzwiller method, we are able to predict the correct Fe-As bond length, and the weakened Fe-As bonding strength is exactly corresponding to the observed ‘soft phonon’[10].

Others:Gang Xu have done many cooperative studies with many collaborators and groups: 1) Large band gap quantum Spin Hall effect in saturated tin-film [13] (citation 450); 2) Dirac fermions in an antiferromagnetic semimetal [15]; 3) Circular photogalvanic effect in WSe2 [16]; 4) Linear dichroism photodetector in black phosphorus [17] (citation 119). 5) Electronic structures studies on KFe2Se2 [12] (citation 260) BaNi2As2, (Tl, Rb)yFe2-xSe2, MoS2 and NaCoO2 et al.

Member of material physics group

Main publications

[1] Gang Xu,Hongming Weng,ZhijunWang,Xi Dai, andZhong Fang,“Chern semimetal and the quantized anomalous Hall effect in HgCr2Se4”,Phys. Rev. Lett 107, 182806 (2011) [Editor's suggestion] (citation 515)

[2] Gang Xu, Biao Lian, Shou-cheng Zhang,“Intrinsic Quantum Anomalous Hall effect in Kagome lattice Cs2LiMn3F12”Phys. Rev. Lett 115, 186802 (2015)

[3] G. Xu, J. Wang, C. Felser, X.-L. Qi, S.-C. Zhang,“Quantum Anomalous Hall Effect in Magnetic Insulator Heterostructure”Nano lett. 15 (3), 2019-2023 (2015)

[4] Gang Xu, Jing Wang, Binghai Yan and Xiao-Liang Qi,“Topological superconductivity at the edge of transition metal dichalcogenides”Phys. Rev. B 90 (10), 100505 (2014)

[5] G. Xu, W. Ming, Y. Yao, X. Dai, S.-C. Zhang and Z. Fang,“Doping-dependent phase diagram of LaOMAs (M=V-Cu) and electron-type superconductivity near ferromagnetic instability”, EuroPhys. Lett. 82, 67002 (June 2008) (citation 262)

[6] Gang Xu, Haijun Zhang, Xi Dai, Zhong Fang“Electron-hole asymmetry and quantum critical point in hole-doped BaFe2As2”EuroPhys. Lett. 84, 67015 (December 2008) (citation 59)

[7] Gang Xu,Biao Lian,Peizhe Tang,Xiao-Liang Qi,Shou-Cheng Zhang“Topological superconductivity on the surface of Fe-based superconductors”Phys. Rev. Lett 117, 047001 (2016)

[8] Hongtao Yuan*, Zhongkai Liu*, Gang Xu*, Bo Zhou, Sanfeng Wu,et al(Co-first author)“Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and their Mono-Layer Limit”Nano letters 16, 4738-4745 (2016)

[9] J. Dong, H. J. Zhang, G. Xu, Z. Li, G. Li,et al“Competing orders and spin-density-wave instability in La(O1-xFx)FeAs”EuroPhysics Letters 83, 27006 (July 2008) (citation 724)

[10] GuangTao Wang，YumingQian, Gang Xu, Xi Dai, Zhong Fang,“Gutzwiller density functional studies of FeAs-based superconductors: Structure optimization and evidence for a three-dimensional Fermi surface”,Phys. Rev. Lett. 104, 047002 (2010) (citation 76)

[11] M. Neupane,P. Richard,Y. M. Xu,K. Nakayama,T. Sato,T. Takahashi,A. V. Fedorov,G. Xu,et al“Electron-Hole Asymmetry in Superconductivity of Pnictides Originated from the Observed Rigid Chemical Potential Shift ”Phys. Rev. B 83, 094522 (2011) (citation 71)

[12] T. Qian,X. P. Wang,W. C. Jin,P. Zhang,P. Richard,G. Xu,et al“Absence of holelike Fermi surface in superconducting K0.8Fe1.7Se2revealed by ARPES”Phys. Rev. Lett. 106, 1 7001 (2011) (citation 261)

[13] Yong Xu, Binghai Yan, Hai-Jun Zhang, Jing Wang, Gang Xu, Peizhe Tang, Wenhui Duan, and Shou-Cheng Zhang“Large-Gap Quantum Spin Hall Insulators in Tin Films” Phys. Rev. Lett. 111, 136804 (2013)(citation 456)

[14] Zhijun Wang,Yan Sun,Xing-Qiu Chen,Cesare Franchini,Gang Xu,et al“Dirac semimetal and topological phase transitions inA3Bi (A=Na, K, Rb)”Phys. Rev. B 85, 195320 2012)(citation 586)

[15] Peizhe Tang,Quan Zhou,Gang Xu&Shou-Cheng Zhang“Dirac fermions in an antiferromagnetic semimetal”Nature Physics 12, 1100-1104 (2016)

[16] Hongtao Yuan,Xinqiang Wang,Biao Lian,Haijun Zhang,Xianfa Fang,Bo Shen,Gang Xu,et al“Generation and Electric Control of Spin-Coupled Valley Current in WSe2” Nature Nanotechnology 9, 851–857(2014)(citation 73)

[17] Hongtao Yuan,Xiaoge Liu,Farzaneh Afshinmanesh,Wei Li,Gang Xu,et al“Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction” Nature Nanotechnology 10, 707–713 (2015)(citation 119)

[18] Zhu Z*, Lin X, Liu J, et al. Quantum oscillations, thermoelectric coefficients and the Fermi surface of semi-metallic WTe2. Physical Review Letters, 2015, 114(17).

[19] Zhu Z, Fauqué B, Malone L, et al. Landau spectrum and twin boundaries of bismuth in the extreme quantum limit. Proceedings of the National Academy of Sciences of the United States of America, 2012, 109(37):14813-8.

[20] Zhu Z, Collaudin A, Fauqué B, et al. Field-induced polarization of Dirac valleys in bismuth. Nature Physics, 2012, 8(1):89-94.

[21] Zhu Z, Yang H, Fauqué B, et al. Nernst effect and dimensionality in the quantum limit. Nature Physics, 2010, 6(1):26-29.