Publications by Yulin Chen

Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family.

Nature materials 15 (2016) 27-31

ZK Liu, LX Yang, Y Sun, T Zhang, H Peng, HF Yang, C Chen, Y Zhang, YF Guo, D Prabhakaran, M Schmidt, Z Hussain, S-K Mo, C Felser, B Yan, YL Chen

Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. Here, by performing angle-resolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spin-orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications.

Weyl semimetal phase in the non-centrosymmetric compound TaAs

Nature Physics Nature Publishing Group 11 (2015) 728-732

Y Chen

Three-dimensional (3D) topologicalWeyl semimetals (TWSs) represent a state of quantum matter with unusual electronic structures that resemble both a ‘3D graphene’ and a topological insulator. Their electronic structure displays pairs of Weyl points (through which the electronic bands disperse linearly along all three momentum directions) connected by topological surface states, forming a unique arc-like Fermi surface (FS). Each Weyl point is chiral and contains half the degrees of freedom of a Dirac point, and can be viewed as a magnetic monopole in momentum space. By performing angle-resolved photoemission spectroscopy on the non-centrosymmetric compound TaAs, here we report its complete band structure, including the unique Fermi-arc FS and linear bulk band dispersion across the Weyl points, in agreement with the theoretical calculations1, 2. This discovery not only confirms TaAs as a 3DTWS, but also provides an ideal platform for realizing exotic physical phenomena (for example, negative magnetoresistance, chiral magnetic effects and the quantum anomalous Hall effect) which may also lead to novel future applications.

Discovery of a three-dimensional topological Dirac semimetal, Na3Bi.

Science 343 (2014) 864-867

ZK Liu, B Zhou, Y Zhang, ZJ Wang, HM Weng, D Prabhakaran, S-K Mo, ZX Shen, Z Fang, X Dai, Z Hussain, YL Chen

Three-dimensional (3D) topological Dirac semimetals (TDSs) represent an unusual state of quantum matter that can be viewed as "3D graphene." In contrast to 2D Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. By investigating the electronic structure of Na3Bi with angle-resolved photoemission spectroscopy, we detected 3D Dirac fermions with linear dispersions along all momentum directions. Furthermore, we demonstrated the robustness of 3D Dirac fermions in Na3Bi against in situ surface doping. Our results establish Na3Bi as a model system for 3D TDSs, which can serve as an ideal platform for the systematic study of quantum phase transitions between rich topological quantum states.

Measurement of Coherent Polarons in the Strongly Coupled Antiferromagnetically Ordered Iron-Chalcogenide Fe_{1.02}Te using Angle-Resolved Photoemission Spectroscopy

PRL American Physical Society 110 (2013) 037003-

ZK Liu, R-H He, DH Lu, M Yi, YL Chen, M Hashimoto, RG Moore, S-K Mo, EA Nowadnick, J Hu, TJ Liu, ZQ Mao, TP Devereaux, Z Hussain, Z-X Shen

Observing electronic structures on ex-situ grown topological insulator thin films

Phys. Status Solidi RRL WILEY-VCH Verlag (2012) n/a-n/a

SH Yao, B Zhou, MH Lu, ZK Liu, YB Chen, JG Analytis, C Brüne, WH Dang, S-K Mo, Z-X Shen, IR Fisher, LW Molenkamp, HL Peng, Z Hussain, YL Chen

Controlling the carriers of topological insulators by bulk and surface doping

Semiconductor Science and Technology 27 (2012)

B Zhou, ZK Liu, JG Analytis, K Igarashi, SK Mo, DH Lu, RG Moore, IR Fisher, T Sasagawa, ZX Shen, Z Hussain, YL Chen

We report a systematic study of bulk and surface chemical doping effects on single Dirac cone topological insulator Bi 2 Se 3 and Bi 2 Te 3 . By bulk doping, we were able to achieve full range control of charge carrier types and concentration, with the exact Fermi energy measured by angle-resolved photoemission spectroscopy (ARPES). Due to the unusual robustness of the topological surface state, we further realized the bi-polar control of the surface carriers by gaseous or alkaline surface doping without affecting the topological nature of these materials. The doping progress monitored by in situ ARPES study clearly demonstrated the switching between different carrier types through the Dirac point. The ability to control the carrier types and the concentration of topological insulators will greatly facilitate future applications. © 2012 IOP Publishing Ltd.

Topological insulator nanostructures for near-infrared transparent flexible electrodes

Nature Chemistry 4 (2012) 281-286

H Peng, W Dang, J Cao, Y Chen, D Wu, W Zheng, H Li, Z-X Shen, Z Liu

Emerging coherence with unified energy, temperature, and lifetime scale in heavy fermion YbRh2Si2

PHYSICAL REVIEW B 85 (2012) ARTN 241103

S-K Mo, WS Lee, F Schmitt, YL Chen, DH Lu, C Capan, DJ Kim, Z Fisk, C-Q Zhang, Z Hussain, Z-X Shen

Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3


JA Sobota, S Yang, JG Analytis, YL Chen, IR Fisher, PS Kirchmann, Z-X Shen

Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate


WS Lee, YD Chuang, RG Moore, Y Zhu, L Patthey, M Trigo, DH Lu, PS Kirchmann, O Krupin, M Yi, M Langner, N Huse, JS Robinson, Y Chen, SY Zhou, G Coslovich, B Huber, DA Reis, RA Kaindl, RW Schoenlein, D Doering, P Denes, WF Schlotter, JJ Turner, SL Johnson, M Foerst, T Sasagawa, YF Kung, AP Sorini, AF Kemper, B Moritz, TP Devereaux, D-H Lee, ZX Shen, Z Hussain

Angle-Resolved Photoemission Studies of Quantum Materials

Annual Review of Condensed Matter Physics Annual Reviews (2011)

D Lu, IM Vishik, M Yi, Y Chen, RG Moore, Z Shen

Controlled synthesis of topological insulator nanoplate arrays on mica

Journal of the American Chemical Society 134 (2012) 6132-6135

H Li, J Cao, W Zheng, Y Chen, D Wu, W Dang, K Wang, H Peng, Z Liu

The orientation- and position-controlled synthesis of single-crystal topological insulator (Bi 2 Se 3 and Bi 2 Te 3 ) nanoplate arrays on mica substrates was achieved using van der Waals epitaxy. Individual ultrathin nanoplates with the lateral dimension up to ∼0.1 mm or uniform thickness down to 1-2 nm were produced. Single-Dirac-cone surface states of nanoplate aggregates were confirmed by angle-resolved photoemission spectroscopy measurements. The large-grain-size, single-crystal nanoplate arrays grown on mica can act as facile platforms for a combination of spectroscopy and in situ transport measurements, which may open up new avenues for studying exotic physical phenomena, surface chemical reactions, and modification in topological insulators. © 2012 American Chemical Society.

Studies on the electronic structures of three-dimensional topological insulators by angle resolved photoemission spectroscopy

Frontiers of Physics 7 (2012) 175-192

Y Chen

Three-dimensional (3D) topological insulators represent a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. The unusual surface states of topological insulators rise from the nontrivial topology of their electronic structures as a result of strong spin-orbital coupling. In this review, we will briefly introduce the concept of topological insulators and the experimental method that can directly probe their unique electronic structure: angle resolved photoemission spectroscopy (ARPES). A few examples are then presented to demonstrate the unique band structures of different families of topological insulators and the unusual properties of the topological surface states. Finally, we will briefly discuss the future development of topological quantum materials. © 2012 Higher Education Press and Springer-Verlag Berlin Heidelberg.

Quantum Hall Effect from the Topological Surface States of Strained Bulk HgTe


C Bruene, CX Liu, EG Novik, EM Hankiewicz, H Buhmann, YL Chen, XL Qi, ZX Shen, SC Zhang, LW Molenkamp

Rapid surface oxidation as a source of surface degradation factor for Bi₂Se₃.

ACS Nano 5 (2011) 4698-4703

D Kong, JJ Cha, K Lai, H Peng, JG Analytis, S Meister, Y Chen, H-J Zhang, IR Fisher, Z-X Shen, Y Cui

Bismuth selenide (Bi(2)Se(3)) is a topological insulator with metallic surface states (SS) residing in a large bulk bandgap. In experiments, synthesized Bi(2)Se(3) is often heavily n-type doped due to selenium vacancies. Furthermore, it is discovered from experiments on bulk single crystals that Bi(2)Se(3) gets additional n-type doping after exposure to the atmosphere, thereby reducing the relative contribution of SS in total conductivity. In this article, transport measurements on Bi(2)Se(3) nanoribbons provide additional evidence of such environmental doping process. Systematic surface composition analyses by X-ray photoelectron spectroscopy reveal fast formation and continuous growth of native oxide on Bi(2)Se(3) under ambient conditions. In addition to n-type doping at the surface, such surface oxidation is likely the material origin of the degradation of topological SS. Appropriate surface passivation or encapsulation may be required to probe topological SS of Bi(2)Se(3) by transport measurements.

Ambipolar field effect in the ternary topological insulator (Bi(x)Sb(1-x))2Te3 by composition tuning.

Nat Nanotechnol 6 (2011) 705-709

D Kong, Y Chen, JJ Cha, Q Zhang, JG Analytis, K Lai, Z Liu, SS Hong, KJ Koski, S-K Mo, Z Hussain, IR Fisher, Z-X Shen, Y Cui

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.

Widespread spin polarization effects in photoemission from topological insulators

PRB American Physical Society 84 (2011) 165113-

C Jozwiak, YL Chen, AV Fedorov, JG Analytis, CR Rotundu, AK Schmid, JD Denlinger, YD Chuang, DH Lee, IR Fisher, RJ Birgeneau, ZX Shen, Z Hussain, A Lanzara

Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family


YL Chen, ZK Liu, JG Analytis, J-H Chu, HJ Zhang, BH Yan, S-K Mo, RG Moore, DH Lu, IR Fisher, SC Zhang, Z Hussain, Z-X Shen

Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator

SCIENCE 329 (2010) 659-662

YL Chen, J-H Chu, JG Analytis, ZK Liu, K Igarashi, H-H Kuo, XL Qi, SK Mo, RG Moore, DH Lu, M Hashimoto, T Sasagawa, SC Zhang, IR Fisher, Z Hussain, ZX Shen

STM Imaging of Electronic Waves on the Surface of Bi2Te3: Topologically Protected Surface States and Hexagonal Warping Effects


Z Alpichshev, JG Analytis, J-H Chu, IR Fisher, YL Chen, ZX Shen, A Fang, A Kapitulnik