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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Visualizing the out-of-plane electronic dispersions in an intercalated transition metal dichalcogenide Xian P. Yang , Harrison Labollita , Zi-Jia Cheng , Hari Bhandari , Tyler A. Cochran , Jia-Xin Yin , Md. Shafayat Hossain , Ilya Belopolski , Qi Zhang , Yuxiao Jiang , Nana Shumiya , Daniel Multer , Maksim Liskevich , Dmitry A. Usanov , Yanliu Dang , Vladimir N. Strocov , Albert V. Davydov , Nirmal J. Ghimire , Antia S. Botana , M. Zahid Hasan Physical Review B 105 DOI: 10.1103/PhysRevB.105.L121107 Diamond Proposal Number(s): [29230] Show Abstract ▼ Abstract: Layered transition metal dichalcogenides have a rich phase diagram and they feature two-dimensionality in numerous physical properties. Co 1 / 3 NbS 2 is one of the newest members of this family where Co atoms are intercalated into the van der Waals gaps between NbS 2 layers. We study the three-dimensional electronic band structure of Co 1 / 3 NbS 2 using both surface and bulk sensitive angle-resolved photoemission spectroscopy. We show that the electronic bands do not fit into the rigid band shift picture after the Co intercalation. Instead, Co 1 / 3 NbS 2 displays a different orbital character near the Fermi level compared to the pristine NbS 2 compound and has a clear band dispersion in the k z direction despite its layered structure. Our photoemission study demonstrates the out-of-plane electronic correlations introduced by the Co intercalation, thus offering a different perspective on this compound. Finally, we propose how Fermi level tuning could lead to exotic phases such as spin density wave instability.	Mar 2022
I05-ARPES	Signatures of weyl fermion annihilation in a correlated kagome magnet Ilya Belopolski , Tyler A. Cochran , Xiaoxiong Liu , Zi-Jia Cheng , Xian P. Yang , Zurab Guguchia , Stepan S. Tsirkin , Jia-Xin Yin , Praveen Vir , Gohil S. Thakur , Songtian S. Zhang , Junyi Zhang , Konstantine Kaznatcheev , Guangming Cheng , Guoqing Chang , Daniel Multer , Nana Shumiya , Maksim Litskevich , Elio Vescovo , Timur K. Kim , Cephise Cacho , Nan Yao , Claudia Felser , Titus Neupert , M. Zahid Hasan Physical Review Letters 127 DOI: 10.1103/PhysRevLett.127.256403 Diamond Proposal Number(s): [17924, 19313] Show Abstract ▼ Abstract: The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal Co 3 Sn 2 S 2 , observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.	Dec 2021
I05-ARPES	Quantum-limit Chern topological magnetism in TbMn6Sn6 Jia-Xin Yin , Wenlong Ma , Tyler A. Cochran , Xitong Xu , Songtian S. Zhang , Hung-Ju Tien , Nana Shumiya , Guangming Cheng , Kun Jiang , Biao Lian , Zhida Song , Guoqing Chang , Ilya Belopolski , Daniel Multer , Maksim Litskevich , Zi-Jia Cheng , Xian P. Yang , Bianca Swidler , Huibin Zhou , Hsin Lin , Titus Neupert , Ziqiang Wang , Nan Yao , Tay-Rong Chang , Shuang Jia , M. Zahid Hasan Nature 583, 533 - 536 DOI: 10.1038/s41586-020-2482-7 Diamond Proposal Number(s): [22332] Show Abstract ▼ Abstract: The quantum-level interplay between geometry, topology and correlation is at the forefront of fundamental physics. Kagome magnets are predicted to support intrinsic Chern quantum phases owing to their unusual lattice geometry and breaking of time-reversal symmetry. However, quantum materials hosting ideal spin–orbit-coupled kagome lattices with strong out-of-plane magnetization are lacking. Here, using scanning tunnelling microscopy, we identify a new topological kagome magnet, TbMn6Sn6, that is close to satisfying these criteria. We visualize its effectively defect-free, purely manganese-based ferromagnetic kagome lattice with atomic resolution. Remarkably, its electronic state shows distinct Landau quantization on application of a magnetic field, and the quantized Landau fan structure features spin-polarized Dirac dispersion with a large Chern gap. We further demonstrate the bulk–boundary correspondence between the Chern gap and the topological edge state, as well as the Berry curvature field correspondence of Chern gapped Dirac fermions. Our results point to the realization of a quantum-limit Chern phase in TbMn6Sn6, and may enable the observation of topological quantum phenomena in the RMn6Sn6 (where R is a rare earth element) family with a variety of magnetic structures. Our visualization of the magnetic bulk–boundary–Berry correspondence covering real space and momentum space demonstrates a proof-of-principle method for revealing topological magnets.	Jul 2020
I05-ARPES	Discovery of magnetic topological crystals Ilya Belopolski Open Access Show Abstract ▼ Abstract: Topological phases of matter have established a new paradigm in physics, bringing quantum phenomena to the macroscopic scale and hosting exotic emergent quasiparticles. In this thesis, I demonstrate with my collaborators the first Weyl semimetal, TaAs, using angle-resolved photoemission spectroscopy (ARPES), directly observing its emergent Weyl fermions and topological Fermi arc surface states [Science 349, 6248 (2015); Physical Review Letters 116, 066802 (2016)]. Next, I consider structurally chiral crystals, which I argue are guaranteed to host exotic chiral fermions leading to giant topological Fermi arcs. I study the chiral crystals RhSi and CoSi and I discover high-degeneracy chiral fermions with wide topological energy window, maximal separation in momentum space and giant Fermi arcs [Nature 567, 500 (2019); Nature Materials 17, 978 (2018)]. I establish a natural relationship between structural and topological chirality, producing a robust topological state which we predict supports a four-unit quantized photogalvanic effect [Physical Review Letters 119, 206401 (2017)]. Next, I discuss the first quantum topological superlattice [Science Advances 3, e1501692 (2017)]. I study multilayer heterostructures of alternating topological and trivial insulators. The Dirac cones at each interface tunnel across layers, realizing a new kind of emergent superlattice, where the interfaces act as lattice sites and the Dirac cones act as atomic orbitals. Adjusting the stacking pattern offers unprecedented control of individual hopping parameters in the atomic chain. I realize a novel topological phase transition and I predict that this platform may allow particle-hole symmetry without superconductivity. Lastly, I present the discovery of a room-temperature topological magnet [arXiv:1712.09992]. I study crystals of Co2MnGa and I observe a topological invariant supported by the material's intrinsic magnetic order [Physical Review Letters 119, 156401 (2017)]. In particular I observe topological Weyl lines and drumhead surface states by ARPES and, through a scaling analysis of the anomalous Hall transport response, I find that the large anomalous Hall effect in Co2MnGa arises from the Weyl lines. I hope that my discovery of Co2MnGa establishes topological magnetism as a new research frontier in condensed matter physics.	Jun 2019
I05-ARPES	Discovery of Lorentz-violating type II Weyl fermions in LaAlGe Su-Yang Xu , Nasser Alidoust , Guoqing Chang , Hong Lu , Bahadur Singh , Ilya Belopolski , Daniel S. Sanchez , Xiao Zhang , Guang Bian , Hao Zheng , Marious-Adrian Husanu , Yi Bian , Shin-Ming Huang , Chuang-Han Hsu , Tay-Rong Chang , Horng-Tay Jeng , Arun Bansil , Titus Neupert , Vladimir N. Strocov , Hsin Lin , Shuang Jia , M. Zahid Hasan Science Advances 3 DOI: 10.1126/sciadv.1603266 Open Access Show Abstract ▼ Abstract: In quantum field theory, Weyl fermions are relativistic particles that travel at the speed of light and strictly obey the celebrated Lorentz symmetry. Their low-energy condensed matter analogs are Weyl semimetals, which are conductors whose electronic excitations mimic the Weyl fermion equation of motion. Although the traditional (type I) emergent Weyl fermions observed in TaAs still approximately respect Lorentz symmetry, recently, the so-called type II Weyl semimetal has been proposed, where the emergent Weyl quasiparticles break the Lorentz symmetry so strongly that they cannot be smoothly connected to Lorentz symmetric Weyl particles. Despite some evidence of nontrivial surface states, the direct observation of the type II bulk Weyl fermions remains elusive. We present the direct observation of the type II Weyl fermions in crystalline solid lanthanum aluminum germanide (LaAlGe) based on our photoemission data alone, without reliance on band structure calculations. Moreover, our systematic data agree with the theoretical calculations, providing further support on our experimental results.	Jun 2017
I05-ARPES	A novel artificial condensed matter lattice and a new platform for one-dimensional topological phases Ilya Belopolski , Su-Yang Xu , Nikesh Koirala , Chang Liu , Guang Bian , Vladimir N. Strocov , Guoqing Chang , Madhab Neupane , Nasser Alidoust , Daniel Sanchez , Hao Zheng , Matthew Brahlek , Victor Rogalev , Timur Kim , Nicholas C. Plumb , Chaoyu Chen , François Bertran , Patrick Le Fèvre , Amina Taleb-Ibrahimi , Maria-Carmen Asensio , Ming Shi , Hsin Lin , Moritz Hoesch , Seongshik Oh , M. Zahid Hasan Science Advances 3 DOI: 10.1126/sciadv.1501692 Diamond Proposal Number(s): [11742] Open Access Show Abstract ▼ Abstract: Engineered lattices in condensed matter physics, such as cold-atom optical lattices or photonic crystals, can have properties that are fundamentally different from those of naturally occurring electronic crystals. We report a novel type of artificial quantum matter lattice. Our lattice is a multilayer heterostructure built from alternating thin films of topological and trivial insulators. Each interface within the heterostructure hosts a set of topologically protected interface states, and by making the layers sufficiently thin, we demonstrate for the first time a hybridization of interface states across layers. In this way, our heterostructure forms an emergent atomic chain, where the interfaces act as lattice sites and the interface states act as atomic orbitals, as seen from our measurements by angle-resolved photoemission spectroscopy. By changing the composition of the heterostructure, we can directly control hopping between lattice sites. We realize a topological and a trivial phase in our superlattice band structure. We argue that the superlattice may be characterized in a significant way by a one-dimensional topological invariant, closely related to the invariant of the Su-Schrieffer-Heeger model. Our topological insulator heterostructure demonstrates a novel experimental platform where we can engineer band structures by directly controlling how electrons hop between lattice sites.	Mar 2017
I05-ARPES	Fermi arc electronic structure and Chern numbers in the type-II Weyl semimetal candidate MoxW1−xTe2 Ilya Belopolski , Su-Yang Xu , Yukiaki Ishida , Xingchen Pan , Peng Yu , Daniel S. Sanchez , Hao Zheng , Madhab Neupane , Nasser Alidoust , Guoqing Chang , Tay-Rong Chang , Yun Wu , Guang Bian , Shin-Ming Huang , Chi-Cheng Lee , Daixiang Mou , Lunan Huang , You Song , Baigeng Wang , Guanghou Wang , Yao-Wen Yeh , Nan Yao , Julien E. Rault , Patrick Le Fèvre , François Bertran , Horng-Tay Jeng , Takeshi Kondo , Adam Kaminski , Hsin Lin , Zheng Liu , Fengqi Song , Shik Shin , M. Zahid Hasan Physical Review B 94 DOI: 10.1103/PhysRevB.94.085127 Diamond Proposal Number(s): [13653] Show Abstract ▼ Abstract: It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that MoxW1−xTe2 may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ɛF to show a Weyl semimetal in MoxW1−xTe2. Then, we study MoxW1−xTe2 by pump-probe ARPES and we directly access the band structure >0.2 eV above ɛF in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of MoxW1−xTe2 by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in MoxW1−xTe2.	Aug 2016
I05-ARPES	Signatures of the Adler–Bell–Jackiw chiral anomaly in a Weyl fermion semimetal Cheng-Long Zhang , Su-Yang Xu , Ilya Belopolski , Zhujun Yuan , Ziquan Lin , Bingbing Tong , Guang Bian , Nasser Alidoust , Chi-Cheng Lee , Shin-Ming Huang , Tay-Rong Chang , Guoqing Chang , Chuang-Han Hsu , Horng-Tay Jeng , Madhab Neupane , Daniel Sanchez , Hao Zheng , Junfeng Wang , Hsin Lin , Chi Zhang , Hai-Zhou Lu , Shun-Qing Shen , Titus Neupert , M. Zahid Hasan , Shuang Jia Nature Communications 7 DOI: 10.1038/ncomms10735 Open Access Show Abstract ▼ Abstract: Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs.	Feb 2016
I05-ARPES	Experimental discovery of a topological Weyl semimetal state in TaP S. Y. Xu , I. Belopolski , D. Sanchez , C. Zhang , G. Chang , C. Guo , G. Bian , Z. Yuan , H. Lu , T. R Chang , P. P Shibayev , Mykhaylo Prokopovych , N. Alidoust , H. Zheng , C. C Lee , S. M. Huang , R. Sankar , F. Chou , C. H. Hsu , H. T Jeng , A. Bansil , T. Neupert , V. N. Strocov , H. Lin , S. Jia , M. Z. Hasan Science Advances 1, e1501092 - e1501092 DOI: 10.1126/sciadv.1501092 Show Abstract ▼ Abstract: Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of tantalum arsenide, tantalum phosphide (TaP), niobium arsenide, and niobium phosphide, was predicted as a Weyl semimetal candidates. We experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk, and the Fermi arcs on the surface. Moreover, we find that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal’s surface. We directly demonstrate the bulk-boundary correspondence and establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.	Nov 2015
I05-ARPES	Discovery of a Weyl fermion semimetal and topological Fermi arcs S. Y. Xu , Ilya Belopolski , N. Alidoust , M. Neupane , G. Bian , C. Zhang , R. Sankar , G. Chang , Z. Yuan , C. C. Lee , S. M. Huang , H. Zheng , J. Ma , D. S. Sanchez , B. Wang , A. Bansil , F. Chou , Pavel Shibayev , H. Lin , M. Zahid Hasan Science 349 (6248), 613 - 617 DOI: 10.1126/science.aaa9297 PMID: 26184916 Diamond Proposal Number(s): [10074] Show Abstract ▼ Abstract: A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.	Aug 2015

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