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Instruments / Technical Area	Publication Details	Published
I10-Beamline for Advanced Dichroism - scattering	Self-intercalation tunable interlayer exchange coupling in a synthetic van der Waals antiferromagnet Xiaoqian Zhang , Wenqing Liu , Wei Niu , Qiangsheng Lu , Wei Wang , Ali Sarikhani , Xiaohua Wu , Chunhui Zhu , Jiabao Sun , Mitchel Vaninger , Paul. F. Miceli , Jianqi Li , David J. Singh , Yew San Hor , Yue Zhao , Chang Liu , Liang He , Rong Zhang , Guang Bian , Dapeng Yu , Yongbing Xu Advanced Functional Materials 2009 DOI: 10.1002/adfm.202202977 Diamond Proposal Number(s): [22532] Show Abstract ▼ Abstract: One of the most promising avenues in 2D materials research is the synthesis of antiferromagnets employing 2D van der Waals (vdW) magnets. However, it has proven challenging, due in part to the complicated fabrication process and undesired adsorbates as well as the significantly deteriorated ferromagnetism at atomic layers. Here, the engineering of the antiferromagnetic (AFM) interlayer exchange coupling between atomically thin yet ferromagnetic CrTe2 layers in an ultra-high vacuum-free 2D magnetic crystal, Cr5Te8 is reported. By self-introducing interstitial Cr atoms in the vdW gaps, the emergent AFM ordering and the resultant giant magnetoresistance effect are induced. A large negative magnetoresistance (10%) with a plateau-like feature is revealed, which is consistent with the AFM interlayer coupling between the adjacent CrTe2 main layers in a temperature window of 30 K below the Néel temperature. Notably, the AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and ferromagnetic states at moderate magnetic fields. This work represents a new route to engineering low-power devices that underpin the emerging spintronic technologies, and an ideal laboratory to study 2D magnetism.	May 2022
I10-Beamline for Advanced Dichroism - scattering	Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films Xiaoqian Zhang , Qiangsheng Lu , Wenqing Liu , Wei Niu , Jiabao Sun , Jacob Cook , Mitchel Vaninger , Paul F. Miceli , David J. Singh , Shang-Wei Lian , Tay-Rong Chang , Xiaoqing He , Jun Du , Liang He , Rong Zhang , Guang Bian , Yongbing Xu Nature Communications 12 DOI: 10.1038/s41467-021-22777-x Diamond Proposal Number(s): [22532] Open Access Show Abstract ▼ Abstract: While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe2, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (TC) up to 300 K, an atomic magnetic moment of ~0.21 𝜇B μ B /Cr and perpendicular magnetic anisotropy (PMA) constant (Ku) of 4.89 × 105 erg/cm3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (TC ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.	May 2021
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
I05-ARPES	Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide Su-Yang Xu , Nasser Alidoust , Ilya Belopolski , Zhujun Yuan , Guang Bian , Tay-Rong Chang , Hao Zheng , Vladimir N. Strocov , Daniel Sanchez , Guoqing Chang , Chenglong Zhang , Daixiang Mou , Yun Wu , Lunan Huang , Chi-Cheng Lee , Shin-Ming Huang , Baokai Wang , Arun Bansil , Horng-Tay Jeng , Titus Neupert , Adam Kaminski , Hsin Lin , Shuang Jia , M. Zahid Hasan Nature Physics 11, 748 - 754 DOI: 10.1038/nphys3437 Diamond Proposal Number(s): [10074] Show Abstract ▼ Abstract: Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics.	Aug 2015

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