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Instruments / Technical Area	Publication Details	Published
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	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 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	Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4 D. Sutter , C. G. Fatuzzo , S. Moser , M. Kim , R. Fittipaldi , A. Vecchione , V. Granata , Y. Sassa , F. Cossalter , G. Gatti , M. Grioni , H. M. Rønnow , N. C. Plumb , C. E. Matt , M. Shi , M. Hoesch , T. K. Kim , T.-R. Chang , H.-T. Jeng , C. Jozwiak , A. Bostwick , E. Rotenberg , A. Georges , T. Neupert , J. Chang Nature Communications 8 DOI: 10.1038/ncomms15176 Diamond Proposal Number(s): [14617, 12926] Open Access Show Abstract ▼ Abstract: A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund’s coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the dxy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund’s coupling that together generate the Mott insulating state of Ca2RuO4.	May 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	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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