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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Direct observation of a uniaxial stress-driven Lifshitz transition in Sr2RuO4 Veronika Sunko , Edgar Abarca Morales , Igor Markovic , Mark E. Barber , Dijana Milosavljević , Federico Mazzola , Dmitry A. Sokolov , Naoki Kikugawa , Cephise Cacho , Pavel Dudin , Helge Rosner , Clifford Hicks , Philip D. C. King , Andrew P. Mackenzie Npj Quantum Materials 4 DOI: 10.1038/s41535-019-0185-9 Diamond Proposal Number(s): [20427] Open Access Show Abstract ▼ Abstract: Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.	Dec 2019
I05-ARPES	Topological electronic structure and its temperature evolution in antiferromagnetic topological insulator MnBi2Te4 Y. J. Chen , L. X. Xu , J. H. Li , Y. W. Li , H. Y. Wang , C. F. Zhang , H. Li , Y. Wu , A. J. Liang , C. Chen , S. W. Jung , C. Cacho , Y. H. Mao , S. Liu , M. X. Wang , Y. F. Guo , Y. Xu , Z. K. Liu , L. X. Yang , Y. L. Chen Physical Review X 9 DOI: 10.1103/PhysRevX.9.041040 Diamond Proposal Number(s): [23648, 24827] Open Access Show Abstract ▼ Abstract: The intrinsic magnetic topological insulator MnBi 2 Te 4 exhibits rich topological effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on MnBi 2 Te 4 and clearly identify its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with diminished gap forming a characteristic Dirac cone. We argue that the topological surface states are mediated by multidomains of different magnetization orientations. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. The investigation of the detailed electronic structure of MnBi 2 Te 4 and its temperature evolution provides important insight into not only the exotic properties of MnBi 2 Te 4 , but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic topological quantum materials.	Nov 2019
I05-ARPES	Surface states and Rashba-type spin polarization in antiferromagnetic MnBi2Te4(0001) R. C. Vidal , H. Bentmann , T. R. F. Peixoto , A. Zeugner , S. Moser , C.-h. Min , S. Schatz , K. Kissner , M. Unzelmann , C. I. Fornari , H. B. Vasili , M. Valvidares , K. Sakamoto , D. Mondal , J. Fujii , I. Vobornik , S. Jung , C. Cacho , T. K. Kim , R. J. Koch , C. Jozwiak , A. Bostwick , J. D. Denlinger , E. Rotenberg , J. Buck , M. Hoesch , F. Diekmann , S. Rohlf , M. Kalläne , K. Rossnagel , M. M. Otrokov , E. V. Chulkov , M. Ruck , A. Isaeva , F. Reinert Physical Review B 100, 121104(R) DOI: 10.1103/PhysRevB.100.121104 Diamond Proposal Number(s): [19278, 22468] Show Abstract ▼ Abstract: The layered van der Waals antiferromagnet MnBi 2 Te 4 has been predicted to combine the band ordering of archetypical topological insulators such as Bi 2 Te 3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi 2 Te 4 (0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.	Sep 2019
I05-ARPES	Magnetic Weyl semimetal phase in a Kagomé crystal D. F. Liu , A. J. Liang , E. K. Liu , Q. N. Xu , Y. W. Li , C. Chen , D. Pei , W. J. Shi , S. K. Mo , P. Dudin , T. Kim , C. Cacho , G. Li , Y. Sun , L. X. Yang , Z. K. Liu , S. S. P. Parkin , C. Felser , Y. L. Chen Science 365, 1282 - 1285 DOI: 10.1126/science.aav2873 Diamond Proposal Number(s): [22367, 20683] Show Abstract ▼ Abstract: Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.	Sep 2019
I05-ARPES	Nanoscale mapping of quasiparticle band alignment Soeren Ulstrup , Cristina E. Giusca , Jill A. Miwa , Charlotte Sanders , Alex Browning , Pavel Dudin , Cephise Cacho , Olga Kazakova , D. Kurt Gaskill , Rachael L. Myers-ward , Tianyi Zhang , Mauricio Terrones , Philip Hofmann Nature Communications 10 DOI: 10.1038/s41467-019-11253-2 Diamond Proposal Number(s): [19260] Open Access Show Abstract ▼ Abstract: Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2.	Jul 2019
I05-ARPES	Chiral topological semimetal with multifold band crossings and long Fermi arcs Niels B. M. Schröter , Ding Pei , Maia G. Vergniory , Yan Sun , Kaustuv Manna , Fernando De Juan , Jonas A. Krieger , Vicky Süss , Marcus Schmidt , Pavel Dudin , Barry Bradlyn , Timur K. Kim , Thorsten Schmitt , Cephise Cacho , Claudia Felser , Vladimir N. Strocov , Yulin Chen Nature Physics 353 DOI: 10.1038/s41567-019-0511-y Diamond Proposal Number(s): [19883, 21400] Show Abstract ▼ Abstract: Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge1, long Fermi arc surface states2,3, unusual magnetotransport4 and lattice dynamics5, as well as a quantized response to circularly polarized light6. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.	May 2019
I05-ARPES	Buried double CuO chains in YBa2Cu4O8 uncovered by nano-ARPES Hideaki Iwasawa , Pavel Dudin , Kyosuke Inui , Takahiko Masui , Timur K. Kim , Cephise Cacho , Moritz Hoesch Physical Review B 99 DOI: 10.1103/PhysRevB.99.140510 Diamond Proposal Number(s): [21083] Show Abstract ▼ Abstract: Identifying the electron dynamics in CuO chains has been elusive in Y-Ba-Cu-O (YBCO) cuprate systems when using standard angle-resolved photoemission spectroscopy (ARPES); a cleaved sample exhibits areas terminated by both CuO chains or BaO layers, and the size of a typical beam results in ARPES signals that are superposed from both terminations. Here, we employ spatially resolved ARPES with a submicrometric beam (nano-ARPES) to reveal the surface-termination-dependent electronic structures of the double CuO chains in YBa 2 Cu 4 O 8 . We present an observation of sharp metallic dispersions and Fermi surfaces of the double CuO chains buried underneath the CuO 2 -plane block on a BaO-terminated surface. While the observed Fermi surfaces of the CuO chains are highly one dimensional, the electrons in the CuO chains do not undergo significant electron correlations and no signature of a Tomonaga-Luttinger liquid or a marginal Fermi liquid is found. Our work represents an important experimental step toward understanding the charge dynamics and provides a starting basis for modeling the high- T c superconductivity in YBCO cuprate systems.	Apr 2019

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