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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Electronic reconstruction forming a C2-symmetric Dirac semimetal in Ca3Ru2O7 M. Horio , Q. Wang , V. Granata , K. P. Kramer , Y. Sassa , S. Jöhr , D. Sutter , A. Bold , L. Das , Y. Xu , R. Frison , R. Fittipaldi , T. K. Kim , C. Cacho , J. E. Rault , P. Le Fèvre , F. Bertran , N. C. Plumb , M. Shi , A. Vecchione , M. H. Fischer , J. Chang Npj Quantum Materials 6 DOI: 10.1038/s41535-021-00328-3 Diamond Proposal Number(s): [20259] Open Access Show Abstract ▼ Abstract: Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.	Mar 2021
I05-ARPES	Orbitally selective breakdown of Fermi liquid quasiparticles in Ca1.8 Sr0.2 RuO4 D. Sutter , M. Kim , C. E. Matt , M. Horio , R. Fittipaldi , A. Vecchione , V. Granata , K. Hauser , Y. Sassa , G. Gatti , M. Grioni , M. Hoesch , T. K. Kim , E. Rienks , N. C. Plumb , M. Shi , T. Neupert , A. Georges , J. Chang Physical Review B 99 DOI: 10.1103/PhysRevB.99.121115 Diamond Proposal Number(s): [15296] Show Abstract ▼ Abstract: We present a comprehensive angle-resolved photoemission spectroscopy study of Ca 1.8 Sr 0.2 RuO 4 . Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to d x z , d y z orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions—associated with a predominantly d x y band character—are shown to display non-Fermi-liquid behavior. We thus demonstrate that Ca 1.8 Sr 0.2 RuO 4 is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.	Mar 2019
I05-ARPES	Measurement of the atomic orbital composition of the near-fermi-level electronic states in the lanthanum monopnictides LaBi, LaSb, and LaAs Thomas J. Nummy , Justin A. Waugh , Stephen P. Parham , Qihang Liu , Hung-Yu Yang , Haoxiang Li , Xiaoqing Zhou , Nicholas C. Plumb , Fazel F. Tafti , Daniel S. Dessau Npj Quantum Materials 3 DOI: 10.1038/s41535-018-0094-3 Open Access Show Abstract ▼ Abstract: Recent debates in the literature over the relationship between topology and Extreme Magnetoresistance (XMR) have drawn attention to the Lanthanum Monopnictide family of binary compounds. Angle resolved photoemission spectroscopy (ARPES) is used to measure the electronic structure of the XMR topological semimetal candidates LaBi, LaSb, and LaAs. The orbital content of the near-E F states in LaBi and LaSb are extracted using varying photon polarizations and both dominant d and p bands are observed near X. The measured bulk bands are shifted in energy when compared to the results of Density Functional Calculations. This disagreement is minor in LaBi, but large in LaSb and LaAs. The measured bulk band structure of LaBi shows a clear band inversion and puts LaBi in the υ = 1 class of Topological Insulators (or semimetals), as predicted by calculations and consistent with the measured Dirac-like surface states. LaSb is on the verge of a band inversion with a less-clear case for any distinctly topological surface states and in disagreement with calculations. Lastly, these same bands in LaAs are clearly non-inverted implying its topological triviality and demonstrating a topological phase transition in the Lanthanum monopnictides. Using a wide range of photon energies the true bulk states are cleanly disentangled from the various types of surface states which are present. These surface states exist due to surface projections of bulk states in LaSb and for topological reasons in LaBi.	May 2018
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	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	Spin orientation of two-dimensional electrons driven by temperature-tunable competition of spin–orbit and exchange–magnetic interactions Alexander Generalov , Mikhail M. Otrokov , Alla Chikina , Kristin Kliemt , Kurt Kummer , Marc Höppner , Monika Guttler , Silvia Seiro , Alexander Fedorov , Susanne Schulz , Steffen Danzenbächer , Evgueni V. Chulkov , Christoph Geibel , Clemens Laubschat , Pavel Dudin , Moritz Hoesch , Timur Kim , Milan Radovic , Ming Shi , Nicholas C. Plumb , Cornelius Krellner , Denis V. Vyalikh Nano Letters DOI: 10.1021/acs.nanolett.6b04036 Diamond Proposal Number(s): [11512] Show Abstract ▼ Abstract: Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin–orbit and exchange–magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.	Jan 2017
I05-ARPES	Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WTe2 F. Y. Bruno , A. Tamai , Q. S. Wu , I. Cucchi , C. Barreteau , A. De La Torre , S. Mckeown Walker , S. Ricco , Z. Wang , T. K. Kim , M. Hoesch , M. Shi , N. C. Plumb , E. Giannini , A. A. Soluyanov , F. Baumberger Physical Review B 94 DOI: 10.1103/PhysRevB.94.121112 Diamond Proposal Number(s): [12404] Show Abstract ▼ Abstract: We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe 2 . On both surfaces, we identify a surface state that forms a large Fermi arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of WTe 2 as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in WTe 2.	Sep 2016
I05-ARPES	Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid Z. Wang , S. Mckeown Walker , A. Tamai , Y. Wang , Z. Ristic , F. Bruno , A. De La Torre , S. Ricco , N. C. Plumb , M. Shi , P. Hlawenka , J. Sánchez-Barriga , A. Varykhalov , Timur Kim , Moritz Hoesch , P. D. C. King , Worawat Meevasana , U. Diebold , J. Mesot , B. Moritz , T. P. Devereaux , M. Radovic , F. Baumberger Nature Materials DOI: 10.1038/nmat4623 Diamond Proposal Number(s): [11741] Show Abstract ▼ Abstract: Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides1, 2, 3, 4. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface5, 6, 7 to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface8, 9, 10, 11, 12, 13, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron–phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs.	Apr 2016

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