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Abstract: The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr2Si2, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.

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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.

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Abstract: The band inversions that generate the topologically non-trivial band gaps of topological insulators and the isolated Dirac touching points of three-dimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbital-manifold band inversions occurring along high-symmetry lines has been demonstrated, stabilising multiple bulk and surface Dirac fermions. Here, we discuss the underlying ingredients necessary to achieve such phases, and discuss their existence within the family of transition metal dichalcogenides. We show how their three-dimensional band structures naturally produce only small k z projected band gaps, and demonstrate how these play a significant role in shaping the surface electronic structure of these materials. We demonstrate, through spin- and angle-resolved photoemission and density functional theory calculations, how the surface electronic structures of the group-X TMDs PtSe2 and PdTe2 are host to up to five distinct surface states, each with complex band dispersions and spin textures. Finally, we discuss how the origin of several recently-realised instances of topological phenomena in systems outside of the TMDs, including the iron-based superconductors, can be understood as a consequence of the same underlying mechanism driving k z -mediated band inversions in the TMDs.

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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.

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Abstract: Using angle-resolved photoemission spectroscopy and density functional theory (DFT) we study the electronic structure of layered BaZnBi2. Our experimental results show no evidence of Dirac states in BaZnBi2 originated either from the bulk or the surface. The calculated band structure without spin-orbit interaction shows linear band dispersions at X along the X−M high-symmetry line. In addition, the calculations suggest a gapless band crossing point along the Γ−M high-symmetry line. However, as soon as the spin-orbit interaction is turned on, the band crossing point is significantly gapped out. These observations suggest that the Dirac fermions in BaZnBi2 are trivial similar to the Dirac states observed in grapheme. The experimental observations are in good agreement with the DFT calculations.