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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Pseudogap in a crystalline insulator doped by disordered metals Sae Hee Ryu , Minjae Huh , Do Yun Park , Chris Jozwiak , Eli Rotenberg , Aaron Bostwick , Keun Su Kim Nature 596, 68 - 73 DOI: 10.1038/s41586-021-03683-0 Diamond Proposal Number(s): [25869] Show Abstract ▼ Abstract: Key to our understanding of how electrons behave in crystalline solids is the band structure that connects the energy of electron waves to their wavenumber. Even in phases of matter with only short-range order (liquid or amorphous solid), the coherent part of electron waves still has a band structure. Theoretical models for the band structure of liquid metals were formulated more than five decades ago, but, so far, band-structure renormalization and the pseudogap induced by resonance scattering have remained unobserved. Here we report the observation of the unusual band structure at the interface of a crystalline insulator (black phosphorus) and disordered dopants (alkali metals). We find that a conventional parabolic band structure of free electrons bends back towards zero wavenumber with a pseudogap of 30–240 millielectronvolts from the Fermi level. This is wavenumber renormalization caused by resonance scattering, leading to the formation of quasi-bound states in the scattering potential of alkali-metal ions. The depth of this potential tuned by different kinds of disordered alkali metal (sodium, potassium, rubidium and caesium) allows the classification of the pseudogap of p-wave and d-wave resonance. Our results may provide a clue to the puzzling spectrum of various crystalline insulators doped by disordered dopants, such as the waterfall dispersion observed in copper oxides.	Aug 2021
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	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	Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides Mingu Kang , Beomyoung Kim , Sae Hee Ryu , Sung Won Jung , Jimin Kim , Luca Moreschini , Chris Jozwiak , Eli Rotenberg , Aaron Bostwick , Keun Su Kim Nano Letters DOI: 10.1021/acs.nanolett.6b04775 Diamond Proposal Number(s): [13946] Show Abstract ▼ Abstract: van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8–2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.	Feb 2017

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