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Instruments / Technical Area	Publication Details	Published
I21-Resonant Inelastic X-ray Scattering (RIXS)	Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens Leonardo Martinelli , Krzysztof Wohlfeld , Jonathan Pelliciari , Riccardo Arpaia , Nicholas B. Brookes , Daniele Di Castro , Mirian Garcia-Fernandez , Mingu Kang , Yoshiharu Krockenberger , Kurt Kummer , Daniel E. Mcnally , Eugenio Paris , Thorsten Schmitt , Hideki Yamamoto , Andrew Walters , Ke-Jin Zhou , Lucio Braicovich , Riccardo Comin , Marco Moretti Sala , Thomas P. Devereaux , Maria Daghofer , Giacomo Ghiringhelli Physical Review Letters 132 DOI: 10.1103/PhysRevLett.132.066004 Diamond Proposal Number(s): [20690] Show Abstract ▼ Abstract: We have investigated the 3 d orbital excitations in CaCuO 2 (CCO), Nd 2 CuO 4 (NCO), and La 2 CuO 4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d x y orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.	Feb 2024
I05-ARPES	Holstein polaron in a valley-degenerate two-dimensional semiconductor Mingu Kang , Sung Won Jung , Woo Jong Shin , Yeongsup Sohn , Sae Hee Ryu , Timur K. Kim , Moritz Hoesch , Keun Su Kim Nature Materials 306 DOI: 10.1038/s41563-018-0092-7 Show Abstract ▼ Abstract: Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density1. Carrier doping to 2D semiconductors can be used to modulate many-body interactions2 and to explore novel composite particles. The Holstein polaron is a small composite particle of an electron that carries a cloud of self-induced lattice deformation (or phonons)3,4,5, which has been proposed to play a key role in high-temperature superconductivity6 and carrier mobility in devices7. Here we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS2, in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently8,9,10,11. Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons12,13,14,15,16,17,18. The short-range nature of electron–phonon (e–ph) coupling in MoS2 can be explained by its valley degeneracy, which enables strong intervalley coupling mediated by acoustic phonons. The coupling strength is found to increase gradually along the superconducting dome up to the intermediate regime, which suggests a bipolaronic pairing in the 2D superconductivity.	May 2018
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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