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Ryu, Hanyoung,Song, Inkyung,Kim, Beomyoung,Cho, Soohyun,Soltani, Shoresh,Kim, Timur,Hoesch, Moritz,Kim, Choong H.,Kim, Changyoung American Physical Society 2017 Physical Review B Vol.95 No.11
<P>We performed angle-resolved photoemission experiments on Au(111) surface with circularly polarized light. Data were taken with photon energies in the range between 20 and 100 eV in order to investigate the photon energy dependent behavior in the circular dichroism (CD). While the magnitude of the normalized CD value varies with a maximum value of about 70%, the sign of CD does not change for the photon energy within the range, inconsistent with the prediction based on the density-functional theory (DFT) calculation. Our calculation of the CD using DFT initial state and free electron final state shows a better consistency with experimental results than an earlier study using the inverse low-energy electron diffraction state as the final state. We briefly discuss the dominating factor that determines the CD from Au(111) states.</P>
Holstein polaron in a valley-degenerate two-dimensional semiconductor
Kang, Mingu,Jung, Sung Won,Shin, Woo Jong,Sohn, Yeongsup,Ryu, Sae Hee,Kim, Timur K.,Hoesch, Moritz,Kim, Keun Su Nature Publishing Group UK 2018 NATURE MATERIALS Vol.17 No.8
<P>Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density(1). Carrier doping to 2D semiconductors can be used to modulate many-body interactions(2) 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)(3-5), which has been proposed to play a key role in high-temperature superconductivity(6) and carrier mobility in devices(7). Here we report the discovery of Holstein polarons in a surfacedoped layered semiconductor, MoS2, in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently(8-11). Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons(12-18). The short-range nature of electronphonon(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.</P>
Soltani, Shoresh,Cho, Soohyun,Ryu, Hanyoung,Han, Garam,Kim, Beomyoung,Song, Dongjoon,Kim, Timur K.,Hoesch, Moritz,Kim, Changyoung American Physical Society 2017 Physical review. B Vol.95 No.12
<P>We performed angle-resolved photoemission spectroscopy (ARPES) experiments on the surface states of SrTiO3(001) using linearly and circularly polarized light to investigate the subband structures of out-of-plane d(xz/yz) orbitals and chiral orbital angular momentum (OAM). The data taken in the first Brillouin zone reveal new subbands for d(xz/yz) orbitals with Fermi wave vectors of 0.25 and 0.45 angstrom(-1) in addition to the previously reported ones. As a result, there are at least two subbands for all the Ti 3d t(2g) orbitals. Our circular dichroism ARPES data are suggestive of a chiral OAM structure in the surface states and may provide clues to the origin of the linear Rashba-like surface band splitting.</P>