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Sandilands, Luke J.,Kyung, Wonshik,Kim, So Yeun,Son, J.,Kwon, J.,Kang, T. D.,Yoshida, Y.,Moon, S. J.,Kim, C.,Noh, Tae Won American Physical Society 2017 Physical Review Letters Vol.119 No.26
<P>The prototypical correlated metal Sr2RhO4 was studied using optical and photoemission spectroscopy. At low energies and temperatures, the optical data reveal a complex, multicomponent response that on the surface points to an unconventional metallic state in this material. Via a comparison with photoemission, the anomalous optical response may be attributed to an unexpectedly strong interband transition near 180 meV between spin-orbit coupled bands that are nearly parallel along Gamma X. This spin-orbit coupling effect is shown to occur in a number of related metallic ruthenates and explains the previously puzzling optical properties reported for these materials.</P>
Optical probe of Heisenberg-Kitaev magnetism inα−RuCl3
Sandilands, Luke J.,Sohn, C. H.,Park, H. J.,Kim, So Yeun,Kim, K. W.,Sears, Jennifer A.,Kim, Young-June,Noh, Tae Won American Physical Society 2016 Physical review. B Vol.94 No.19
<P>We report a temperature-dependent optical spectroscopic study of the Heisenberg-Kitaev magnet alpha-RuCl3. Our measurements reveal anomalies in the optical response near the magnetic ordering temperature. At higher temperatures, we observe a redistribution of spectral weight over a broad energy range that is associated with nearest-neighbor spin-spin correlations. This finding is consistent with highly frustrated magnetic interactions and in agreement with theoretical expectations for this class of material. The optical data also reveal significant electron-hole interaction effects, including a bound excitonic state. These results demonstrate a clear coupling between charge and spin degrees of freedom and provide insight into the properties of thermally disordered Heisenberg-Kitaev magnets.</P>
Park, H. J.,Sandilands, Luke J.,You, J. S.,Ji, Hyo Seok,Sohn, C. H.,Han, J. W.,Moon, S. J.,Kim, K. W.,Shim, J. H.,Kim, Jun Sung,Noh, T. W. American Physical Society 2016 Physical Review B Vol.93 No.20
<P>We report an optical spectroscopic study of SrMnSb2, a low-carrier-density metal. As temperature is decreased, our measurements reveal a large increase in the quasiparticle plasma frequency, which is highly unusual for a metal. This seemingly anomalous behavior can be accounted for using a 'three-band' model of the multiband electronic structure of SrMnSb2 that includes two conduction bands and one valence band. The second conduction band is assumed to be heavy and its minimum is taken to be close to, but not intersecting, the Fermi level. At finite temperature, quasiparticles are thermally redistributed between the two conduction bands, leading to an increase in the optical effective mass and a decrease in the plasma frequency. The temperature dependence of the low-lying interband optical transitions and the Hall number can also be understood using our model. The phenomenology of such a three-band scenario has not been widely considered to date in optical spectroscopic studies. Our results provide an explanation for the puzzling optical properties that have previously been reported in a number of topical low-carrier-density metals and semimetals and lay a foundation for future optical studies of these materials.</P>
Charge-Spin Correlation in van der Waals Antiferromagnet NiPS3
Kim, So Yeun,Kim, Tae Yun,Sandilands, Luke J.,Sinn, Soobin,Lee, Min-Cheol,Son, Jaeseok,Lee, Sungmin,Choi, Ki-Young,Kim, Wondong,Park, Byeong-Gyu,Jeon, C.,Kim, Hyeong-Do,Park, Cheol-Hwan,Park, Je-Geun American Physical Society 2018 Physical Review Letters Vol.120 No.13
Kim, So Yeun,Kim, Choong H.,Sandilands, L. J.,Sohn, C. H.,Matsuno, J.,Takagi, H.,Kim, K. W.,Lee, Y. S.,Moon, S. J.,Noh, T. W. American Physical Society 2016 Physical Review B Vol.94 No.24
<P>We investigated the electronic structure of [(SrIrO3)(m),(SrTiO3)](m = 1, 2, and infinity) superlattice (SL) thin films with optical spectroscopy and first principles calculations. Our optical results confirmed the existence of the J(eff) = 1/2 states in SL samples, similar to the bulk Ruddlesden-Popper series Sr Ir-n+ 1(n) O3n+1 iridates. Apart from this similarity, in the SL samples, we observed red shifts of the characteristic optical excitations in the J(eff) = 1/2 state and an enhancement of the low-energy spectral weight, which implies a reduction in the effective electron correlation for bands near the Fermi energy. The density functional theory plus Coulomb interactions (DFT + U) calculations suggested that the SrTiO3 layer intervened between SrIrO3 layers in the SLs activated additional hopping channels between the Ir ions, thus increasing the bandwidth and reducing the effective strength of the correlations. This paper demonstrates that fabrication of iridium-based heterostructures can be used to finely tune electronic structures via alteration of their local orbital environments.</P>