0428同步年報-2021-全

and Cheng-Maw Cheng and their collaborators published in ACS Nano 15 , 15085 (2021). TLS 21B1 Angle-resolved UPS • ARPES • Materials Science, Condensed-matter Physics Reference 1. S. H. Su, P.-Y. Chuang, H.-Y. Chen, S.-C. Weng, W.-C. Chen, K.-D. Tsuei, C.-K. Lee, S.-H. Yu, M. M.-C. Chou, L.-W. Tu, H.-T. Jeng, C.-M. Tu, C.-W. Luo, C.-M. Cheng, T.-R. Chang, J.-C. A. Huang, ACS Nano 15 , 15085 (2021). Physics and Materials Science 015 A Hallmark of Hund’s Physics in the Multi-Orbital System An abrupt deviation from Fermi liquid behavior has been directly observed in electron self-energy results as the kink feature at a low-energy scale. The results of the evolution of the characteristic temperature scale via a kink features, which is a hallmark of Hund’s physics in a multi-orbital system. S ince Mott’s initial proposal that an insulating ground state can appear due to the electron–electron correlation, the metal-insulator transition (MIT) has been at the core of condensed-matter physics. The Coulomb interaction U is the most important parameter; thus finding how the spectral function and energy scale evolve as a function of U has been a fundamental issue in MIT studies. The Brinkman-Rice picture and dynamical mean-field theory (DMFT) for the half-filled one-band Hubbard model show that the overall quasi-particle (QP) peak and Kondo temperature T K gradually become renormalized as U increases. At the MIT, the QP mass diverges with vanishing T K . Most realistic materials are, however, multi-orbital systems in which not only U but also Hund’s coupling J H is a critical parameter for the ground state. During the past decade, there has been a remarkable progress in the theoretical description of Hund’s physics in correlated electron systems. It was found that J H can enhance the effective correlation strength of multi-orbital systems by weakening the Kondo screening channel. The most drastic effect occurs in non-singly-occupied and non-half-filled cases such as iron pnictides, chalcogenides and ruthenates. Although these materials are metallic and are located far from the Mott insulating state, their small coherence energy scale due to J H induces incoherent transport properties. These new phases are classified as Hund’s metal; their correlated electronic structures have been intensively studied through both experimental and theoretical approaches. An important remaining question is how J H affects the evolution of the spectral function and the energy scale of multiband systems. Considering these aspects, NiS 2−x Se x , a half-filled system with degenerate Ni e g orbitals, is probably the most suitable multi-orbital system for an investigation of the evolution in the presence of J H . On varying the Se content, the correlation strength can be easily tuned in the existence of J H . To address the role of J H during the MIT, Changyoung Kim (Seoul National University, Korea), Cheng-Maw Cheng (NSRRC) and their teams reexamined the band structure of NiS 2−x Se x not only with angle-resolved photoemission spectra (ARPES) with finer doping steps and higher resolution but also via density-functional theory (DFT) plus DMFT with and without J H . They utilized ARPES to achieve the high resolution needed to observe clearly the QP of NiS 2−x Se x . Their results reveal clear QP dispersions as well as doping-dependent low-energy kink structures. The DFT+DMFT calculations also identify the kink structures, which explain the strongly suppressed temperature scale due to J H . The evolution of a kink observed in their ARPES data provides direct spectroscopic evidence for the evolution of the energy scale in the presence of J H . The α hole pocket is the most representative QP band for the Mott transition in NiS 2−x Se x . As the Fermi-surface volume of the α band is much larger than the others, the transport properties of NiS 2−x Se x should be dominated by the hole pocket. To study how the α band dispersion varies across the MIT, they performed ARPES experiments at TLS 21B1 beamline of the NSRRC. ARPES spectra along the Γ-X line were recorded at 16 K for diverse Se doping shown in Fig. 1 . A QP band, distinct from the incoherent band ( Fig. 1(g) ), is clearly observed in all metallic samples, whereas the QP was not clearly discernible as it was buried under an incoherent spectral weight in previous reports (grey filled curve in Fig. 1(g) ). The appearance or disappearance of the QP follows the MIT behavior along the Se doping; the QP is seen for the metallic phase ( x ≥ 0.43) whereas it is absent in the insulating phase ( x = 0.3) (see Fig. 1(a) for the phase diagram).