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Recently, spin-triplet superconductivity, in which individual electron spins are aligned in parallel in the electron pairs that form the superconductivity, has attracted much attention. Since most superconductors are spin-singlet superconductors with antiparallel electron spins, the elucidation of the mechanism of electron pair formation in spin-triplet superconductors is expected to provide essential guidelines for designing room-temperature superconductors. The ruthenium oxide superconductor Sr2RuO4 has been extensively studied as
a strong candidate for the spin-triplet superconductors. In particular, angle-resolved photoemission spectroscopy (ARPES) experiments have been widely believed to observe the electron-boson (phonon, magnetic fluctuation, etc.) coupling as a sudden change of the band dispersion, which might be involved in pair formation. Indeed, many previous ARPES results reported the existence of electron-boson interactions fingerprinted by a “kink” structure in the γ-band derived from Ru 4dxy orbital. On the other hand, the latest high-resolution ARPES study, which measured the electronic states near the Fermi energy (EF), reported that the electrons and bosons are not strongly coupled [1]. Such a discrepancy could be attributed to the fact that the existing ARPES measurements only focused on the near-EF region and could not accurately evaluate electron correlations with an energy scale 10 to 100 times larger than that of the boson mode (≲100 meV). This suggests that we need to observe not only the band structures in the vicinity of the EF but also the entire band structure for the accurate identification of coupling components by ARPES. In this regard, to separate and evaluate multiple types of the many-body effects, we have performed both soft X-ray (SX-) and vacuum ultra-violet (VUV-) ARPES by observing the entire band structure and detailed band structure in the vicinity of the EF, respectively. SX- and VUV-ARPES experiments were performed at SPring-8 BL25SU and SSRL BL5-2, respectively, and preliminary VUV-ARPES experiments were performed at HiSOR BL-1.
Figures 1(a) and 1(b) show the Fermi surface and ARPES images using ℎ???????? = 450 eV (SX) and 65 eV (VUV), respectively. In both SX-ARPES and VUV-ARPES results, the observed Fermi surfaces clearly show the three band-structures (α, β, and γ), originating in the bulk Ru 4d t2g orbitals. First, we focus on the SX-ARPES image in the X-M-X direction, where the α band was observed entirely and showed a parabolic dispersion shape. This enables us to evaluate the electron correlation effects accurately because the α band shows negligible dependence on the perpendicular momentum (kz) as well as the spin-orbit coupling in this direction. Next, we focus on the γ band in the Γ/Z-M direction because it shows the largest effective mass, namely, the electrons most strongly affected by the many-body interaction there. Then, we have determined the γ band dispersion by fitting the momentum distribution curves (MDCs) as shown in the blue-filled circles in Fig. 1(c), where a phenomenological model dispersion is also shown. The model dispersion was calculated by including the electron correlation effects with the estimated strength from the SX-ARPES results. One
can see that the experimental dispersion is not reproduced by the model calculations, including the electron correlation effects. This indicates that the electron-boson coupling should be considered to explain the mass renormalization effects in the γ band. The electron-boson coupling could be more visualized in the real part of the self-energy shown in Fig. 1(d), induced by taking the energy difference between the experimental and model dispersion. Interestingly, we found a clear shoulder structure at ~ -8 meV, corresponding to the energy of the kink structure in the band dispersion as indicated by the orange line in Fig. 1(c). The kink energy is consistent with the Σ3 phonon mode, the in-plane rotation of the RuO6 octahedron, as observed by a neutron scattering experiment [2]. In addition, a strong coupling between the Σ3 phonon mode and ferromagnetic fluctuations has been suggested by a scanning tunneling microscopy and theoretical study [3]. Therefore, we believe that our results reveal the strong electronic coupling to the ferromagnetic fluctuations mediated by the Σ3 phonon mode. This may suggest the formation of the spin-triplet electron pair mediated by a magnon-phonon coupling.
会議概要（会議名, 開催地, 会期, 主催者等）
The 25th Hiroshima International Symposium on Synchrotron Radiation
Low-energy electron-boson coupling in Sr2RuO4
