Takayoshi Yokoya

Half-metals (HMs), characterized by 100% spin polarization at the Fermi level ( E F ), are ideal candidates for future spintronic memory devices. However, their spin polarization rapidly decreases with temperature, limiting their practical implementation. While this degradation has been theoretically attributed to nonquasiparticle (NQP) states—many-body electronic states arising from electron-magnon interactions that reduce spin polarization near E F —experimental verification remains limited. Here, we report the temperature evolution of the electronic states in epitaxial Ni Co 2 O 4 (NCO) thin films, a predicted type I B HM where the main spectral weight of NQP states lies below E F . Using high-resolution bulk-sensitive laser-based spin-resolved photoemission spectroscopy, we reveal that at 20 K, a metallic Fermi edge is observed in the minority-spin states but is absent in the majority-spin states, providing spectroscopic evidence for the half-metallicity of NCO. As temperature increases, majority-spin spectral weight emerges at E F , and spin polarization near E F decreases—consistent with the emergence of NQP states. The onset of this behavior occurs around 100 K. Our results suggest that NCO provides a promising platform for investigating NQP-related many-body effects in type I B HMs, guiding the design of spintronic materials with robust high-temperature spin polarization.

Pb-substituted La(O, F)BiS₂ (Pb-LaOFBiS₂) exhibits improved superconducting properties and a resistivity anomaly around 100 K that is attributed to a structural transition. We have performed temperature(T)-dependent photoelectron holography (PEH) to study dopant incorporation sites and the local structure change across the anomaly. The PEH study of Pb-LaOFBiS₂ provided evidence for the dominant incorporation sites of Pb and F: Pb atoms are incorporated into the Bi sites and F atoms are incorporated into the O site. No remarkable difference in the local structures around Pb and Bi atoms was observed. Across the temperature of the resistivity anomaly (T ^*), photoelectron holograms of Bi 4f changed. Comparisons of holograms with those of non-substituted LaOFBiS₂ sample, as well as simulated holograms, suggested that (1), above T ^*, the tetragonal structure of Pb-LaOFBiS₂ is different from the tetragonal structure of LaOFBiS₂ and (2), below T ^*, the tetragonal structure still remains in Pb-LaOFBiS₂. We discuss a possible origin of the difference in the structure above T ^* and the implication of the result below T ^*, which are necessary ingredients to understand the physical properties of Pb-LaOFBiS₂.

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (lambda = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-x Ti x O2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

The structural and superconducting properties of the Bi-based superconductor Bi₂Rh₃(Se_1-xS_x)₂ were investigated over a wide pressure range. Bi₂Rh₃Se₂ is a superconductor with a superconducting transition temperature, T_c, of less than ∼1.0 K, whereas Bi₂Rh₃S₂ is a nonsuperconductor. The former shows a clear charge density wave (CDW) phase transition at 240 K and the latter shows a first-order structural phase transition at 165 K, providing the supercell structure. Both compounds have the same crystal structure at ambient temperature and pressure (monoclinic, space group of C2/m (No. 12)). This implies the possible formation of solid solution-like phases Bi₂Rh₃(Se_1-xS_x)₂. In this study, we prepared Bi₂Rh₃(Se_1-xS_x)₂ (x = 0-1.0) and investigated its superconducting properties and phase transitions at different x values. Because the superconducting properties and temperature causing the phase transition (T_pt) are quite different between Bi₂Rh₃Se₂ and Bi₂Rh₃S₂, their variation against x is quite intriguing from the viewpoint of the pursuit of physical properties via the replacement of Se with S. In this study, the features of the superconductivity and crystal structure against x were fully explored to depict the phase diagram. Moreover, we investigated the pressure (p) dependence of superconductivity and phase transition (CDW or the first-order structural transition) to clarify the interplay between the two ordered states.

La(O,F)BiS_2-x Se _x is a layered material that is considered to be a candidate exotic superconductor as well as a promising thermoelectrical material. We performed soft X-ray photoelectron holography to study the Se incorporation site and the local atomic arrangement of the conducting layer. A comparison of the experimental holograms with the simulated holograms indicates that Se atoms preferentially occupy the S sites in the conducting Bi–S plane of La(O,F)BiS₂. A comparison between the state-of-the-art holographic reconstructions of La(O,F)BiSSe and La(O,F)BiS₂ suggests that Se substitution suppresses the displacement of S atoms in La(O,F)BiS₂. These results provide photoelectron holographic evidence for the Se incorporation site and the Se-induced suppression of in-plane disorder.

Depending on thermal and pressure conditions, AgInS₂ exhibits various crystal structures. In this study, we synthesized a high-purity polycrystalline sample of trigonal AgInS₂, which is a layered compound, using a high-pressure synthesis technique. The crystal structure was investigated by synchrotron powder X-ray diffraction and the Rietveld refinement. On the basis of band calculation, X-ray photoelectron spectroscopy, and electrical resistance measurements, we found that the obtained trigonal AgInS₂ is a semiconductor. Temperature dependencies of electrical resistance of AgInS₂ were measured by a diamond anvil cell up to 31.2 GPa. Although semiconducting behavior was suppressed with pressure, metallic behavior was not observed within the pressure range investigated in this study.

Photoelectron holography (PEH) is an experimental technique that can be used to study the three-dimensional (3D) local structure around a focused atom. Recent advancements in reconstruction algorit...