In order to investigate the electronic properties of the semiconducting van der Waals ferromagnet Cr 2 Ge 2 Te 6 (CGT), where ferromagnetic layers are bonded through van der Waals forces, we have performed angle-resolved photoemission spectroscopy (ARPES) measurements and density-functional-theory (DFT+U) calculations. The valence-band maximum at the Γ point is located ~ 0.2 eV below the Fermi level, consistent with the semiconducting property of CGT. Comparison of the experimental density of states with the DFT calculation has suggested that Coulomb interaction between the Cr 3d electrons U eff ~ 1.1 eV. The DFT+U calculation indicates that magnetic coupling between Cr atoms within the layer is ferromagnetic if Coulomb U eff is smaller than 3.0 eV and that the inter-layer coupling is ferromagnetic below U eff ~ 1.0 eV. We therefore conclude that, for U eff deduced by the experiment, the intra-layer Cr-Cr coupling is ferromagnetic and the inter-layer coupling is near the boundary between ferromagnetic and antiferromagnetic, which means experimentally deduced U eff is consistent with theoretical ferromagnetic condition.

We have studied the anisotropy in the in-plane resistivity and the electronic structure of isovalent Ru-substituted BaFe 2 As 2 in the antiferromagnetic-orthorhombic phase using well-annealed crystals. The anisotropy in the residual resistivity component increases in proportional to the Ru dopant concentration, as in the case of Co-doped compounds. On the other hand, both the residual resistivity and the resistivity anisotropy induced by isovalent Ru substitution is found to be one order of magnitude smaller than those induced by heterovalent Co substitution. Combined with angle-resolved photoemission spectroscopy results, which show almost the same anisotropic band structure both for the parent and Ru-substituted compounds, we confirm the scenario that the anisotropy in the residual resistivity arises from anisotropic impurity scattering in the magnetostructurally ordered phase rather than directly from the anisotropic band structure of that phase.

The electronic structure of the new diluted magnetic semiconductor Ba 1−x K x (Zn 1−y Mn y) 2 As 2 (x = 0.30, y = 0.15) in single crystal form has been investigated by angle-resolved photoemission spectroscopy (ARPES). Measurements with soft x-rays clarify the host valence-band electronic structure primarily composed of the As 4p states. Two hole pockets around the Γ point, a hole corrugated cylinder surrounding the Γ and Z points, and an electron pocket around the Z point are observed, and explain the metallic transport of Ba 1−x K x (Zn 1−y Mn y) 2 As 2. This is contrasted with Ga 1−x Mn x As (GaMnAs), where it is located above the As 4p valence-band maximum (VBM) and no Fermi surfaces have been clearly identified. Resonance soft x-ray ARPES measurements reveal a nondispersive (Kondo resonance-like) Mn 3d impurity band near the Fermi level, as in the case of GaMnAs. However, the impurity band is located well below the VBM, unlike the impurity band in GaMnAs, which is located around and above the VBM. We conclude that, while the strong hybridization between the Mn 3d and the As 4p orbitals plays an important role in creating the impurity band and inducing high temperature ferromagnetism in both systems, the metallic transport may predominantly occur in the host valence band in Ba 1−x K x (Zn 1−y Mn y) 2 As 2 and in the impurity band in GaMnAs.

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In order to realize superconductivity in cuprates with the T^{'}-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T^{'}-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T^{'}-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd_{2-x}Ce_{x}CuO_{4} (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO_{2} plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd_{2}CuO_{4} film is doped with electrons despite the absence of the Ce substitution.