Kazumasa Horigane

Spin and Charge Orders and Their Hole-Doping Dependence in Single Layered Cobaltate La2-xCaxCoO4 (0.3≦x≦0.8)

Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La 2- x Ca x CoO 4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3≦ x ...

Crystal Structure and Charge-Ordering in La1.5Ca0.5CoO4 Studied by Neutron and Resonant X-ray Scattering

Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La 1.5 Ca 0.5 CoO 4 . Taking A 2 m m with a – b type twin structure into account, Co 2+ /Co 3+ checkerboard charge ordering pattern was suggested in La 1.5 Ca 0.5 CoO 4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K -edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co 2+ /Co 3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E =7.710 and 7.713 keV, corresponding to 1 s → t 2 g and 1 s → e g transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e g orbital is realized in La 1.5 Ca 0.5 CoO 4 .

Antiferromagnetic order of the Co2+ high-spin state with a large orbital angular momentum in La1.5Ca0.5CoO4

The antiferromagnetic insulator La1.5Ca0.5CoO4 has been investigated by Co L2,3-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements and Co L2,3-edge resonant soft X-ray magnetic scattering (RXMS) measurement to determine the Co electronic structures associated with magnetic ordering. Co L2,3-edge linear-dichroic XAS shows that Co2+ takes a high-spin (HS) state and Co3+ takes a low-spin (LS) state. Using Co L2,3-edge RXMS, we directly determined that an antiferromagnetic order is formed with a HS state of Co2+ ions. Moreover, the spin and orbital angular momenta of the Co2+ HS state are quantitatively estimated to be 1.1 ± 0.1 and 1.0 ± 0.1, respectively, and to align parallel in the ab plane by utilizing the cluster model calculation. The large orbital angular momentum of the Co2+ HS state originates from the small D4h-symmetry crystal field splitting of t2g levels, which is comparable with the spin–orbit coupling constant of the Co 3d orbital.

Suppression of spin-exciton state in hole overdoped iron-based superconductors

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δs), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1−xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δs in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2Δs to higher energies, peaking at values of 3Δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.

Spin excitations in hole-overdoped iron-based superconductors

Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0; Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors.

Single-Crystal Growth of Ba1−xKxFe2As2 by KAs Self-Flux Method

Single crystals of Ba1−xKxFe2As2 with 0.15 ≤ x ≤ 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38 K at x = 0.4 to 3.4 K at x = 1.0, in good agreement with the results for polycrystalline samples.

Evolution of the remnant Fermi-surface state in the lightly doped correlated spin-orbit insulator Sr2-xLaxIrO4

Electronic structure has been studied in lightly electron doped correlated spin-orbit insulator Sr

Metal?semiconductor transition in novel layered oxychalcogenides

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Resonant soft X-ray scattering study of the magnetic structures in La1.5Ca0.5CoO4 using a high vacuum diffractometer with a 4-blade-slit detector system

IrO

Single crystal growth and magnetic properties of La2-xMxCoO4 (M=Ca, Ba)

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