Miyasaka Shigeki

Magnetic, Optical, and Magnetooptical Properties of Spinel-Type ACr2X4 (A=Mn, Fe, Co, Cu, Zn, Cd; X=O, S, Se)

A comprehensive study of magnetic, optical, and magnetooptical properties was carried out for single crystals of the spinel-type A Cr 2 X 4 ( A =Mn, Fe, Co, Cu, Zn, and Cd; X =O, S, and Se). The optical reflectivity measurements for 0.1–30 eV revealed a wide variation in electronic structures on a large energy scale between oxides ( X =O) and chalcogenides ( X =S and Se). For A =Fe and Co, we observed the intra-atomic d – d transitions of A 2+ ions with a tetrahedral coordination, and successfully deduced the crystal field splitting Δ E , the Racah parameter B , and the spin–orbit coupling constant ζ by analysis based on the ligand field theory. A comparison of these optical parameters between oxides and chalcogenides indicated the strong covalency effect in the chalcogenides. In A =Cu, the insulator–metal transition between X =O and Se was clearly demonstrated by optical conductivity spectra. Magnetic properties were discussed in relation to electronic structures. A compound with a small optical gap is t...

Dopant-Dependent Impact of Mn-Site Doping on Critical-State Manganites R0.6Sr0.4MnO3 (R = La, Nd, Sm, and Gd)

Versatile features of impurity doping effects on perovskite manganites, R 0.6 Sr 0.4 MnO 3 , have been investigated by varying the doping species as well as the R -dependent one-electron bandwidth in the single-crystalline specimens. In ferromagnetic-metallic (FM) manganites ( R =La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating (SGI) phase for each R species. There, the correlation of charge-orbital ordering in competition with metallic ferromagnetism is effectively induced, which has been observed as an evolution of diffuse electron scattering even in the R =La system with the largest bandwidth. Such a marked impact of Fe doping, widely observed in a variety of FM manganites, virtually results in the reduction in the bandwidth for the system in the critical region near the SGI phase. We have also found a contrastive impact of Cr (or Ru) doping on a SGI manganite ( R =Gd). For these dopants, the impurity-i...

Correlation between Tc and Transport Properties in PrFeP1-xAsxO0.9F0.1

We investigated the isovalent P/As substitution effect in iron pnictide superconductor, PrFeP 1- x As x O 0.9 F 0.1 to find the relationship among superconducting transition temperature T c , crystal structure and electronic properties. The lattice constants and the pnictogen height from iron layer show linear x dependence, indicating the continuous change of chemical pressure without the variation of band filling. In contrast to the monotonic change of crystal structure, T c , the temperature dependence of resistivity and Hall coefficient R H show anomalous x dependence. In the region of the As concentration of 0≤ x ≤0.6, the T c continuously increases, and the power of temperature of resistivity ( n ) is gradually changed from 2 to 1 with increasing x . The T c and the power n show distinct correlation, indicating that the antiferromagnetic fluctuation enhances T c . On the other hand, T c and the power n have no correlation in the region of 0.6 < x ≤1.0. These different behaviors in the regions of x < ...

Magnetic Phase Diagrams of YVO3 and TbVO3 under High Pressure

Magnetic states in R VO 3 ( R = Y, Tb) were investigated using low-temperature and high-pressure neutron-diffraction techniques, and the pressure–temperature magnetic phase diagrams were clarified up to 6.2 GPa. We elucidated that, on application of pressure, the magnetic ground state changes from C -type spin ordering ( C -SO) to G -type spin ordering ( G -SO); this corresponds to the orbital ground-state switching from G -type orbital ordering ( G -OO) to C -type orbital ordering ( C -OO). It is also interesting that the G -SO transition occurs simultaneously with the C -OO transition, but not with the G -OO one. When the transition temperature of C -OO exceeds that of G -OO under high pressure, the G -OO phase vanishes completely, and a simultaneous spin and orbital order–disorder phase transition occurs. Such a transition is the first case in perovskite-type transition-metal oxides.

Single Crystal Growth and Physical Properties of SrFe2(As1-xPx)2

We report a crystal growth and physical properties of SrFe 2 (As 1- x P x ) 2 . The single crystals for various x s were grown by a self flux method. For x = 0.35, T c reaches the maximum value of 30 K and the electrical resistivity ρ( T ) shows T -linear dependence. As x increases, T c decreases and ρ( T ) changes to T 2 -behavior, indicating a standard Fermi liquid. These results suggest that a magnetic quantum critical point exists around x =0.35.

Reflective Terahertz Time-Domain Spectroscopy Measurement on the Stripe-Ordered Superconductor La1.84-yNdySr0.16CuO4

We measured reflectivity on static stripe-ordered La 1.84−yNdySr0.16CuO4 (LNSCO)y=0.1, 0.2, 0.3, and 0.4 by means of reflective terahertz time-domain spectro scopy (THz-TDS) with electric field polarized alongc-axis, in which one can obtain lower frequency information t han the conventional Fourier transform type spectrometer. Recently, two-dimen sional superconducting (2DSC) property [1–3], intra-layer perfect conductivity not accompanied w ith inter-layer superconducting behavior, was reported on LNSCO ( x=0.15 ,y≥ 0.2). We observed the existence of Josephson plasma edge, which suggests that 2DSC is not realized in these compounds.We measured reflectivity on static stripe-ordered La 1.84- y Nd y Sr 0.16 CuO 4 (LNSCO) y =0.1, 0.2, 0.3, and 0.4 by means of reflective terahertz time-domain spectroscopy (THz-TDS) with electric field polarized along c -axis, in which one can obtain lower frequency information than the conventional Fourier transform type spectrometer. Recently, two-dimensional superconducting (2DSC) property [tajima,schafgans,li], intra-layer perfect conductivity not accompanied with inter-layer superconducting behavior, was reported on LNSCO ( x =0.15, y ≥0.2). We observed the existence of Josephson plasma edge, which suggests that 2DSC is not realized in these compounds.

Soft- and Hard-X-ray Photoemission Spectroscopy of La2-2xSr1+2xMn2O7

We have performed the photoemission spectroscopy of the bilayered manganese oxides La 2-2 x Sr 1+2 x Mn 2 O 7 ( x = 0.38, 0.40) with the photon energies ( h ν) of 640–700 and 8150 eV. The difference between the surface and the bulk electronic structures is revealed from the polar-angle dependence of the Sr 3 d core level and the valence band spectra with h ν=700 eV. Moreover, we have reconfirmed that the spectral weight at the Fermi level is suppressed even in the bulk electronic states by means of the valence band spectra obtained by the Mn 2 p –3 d resonant photoemission with h ν∼643 eV and by the hard-X-ray photoemission spectroscopy with 8175 eV photons.

Cr- and Mo-Doping Effects on Structural and Orbital Order Phase Transition in Spinel-Type MnV2O4

Spinel-type vanadium oxides have been studied from the viewpoints of spin and orbital order in a geometrically frustrated system. We have investigated the doping effects of Cr 3+ (3 d 3 ) and Mo 3+ (4 d 3 ), which have no orbital degrees of freedom, to the V 3+ (3 d 2 ) site of MnV 2 O 4 , in order to clarify the structural, transport and optical properties of Cr- and Mo-doped samples. The orbital order is suppressed by Cr and Mo doping and the electronic structure depends on the doped elements. The results of transport and optics indicate that electrons are localized in the Cr-doped system, while they become nearly itinerant in the Mo-doped one.

Observation of a Pseudogap in the Vicinity of the Metal–Insulator Transition in the Perovskite-type Vanadium Oxides Nd1−xSrxVO3

We have performed a photoemission study of the Mott–Hubbard system Nd1−xSrxVO3 (x = 0.20 and 0.30) to investigate the electronic structure in the vicinity of the metal–insulator transition. By using bulk sensitive hard X-ray photoemission spectroscopy, we have observed a large coherent spectral weight near the Fermi level compared to those observed with surface-sensitive low photons. In particular, a pseudogap with an energy of ∼0.2 eV has been observed near the Fermi level, which is consistent with a prediction with a dynamical cluster approximation calculation. In order to understand the characteristic features in the Mott–Hubbard-type metal–insulator transition, particularly the pseudogap opening at x = 0.2 and 0.3, a phenomenological model of the self-energy has been proposed.

Optical Study of Electron-Doped Cuprate Pr1.3−xLa0.7CexCuO4+δ in Under-Doped Regime: Revisit the Phase Diagram

A recent progress of reduction process for electron-doped cuprates enabled us to get superconducting samples at very low doping levels. In order to clarify the electronic state of strongly reduced Pr1.3−xLa0.7CexCuO4+δ (x = 0.05, 0.10) which exhibit high Tc (∼27 K) superconductivity, we have measured their optical spectra. The reflectivity of these samples was found much higher than the published data for the moderately reduced and non-superconducting samples with the same Ce concentrations. Moreover, the estimated effective electron numbers Neff for x = 0.05 and 0.10 were close to that of the optimally doped and superconducting sample with x = 0.15. Given that the parent compound is a Mott insulator, these results indicate that in the electron-doped cuprates only a small amount of carrier doping changes the system to a high Tc superconductor with a large Fermi surface. At low temperatures, a broad mid-infrared peak appeared even in the superconducting samples.