arXiv subjects

S. Miyasaka

Publications and source records attributed to S. Miyasaka.

51 records · Page 3Linked to original sources

arXiv2010

Thermo-physical properties of 162173 (1999 JU3), a potential flyby and rendezvous target for interplanetary missions

Near-Earth asteroid 162173 (1999 JU3) is a potential flyby and rendezvous target for interplanetary missions because of its easy to reach orbit. The physical and thermal properties of the asteroid are relevant for establishing the scientific mission goals and also important in the context of near-Earth object studies in general. Our goal was to derive key physical parameters such as shape, spin-vector, size, geometric albedo, and surface properties of 162173 (1999 JU3). With three sets of published thermal observations (ground-based N-band, Akari IRC, Spitzer IRS), we applied a thermophysical model to derive the radiometric properties of the asteroid. The calculations were performed for the full range of possible shape and spin-vector solutions derived from the available sample of visual lightcurve observations. The near-Earth asteroid 162173 (1999 JU3) has an effective diameter of 0.87 +/- 0.03 km and a geometric albedo of 0.070 +/- 0.006. The chi2-test reveals a strong preference for a retrograde sense of rotation with a spin-axis orientation of lambda_ecl = 73 deg, beta_ecl = -62 deg and P_sid = 7.63 +/- 0.01 h. The most likely thermal inertia ranges between 200 and 600 Jm-2s-0.5K-1, about a factor of 2 lower than the value for 25143 Itokawa. This indicates that the surface lies somewhere between a thick-dust regolith and a rock/boulder/cm-sized, gravel-dominated surface like that of 25143 Itokawa. Our analysis represents the first time that shape and spin-vector information has been derived from a combined data set of visual lightcurves (reflected light) and mid-infrared photometry and spectroscopy (thermal emission).

astro-ph.EP↗

arXiv2009

Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx

We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carriers in this system. Temperature dependence of the upper critical field suggests that the system is a two gap superconductor. The F-doping dependence of these properties in this system are very weak, while in the FeAs system (LaFeAsO), the F doping induces the large changes in electronic properties. This difference is probably due to the different F-doping dependence of the lattice in these two systems. It has been revealed that a pure effect of electron doping on electronic properties is very weak in this Fe pnictide compound.

cond-mat.supr-con↗

arXiv2009

Doping dependence of Normal-state Property in the Iron-based Oxypnictide Superconductors LaFeAsO1-y probed by 57Fe-NMR and 75As-NMR/NQR

We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdoped sample (T_c=20 K), an optimally doped sample (T_c=28 K), and an overdoped sample (T_c=22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO_{1-y}$. A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, 1/T_1T in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe and As sites regardless of the electron-doping level in LaFeAsO_{1-y}. On the basis of this result, we claim that $1/T_1T$ is not always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic phase in the underdoped superconducting sample. This contrasts with the behavior in hole-doped Ba_{0.6}K_{0.4}Fe2As2(T_c= 38 K), which exhibits a significant increase in $1/T_1T$ upon cooling. We remark that the crucial difference between the normal-state properties of LaFeAsO_{1-y} and Ba_{0.6}K_{0.4}Fe2As2 originates from the fact that the relevant Fermi surface topologies are differently modified depending on whether electrons or holes are doped into the FeAs layers.

cond-mat.supr-con↗

arXiv2008

75As NQR/NMR Studies on Oxygen-deficient Iron-based Oxypnictide Superconductors LaFeAsO_{1-y} (y=0,0.25,0.4) and NdFeAsO_{0.6}

We report 75As-NQR/NMR studies on the oxygen-deficient iron(Fe)-based oxypnictide superconductors LaFeAsO_{0.6} (T_c=28 K) along with the results on LaFeAsO, LaFeAsO_{0.75}(T_c=20 K) and NdFeAsO_{0.6}(T_c=53 K). Nuclear spin-lattice relaxation rate 1/T_1 of 75As NQR at zero field on LaFeAsO_{0.6} has revealed a T^3 dependence below T_c upon cooling without the coherence peak just below T_c, evidencing the unconventional superconducting state with the line-node gap. We have found an intimate relationship between the nuclear quadrupole frequencyof 75As and T_c for four samples used in this study. It implies microscopically that the local configuration of Fe and As atoms is significantly related to the T_c of the Fe-oxypnictide superconductors, namely, the T_c can be enhanced up to 50 K when the local configuration of Fe and As atoms is optimal, in which the band structure may be also optimized through the variation of hybridization between As 4p orbitals and Fe 3d orbitals.

cond-mat.supr-con↗

arXiv2008

Dopant-dependent impact of Mn-site doping on the 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 with varying the doing species as well as the $R$-dependent one-electron bandwidth. In ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating state with short-range charge-orbital correlation. For each $R$ species, the phase diagram as a function of Fe concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping in the phase-competing region weakens the ferromagnetic double-exchange interaction, relatively to the charge-orbital ordering instability. We have also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is observed at low temperatures as a consequence of the collapse of the inherent short-range charge-orbital ordering, while Fe doping plays only a minor role. The observed opposite nature of impurity doping may be attributed to the difference in magnitude of the antiferromagnetic interaction between the doped ions.

cond-mat.str-el↗

arXiv2008

MuSR studies of RE(O,F)FeAs (RE = La, Nd, Ce) and LaOFeP systems: possible incommensurate/stripe magnetism and superfluid density

Muon spin relaxation (MuSR) measurements in iron oxy-pnictide systems have revealed: (1) commensurate long-range order in undoped LaOFeAs; (2) Bessel function line shape in La(O0.97F0.03)FeAs which indicates possible incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in superconducting LaOFeP, Ce(O0.84F0.16)FeAs, and Nd(O0.88F0.12)FeAs but absent in superconducting La(O0.92F0.08)FeAs; and (4) scaling of superfluid density and Tc in the Ce, La, and Nd-FeAs superconductors following a nearly linear relationship found in cuprates.

cond-mat.supr-con↗

arXiv2008

Doping variation of anisotropic charge and orbital dynamics in Y1-xCaxVO3 : Comparison with La1-xSrxVO3

The doping variation of charge and orbital dynamics in perovskite-type Y1-xCaxVO3 (0<x<0.1) is investigated by measurements of the optical conductivity and Raman scattering spectra in comparison with the larger-bandwidth system La1-xSrxVO3. We also take into consideration the magnitude of the GdFeO3-type orthorhombic lattice distortion, which is large and small in Y1-xCaxVO3 and La1-xSrxVO3, respectively, and discuss its effect on the evolution of charge dynamics. The optical conductivity spectra show that the doped hole is well localized and forms the small polaron like state. The hole dynamics in Y1-xCaxVO3 is nearly isotropic up to the doping level of the orbital order-disorder transition, while that in La1-xSrxVO3 is anisotropic in the lightly doped region due to the one-dimensional orbital exchange interaction. The possible origin of the difference in the hole dynamics is discussed in terms of the local lattice distortion, which is induced by the formation of the small polaron like state and becomes more significant for the reduced one-electron bandwidth. In addition, the optical Mott-gap excitation in the nominally C-type spin and $G$-type orbital ordered phase is distinct from that for La1-xSrxVO3 in its intensity and spectral shape. This suggests that the orthorhombic lattice distortion enhances the modification of the spin and orbital ordering from the pure C-type and G-type, respectively. The systematic study of Raman scattering spectra has shown that the dynamic G-type spin and C-type orbital correlation subsists at low temperatures in the doping induced phase of the nominally C-type SO and G-type OO.

cond-mat.str-el↗

arXiv2007

Optical phase diagram of perovskite-type colossal magnetoresistance manganites with near-half doping

We present a systematic optical study for a bandwidth-controlled series of nearly half doped colossal magnetoresistive manganites RE$_{0.55}$AE$_{0.45}$MnO$_3$ (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region $T=10-800$ K. The ground state of the compounds ranges from the charge and orbital ordered insulator through the spin glass to the ferromagnetic metal. The enhanced phase fluctuations, namely the short-range charge and orbital correlations dominate the paramagnetic region of the phase diagram above all the ground-state phases. This paramagnetic region is characterized by a full-gap to pseudo-gap crossover towards elevated temperatures where a broad low-energy electronic structure appears in the conductivity spectra over a large variation of the bandwidth. This pseudo-gap state with local correlations is robust against thermal fluctuations at least up to T=800 K. For small bandwidth the onset of the long-range charge order is accompanied by an instantaneous increase of the gap. The emergence of the ferromagnetic state is manifested in the optical spectra as a first-order insulator to metal transition for compounds with moderate bandwidth while it becomes a second-order transition on the larger bandwidth side. Unusually large scattering rate of the metallic carriers is observed in the ferromagnetic state which is attributed to orbital correlation with probably rod-like ($3z^2-r^2$-like) character.

cond-mat.str-el↗

arXiv2007

Universal observation of multiple order parameters in cuprate superconductors

The temperature dependence of the London penetration depth λwas measured for an untwined single crystal of YBa_2Cu_3O_{7-δ} along the three principal crystallographic directions (a, b, and c). Both in-plane components (λ_a and λ_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (λ_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of s+d-wave symmetry whereas it is exclusively s-wave along the c-direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.

cond-mat.supr-con↗

arXiv2007

Quantum criticality and disorder in the antiferromagnetic critical point of NiS$_{2}$ pyrite

A quantum critical point (QCP) between the antiferromagnetic and the paramagnetic phases was realized by applying a hydrostatic pressure of ~ 7 GPa on single crystals of NiS_{2} pyrite with a low residual resistivity, rho_{0}, of 0.5 mu-Omega-cm. We found that the critical behavior of the resistivity, rho, in this clean system contrasts sharply with those observed in its disordered analogue, NiS_{2-x}Se_{x} solid-solution, demonstrating the unexpectedly drastic effect of disorder on the quantum criticality. Over a whole paramagnetic region investigated up to P = 9 GPa, a crossover temperature, defined as the onset of T^{2} dependence of rho, an indication of Fermi liquid, was suppressed to a substantially low temperature T sim 2 K and, instead, a non Fermi liquid behavior of rho, T^{3/2}-dependence, robustly showed up.

cond-mat.str-el↗

arXiv2005

Change in lattice modulation upon gigantic magnetoelectric transition in GdMnO3 and TbMnO3

Single-crystal synchrotron x-ray diffraction measurements in strong magnetic fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It has been found that the P || a ferroelectric phase induced by the application of a magnetic field at low temperatures is characterized by commensurate lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The lattice modulation is ascribed to antiferromagnetic spin alignment with a modulation vector of (0 1/4 1). The change of the spin structure is directly correlated with the magnetic-field-induced electric phase transition, because any commensurate spin modulation with (0 1/4 1) should break glide planes normal to the a axis of the distorted perovskite with the Pbnm space group.

cond-mat.str-el↗

arXiv2005

Orbital Disordering and metal-insulator transition with hole-doping in perovskite-type vanadium oxides

Filling-control metal-insulator transitions (MITs) and related electronic phase diagrams have been investigated for hole-doped vanadium oxides, Pr_{1-x}Ca_xVO_3, Nd_{1-x}Sr_xVO_3 and Y_{1-x}Ca_xVO_3, with perovskite structure. The increase of the doping level x causes the melting of the G-type (and C-type) orbital order, prior to or concomitantly with the MIT, due partly to the doped-hole motion and partly to the ramdom potential arising from the quenched disorder. In particular, the G-type spin- and C-type orbital-ordered phase present in Y_{1-x}Ca_xVO_3 disappears immediately upon hole doping, around x=0.02. On the other hand, the critical doping level x for MIT is governed by the electron-correlation strength of the undoped parent compound.

cond-mat.str-el↗

arXiv2004

Ferroelectric properties controlled by magnetic fields in DyMn2O5

Cross-correlation between magnetic and dielectric properties has been attracting renewed interest because of the fundamental as well as technological importance of controlling the electric (magnetic) polarization by an external magnetic (electric) field. Here, we report the novel phenomenon that an external magnetic field induces and/or modifies ferroelectric states in a magnetic material, DyMn2O5. Measurements of temperature dependence of hysteretic polarization curves, pyroelectric current, specific heat, optical second harmonic generation, and x-ray superlattice peaks have revealed successive phase transitions between 43 K and 4 K, accompanying three ferroelectric phases. The zero-field lowest-temperature phase (<8 K) induced by the Dy-moment ordering is a reentrant paraelectric state, but is turned to a ferroelectric state with increasing the magnetic field. The phenomenon is closely related to the metamagnetic transitions of the Dy f-moment, indicating that all the ferroelectric phases of this material are strongly tied to the antiferromagnetic Mn spin structure affected by the f-d exchange interaction. The electric phase diagram for DyMn2O5 is presented in the plane of temperature and magnetic field.

cond-mat.str-el↗

arXiv2003

Early ($<$0.3 day) R-band light curve of the optical afterglow of GRB030329

We observed the optical afterglow of the bright gamma-ray burst GRB030329 on the nights of 2003 March 29, using the Kiso observatory (the University of Tokyo) 1.05 m Schmidt telescope. Data were taken from March 29 13:21:26 UT to 17:43:16 (0.072 to 0.253 days after the burst), using an $Rc$-band filter. The obtained $Rc$-band light curve has been fitted successfully by a single power law function with decay index of $0.891\pm0.004$. These results remain unchanged when incorporating two early photometric data points at 0.065 and 0.073 days, reported by Price et al.(2003) using the SSO 40 inch telescope, and further including RTT150 data (Burenin et al. 2003) covering at about 0.3 days. Over the period of 0.065-0.285 days after the burst, any deviation from the power-law decay is smaller than $\pm$0.007 mag. The temporal structure reported by Uemura et al. (2003) does not show up in our $R$-band light curve.

astro-ph↗

arXiv2003

Magnetic neutron scattering study of YVO3: Evidence for an orbital Peierls state

Neutron spectroscopy has revealed a highly unusual magnetic structure and dynamics in YVO$_3$, an insulating pseudocubic perovskite that undergoes a series of temperature induced phase transitions between states with different spin and orbital ordering patterns. A good description of the neutron data is obtained by a theoretical analysis of the spin and orbital correlations of a realistic one-dimensional model. This leads to the tentative identification of one of the phases of YVO$_3$ with the ``orbital Peierls state'', a theoretically proposed many-body state comprised of orbital singlet bonds.

cond-mat.str-el↗