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M. Uehara

Publications and source records attributed to M. Uehara.

At least 19 recordsLinked to original sources
arXiv2016

The Atacama Cosmology Telescope: The polarization-sensitive ACTPol instrument

The Atacama Cosmology Telescope (ACT) is designed to make high angular resolution measurements of anisotropies in the Cosmic Microwave Background (CMB) at millimeter wavelengths. We describe ACTPol, an upgraded receiver for ACT, which uses feedhorn-coupled, polarization-sensitive detector arrays, a 3 degree field of view, 100 mK cryogenics with continuous cooling, and meta material anti-reflection coatings. ACTPol comprises three arrays with separate cryogenic optics: two arrays at a central frequency of 148 GHz and one array operating simultaneously at both 97 GHz and 148 GHz. The combined instrument sensitivity, angular resolution, and sky coverage are optimized for measuring angular power spectra, clusters via the thermal Sunyaev-Zel'dovich and kinetic Sunyaev-Zel'dovich signals, and CMB lensing due to large scale structure. The receiver was commissioned with its first 148 GHz array in 2013, observed with both 148 GHz arrays in 2014, and has recently completed its first full season of operations with the full suite of three arrays. This paper provides an overview of the design and initial performance of the receiver and related systems.

astro-ph.IM↗
arXiv2010

Intrinsic pinning property of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe0.5Te0.5, which is the superconductor with Tc of about 14 K, were studied by the analysis of magnetization curves by the extended critical state model. In the magnetization measurements by SQUID magnetometer, the external magnetic fields were applied parallel and perpendicular to c-axis of the sample. The critical current density Jc's under the perpendicular field of 1 T were estimated by using the Kimishima model as about 1.6 x 10^4, 8.8 x 10^3, 4.1 x 10^3, and 1.5 x 10^3 A/cm2 at 5, 7, 9, and 11 K, respectively, and the temperature dependence of Jc could be fitted with the exponential law of Jc(0)xexp(-αT /Tc) up to 9 K and power law of Jc(0)x(1-T / Tc)n near Tc.

cond-mat.supr-con↗
arXiv2004

A Unitarized Chiral Approach to f0(980) and a0(980) States and Nature of Light Scalar Resonances

We show that the f_0(980) and a_0(980) states appearing in scattering and production processes can be described quite well by the two-channel Oller-Oset-Peláez (OOP) version of unitarized chiral theories. It is impossible, however, to deduce a conclusion on the nature of them from the fitting to experimental data alone. Using explicit N_c dependence of parameters in OOP amplitudes we demonstrate that light scalar resonances are not of q\bar q states but dynamical rescattering effects generated under chiral symmetry, unitarity and channel couplings.

hep-ph↗
arXiv2004

Large Nc Behavior of Light Scalar Meson Nonet Revisited

We study whether light scalar meson nonet can survive in large values of Nc through observing how scattering amplitudes behave as Nc increases from 3 within a unitarized chiral approach. We obtain the result that vector mesons such as rho and K* survive as narrow width resonances, but all of the scalar meson nonet below 1 GeV fade out as Nc exceeds a rather small number about 6.

hep-ph↗
arXiv2003

Scalar Dipion States Produced in Heavy Quarkonium Decays and the Final State Interaction

We study phenomenologically the invariant mass spectra of scalar dipion states produced in cascade decays of excited $Υ$ and $ψ$, and $J/ψ$ decays into light vector meson. Dipion production amplitude is given as a product of the Born term and an effective scalar form factor written in terms of a unitarized chiral theory. The model reproduces the experimental mass spectra very well.

hep-ph↗
arXiv2002

Multi-scale Phase Modulations in Colossal Magnetoresistance Manganites

Extensive experimental results are presented on the multi-scale phase modulation phenomena observed in colossal magnetoresistance manganites. Two key types of phase inhomogeneities directly relevant to the colossal magnetoresistance (CMR) are discussed. The first type involves micrometer-scale coexistence of structurally and electronically different phases. We present extensive experimental data for a prototypical system exhibiting such a phase coexistence, (La,Pr)$_{5/8}$Ca$_{3/8}$MnO$_{3}$. These data reveal that percolative transport phenomena play the key role in the metal-insulator transition in this system, and are largely responsible for the significant magnitude of the magnetoresistance. The phase composition of the multiphase states is governed by both the electron correlations and the effects of martensitic accommodation strain. The second type of an inhomogeneous state is realized in the paramagnetic state commonly found in manganites at high-temperatures. In this state, nanometer-scale structural correlations associated with nanoscale charge/orbital ordered regions are observed. Experimental investigation of numerous manganite systems indicates that these correlations are generic in orthorhombic mixed-valent manganites, and that the correlated regions play an essential role in the CMR effect.

cond-mat.str-el↗
arXiv2002

The f_0(980) and a_0(980) Productions in Two-Photon Collisions and Radiative phi Meson Decays

We study how the scalar f_0(980) and a_0(980) states are produced in the two-photon collisions and radiative decays of the phi meson through unitarized Born amplitudes with the charged pion and kaon loops followed by S-wave meson-meson scattering amplitudes. We found in a previous paper that f_0(980) is generated as the bound state resonance, but a_0(980) as the cusp, and find herre that the nature of the generation of both states is consistent with the features of the production processes.

hep-ph↗
arXiv2002

Revisit to Low Mass Scalar Mesons via Unitarized Chiral Perturbation Theory

We study how the scalar mesons below 1 GeV are generated through the Oller-Oset-Pelaez version of the multichannel inverse amplitude method applied to the chiral perturbation theory. We find out that the f0(980) state is certainly generated as a bound state resonance below the K-Kbar threshold, while the a0(980) state, generated through the channel coupling between the pi-eta and K-Kbar channels, appears as a cusp at the K-Kbar threshold. The so-called sigma(500) and kappa(900) need not be interpreted as the conventional resonances.

hep-ph↗
arXiv2000

Hole distribution for (Sr,Ca,Y,La)_14 Cu_24 O_41 ladder compounds studied by x-ray absorption spectroscopy

The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr^2+ by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.

cond-mat.str-el↗
arXiv2000

High Temperature Charge-Ordering Fluctuation in Manganites

High temperature (T) studies of La_{1-x}Ca_{x}MnO_{3} show that charge and phonon transport is significantly suppressed in a narrow doping region of x~1/2 and in a wide T range up to 900 K, which accompanies anomalous broadening of x-ray Bragg peaks. Contrarily, ferromagnetic (FM) correlation above Curie T is sharply enhanced at x~3/8. All these observations indicate the presence of short-range charge ordering (CO) correlation at high T, possibly in the form of a FM "zigzag", a small segment of the CE-type CO state. The decoupling and coupling of the FM zigzags for x~3/8 and 1/2, respectively, is consistent with our results.

cond-mat.str-el↗
arXiv2000

Thermal/Electronic Transport Properties and Two-Phase Mixtures in La_{5/8-x}Pr_{x}Ca_{3/8}MnO_{3}

We measured thermal conductivity, k, thermoelectric power, S, and dc electric conductivity, sigma, of La_{5/8-x}Pr_{x}Ca_{3/8}MnO_{3}, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of k, S and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.

cond-mat.str-el↗
arXiv1999

Giant Peak of the 1/f Noise at the Metal-Insulator Transitions in Low-Tc CMR Manganites: Evidence of the Percolation Threshold at Tc

We observed a dramatic peak in the 1/f noise at the metal-insulator transition (MIT) in low-Tc manganites. This many-orders-of-magnitude noise enhancement is observed for both polycrystalline and single-crystal samples of La{5/8-y}Pr{y}Ca{3/8}MnO{3} (y=0.35-0.4) in zero magnetic field and Pr{1-x}Ca{x}MnO{3} (x=0.35-0.5) in magnetic field. This observation strongly suggests that the microscopic phase separation in the low-Tc manganites causes formation of a percolation network, and that the observed MIT is a percolation threshold.

cond-mat.str-el↗
arXiv1999

One-loop calculations of hyperon polarizabilities under the large N_c consistency condition

The spin-averaged electromagnetic polarizabilities of the hyperons $Λ$ and $Σ$ are calculated within the one-loop approximation by use of the dispersion theory. The photon and meson couplings to hyperons are determined so as to satisfy the large N_c consistency condition. It is shown that in order for the large N_c consistency condition to hold exotic hyperon states such as $Σ^{**}$ with I=2 and J=3/2 are required in the calculation of the magnetic polarizability of the $Σ$ state.

nucl-th↗
arXiv1999

Giant 1/f noise in perovskite manganites: evidence of the percolation threshold

We discovered an unprecedented magnitude of the 1/f noise near the Curie temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and the resistance provides strong evidence of the percolation nature of the ferromagnetic transition in the polycrystalline samples. The step-like changes of the resistance with temperature, observed for single crystals, suggest that the size of the ferromagnetic domains depends on the size of crystallites.

cond-mat.str-el↗
arXiv1999

Spin-Polarizabilities of the Nucleon in Chiral Soliton Model with Dispersion Relation

We calculate the spin-polarizabilities of the nucleon by means of dispersion relation, where the pion photoproduction amplitude predicted by chiral soliton model is utilized. We consider the $Nπ$ and $Δπ$ channels in the pion-photoproduction amplitude, and also evaluate the contribution of the anomalous term of the $π^0γγ$ process. In a narrow decay-width limit of the $Δ$ particle the result coincides with that in heavy baryon chiral perturbation theory (HBChPT) except for the interference part of the electric and magnetic amplitudes. A comparison with the multipole analysis is given.

nucl-th↗
arXiv1997

Spin-dependent Polarizability of Nucleon with Dispersion Relation in the Skyrme Model

We calculate the spin-dependent polarizability of the nucleon in the Skyrme model. The result is compared with that of a heavy baryon chiral perturbation theory(HBChPT), and is shown to be the same as that of HBChPT up to the $Δ$-pole terms in the narrow width limit of the $Δ$ state and with the experimental physical constants. The effect of the $Δ+π$ channel is rather small and is numerically quite similar to that of the $Δ$ loop in the HBChPT. The electric and magnetic polarizabilities are recalculated using the transverse photon and a consistent inclusion of the $Δ$ width.

hep-ph↗