arXiv subjects

T. Saito

Publications and source records attributed to T. Saito.

At least 145 records · Page 8Linked to original sources

arXiv2007

Two-Staged Magnetoresistance Driven by Ising-like Spin Sublattice in SrCo6O11

A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo6O11 having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field-dependence of magnetization. The stepwise field-dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.

cond-mat.str-el↗

arXiv2007

Infrared behavior of the Faddeev-Popov operator in Coulomb gauge QCD

We calculate the eigenvalue distribution of the Faddeev-Popov operator in Coulomb gauge QCD using quenched SU(3) lattice simulation. In the confinement phase, the density of the low-lying eigenvalues increases with lattice volume, and the confinement criterion is satisfied. Moreover, even in the deconfinement phase, the behavior of the FP eigenvalue density is qualitatively the same as in the confinement phase. This is consistent with the fact that the color-Coulomb potential is not screened in the deconfined phase.

hep-lat↗

arXiv2006

Infrared divergence of the color-Coulomb self-energy in Coulomb gauge QCD

We investigate the spectrum of the Faddeev-Popov operator in Coulomb gauge QCD using quenched SU(3) lattice simulation. In the confinement phase, we observe the accumulation of the near-zero modes of the FP operator at large lattice volumes, and the color-Coulomb self-energy diverges in the infrared limit. Moreover, even in the deconfinement phase, the behavior of the FP eigenvalue density is qualitatively the same as in the confinement phase and the color-Coulomb self-energy is infrared divergent.

hep-lat↗

arXiv2006

Muon spin relaxation and rotation study on the solid solution of the two spin-gap systems (CH3)2CHNH3-CuCl3 and (CH3)2CHNH3-CuBr3

Muon-spin-rotation and relaxation studies have been performed on (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ with $x$=0.85 and 0.95, which are solid solutions of the two isomorphic spin-gap systems (CH$_3$)$_2$CHNH$_3$CuCl$_3$ and (CH$_3$)$_2$CHNH$_3$CuBr$_3$ with different spin gaps. The sample with $x$=0.85 showed a clear muon spin rotation under zero-field below $T_{\rm N}$=11.65K, indicating the existence of a long-range antiferromagnetic order. A critical exponent of the hyperfine field was obtained to be $β$=0.33, which agrees with 3D-Ising model. In the other sample with $x$=0.95, an anomalous enhancement of the muon spin relaxation was observed at very low temperatures indicating a critical slowing down due to a magnetic instability of the ground state.

cond-mat.str-el↗

arXiv2006

Properties of Color-Coulomb String Tension

We study the properties of the color-Coulomb string tension obtained from the instantaneous part of gluon propagators in Coulomb gauge using quenched SU(3) lattice simulation. In the confinement phase, the dependence of the color-Coulomb string tension on the QCD coupling constant is smaller than that of the Wilson loop string tension. On the other hand, in the deconfinement phase, the color-Coulomb string tension does not vanish even for $T/T_c = 1 \sim 5$, the temperature dependence of which is comparable with the magnetic scaling, dominating the high temperature QCD. Thus, the color-Coulomb string tension is not an order parameter of QGP phase transition.

hep-lat↗

arXiv2006

Efficient propagation of the polarization from laser photons to positrons through Compton scattering and electron-positron pair creation

We demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scatterings and electron-positron pair creations. Using a circularly polarized laser beam of 532 nm scattered off a high-quality electron beam with the energy of 1.28 GeV, we obtained polarized positrons with an intensity of 10^4 e+/bunch. Magnitude of positron polarizations was determined as 73+-15(sta) +-19(sys)% by means of a newly designed positron polarimeter.

hep-ex↗

arXiv2006

Recent progress of avalanche photodiodes in high-resolution X-rays and Gamma-rays detection

We have studied the performance of large area avalanche photodiodes (APDs) recently developed by Hamamatsu Photonics K.K, in high-resolution X-rays and Gamma-rays detections. We show that reach-through APD can be an excellent soft X-ray detector operating at room temperature or moderately cooled environment. We obtain the best energy resolution ever achieved with APDs, 6.4 % for 5.9 keV X-rays, and obtain the energy threshold as low as 0.5 keV measured at -20deg. Thanks to its fast timing response, signal carriers in the APD device are collected within a short time interval of 1.9 nsec (FWHM). This type of APDs can therefore be used as a low-energy, high-counting particle monitor onboard the forthcoming Pico-satellite Cute1.7. As a scintillation photon detector, reverse-type APDs have a good advantage of reducing the dark noise significantly. The best FWHM energy resolutions of 9.4+-0.3 % and 4.9+-0.2 % were obtained for 59.5 keV and 662 keV Gamma-rays, respectively, as measured with a CsI(Tl) crystal. Combination of APDs with various other scintillators (BGO, GSO, and YAP) also showed better results than that obtained with a photomultiplier tube (PMT). These results suggest that APD could be a promising device for replacing traditional PMT usage in some applications. In particular 2-dim APD array, which we present in this paper, will be a promising device for a wide-band X-ray and Gamma-ray imaging detector in future space research and nuclear medicine.

astro-ph↗

arXiv2006

An Active Gain-control System for Avalanche Photo-Diodes under Moderate Temperature Variations

Avalanche photodiodes (APDs) are promising light sensor for various fields of experimental physics. It has been argued, however, that variation of APD gain with temperature could be a serious problem preventing APDs from replacing traditional photomultiplier tubes (PMTs) in some applications. Here we develop an active gain-control system to keep the APD gain stable under moderate temperature variations. As a performance demonstration of the proposed system, we have tested the response of a scintillation photon detector consisting of a 5x5 mm^2 reverse-type APD optically coupled with a CsI(Tl) crystal. We show that the APD gain was successfully controlled under a temperature variation of DT = 20deg, within a time-cycle of 6000 sec. The best FWHM energy resolution of 6.1+-0.2 % was obtained for 662 keV gamma-rays, and the energy threshold was as low as 6.5 keV, by integrating data from +20deg - 0deg cycles. The corresponding values for -20deg - 0deg cycles were 6.9+-0.2 % and 5.2 keV, respectively. These results are comparable, or only slightly worse than that obtained at a fixed temperature. Our results suggest new potential uses for APDs in various space researches and nuclear physics. As examples, we briefly introduce the NeXT and Cute-1.7 satellite missions that will carry the APDs as scientific instruments for the first time.

astro-ph↗

arXiv2005

QCD color interactions between two quarks

We study the QCD color interactions between static two heavy quarks at zero temperature in a quenched SU(3) lattice gauge simulation: in addition to the standard singlet $q\bar{q}$ potentials, we calculate octet $q\bar{q}$ potentials, symmetric and antisymmetric $qq$ potentials. It is shown that the antisymmetric $qq$ channel behaves as a linearly rising potential at large quark separations. We further find that the $q\bar{q}$ octet and $qq$ symmetric channels have the complex dependence on the distance; at short distances they are repulsive forces, while at large distances, they show linearly rising feature. Ratio of string tensions between $q\bar{q}$ singlet and $qq$ antisymmetric potentials is described in terms of the Casimir factor.

hep-lat↗

arXiv2005

Color confinement in Coulomb gauge QCD

We study the long-range behavior of the heavy quark potential in Coulomb gauge using a quenched SU(3) lattice gauge simulation with partial-length Polyakov line correlators. We show that the Coulomb heavy quark potential associated with the instantaneous part of gluon propagators in Coulomb gauge, presents a linearly rising behavior at large distances, and the resulting Coulomb string tension is greater than the Wilson loop string tension, which can be explained by Zwanziger's inequality. The linearly rising behavior of the Coulomb heavy quark potential persists even in the deconfinement phase. The heavy quark potential in Lorentz gauge shows completely different behavior than that in Coulomb gauge. Our SU(3) result, i.e., the Coulomb heavy quark potential is confining, qualitatively agrees with that of the SU(2) analysis carried out by Greensite, Olejnik and Zwanziger.

hep-lat↗

arXiv2005

Definitive Identification of the Transition between Small- to Large-Scale Clustering for Lyman Break Galaxies

We report angular correlation function (ACF) of Lyman Break Galaxies (LBGs) with unprecedented statistical quality on the basis of 16,920 LBGs at z=4 detected in the 1 deg^2 sky of the Subaru/XMM-Newton Deep Field. The ACF significantly departs from a power law, and shows an excess on small scale. Particularly, the ACF of LBGs with i'<27.5 have a clear break between the small and large-scale regimes at the angular separation of ~7'' whose projected length corresponds to the virial radius of dark halos with a mass of 10^11-12 Mo, indicating multiple LBGs residing in a single dark halo. Both on small (2''<theta<3'') and large (40''<theta<400'') scales, clustering amplitudes monotonically increase with luminosity for the magnitude range of i'=24.5-27.5, and the small-scale clustering shows a stronger luminosity dependence than the large-scale clustering. The small-scale bias reaches b~10-50, and the outskirts of small-scale excess extend to a larger angular separation for brighter LBGs. The ACF and number density of LBGs can be explained by the cold dark matter model.

astro-ph↗

arXiv2005

Resolving the Discrepancy of 135 MeV pd Elastic Scattering Cross Sections and Relativistic Effects

Three precise measurements for elastic pd scattering at 135 MeV/A have been performed with the three different experimental setups. The cross sections are described well by the theoretical predictions based on modern nucleon-nucleon forces combined with three nucleon forces. Relativistic Faddeev calculations show that relativistic effects are restricted to backward angles. This result supports the two measurements recently reported by RIKEN and contradicts the KVI data.

nucl-ex↗

arXiv2005

Determination of the Gamow-Teller Quenching Factor from Charge Exchange Reactions on 90Zr

Double differential cross sections between 0-12 degrees were measured for the 90Zr(n,p) reaction at 293 MeV over a wide excitation energy range of 0-70 MeV. A multipole decomposition technique was applied to the present data as well as the previously obtained 90Zr(p,n) data to extract the Gamow-Teller (GT) component from the continuum. The GT quenching factor Q was derived by using the obtained total GT strengths. The result is Q=0.88+/-0.06 not including an overall normalization uncertainty in the GT unit cross section of 16%.

nucl-ex↗

arXiv2005

Performance of the neutron polarimeter NPOL3 for high resolution measurements

We describe the neutron polarimeter NPOL3 for the measurement of polarization transfer observables $D_{ij}$ with a typical high resolution of $\sim$300 keV at $T_n$ $\simeq$ 200 MeV. The NPOL3 system consists of three planes of neutron detectors. The first two planes for neutron polarization analysis are made of 20 sets of one-dimensional position-sensitive plastic scintillation counters with a size of 100 cm $\times$ 10 cm $\times$ 5 cm, and they cover the area of 100 $\times$ 100 $\mathrm{cm}^2$. The last plane for detecting doubly scattered neutrons or recoiled protons is made of the two-dimensional position-sensitive liquid scintillation counter with a size of 100 cm $\times$ 100 cm $\times$ 10 cm. The effective analyzing powers $A_{y;\mathrm{eff}}$ and double scattering efficiencies $ε_{\mathrm{D.S.}}$ were measured by using the three kinds of polarized neutrons from the ${}^{2}{\rm H}(\vec{p},\vec{n})pp$, ${}^{6}{\rm Li}(\vec{p},\vec{n}){}^{6}{\rm Be}(\mathrm{g.s.})$, and ${}^{12}{\rm C}(\vec{p},\vec{n}){}^{12}{\rm N}(\mathrm{g.s.})$ reactions at $T_p$ = 198 MeV. The performance of NPOL3 defined as $ε_{\mathrm{D.S.}}(A_{y;\mathrm{eff}})^2$ are similar to that of the Indiana Neutron POLarimeter (INPOL) by taking into account for the counter configuration difference between these two neutron polarimeters.

nucl-ex↗

arXiv2004

The Discovery of Primeval Large-Scale Structures with Forming Clusters at Redshift 6

We report the discovery of primeval large-scale structures (LSSs) including two proto-clusters in a forming phase at z=5.7. We carried out extensive deep narrow-band imaging in the 1 deg^2 sky of the Subaru/XMM-Newton Deep Field, and obtained a cosmic map of 515 Ly alpha emitters (LAEs) in a volume with a transverse dimension of 180 Mpc x 180 Mpc and a depth of ~40 Mpc in comoving units. This cosmic map shows filamentary LSSs, including clusters and surrounding 10-40 Mpc scale voids, similar to the present-day LSSs. Our spectroscopic follow-up observations identify overdense regions in which two dense clumps of LAEs with a sphere of 1-Mpc diameter in physical units are included. These clumps show about 130 times higher star-formation rate density, mainly due to a large overdensity, ~80, of LAEs. These clumps would be clusters in a formation phase involving a burst of galaxy formation.

astro-ph↗

arXiv2004

Study of the Sigma-nucleus potential by the (pi^-,K^+) reaction on medium-to-heavy nuclear targets

In order to study the Sigma-nucleus optical potential, we measured inclusive (pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi. The CH_2 target was used to calibrate the excitation energy scale by using the elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also extracted. The calibration was done with +-0.1 MeV precision. The angular distribution of the elementary cross section was measured, and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Sigma^- binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the Distorted Wave Impulse Approximation (DWIA). It has been demonstrated that a strongly repulsive \sig-nucleus potential with a non-zero size of the imaginary part is required to reproduce the shape of the measured spectra.

nucl-ex↗

arXiv2004

Measurement of the tensor analyzing power T20 in the dd->^3Hen and dd->^3Hp at intermediate energies and at zero degree

The data on the tensor analyzing power T20 in the dd->^3Hen and dd-> ^3Hp reactions at 140, 200 and 270 MeV of the deuteron kinetic energy and at zero degree obtained at RIKEN Accelerator Research Facility are presented. The observed positive sign of T20 clearly demonstrates the sensitivity to the D/S wave ratios in the ^3He and ^3H in the energy domain of the measurements. The T20 data for the ^3He-n and ^3H-p channels are in agreement within experimental accuracy.

nucl-ex↗

arXiv2004

Polarization Transfer Measurement for $^1H(\vec{d},\vec{p})^2 H$ Elastic Scattering at 135 MeV/u and Three Nucleon Force Effects

The deuteron to proton polarization transfer coefficients for the $d$--$p$ elastic scattering were precisely measured with an incoming deuteron energy of 135 MeV/u at the RIKEN Accelerator Research Facility. The data are compared to theoretical predictions based on exact solutions of three-nucleon Faddeev equations with high--precision nucleon--nucleon forces combined with different three-nucleon forces (3NFs), representing the current, most popular models: the $2π$-exchange Tucson-Melbourne model, a modification thereof closer to chiral symmetry TM'(99), and the Urbana IX 3NF. Theory predicts large 3NF effects, especially in the angular range around the cross section minimum, but the present data only partially concurs, predominantly for $K_{xx}^{y'}-K_{yy}^{y'}$ ($K_{xx}^{y'}$, $K_{yy}^{y'}$). For the induced polarization, $P^{y'}$, the TM$'$(99) and Urbana IX 3NFs reproduce the data, but the Tucson-Melbourne 3NF fails to describe the data. For the polarization transfer coefficients, $K_{y}^{y'}$ and $K_{xz}^{y'}$, the predicted 3NF ffects are in drastic conflict to the data. These facts clearly reveal the defects of the 3NF models currently used.

nucl-ex↗