arXiv subjects

T. Ishikawa

Publications and source records attributed to T. Ishikawa.

At least 127 records · Page 7Linked to original sources

arXiv2008

Theoretical evidences for enhanced superconducting transition temperature of CaSi$_2$ in a high-pressure AlB$_2$ phase

By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi$_2$ at high pressure. Our simulation showed stability of the AlB$_2$ structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron-phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB$_2$ structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher $T_{\rm c}$ is expected in the AlB$_2$ structure than the trigonal structures which are known so far.

cond-mat.supr-con↗

arXiv2008

Proton elastic scattering from tin isotopes at 295 MeV and systematic change of neutron density distributions

Cross sections and analyzing powers for proton elastic scattering from $^{116,118,120,122,124}$Sn at 295 MeV have been measured for a momentum transfer of up to about 3.5 fm$^{-1}$ to deduce systematic changes of the neutron density distribution. We tuned the relativistic Love-Franey interaction to explain the proton elastic scattering of a nucleus whose density distribution is well known. Then, we applied this interaction to deduce the neutron density distributions of tin isotopes. The result of our analysis shows the clear systematic behavior of a gradual increase in the neutron skin thickness of tin isotopes with mass number.

nucl-ex↗

arXiv2008

Forward coherent $ϕ$-meson photoproduction from deuterons near threshold

Differential cross sections and decay asymmetries for coherent $ϕ$-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at $E_γ$= 1.5-2.4 GeV. This reaction offers a unique way to directly access natural-parity Pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity $π$-meson exchange is small. Nevertheless the deduced cross sections of $ϕ$-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional Pomeron model.

nucl-ex↗

arXiv2007

$Λd$ correlations from the $^4$He(stopped-$K^-$,$d$) reaction

We have observed an intense high-energy component in an inclusive $^4$He(stopped $K^-$, $d$) spectrum. For back-to-back $Λd$ pairs a prominent event cluster has been found just below the $m_{^4\textrm{He}}+m_{K^-}-m_{n}$ mass threshold in the $Λd$ invariant mass spectrum of $Λdn$ events, which is evidence for a three-nucleon absorption process of $K^-$ in $^4$He. In addition, an appreciable strength is revealed below $\sim$3220 MeV/$c^2$. Well separated $Σ^0 dn$ events show a peak similar to the case of $Λdn$.

nucl-ex↗

arXiv2007

Electronic structures of CeRu$_2X_2$ ($X$ = Si, Ge) in the paramagnetic phase studied by soft X-ray ARPES and hard X-ray photoelectron spectroscopy

Soft and hard X-ray photoelectron spectroscopy (PES) has been performed for one of the heavy fermion system CeRu$_2$Si$_2$ and a $4f$-localized ferromagnet CeRu$_2$Ge$_2$ in the paramagnetic phase. The three-dimensional band structures and Fermi surface (FS) shapes of CeRu$_2$Si$_2$ have been determined by soft X-ray $hν$-dependent angle resolved photoelectron spectroscopy (ARPES). The differences in the Fermi surface topology and the non-$4f$ electronic structures between CeRu$_2$Si$_2$ and CeRu$_2$Ge$_2$ are qualitatively explained by the band-structure calculation for both $4f$ itinerant and localized models, respectively. The Ce valences in CeRu$_2X_2$ ($X$ = Si, Ge) at 20 K are quantitatively estimated by the single impurity Anderson model calculation, where the Ce 3d hard X-ray core-level PES and Ce 3d X-ray absorption spectra have shown stronger hybridization and signature for the partial $4f$ contribution to the conduction electrons in CeRu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2007

Photoproduction of neutral kaons on the liquid deuterium target in the threshold region

The photoproduction process of neutral kaons on a liquid deuterium target is investigated near the threshold region, Egamma = 0.8-1.1 GeV. K0 events are reconstructed from positive and negative pions, and differential cross sections are derived. Experimental momentum spectra are compared with those calculated in the spectator model using a realistic deuteron wave function. Elementary amplitudes as given by recent isobar models and a simple phenomenological model are used to study the effect of the new data on the angular behavior of the elementary cross section. The data favor a backward-peaked angular distribution of the elementary n(gamma,K0)Lambda process, which provides additional constraints on current models of kaon photoproduction. The present study demonstrates that the n(gamma,K0)Lambda reaction can provide key information on the mechanism of the photoproduction of strangeness.

nucl-ex↗

arXiv2007

$ΛN$ correlations from the stopped $K^-$ reaction on ${}^4$He

We have investigated correlations of coincident $ΛN$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $Λp$ and $Λn$ branches of the two-nucleon absorption process in the $ΛN$ invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.

nucl-ex↗

arXiv2007

Status reports from the GRACE Group

We discuss a new approach for the numerical evaluation of loop integrals. The fully numerical calculations of an infrared one-loop vertex and a box diagram are demonstrated. To perform these calculations, we apply an extrapolation method based on the $ε$-algorithm. In our approach, the super high precision control in the numerical manipulation is essential to handle the infrared singularity. We adopt a multi-precision library named {\tt HMLib} for the precision control in the calculations.

hep-ph↗

arXiv2007

Precise Numerical Evaluation of the Scalar One-Loop Integrals with the Infrared Divergence

We present a new approach for obtaining very precise integration results for infrared vertex and box diagrams, where the integration is carried out directly without performing any analytic integration of Feynman parameters. Using an appropriate numerical integration routine with an extrapolation method, together with a multi-precision library, we have obtained integration results which agree with the analytic results to 10 digits even for such a very small photon mass as $10^{-150}$ GeV in the infrared vertex diagram.

hep-ph↗

arXiv2007

Search for strange tribaryon states in the inclusive ^4He(K^{-}_{stopped}, p) reaction

A search for tribaryon states was performed at KEK-PS. We adopted the $^4$He($K^-_{stopped}, p)$ reaction to populate the states with strangeness -1, charge 0 and isospin 1. No significant narrow structure was observed in the mass region from 3000 to 3200 MeV/c^2 in an inclusive missing mass spectrum. The upper limit of the formation branching ratio was determined to be ($1\sim3)\times 10^{-4}$, ($0.7\sim2)\times 10^{-3}$ and ($2\sim8) \times 10^{-3}$/($stopped K^{-}$) with 95 % confidence level for narrow states with an assumed width of 0, 20 and 40 MeV/c^2, respectively.

nucl-ex↗

arXiv2007

Backward-angle photoproduction of $π^0$ mesons on the proton at $E_γ$ = 1.5--2.4 GeV

Differential cross sections and photon beam asymmetries for $π^0$ photoproduction have been measured at $E_γ$ = 1.5--2.4 GeV and at the $π^0$ scattering angles, --1 $<$ cos$Θ_{c.m.} <$ --0.6. The energy-dependent slope of differential cross sections for $u$-channel $π^0$ production has been determined. An enhancement at backward angles is found above $E_γ$ = 2.0 GeV. This is inferred to be due to the $u$-channel contribution and/or resonances. Photon beam asymmetries have been obtained for the first time at backward angles. A strong angular dependence has been found at $E_γ>$ 2.0 GeV, which may be due to the unknown high-mass resonances.

nucl-ex↗

arXiv2007

Precision measurement of the $3d \to 2p$ x-ray energy in kaonic $^4$He

We have measured the Balmer-series x-rays of kaonic $^4$He atoms using novel large-area silicon drift x-ray detectors in order to study the low-energy $\bar{K}$-nucleus strong interaction. The energy of the $3d \to 2p$ transition was determined to be 6467 $\pm$ 3 (stat) $\pm$ 2 (syst) eV. The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment.

nucl-ex↗

arXiv2007

Search for a kaonic nuclear state via $^4$He$(K^-, N)$

Very recently, we have performed a couple of experiments, {\it{KEK PS-E549/E570}}, for the detailed study of the strange tribaryon $S^0(3115)$ obtained in {\it{KEK PS-E471}}. These experiments were performed to accumulate much higher statistics with improved experimental apparatusespecially for the better proton spectroscopy of the $^4$He({\it{stopped K}}$^-$, {\it{N}}) reaction. In contrast to the previous proton spectrum, no narrow ($\sim$ 20 MeV) peak structure was found either in the inclusive $^4$He({\it{stopped K}}$^-$, {\it{p}}) or in the semi-inclusive $^4$He({\it{stopped K}}$^-$, {\it{p}}$X^\pm$) reaction channel, which is equivalent to the previous $E471$ event trigger condition. Detailed analysis of the present data and simulation shows that the peak, corresponding to $S^0(3115)$, has been an experimental artifact. Present analysis does not exclude the possible existence of a much wider structure. To be sensitive to such structure and for better understanding of the non-mesonic $K^-$ absorption reaction channel, detailed analysis of the data is in progress.

nucl-ex↗

arXiv2007

2+1 flavor light hadron spectrum and quark masses with the O(a) improved Wilson-clover quark formalism

We present a summary of results of the joint CP-PACS and JLQCD project toward a 2+1 flavor full QCD simulation with the O(a)-improved Wilson quark formalism and the Iwasaki gauge action. Configurations were generated during 2002-2005 at three lattice spacings, a~0.076, 0.100 and 0.122 fm, keeping the physical volume constant at (2.0fm)^3. Up and down quark masses are taken in the range m_{PS}/m_V~0.6-0.78. We have completed the analysis for the light meson spectrum and quark masses in the continuum limit using the full configuration set. The predicted meson masses reproduce experimental values in the continuum limit at a 1% level. The average up and down, and strange quark masses turn out to be m_{ud}^{\bar{MS}}(μ=2 GeV)=3.50(14)({}^{+26}_{-15}) MeV and m_s^{\bar{MS}}(μ=2 GeV)=91.8(3.9)({}^{+6.8}_{-4.1}) MeV. We discuss our future strategy toward definitive results on hadron spectroscopy with the Wilson-clover formalism.

hep-lat↗

arXiv2007

Hard x-ray photoemission study of LaAlO3/LaVO3 multilayers

We have studied the electronic structure of multilayers composed of a band insulator LaAlO$_3$ (LAO) and a Mott insulator LaVO$_3$ (LVO) by means of hard x-ray photoemission spectroscopy, which has a probing depth as large as $\sim 60 Å$. The Mott-Hubbard gap of LVO remained open at the interface, indicating that the interface is insulating unlike the LaTiO$_3$/SrTiO$_3$ multilayers. We found that the valence of V in LVO were partially converted from V$^{3+}$ to V$^{4+}$ only at the interface on the top side of the LVO layer and that the amount of V$^{4+}$ increased with LVO layer thickness. We suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar heterostructure is the origin of the highly asymmetric valence change at the LVO/LAO interfaces.

cond-mat.str-el↗

arXiv2006

Hard X-ray Cu $2p$ Core-Level Photoemission of High-$T_c$ Cuprate Superconductors

We have performed a detailed study of Cu $2p$ core-level spectra in single layer La$_{2-x}$Sr$_{x}$CuO$_{4}$, La doped Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ (Bi2201) and bilayer Bi$_2$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) high-temperature superconductors by using hard x-ray photoemission (HX-PES). We identify the Cu$^{2+}$ derived (i) the Zhang-Rice singlet (ZRS) feature, (ii) the $d^{n+1}\underline{L}$ (ligand screened) feature, (iii) the $d^{n}$ satellite feature, as well as the hole-doping derived high binding energy feature in the main peak. In Bi-based cuprates, intensities of the $d^{n}$ satellite features seem to be strongly enhanced compared to La$_{2-x}$Sr$_{x}$CuO$_{4}$. From x-ray photon energy dependent measurements, it is shown that the increased intensity in the satellite region is associated with Bi $4s$ core-level spectral intensity. The corrected $d^{n}$ satellite intensity is independent of the doping content or number of Cu-O layers. Our results suggest a correlation of the relative intensity of ZRS feature and hole-doping induced high binding energy spectral changes in the main peak with superconductivity.

cond-mat.str-el↗

arXiv2006

Photo-production of neutral kaons on 12C in the threshold region

Kaon photo-production process on $^{12}$C has been studied by measuring neutral kaons in a photon energy range of 0.8$-$1.1 GeV. Neutral kaons were identified by the invariant mass constructed from two charged pions emitted in the $K^{0}_{S}\toπ^{+}π^{-}$ decay channel. The differential cross sections as well as the integrated ones in the threshold photon energy region were obtained. The obtained momentum spectra were compared with a Spectator model calculation using elementary amplitudes of kaon photo-production given by recent isobar models. Present result provides, for the first time, the information on $n(γ,K^{0})Λ$ reaction which is expected to play an important role to construct models for strangeness production by the electromagnetic interaction. Experimental results show that cross section of $^{12}{\rm C}(γ,K^0)$ is of the same order to that of $^{12}{\rm C}(γ,K^+)$ and suggest that slightly backward $K^0$ angular distribution is favored in the $γn\to K^0Λ$ process.

nucl-ex↗

arXiv2006

An application of the UV-filtering preconditioner to the Polynomial Hybrid Monte Carlo algorithm

We apply the UV-filtering preconditioner, previously used to improve the Multi-Boson algorithm, to the Polynomial Hybrid Monte Carlo (UV-PHMC) algorithm. The performance test for the algorithm is given for the plaquette gauge action and the $O(a)$-improved Wilson action at $β=5.2, c_{\mathrm{sw}}=2.02, M_π/M_ρ\sim 0.8$ and 0.7 on a $16^3\times 48$ lattice. We find that the UV-filtering reduces the magnitude of the molecular dynamics force from the pseudo fermion by a factor 3 by tuning the UV-filter parameter. Combining with the multi-time scale molecular dynamics integrator we achieve a factor 2 improvement.

hep-lat↗