arXiv subjects

Y. Asano

Publications and source records attributed to Y. Asano.

At least 37 records · Page 2Linked to original sources

arXiv2008

Evidence of the Θ^+ in the γd \to K^+K^-pn reaction

The γd \to K^+K^-pn reaction has been studied to search for the evidence of the Θ^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 σ, and the differential cross-section for the γn \to K^- Θ^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Θ^+ in the γn center-of-mass system. The obtained results support the existence of the Θ^+.

nucl-ex↗

arXiv2008

Anomalous Transport through the p-Wave Superconducting Channel in the 3-K Phase of Sr2RuO4

Using micro fabrication techniques, we extracted individual channels of 3-Kelvin (3-K) phase superconductivity in Sr2RuO4-Ru eutectic systems and confirmed odd-parity superconductivity in the 3-K phase, similar to pure Sr2RuO4. Unusual hysteresis in the differential resistance-current and voltage-current characteristics observed below 2 K indicates the internal degrees of freedom of the superconducting state. A possible origin of the hysteresis is current-induced chiral-domain-wall motion due to the chiral p-wave state.

cond-mat.supr-con↗

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

Cooper Pair Shape in Normal-metal/Superconductor Junctions

In s-wave superconductors the Cooper pair wave function is isotropic in momentum space. This property may also be expected for Cooper pairs entering a normal metal from a superconductor due to the proximity effect. We show, however, that such a deduction is incorrect and the pairing function in a normal metal is surprisingly anisotropic because of quasiparticle interference. We calculate angle resolved quasiparticle density of states in NS bilayers which reflects such anisotropic shape of the pairing function. We also propose a magneto-tunneling spectroscopy experiment which could confirm our predictions.

cond-mat.supr-con↗

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↗

arXiv2006

New result on the measurement of the direct photon emission in K+ --> pi^+ pi^0 gamma decay

We present a new result on the K^+ --> pi^+ pi^0 gamma decay measurement using stopped kaons. The best fit to the decay spectrum comprised of 10k events gives a branching ratio for the direct photon emission of [3.8+-0.8(stat)+-0.7(syst)] times 10^{-6} in the pi^+ kinetic energy region of 55 to 90 MeV. This result has been obtained with the assumption that there is no component due to interference with the inner bremsstrahlung.

hep-ex↗

arXiv2006

Differential cross section and photon beam asymmetry for the gamma n -> K+ Sigma- reaction at Egamma=1.5-2.4 GeV

Differential cross sections and photon beam asymmetries have been measured for the gamma n -> K+ Sigma- and gamma p -> K+ Sigma0 reactions separately using liquid deuterium and hydrogen targets with incident linearly polarized photon beams of Egamma=1.5-2.4 GeV at 0.6<cosTheta<1. The cross section ratio of sigma(K+Sigma-)/sigma(K+Sigma0), expected to be 2 on the basis of the isospin 1/2 exchange, is found to be close to 1. For the K+Sigma- reaction, large positive asymmetries are observed indicating the dominance of the K*-exchange. A large difference between the asymmetries for the K+Sigma- and K+Sigma0 reactions can not be explained by simple theoretical considerations.

hep-ex↗

arXiv2006

Theory of tunneling spectroscopy in the Larkin-Ovchinnikov state

A theory of the tunneling spectroscopy of normal metal / Larkin-Ovchinnikov (LO) state is presented by fully taking account of the periodic modulation of pair potentials in real space. The resulting tunneling spectra have characteristic line shapes with several maxima and minima reflecting minigap structures due to the periodic pair potentials. These features are qualitatively different from tunneling spectra of Fulde-Ferrell state and those of uniform $s$- and d-wave superconductors. On the basis of calculated results, we propose a method to identify the LO state in actual experiments.

cond-mat.supr-con↗

arXiv2006

Nonmonotonic temperature dependence of critical current in diffusive d-wave junctions

We study the Josephson effect in D/I/DN/I/D junctions, where I, DN and D denote an insulator, a diffusive normal metal and a d-wave superconductor, respectively.The Josephson current is calculated based on the quasiclassical Green's function theory with a general boundary condition for unconventional superconducting junctions. In contrast to s-wave junctions, the product of the Josephson current and the normal state resistance is enhanced by making the interface barriers stronger. The Josephson current has a nonmonotonic temperature dependence due to the competition between the proximity effect and the midgap Andreev resonant states.

cond-mat.supr-con↗

arXiv2005

Measurement of K^+ \to π^0 μ^+ νγdecay using stopped kaons

The K^+ \to π^0 μ^+ νγ($K_{μ3 γ}$) decay has been measured with stopped positive kaons at the KEK 12 GeV proton synchrotron. A $K_{μ3 γ}$ sample containing 125 events was obtained. The partial branching ratio $Br(K_{μ3 γ}, E_γ>30 {\rm MeV}, θ_{μ^+ γ}>20^{\circ})$ was found to be $[2.4 \pm 0.5(stat) \pm 0.6(syst)]\times 10^{-5}$, which is in good agreement with theoretical predictions.

hep-ex↗

arXiv2005

Diffractive $ϕ$-meson photoproduction on proton near threshold

Photoproduction of $ϕ$-meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to $E_γ$= 2.37 GeV. The differential cross sections at $t = -|t|_{min}$ do not increase smoothly as $E_γ$ increases, but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that $ϕ$-mesons are photo-produced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.

nucl-ex↗

arXiv2005

Meissner effect in diffusive normal metal / d-wave superconductor junctions

The Meissner effect in diffusive normal metal / insulator / d-wave superconductor junctions is studied theoretically in the framework of the Usadel equation under the generalized boundary condition. The effect of midgap Andreev resonant states (MARS) formed at the interface of d-wave superconductor is taken into account. It is shown that the formation of MARS suppresses the susceptibility of the diffusive normal metal.

cond-mat.supr-con↗

arXiv2005

Theory of thermal and charge transport in diffusive normal metal / superconductor junctions

Thermal and charge transport in the diffusive normal metal(DN) / insulator / $s$-, $d$- and p-wave superconductor junctions are studied for various situations, where we have used the Usadel equation with Nazarov's generalized boundary condition. Thermal and electrical conductance of the junction and the Lorentz ratio are calculated by varying the magnitudes of the resistance, the Thouless energy and the magnetic scattering rate in DN, the transparency of the insulating barrier, and the angle between the normal to the interface and the crystal axis of d-wave superconductors or the angle between the normal to the interface and the lobe direction of the p-wave pair potential. New general expression is derived for the calculation of the thermal conductance. It is demonstrated that the proximity effect doesn't influence the thermal conductance while the mid gap Andreev resonant states suppress it. We have also discussed a possibility of distinguishing pairing symmetries based on the dependencies of the electrical and thermal conductance on temperatures.

cond-mat.supr-con↗

arXiv2005

Anomalous features of the proximity effect in triplet superconductors

Anomalous features of the proximity effect specific to diffusive normal metal / triplet superconductor (DN/TS) junctions are studied within the quasiclassical Green's function formalism. The pair amplitude $f_{N}$ as a function of energy $ε$ in DN, satisfies an anomalous relation, $% f_{N}(ε)=-f^{*}_{N}(-ε)$, contrary to that in singlet superconductor junctions case where $f_{N}(ε)=f^{*}_{N}(-% ε)$. Such an unusual $ε$ dependence is responsible for a zero energy peak in the local density of states and is a source of an anomalous penetration of the applied magnetic field into DN. % Our results are relevant to Sr$_{2}$RuO$_{4}$, which is considered to have a $p_{x}+ip_{y}$-wave symmetry.

cond-mat.supr-con↗

arXiv2005

Spin Current in p-wave Superconducting Rings

A formula of the spin current in mesoscopic superconductors is derived from the mean-field theory of superconductivity. The spin flow is generated by the spatial fluctuations of $\vec{d}$ which represents a spin state of spin-triplet superconductors. We discuss a possibility of the circulating spin current in isolated p-wave superconducting rings at the zero magnetic field. The direction of the spin current depends on topological numbers which characterize the spatial configuration of $\vec{d}$ on the ring.

cond-mat.supr-con↗

arXiv2005

Josephson interferometer in a ring topology as a symmetry prove of Sr_2RuO_4

The Josephson effect is theoretically studied in two types of SQUIDs consisting of $s$ wave superconductor and Sr$_2$RuO$_4$. Results show various response of the critical Josephson current to applied magnetic fields depending on the type of SQUID and on the pairing symmetries. In the case of a $p_x+ip_y$ wave symmetry, the critical current in a corner SQUID becomes an asymmetric function of magnetic fields near the critical temperatures. Our results well explain a recent experimental finding [Nelson et. al, Science \textbf{306}, 1151 (2004)]. We also discuss effects of chiral domains on the critical current.

cond-mat.supr-con↗

arXiv2004

Influence of magnetic impurities on charge transport in diffusive-normal-metal / superconductor junctions

Charge transport in the diffusive normal metal (DN) / insulator / $s$- and $% d $-wave superconductor junctions is studied in the presence of magnetic impurities in DN in the framework of the quasiclassical Usadel equations with the generalized boundary conditions. The cases of $s$- and d-wave superconducting electrodes are considered. The junction conductance is calculated as a function of a bias voltage for various parameters of the DN metal: resistivity, Thouless energy, the magnetic impurity scattering rate and the transparency of the insulating barrier between DN and a superconductor. It is shown that the proximity effect is suppressed by magnetic impurity scattering in DN for any value of the barrier transparency. In low-transparent s-wave junctions this leads to the suppression of the normalized zero-bias conductance. In contrast to that, in high transparent junctions zero-bias conductance is enhanced by magnetic impurity scattering. The physical origin of this effect is discussed. For the d-wave junctions, the dependence on the misorientation angle $α$ between the interface normal and the crystal axis of a superconductor is studied. The zero-bias conductance peak is suppressed by the magnetic impurity scattering only for low transparent junctions with $α\sim 0$. In other cases the conductance of the d-wave junctions does not depend on the magnetic impurity scattering due to strong suppression of the proximity effect by the midgap Andreev resonant states.

cond-mat.supr-con↗

arXiv2004

Measurement of $K^+ \to π^0 π^0 e^+ ν$ ($K_{e4}^{00}$) decay using stopped positive kaons

The $K^+ \to π^0 π^0 e^+ ν$ ($K_{e4}^{00}$) decay has been measured with stopped positive kaons for a data sample of 216 events. A comparison of the observed spectra with a Monte Carlo simulation determined the $K_{e4}^{00}$ form factor. The results are compatible with the $K^+ \to π^+ π^- e^+ ν$ data, as estimated from the $ΔI=1/2$ rule. We also established that the $K_{e4}^{00}$ channel can be used to determine the $π$-$π$ scattering lengths.

hep-ex↗