arXiv subjects

H. Fujii

Publications and source records attributed to H. Fujii.

66 records · Page 4Linked to original sources

arXiv2002

Study of superconducting properties of MgB2

We synthesized single crystalline and policrystalline MgB_2 under ambient pressures. The single crystals of MgB_2 were of good quality, where the crystal structure refinements were successfully converged with R = 0.020. The specific heat of policrystalline MgB_2 samples has been measured in a temperature range between 2 and 60 K in magnetic field up to 6 T. The measurement gave the coefficient of the linear term in the electronic specific heat, gamma = 3.51 mJ/K^2 mol, and the jump of the specific heat, 2.8 mJ/K^2 mol at 38.5 K. It is shown from the analysis of the specific heat that the electronic specific heat in the superconducting state differs largely from the conventional BCS weak coupling theory. From the results of measurements of the magnetic properties on single crystal samples, we found a sharp superconducting transition at 38 K with transition width Delta Tc = 0.8 K and the superconducting anisotropy ratio gamma increasing from about 1 near Tc to 4.0 at 25 K.

cond-mat.supr-con↗

arXiv2002

Penetration of a high energy QQbar bound state through SU(N) color background

Attenuation of the singlet quark--anti-quark bound state passing through a random external SU(N) color field at high energy is studied within the eikonal framework as a model for the absorption of a quarkonium by nuclear target. For a thick target of length L the penetration probability is suppressed non-exponentially and behaves asymptotically $\propto 1/LN^2$ with 1/N^2 indicating the equi-partitioning in the color space. An approximate rational expression for the penetration of the small Gaussian bound state is found which allows the explicit L and N dependences, and shows the attenuation is in the powers of L although the approach to the asymptote is rather slow.

nucl-th↗

arXiv2002

A New Approach for the Fabrication of MgB2 Superconducting Tape with Large In-field Transport Critical Current Density

The paper reports the first successful fabrication of MgB2 superconducting tape using a flexible metallic substrate as well as its strong pinning force, which was verified by direct measurement of transport critical current density. The tape was prepared by depositing MgB2 film on a Hastelloy tape buffered with an YSZ layer. The Jc of the tape exceeds 105A/cm2 at 4.2K and 10T, which is considered as a common benchmark for magnet application. The Jc dependence on magnetic field remains surprisingly very small up to 10T, suggesting that the tape has much better magnetic field characteristic than conventional Nb-Ti wires in liquid helium.

cond-mat.supr-con↗

arXiv2001

A Control System of the Joint-Project Accelerator Complex

The current status of the control system for a new high intensity proton accelerator, the JAERI-KEK Joint Project, is presented. The Phase 1 of the Joint-Project has been approved and recently started its construction at JAERI site at Tokai. The first beam commissioning is scheduled in 2006. In parallel with it, a 60-MeV Linac is now being constructed at KEK site at Tsukuba for R&D purpose. Recently the Project has officially decided to use the Experimental Physics and Industrial Control System (EPICS). Under the EPICS environment, we are challenging to implement the Ethernet/IP network for all communication, even at the level of end-point controllers which are so far connected via a field bus. In order to realize such a system, three new controllers (PLCs, WE7000 stations and general-purpose Ethernet boards) are being developed. A prototype EPICS driver for the PLCs works fine and is used to control the ion-source at the KEK Linac.

physics.acc-ph↗

arXiv2001

High transport critical current density obtained for Powder-In-Tube-processed MgB2 tapes and wires using stainless steel and Cu-Ni tubes

MgB2 tapes and wires were fabricated by the Powder-In-Tube method. Stainless steel and Cu-Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10,000A/cm2 at 4.2K and 5T. A high Jc of about 300,000A/cm2 was obtained by extrapolating the Jc-B curves to zero field. Multifilamentary(7-core) MgB2 wire was successfully fabricated using Cu-Ni tubes. For both tapes and wires the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu-Ni sheathed wire was lower than the stainless steel sheathed tape due to the lower packing density of MgB2.

cond-mat.supr-con↗

arXiv2001

Superconducting Properties of MgB2 Bulk Materials Prepared by High Pressure Sintering

High-density bulk materials of a newly discovered 40K intermetallic MgB2 superconductor were prepared by high pressure sintering. Superconducting transition with the onset temperature of 39K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M-H) curve shows the behavior of a typical Type II superconductor and the lower critical field Hc1(0) estimated from M-H curve is 0.032T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 2x104A/cm2 at 20K and 1T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18T.

cond-mat.supr-con↗

arXiv1999

Test by JANZOS of the Standard Model of Cosmic Ray Acceleration in the COMPTEL/ROSAT Supernova Renmant

A search for ultra-high energy gamma-rays emitted by the young, nearby supernova remnant that was discovered recently by the COMPTEL and ROSAT satellites was made using the JANZOS database for the period 1987-1993. A 95% confidence upper limit on the flux above 100 TeV of 3x10^(-13)cm^(-2)sec^(-1) was obtained. This is an order of magnitude below the expected flux based on the standard model of cosmic ray acceleration in supernova shocks. An optical survey of the region that has been commenced is also reported. This uses UK and ESO Schmidt plates, and CCD images by a NZ/Japan microlensing group.

astro-ph↗

arXiv1999

Scalar Glueball--Quarkonium Mixing and the Structure of the QCD Vacuum

We use Ward identities of broken scale invariance to infer the amount of scalar glueball--$\bar{q}q$ meson mixing from the ratio of quark and gluon condensates in the QCD vacuum. Assuming dominance by a single scalar state, as suggested by a phase-shift analysis, we find a mixing angle $γ\sim 36^{\circ}$, corresponding to near-maximal mixing of the glueball and $\bar{s}s$ components.

hep-ph↗

arXiv1999

Long-Range Forces of QCD

We consider the scattering of two color dipoles (e.g., heavy quarkonium states) at low energy - a QCD analog of Van der Waals interaction. Even though the couplings of the dipoles to the gluon field can be described in perturbation theory, which leads to the potential proportional to (N_c^2-1)/R^{7}, at large distances R the interaction becomes totally non-perturbative. Low-energy QCD theorems are used to evaluate the leading long-distance contribution \sim (N_f^2-1)/(11N_c - 2N_f)^2 R^{-5/2} exp(-2 μR) (μis the Goldstone boson mass), which is shown to arise from the correlated two-boson exchange. The sum rule which relates the overall strength of the interaction to the energy density of QCD vacuum is derived. Surprisingly, we find that when the size of the dipoles shrinks to zero (the heavy quark limit in the case of quarkonia), the non-perturbative part of the interaction vanishes more slowly than the perturbative part as a consequence of scale anomaly. As an application, we evaluate elastic πJ/ψand πJ/ψ\to πψ' cross sections.

hep-ph↗

arXiv1999

Interactions of quarkonium at low energies

We present two examples of the short-distance QCD calculations for the quarkonium (Φ) interactions; 1) πΦelastic and πΦ-> πΦ' cross sections, and 2) the potential between two Φ's. The former is relevant for the Φsuppression in heavy ion collisions; we find that the corresponding cross sections are very small. For the latter we derive the sum rule which relates the strength of the potential to the energy density of the QCD vacuum. The key starting point in both cases is that the leading operator in the operator product expansion (OPE) for the low--energy scattering amplitude is the trace of the QCD stress tensor whose matrix elements are unambiguously fixed by the low energy theorem. The resonances in the two-pion scalar channel are taken into account as a formfactor.

nucl-th↗

arXiv1998

Long-Range Interactions of Small Color Dipoles

We study the scattering of small color dipoles (e.g., heavy quarkonium states) at low energies. We find that even though the couplings of color dipoles to the gluon field can be described in perturbation theory, at large distances the interaction becomes totally non--perturbative. The structure of the scattering amplitude, however, is fixed by the (broken) chiral and scale symmetries of QCD; the leading long--distance contribution arises from the correlated two--pion exchange. We use the spectral representation technique to evaluate both perturbative and non-perturbative contributions to the scattering amplitude. Our main result is the sum rule which relates the overall strength of the non--perturbative interaction between color dipoles to the energy density of QCD vacuum.

hep-ph↗

arXiv1993

Muonium Hyperfine Structure and the Decay $ μ^+\to e^+ +\overline ν_e +ν_μ$ in Models with Dilepton Gauge Bosons

We examine the muonium ($μ^+e^-$)-antimuonium ($μ^-e^+$) system in the models which accomodate the dilepton gauge bosons, and study their contributions to the ground state hyperfine splitting in ``muonium''. We also consider the exotic muon decay $μ^+\to e^+ +\overline ν_e +ν_μ$ mediated by the dilepton gauge boson, and obtain a lower bound $(M_{X^{\pm }}/g_{3l})>550 \rm GeV$ at 90\% confidence level for the singly-charged dilepton mass using the unitarity relation of the Kobayashi-Maskawa matrix for the 3-family case.

hep-ph↗