arXiv subjects

K. Nakao

Publications and source records attributed to K. Nakao.

11 recordsLinked to original sources

arXiv2017

The status of KAGRA underground cryogenic gravitational wave telescope

KAGRA is a 3-km interferometric gravitational wave telescope located in the Kamioka mine in Japan. It is the first km-class gravitational wave telescope constructed underground to reduce seismic noise, and the first km-class telescope to use cryogenic cooling of test masses to reduce thermal noise. The construction of the infrastructure to house the interferometer in the tunnel, and the initial phase operation of the interferometer with a simple 3-km Michelson configuration have been completed. The first cryogenic operation is expected in 2018, and the observing runs with a full interferometer are expected in 2020s. The basic interferometer configuration and the current status of KAGRA are described.

gr-qc↗

arXiv2010

Electrical transport properties of high quality Bi-2223 crystal

We have studied electrical transport properties of a high quality Bi2Sr2Ca2Cu3O10+x crystal below a superconducting critical temperature, Tc. In magnetic fields B parallel to the c-axis just above a voltage response onset resistance vs temperature data are well fitted to the vortex-glass model. Obtained from the vortex-glass analysis a melting transition boundary for Bi-2223 crystal shifted towards lower temperatures compared to previously reported data for a (Bi,Pb)-2223 single crystal. The critical current density, Jc, of Bi-2223 crystals is close to presented elsewhere Jc values for Bi-2223 tapes suggesting a principal role of weak intrinsic pinning properties of Bi-2223 as a main limiting factor of Jc of Bi-2223 conductors.

cond-mat.supr-con↗

arXiv2009

Electric transport properties of YBCO bicrystal films with 45o misorientation angle grown by liquid phase epitaxy

We report on transport properties of grain boundaries fabricated in YBa2Cu3O7-x thin films grown by the liquid phase epitaxy (LPE) technique on MgO asymmetrical bicrystal substrate with 45o misorientation angle. In total around 10 samples have been studied. Substantial scatter of zero field values of the critical current density at 5K has been observed. The upper limit of Jc of the order of 104 A/cm2 found in our study is close to previously reported data for 45o bicrystals grown by various physical vapour deposition methods while the minimal value of Jc for the LPE grown bicrystals in striking difference to the results published before is exactly equal to zero. For samples with non-zero Jc we have found a few different types of critical current dependence on magnetic field ranging from pattern reminiscent Fraunhover-like Ic(H) to Ic(H) profile with Ic minimum at zero field.

cond-mat.supr-con↗

arXiv2009

Vortex glass scaling in Pb-doped Bi2223 single crystal

We report on study of the vortex liquid in Pb-doped Bi-2223 single crystal using the in-plane resistivity measurements as a function of temperature and magnetic field up to 6T applied perpendicular to CuO planes. Below Tc at the upper part of superconducting transition we found Arrhenius-like resistivity behavior. With further temperature decrease close to resistivity onset resistivity shows power law dependence on temperature signaling approaching vortex-glass transition. The critical exponents nu(z-1)=4.6 plus-minus 0.5 are found to be field independent within experimental errors. We also present magnetic phase diagram defining region of nonzero critical current for Pb-doped Bi-2223 single crystal.

cond-mat.supr-con↗

arXiv2009

Quantum circuits for solving one-dimensional Schrödinger equations

We construct quantum circuits for solving one-dimensional Schrödinger equations. Simulations of three typical examples, i.e., harmonic oscillator, square-well and Coulomb potential, show that reasonable results can be obtained with eight qubits. Our simulations show that simple quantum circuits can solve the standard quantum mechanical problems.

quant-ph↗

arXiv2007

Compensation of the Crossing Angle with Crab Cavities at KEKB

Crab cavities have been installed in the KEKB B--Factory rings to compensate the crossing angle at the collision point and thus increase luminosity. The beam operation with crab crossing has been done since February 2007. This is the first experience with such cavities in colliders or storage rings. The crab cavities have been working without serious issues. While higher specific luminosity than the geometrical gain has been achieved, further study is necessary and under way to reach the prediction of simulation.

physics.ins-det↗

arXiv2002

Anisotropic resistivity and Hall effect in MgB2 single crystals

We report resistivity and the Hall effect measurements in the normal and superconducting states of MgB2 single crystal. The resistivity has been found to be anisotropic with slightly temperature dependent resistivity ratio of about 3.5. The Hall constant, with a magnetic field parallel to the Mg and B sheets is negative in contrast to the hole-like Hall response with a field directed along the c-axis indicating presence of both types of charge carriers and, thus, multi-band electronic structure of MgB2. The Hall effect in the mixed state shows no sign change anomaly reproducing the Hall effect behavior in clean limit type-II superconductors.

cond-mat.supr-con↗

arXiv2002

Anisotropic Superconducting Properties of MgB2 Single Crystals

In-plane electrical transport properties of MgB2 single crystals grown under high pressure of 4-6 GPa and temperature of 1400-1700oC in Mg-B-N system have been measured. For all specimens we found sharp superconducting transition around 38.1-38.3K with transition width within 0.2-0.3K. Estimated resistivity value at 40K is about 1 mkOhmcm and resistivity ratio R(273K)/R(40K) of about 4.9. Results of measurements in magnetic field up to 5.5T perpendicular to Mg and B planes and up to 9T in parallel orientation show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Strong deviation of the angular dependence of Hc2 from anisotropic mass model has been also found.

cond-mat.supr-con↗

arXiv2002

Anisotropic superconducting properties of MgB2 single crystals probed by in-plane electrical transport measurements

We report on study of the electronic anisotropy of the newly discovered MgB2 superconductor using the in-plane resistivity measurements in magnetic field applied perpendicular and parallel to Mg and B planes of MgB2 single crystals. The results show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Our estimation of the in-plane and out-of-plane coherence length of about 68A and 23A and electronic mean-free path 240A and 60A respectively indicates MgB2 single crystal as approaching clean limit type-II superconductor.

cond-mat.supr-con↗

arXiv2001

Two-Dimensional Sigma-Hole Systems in Boron Layers: A First-Principles Study on Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2

We study two-dimensional sigma-hole systems in boron layers by calculating the electronic structures of Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2. In Mg_{1-x}Na_xB_2, it is found that the concentration of sigma holes is approximately described by (0.8 + 0.8 x) * 10^{22} cm^{-3} and the largest attainable concentration is about 1.6 * 10^{22} cm^{-3} in NaB_2. In Mg_{1-x}Al_xB_2, on the other hand, it is found that the concentration of sigma holes is approximately described by (0.8 - 1.4 x) * 10^{22} cm^{-3} and sigma holes are disappeared at x of about 0.6. These relations can be used for experimental studies on the sigma-hole systems in these materials.

cond-mat.supr-con↗

arXiv2000

Newtonian Analysis of Gravitational Waves from Naked Singularity

Spherical dust collapse generally forms a shell focusing naked singularity at the symmetric center. This naked singularity is massless. Further the Newtonian gravitational potential and speed of the dust fluid elements are everywhere much smaller than unity until the central shell focusing naked singularity formation if an appropriate initial condition is set up. Although such a situation is highly relativistic, the analysis by the Newtonian approximation scheme is available even in the vicinity of the space-time singularity. This remarkable feature makes the analysis of such singularity formation very easy. We investigate non-spherical even-parity matter perturbations in this scheme by complementary using numerical and semi-analytical approaches, and estimate linear gravitational waves generated in the neighborhood of the naked singularity by the quadrupole formula. The result shows good agreement with the relativistic perturbation analysis recently performed by Iguchi et al. The energy flux of the gravitational waves is finite but the space-time curvature carried by them diverges.

gr-qc↗