arXiv subjects

T. Ozaki

Publications and source records attributed to T. Ozaki.

At least 37 records · Page 2Linked to original sources

arXiv2016

Aligned copper nanorod arrays for highly efficient generation of intense ultra-broadband THz pulses

We demonstrate an intense broadband terahertz (THz) source based on the interaction of relativistic-intensity femtosecond lasers with aligned copper nanorod array targets. For copper nanorod targets with length 5 μm, a maximum 13.8 times enhancement in the THz pulse energy (in $\leq$ 20 THz spectral range) is measured as compared to that with a thick plane copper target under the same laser conditions. A further increase in the nanorod length leads to a decrease in the THz pulse energy at medium frequencies ($\leq$ 20THz) and increase of the electromagnetic pulse energy in the high-frequency range (from 20 - 200 THz). For the latter, we measure a maximum energy enhancement of 28 times for the nanorod targets of length 60 μm . Particle-in-cell simulations reveal that THz pulses are mostly generated by coherent transition radiation of laser produced hot electrons, which are efficiently enhanced with the use of nanorod targets. Good agreement is found between the simulation and experimental results.

physics.plasm-ph↗

arXiv2015

Substitution of Ni for Fe in superconducting Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ depresses the normal-state conductivity but not the magnetic spectral weight

We have performed systematic resistivity and inelastic neutron scattering measurements on Fe$_{0.98-z}$Ni$_z$Te$_{0.5}$Se$_{0.5}$ samples to study the impact of Ni substitution on the transport properties and the low-energy ($\le$ 12 meV) magnetic excitations. It is found that, with increasing Ni doping, both the conductivity and superconductivity are gradually suppressed; in contrast, the low-energy magnetic spectral weight changes little. Comparing with the impact of Co and Cu substitution, we find that the effects on conductivity and superconductivity for the same degree of substitution grow systematically as the atomic number of the substituent deviates from that of Fe. The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration. The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic correlations does not depend on electronic states close to the Fermi energy.

cond-mat.supr-con↗

arXiv2012

Superconductivity in Fe1+dTe0.9Se0.1 induced by deintercalation of excess Fe using alcoholic beverage treatment

Superconductivity in polycrystalline Fe1+dTe0.9Se0.1 was enhanced by heating in alcoholic beverages. We found that part of the excess Fe was deintercalated from the interlayer sites by the treatment and the shielding volume fractions were increased with the reduction of excess Fe content in sample. This behavior is similar to the case of Fe1+dTe1-xSx. Thus the technique of alcoholic beverage treatment can be applied generally to the Fe-chalcogenide superconductors.

cond-mat.supr-con↗

arXiv2012

Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighbouring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment.

physics.atom-ph↗

arXiv2012

Evolution of superconductivity in LaO1-xFxBiS2 prepared by high pressure technique

Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure synthesis technique were systematically studied. It was found that the high pressure annealing strongly the lattice as compared to the LaO1-xFxBiS2 samples prepared by conventional solid state reaction at ambient pressure. Bulk superconductivity was observed within a wide F-concentration range of x = 0.2 ~ 0.7. On the basis of those results, we have established a phase diagram of LaO1-xFxBiS2.

cond-mat.supr-con↗

arXiv2012

Fermiological Interpretation of Superconductivity/Non-superconductivity of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation Measurement

We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}. QO measurements were performed using non-superconducting and superconducting thin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We show that the Fermi surfaces (FS) of the non-superconducting crystal are in good agreement with the rigid band shift model based on electron doping by excess Fe while that of the superconducting crystal is in good agreement with the calculated FS with no shift. From the FS comparison of both crystals, we demonstrate the change of the cross-sectional area of the FS, suggesting that the suppression of the FS nesting with the vector Q_{s} = (π, π) due to excess Fe results in the disappearance of the superconductivity.

cond-mat.supr-con↗

arXiv2012

Phase diagram and oxygen annealing effect of FeTe1-xSex iron-based superconductor

Phase diagrams of as-grown and O2-annealed FeTe1-xSex determined from magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the range region x \geq 0.15, and weak superconductivity appeared when x \geq 0.1. Beginning at x = 0.5, weak superconductivity was found to evolve into bulk superconductivity. Interestingly, for O2-annealed samples, complete suppression of magnetic order and the occurrence of bulk superconductivity were observed when x \geq 0.1. We found that O2-annealing induces bulk superconductivity for FeTe1-xSex. Oxygen probably plays a key role in the suppression of the magnetic order and the appearance of bulk superconductivity.

cond-mat.supr-con↗

arXiv2012

Evidence of Inhomogeneous Superconductivity in FeTe$_{1-x}$Se$_x$ Thin Film Using Scotch-Tape Method

We have fabricated thin films of FeTe$_{1-x}$Se$_x$ using a scotch-tape method. The superconductivities of the thin films are different from each other although these films were fabricated from the same bulk sample. The result clearly presents the inhomogeneous superconductivity in FeTe$_{1-x}$Se$_x$. The difference comes from inhomogeneity due to the excess Fe concentration. The resistivity of a thin film with low excess Fe shows good superconductivity with the sharp superconducting-transition width and more isotropic superconductivity.

cond-mat.supr-con↗

arXiv2012

Clarification as to why alcoholic beverages have the ability to induce superconductivity in Fe_{1+d}Te_{1-x}S_x

To elucidate the mechanism as to why alcoholic beverages can induce superconductivity in Fe_{1+d}Te_{1-x}S_x samples, we performed component analysis and found that weak acid such as organic acid has the ability to induce superconductivity. Inductively-coupled plasma spectroscopy was performed on weak acid solutions post annealing. We found that the mechanism of inducement of superconductivity in Fe_{1+d}Te_{1-x}S_x is the deintercalation of excess Fe from the interlayer sites.

cond-mat.supr-con↗

arXiv2011

Transport properties of iron-based FeTe0.5Se0.5 superconducting wire

FeTe0.5Se0.5 superconducting wires have been fabricated using ex-situ PIT method with an Fe sheath. Among the other FeAs-based superconductor, FeTe0.5Se0.5 has great advantage for application due to binary composition and less toxic. Surprisingly, superconducting current was observed in the as-fabricated wire without any heat treatments. Zero resistivity and transport critical current density (Jc) were 3.2 K and 2.8 A/cm2, respectively. In addition, by annealing at 200 degC for 2 h, critical temperature Tczero and Jc were enhanced up to 9.1 K, and 64.1 A/cm2, respectively. This suggests that the grain connectivity was improved by heat treatment, and superconducting property of FeTe0.5Se0.5 wire was enhanced.

cond-mat.supr-con↗

arXiv2011

Microstructure and transport properties of FeTe0.5Se0.5 superconducting wires fabricated by ex-situ Powder-in-tube process

We fabricated FeTe0.5Se0.5 superconducting wires using ex-situ Powder-in-tube method with an Fe sheath. It was amazing that superconducting current was observed in the as-drawn wire without any heat treatments. By heat treatment at 200C for 2 hours, Tczero and Jc at 4.2 K were enhanced up to 9.1 K and 64.1 A/cm2 (Ic = 182.6 mA), respectively. Furthermore, the Jc of FeTe0.5Se0.5 wire heat treated at 200C for 2 hours was not much sensitive to the applied magnetic fields. Therefore, FeTe0.5Se0.5 wires have a great potential for applications.

cond-mat.supr-con↗

arXiv2011

High-precision CTE measurement of hybrid C/SiC composite for cryogenic space telescopes

This paper presents highly precise measurements of thermal expansion of a "hybrid" carbon-fiber reinforced silicon carbide composite, HB-Cesic\textregistered - a trademark of ECM, in the temperature region of \sim310-10K. Whilst C/SiC composites have been considered to be promising for the mirrors and other structures of space-borne cryogenic telescopes, the anisotropic thermal expansion has been a potential disadvantage of this material. HB-Cesic\textregistered is a newly developed composite using a mixture of different types of chopped, short carbon-fiber, in which one of the important aims of the development was to reduce the anisotropy. The measurements indicate that the anisotropy was much reduced down to 4% as a result of hybridization. The thermal expansion data obtained are presented as functions of temperature using eighth-order polynomials separately for the horizontal (XY-) and vertical (Z-) directions of the fabrication process. The average CTEs and their dispersion (1σ) in the range 293-10K derived from the data for the XY- and Z-directions were 0.805$\pm$0.003\times10$^{-6}$ K$^{-1}$ and 0.837\pm0.001\times10$^{-6}$ K$^{-1}$, respectively. The absolute accuracy and the reproducibility of the present measurements are suggested to be better than 0.01\times10$^{-6}$ K$^{-1}$ and 0.001\times(10)^{-6} K^{-1}, respectively. The residual anisotropy of the thermal expansion was consistent with our previous speculation regarding carbon-fiber, in which the residual anisotropy tended to lie mainly in the horizontal plane.

astro-ph.IM↗

arXiv2011

Semianalytical treatment of current density of particles injected by a monoenergetic source

This paper extends the semianalytical treatment of fast ion current density to take into account time dependence and velocity diffusion using solutions of Boltzmann equation with a complete Coulomb collision term (Goncharov et al. 2010 Phys. Plasmas 17 112313). An arbitrary angle distribution of the fast ion source is assumed. The results are applicable to multi-ion-species plasma. Since analytical results provide an extra physical insight and constitute a reliable basis for verification of numerical codes, it is desirable to obtain exact solutions where possible. New results clarify the discrepancies between analytical formulae in earlier bibliography and improve the physical basis of neutral beam injection current drive.

physics.plasm-ph↗

arXiv2011

Pressure study of the new iron-based superconductor K0.8Fe2Se2

We investigated pressure effects on transition temperature (Tc) of the new iron-based superconductor K0.8Fe2Se2 using a BeCu/NiCrAl hybrid-type clamped piston-cylinder cell. The Tc(onset) was 33K at 0.85 GPa. With increasing pressure, Tc(onset) gradually increased and reached 36.6 K at 2.03 GPa.

cond-mat.supr-con↗

arXiv2010

Anisotropic ultrafast electron dynamics induced by high-field terahertz pulses in n-doped InGaAs

The anisotropic effective mass of electrons is directly measured using time-resolved THz- pump/THz-probe techniques in a n-doped InGaAs semiconductor thin film. A microscopic theory is used to attribute this anisotropy in the THz probe transmission to the nonparabolicity of the conduction band. Self-consistent light-matter coupling is shown to contribute significantly to the THz response.

cond-mat.mtrl-sci↗

arXiv2010

Design and performance of the muon monitor for the T2K neutrino oscillation experiment

This article describes the design and performance of the muon monitor for the T2K (Tokaito-Kamioka) long baseline neutrino oscillation experiment. The muon monitor consists of two types of detector arrays: ionization chambers and silicon PIN photodiodes. It measures the intensity and profile of muons produced, along with neutrinos, in the decay of pions. The measurement is sensitive to the intensity and direction of the neutrino beam. The linearity and stability of the detectors were measured in beam tests to be within 2.4% and 1.5%, respectively. Based on the test results, the precision of the beam direction measured by the muon monitor is expected to be 0.25 mrad.

physics.ins-det↗

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↗

arXiv2001

Convergent recursive O(N) calculations for ab initio tight-binding

A theory is presented for a novel recursion method for O(N) ab initio tight-binding calculations. A long-standing problem of generalizing the recursion method to a non-orthogonal basis, which is a crucial step to make the recursion method applicable to ab initio calculations, is solved by the introduction of the hybrid representation and the two-sided block Lanczos algorithm. As a test of efficiency of the new method, convergence properties in energy and force of insulators, semiconductors, metals, and molecules are studied for not only simple model systems but also some real materials within the density functional theory. The present O(N) method possesses good convergence properties for metals as well as insulators.

cond-mat↗