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T. Yamamoto

Publications and source records attributed to T. Yamamoto.

120 records · Page 7Linked to original sources
arXiv2002

Physical properties of misfit-layered (Bi,Pb)-Sr-Co-O system: Effect of hole doping into triangular lattice formed by low-spin Co ions

Pb-doping effect on physical properties of misfit-layered (Bi,Pb)-Sr-Co-O system, in which Co ions form a two-dimensional triangular lattice, was investigated in detail by electronic transport, magnetization and specific-heat measurements. Pb doping enhances the metallic behavior, suggesting that carriers are doped. Pb doping also enhances the magnetic correlation in this system and increases the magnetic transition temperature. We found the existence of the short-range magnetic correlation far above the transition temperature, which seems to induce the spin-glass state coexisting with the ferromagnetic long-range order at low temperatures. Specific-heat measurement suggests that the effective mass of the carrier in (Bi,Pb)-Sr-Co-O is not enhanced so much as reported in NaCo${}_2$O${}_4$. Based on these experimental results, we propose a two-bands model which consists of narrow $a_{1g}$ and rather broad $e'{}_g$ bands. The observed magnetic property and magnetotransport phenomena are explained well by this model.

cond-mat.str-el↗
arXiv2001

Charge echo in a Cooper-pair box

A spin-echo-type technique is applied to an artificial two-level system that utilizes charge degree of freedom in a small superconducting electrode. Gate-voltage pulses are used to produce the necessary pulse sequence in order to eliminate the inhomogeneity effect in the time-ensemble measurement and to obtain refocused echo signals. Comparison of the decay time of the observed echo signal with estimated decoherence time suggests that low-frequency energy-level fluctuations due to the 1/f charge noise dominate the dephasing in the system.

cond-mat.mes-hall↗
arXiv2001

Universal relation between the dispersion curve and the ground-state correlation length in 1D antiferromagnetic quantum spin systems

We discuss an universal relation $ε(iκ)=0$ with ${\rm Re} κ=1/ξ$ in 1D quantum spin systems with an excitation gap, where $ε(k)$ is the dispersion curve of the low-energy excitation and $ξ$ is the correlation length of the ground-state. We first discuss this relation for integrable models such as the Ising model in a transverse filed and the XYZ model. We secondly make a derivation of the relation for general cases, in connection with the equilibrium crystal shape in the corresponding 2D classical system. We finally verify the relation for the S=1 bilinear-biquadratic spin chain and $S=1/2$ zigzag spin ladder numerically.

cond-mat.stat-mech↗
arXiv2000

Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound

Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+δ}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at $T$ = 3.2 K with small spontaneous magnetization along the $c$ axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO${}_2$ layer.

cond-mat.str-el↗
arXiv2000

Updated AGASA event list above 4x10^19eV

After our Ap.J. publication of the Akeno Giant Air Shower Array (AGASA) results in 1999 (Takeda et al., 1999), we observed nine events with energies above 4 $\times$ 10$^{19}$eV until May 2000. This short report lists the coordinates of these events, and shows the updated energy spectrum and arrival direction map. The analysis was carried out with the same procedure employed in the Ap.J. publication.

astro-ph↗
arXiv2000

Small-scale anisotropy of cosmic rays above 10^19eV observed with the Akeno Giant Air Shower Array

With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV, 47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998. Arrival direction distribution of these extremely high energy cosmic rays has been studied. While no significant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 x 10^19eV, there are one triplet and three doublets within separation angle of 2.5^o and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1 %. Especially the triplet is observed against expected 0.05 events. The cos(θ_GC) distribution expected from the Dark Matter Halo model fits the data as well as an isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer fit than isotropy above 10^19eV. Arrival direction distribution of seven 10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of seven are members of doublets above about 4 x 10^19eV.

astro-ph↗
arXiv1999

The Anisotropy of Cosmic Ray Arrival Directions around 10$^{18}$eV

Anisotropy in the arrival directions of cosmic rays with energies above 10$^{17}$eV is studied using data from the Akeno 20 km$^2$ array and the Akeno Giant Air Shower Array (AGASA), using a total of about 117,000 showers observed during 11 years. In the first harmonic analysis, we have found strong anisotropy of $\sim$ 4% around 10$^{18}$eV, corresponding to a chance probability of 0.2%. With two dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region.

astro-ph↗
arXiv1998

Extension of the Cosmic-Ray Energy Spectrum Beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff

The cosmic-ray energy spectrum above 10^{18.5} eV is reported using the updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990 to October 1997. The energy spectrum extends beyond 10^{20} eV and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff.

astro-ph↗
arXiv1998

Universality of the Tangential Shape Exponent at the Facet Edge of a Crystal

Below the roughening temperature, the equilibrium crystal shape (ECS) is composed of both facets and a smoothly curved surface. As for the ``normal'' profile (perpendicular to the facet contour), the ECS has the exponent 3/2 which is characteristic of systems in the Gruber-Mullins-Pokrovsky-Talapov (GMPT) universality class. Quite recently,it was pointed out that the ECS have a ``new'' exponent 3 for ``tangential'' profile. We first show that this behavior is universal because it is a direct consequence of the well-establised universal form of the vicinal-surface free energy. Second, we give a universal relation between the amplitudes of the tangential and the normal profiles, in close connection with the universal Gaussian curvature jump at the facet edge in systems with short-range inter-step interactions. Effects of the long-range interactions are briefly discussed.

cond-mat.stat-mech↗
arXiv1995

Collective Field Description of Spin Calogero-Sutherland Models

Using the collective field technique, we give the description of the spin Calogero-Sutherland Model (CSM) in terms of free bosons. This approach can be applicable for arbitrary coupling constant and provides the bosonized Hamiltonian of the spin CSM. The boson Fock space can be identified with the Hilbert space of the spin CSM in the large $N$ limit. We show that the eigenstates corresponding to the Young diagram with a single row or column are represented by the vertex operators. We also derive a dual description of the Hamiltonian and comment on the construction of the general eigenstates.

hep-th↗