arXiv subjects

H. Matsui

Publications and source records attributed to H. Matsui.

11 recordsLinked to original sources

arXiv2016

Galactic-Center Hyper-Shell Model for the North Polar Spurs

The bipolar-hyper shell (BHS) model for the North Polar Spurs (NPS-E, -W, and Loop I) and counter southern spurs (SPS-E and -W) is revisited based on numerical hydrodynamical simulations. Propagations of shock waves produced by energetic explosive events in the Galactic Center are examined. Distributions of soft X-ray brightness on the sky at 0.25, 0.7, and 1.5 keV in a +/-50 deg x +/-50 deg region around the Galactic Center are modeled by thermal emission from high-temperature plasma in the shock-compressed shell considering shadowing by the interstellar HI and H2 gases. The result is compared with the ROSAT wide field X-ray images in R2, 4 and 6 bands. The NPS and southern spurs are well reproduced by the simulation as shadowed dumbbell-shaped shock waves. We discuss the origin and energetics of the event in relation to the starburst and/or AGN activities in the Galactic Center. [ High resolution pdf is available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016bhs/ ]

astro-ph.GA↗

arXiv2010

Cupric chloride CuCl2 as an S=1/2 chain multiferroic

Magnetoelectric properties were investigated for an S=1/2 chain antiferromagnet CuCl2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. Upon the onset of helimagnetic order propagating along the b-axis under zero magnetic field (H), we found emergence of ferroelectric polarization along the c-axis. Application of H along the b-axis leads to spin-flop transition coupled with drastic suppression of ferroelectricity, and rotation of H around the b-axis induces the rotation of spin-spiral plane and associated polarization direction. These behaviors are explained well within the framework of the inverse Dzyaloshinskii-Moriya model, suggesting the robustness of this magnetoelectric coupling mechanism even under the strong quantum fluctuation.

cond-mat.str-el↗

arXiv2008

Non-thermal Electrons at the Earth's Bow Shock: A `Gradual' Event

Earth's bow shock is known to produce non-thermal electrons which are generally observed as a `spike' in their flux profile. Here, in this paper, we present an analysis of electron and whistler wave properties for a quasi-perpendicular shock crossing that is supercritical, but subcritical to the so-called whistler critical Mach number, M$^w_{\rm crit}$, above which whistler waves cannot propagate upstream. We have found that the amplitudes of whistler waves increased exponentially as a function of time prior to the shock encounter, while the suprathermal ($>$ 2 keV) electron flux similarly increased with time, although with differing $e$-folding time scales. Comparison of the electron energy spectrum measured within the ramp with predictions from diffusive shock acceleration theory was poor, but the variation of pitch angle distribution showed scattering of non-thermal electrons in the upstream region. While not finding a specific mechanism to account for the electron diffusion, we suggest that the whistlers seen probably account for the differences observed between this `gradual' event and the `spike' events seen at shocks with no upstream whistlers.

astro-ph↗

arXiv2006

Bulk and surface low-energy excitations in YBa2Cu3O7-d studied by high-resolution angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-Tc cuprates. We unambiguously identified surface-bilayer-derived bonding and antibonding bands, together with their bulk counterparts. The surface bands are highly overdoped (hole concentration x = 0.29), showing no evidence for the gap opening or the dispersion anomaly in the antinodal region, while the bulk bands show a clear dx2-y2-wave superconducting gap and the Bogoliubov quasiparticle-like behavior with a characteristic energy scale of 50-60 meV indicative of a strong electron-boson coupling in the superconducting state. All these results suggest that the metallic and superconducting states coexist at the adjacent bilayer of Y123 surface.

cond-mat.supr-con↗

arXiv2006

Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

cond-mat.supr-con↗

arXiv2004

Angle-Resolved Photoemission Spectroscopy of the Antiferromagnetic Superconductor Nd1.87Ce0.13CuO4: Anisotropic Spin-Correlation Gap, Pseudogap, and the Induced Quasiparticle Mass Enhancement

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi, 0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Neel temperature (TN) while the AF pseudogap is gradually filled up at much higher temperatures, possibly due to the short-range AF correlation.

cond-mat.supr-con↗

arXiv2004

Direct Observation of Non-Monotonic dx2-y2-Wave Superconducting Gap in Electron-Doped High-Tc Superconductor Pr0.89LaCe0.11CuO4

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2-wave symmetry, but obviously deviates from the monotonic dx2-y2 gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results unambiguously indicate the spin-mediated pairing mechanism in electron-doped high-Tc superconductors.

cond-mat.supr-con↗

arXiv2004

Fermi surface topology of Ca_{1.5}Sr_{0.5}RuO_{4} determined by ARPES

We report ARPES results of the Fermi surface of Ca_{1.5}Sr_{0.5}RuO$_{4}, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t_{2g} energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr_{2}RuO$_{4}. We find that while the Fermi surface topology of the alpha, beta (d_{yz, zx}) sheets remains almost the same in these two materials, the gamma (d_{xy}) sheet exhibits a hole-like Fermi surface in Ca_{1.5}Sr_{0.5}RuO_{4} in contrast to being electron-like in Sr_{2}RuO_{4}. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca_{1.5}Sr_{0.5}RuO_{4}.

cond-mat.str-el↗

arXiv2003

ARPES on Na$_{0.6}$CoO$_{2}$: Fermi surface, extended flat dispersion, and unusual band splitting

The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c \sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($Γ$-$K$, $Γ$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $Γ$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $Γ$-$M$ direction, which differs from the predicted bilayer splitting.

cond-mat.supr-con↗

arXiv2003

BCS-like Bogoliubov Quasiparticles in High-Tc Superconductors Observed by Angle-Resolved Photoemission Spectroscopy

We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-Tc cuprate Bi2Sr2Ca2Cu3O10+delta. We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below EF as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.

cond-mat.supr-con↗

arXiv2001

Quasiparticles and Energy Scaling in Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4}$ ($\it{n}$=1-3): Angle-Resolved Photoemission Spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) has been performed on the single- to triple-layered Bi-family high-{\it T$_c$} superconductors (Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4}$, $\it{n}$=1-3). We found a sharp quasiparticle peak as well as a pseudogap at the Fermi level in the triple-layered compound. Comparison among three compounds has revealed a universal rule that the characteristic energies of superconducting and pseudogap behaviors are scaled with the maximum {\it T$_c$}.

cond-mat.supr-con↗