arXiv subjects

T. Sasaki

Publications and source records attributed to T. Sasaki.

At least 127 records · Page 7Linked to original sources

arXiv2003

Impurity Effect on the In-plane Penetration Depth of the Organic Superconductors $κ$-(BEDT-TTF)$_2X$ ($X$ = Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br)

We report the in-plane penetration depth $λ_{\parallel}$ of single crystals $κ$-(BEDT-TTF)$_2X$ ($X=$ Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br) by means of the reversible magnetization measurements under the control of cooling-rate. In $X$ = Cu(NCS)$_2$, $λ_{\parallel}(0)$ as an extrapolation toward $T$ = 0 K does not change by the cooling-rate within the experimental accuracy, while $T_{\textrm{c}}$ is slightly reduced. On the other hand, in $X$ = Cu[N(CN)$_2$]Br, $λ_{\parallel}(0)$ indicates a distinct increase by cooling faster. The different behavior of $λ_{\parallel}(0)$ on cooling-rate between the two salts is quantitatively explained in terms of the local-clean approximation (London model), considering that the former salt belongs to the very clean system and the later the moderate clean one. The good agreement with this model demonstrates that disorders of ethylene-group in BEDT-TTF introduced by cooling faster increase the electron(quasiparticle)-scattering, resulting in shorter mean free path.

cond-mat.supr-con↗

arXiv2003

The role of the water molecules in novel superconductor, Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O

In order to investigate the role of the water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O, we synthesized superconducting Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O and nonsuperconducting Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O, and measured their normal-state magnetic susceptibilities. The susceptibility of Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O has an enhancement below ~150 K probably caused by ferromagnetic fluctuation, whereas no such enhancement was observed in Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O. The water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O may work to shield random coulomb potential of the Na ions with smoother potential at the CoO$_{2}$ layer. This effect may account for the appearance of superconductivity in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O.

cond-mat.supr-con↗

arXiv2003

Electron correlation in the two-dimensional triangle lattice of Na$_x$CoO$_2$

Magnetism of layered cobaltites Na$_x$CoO$_2$ with $x$ = 0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation ($μ^+$SR) spectroscopy together with magnetic susceptibility and specific heat measurements, using single crystal samples in the temperature range between 250 and 1.8 K. Zero-field (ZF-) $μ^+$SR measurements on Na$_{0.9}$CoO$_2$ indicates a transition from a paramagnetic to an incommensurate spin density wave state at 19 K(=$T_{\sf SDW}$). The anisotropic ZF-$μ^+$SR spectra suggest that the oscillating moments of the {\sf IC-SDW} directs along the c-axis. Since Na$_{0.6}$CoO$_2$ is paramagnetic down to 1.8 K, the magnitude of $T_{\sf SDW}$ is found to strongly depend on $x$.This behavior is well explained using the Hubbard model within a mean field approximation on two-dimensional triangle lattice in the CoO$_2$ plane. Also, both the appearance of the {\sf IC-SDW} state by the change in $x$ and the magnitude of the electronic specific heat parameter of Na$_{0.6}$CoO$_2$ indicate that Na$_x$CoO$_2$ is unlikely to be a typical strongly correlated electron system.

cond-mat.str-el↗

arXiv2003

Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites

Using muon spin spectroscopy we have found that, for both Na$_x$CoO$_2$ (0.6 $\leq x \leq$ 0.9) and 3- and 4-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO$_2$ planes. Here we summarize those results and report a dome-shaped relation between the transition temperature into the low-$T$ magnetic state and the composition $x$ for Na$_x$CoO$_2$ and/or the high-temperature asymptotic limit of thermopower in the more complex 3- and 4-layer cobaltites. This behavior is explained using the Hubbard model on two-dimensional triangular lattice in the CoO$_2$ plane.

cond-mat.str-el↗

arXiv2003

Subaru Deep Spectroscopy of the Very Extended Emission-Line Region of NGC 4388: Ram Pressure Stripped Gas Ionized by the Nuclear Radiation

We report here the results of deep optical spectroscopy of the very extended emission-line region (VEELR) found serendipitously around the Seyfert 2 galaxy NGC 4388 in the Virgo cluster using the Subaru Telescope. The H-alpha recession velocities of most of the filaments of the region observed are highly blue-shifted with respect to the systemic velocity of the galaxy. The velocity field is complicated, and from the kinematic and morphological points of view, there seem to be several streams of filaments: low velocity (v = -100 km/s) filaments, high velocity (v = -300 km/s) filaments, and a very high velocity (v > -500 km/s) cloud complex. The emission-line ratios of the VEELR filaments are well explained by power-law photoionization models with solar abundances, suggesting that the Seyfert nucleus of NGC 4388 is the dominant ionization source of the VEELR and that the VEELR gas has moderate metallicity. In addition to photoionization, shock heating probably contributes to the ionization of the gas. In particular, the filaments outside the ionization cone of the Seyfert nucleus are mainly excited by shocks. We conclude that the VEELR was formerly the disk gas of NGC 4388, which has been stripped by ram pressure due to the interaction between the hot intra-cluster medium (ICM) and the galaxy. The velocity field and the morphology of the VEELR closely resemble snapshots from some numerical simulations of this process. In the case of NGC 4388, the ram pressure-stripped gas, which is normally seen as extended HI filaments, happens to be exposed and ionized by the radiation from the AGN, and so can be seen as optical emission-line gas.

astro-ph↗

arXiv2003

Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

We report on a Raman scattering study on the superconducting cobaltite $Na_xCoO_2\cdot yH_2O$ as function of Na content and hydration (x$\approx$1/3, 3/4 and y$\approx$0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two $H_2O$ layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the $a_{1g}$ level from the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.

cond-mat.str-el↗

arXiv2003

Disorder Effect on the Vortex Pinning by the Cooling Process Control in the Organic Superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br

We investigate the influence of disorders in terminal ethylene groups of BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br. Magnetization measurements are performed under different cooling-processes. The second peak in the magnetization hysteresis curve is observed for all samples studied, and the hysteresis width of the magnetization becomes narrower by cooling faster. In contradiction to the simple pinning effect of disorder, this result shows the suppression of the vortex pinning force by introducing more ethylene-disorders. The ethylene-disorder domain model is proposed for explaining the observed result. In the case of the system containing a moderate number of the ethylene-disorders, the disordered molecules form a domain structure and it works as an effective pinning site. On the contrary, an excess number of the ethylene-disorders may weaken the effect of the domain structure, which results in the less effective pinning force on the vortices.

cond-mat.supr-con↗

arXiv2003

Electronic correlation in the infrared optical properties of the quasi two dimensional $κ$-type BEDT-TTF dimer system

The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]$Y$, $Y =$ Br and Cl are measured in the infrared region. The former shows the superconductivity at $T_{\rm c} \simeq$ 11.6 K and the latter does the antiferromagnetic insulator transition at $T_{\rm N} \simeq$ 28 K. Both the specific molecular vibration mode $ν_{3}(a_{g})$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at $T^{*} \simeq$ 38 K of $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, although no indication of $T^{*}$ but the insulating behaviour below $T_{\rm ins} \simeq$ 50-60 K are found in $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl. The results suggest that the electron-molecular vibration coupling on the $ν_{3}(a_{g})$ mode becomes weak due to the enhancement of the itinerant nature of the carriers on the dimer of the BEDT-TTF molecules below $T^{*}$, while it does strong below $T_{\rm ins}$ because of the localized carriers on the dimer. These changes are in agreement with the reduction and the enhancement of the mid-infrared conductivity hump below $T^{*}$ and $T_{\rm ins}$, respectively, which originates from the transitions between the upper and lower Mott-Hubbard bands. The present observations demonstrate that two different metallic states of $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br are regarded as {\it a correlated good metal} below $T^{*}$ including the superconducting state and {\it a half filling bad metal} above $T^{*}$. In contrast the insulating state of $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl below $T_{\rm ins}$ is the Mott insulator.

cond-mat.supr-con↗

arXiv2003

Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na$_x$CoO$_2$$\cdot y$H$_2$O

Co NQR studies were performed in recently discovered superconductor Na$_x$CoO$_2$$\cdot y$H$_2$O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$ were examined, SC bilayer-hydrate sample with $T_c \sim 4.7$ K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak is absent just below $T_c$ and that $1/T_1$ is proportional to temperature far below $T_c$. These results, which are in qualitatively agreement with the previous result by Fujimoto {\it et al.}, suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, $1/T_1T$ of the SC sample shows gradual increase below 100K down to $T_c$, whereas $1/T_1T$ of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between $1/T_1T$ and $χ_{\rm bulk}$ in the SC sample, the increase of $1/T_1T$ is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.

cond-mat.supr-con↗

arXiv2003

On The Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329

Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB 030329 obtained in 2003 May using the Subaru 8.2 m telescope are presented. The properties of the SN are investigated through a comparison with spectra of the Type Ic hypernovae SNe 1997ef and 1998bw. (Hypernovae being a tentatively defined class of SNe with very broad absorption features: these features suggest a large velocity of the ejected material and possibly a large explosion kinetic energy.) Comparison with spectra of other hypernovae shows that the spectrum of SN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame days after the GRB (for z=0.1685), are similar to those of SN 1997ef obtained ~34-42 days after the fiducial time of explosion of that SN. The match with SN 1998bw spectra is not as good (at rest 7300-8000 A, but again spectra obtained ~33-43 days after GRB 980425 are preferred. This indicates that the SN may have intermediate properties between SNe 1997ef and 1998bw. Based on the analogy with the other hypernovae, the time of explosion of SN 2003dh is then constrained to be between -8 and +2 days of the GRB. The Si and O P-Cygni lines of SN 2003dh seem comparable to those of SN 1997ef, which suggests that the ejected mass in SN 2003dh may match that in SN 1997ef. Polarization was marginally detected at optical wavelengths. This is consistent with measurements of the late afterglow, implying that it mostly originated in the interstellar medium of the host galaxy.

astro-ph↗

arXiv2003

Interfacing the JQMD and JAM Nuclear Reaction Codes to Geant4

Geant4 is a toolkit for the simulation of the passage of particles through matter. It provides a comprehensive set of tools for geometry, tracking, detector response, run, event and track management, visualization and user interfaces. Geant4 also has an abundant set of physics models that handle the diverse interactions of particles with matter across a wide energy range. However, there are also many well-established reaction codes currently used in the same fields where Geant4 is applied. In order to take advantage of these codes, we began to investigate their use from within the framework of Geant4. The first codes chosen for this investigation were the Jaeri Quantum Molecular Dynamics (JQMD) and Jet AA Microscopic Transportation Model (JAM) codes. JQMD is a QMD model code which is widely used to analyze various aspects of heavy ion reactions. JAM is a hadronic cascade model code which explicitly treats all established hadronic states, including resonances with explicit spin and isospin, as well as their anti-particles. We successfully developed interfaces between these codes and Geant4. These allow a user to construct a detector using the powerful material and geometrical capabilities of Geant4, while at the same time implementing nuclear reactions handled by the JQMD and JAM models and the Hadronic framework of Geant4 proved its flexibility and expandability.

physics.comp-ph↗

arXiv2003

Electron correlation and two dimensionality in the spin-density-wave phase of (TMTTF)$_2$Br under pressure

The incommensurate spin-density-wave (SDW) phase in (TMTTF)$_2$Br was investigated through transport measurements under pressure and magnetic fields parallel to the $c^{\ast}$ axis. For the incommensurate SDW phase of (TMTTF)$_2$Br stabilized above 0.5 GPa, the SDW transition temperature $T_{\rm SDW}$ increases with the applied magnetic field. The field dependence of $T_{\rm SDW}$ is described by a quadratic behavior and the coefficient of the quadratic term increases with increasing pressure. These results are consistent with the prediction of the mean-field theory based on the suppression of the SDW transition by two-dimensionality. From the relation between the coefficient of the quadratic term and $T_{\rm SDW}$ at zero magnetic field, we determined the role of electron correlation and two dimensionality in the SDW phase of (TMTTF)$_2$Br under pressure and found that the SDW transition in (TMTTF)$_2$Br can be well explained within the mean field theory by taking into account the reduction of the coupling constant $N(0)I$ by pressure.

cond-mat.str-el↗

arXiv2003

Development of an Interface for Using EGS4 Physics Processes in Geant4

As simulation system, the variety of physics processes implemented is one of the most important functionalities. In that sense, Geant4 is one of the most powerful simulation toolkits. Its flexibility and expansibility brought by object-oriented approach make it possible for us to easily assimilate external simulation packages into the Geant4 system as modules of physics processes. We developed an interface for using EGS4, which is another of the most well-known simulation package for electromagnetic physics, in Geant4. By means of this interface, EGS4 users can share Geant4 powerful resources, such as geometry, tracking, etc. It is also important that it can provide a common environment for comparison tests between EGS4 and Geant4. In this paper, we describe our design and implementation of the interface.

physics.comp-ph↗

arXiv2003

Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping

The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

cond-mat.supr-con↗

arXiv2003

XMM-Newton observations expose AGN in apparently normal galaxies

We have performed a detailed analysis of 3 optically normal galaxies extracted from the XMM Bright Serendipitous Source Sample. Thanks to the good statistics of the XMM-Newton data, we have unveiled the presence of an AGN in all of them. In particular, we detect both X-ray obscured (N_H>10^{22} cm^{-2}) and unobscured (N_H<10^{22} cm^{-2}) AGN with intrinsic 2--10 keV luminosities in the range between 10^{42} -- 10^{43} erg s^{-1}. We find that the X-ray and optical properties of the sources discussed here could be explained assuming a standard AGN hosted by galaxies with magnitudes M_R<M^*, taking properly into account the absorption associated with the AGN, the optical faintness of the nuclear emission with respect to the host galaxy, and the inadequate set--up and atmospheric conditions during the optical spectroscopic observations. Our new spectroscopic observations have revealed the expected AGN features also in the optical band. These results clearly show that optical spectroscopy sometimes can be inefficient in revealing the presence of an AGN, which instead is clearly found from an X-ray spectroscopic investigation. This remarks the importance of being careful in proposing the identification of X-ray sources (especially at faint fluxes) when only low quality optical spectra are in hand. This is particularly important for faint surveys (such as those with XMM-Newton and Chandra), in which optically dull but X-ray active objects are being found in sizeable numbers.

astro-ph↗

arXiv2003

Variable-range hopping conductivity in the copper-oxygen chains of La_3Sr_3Ca_8Cu_24O_41

We show that the spin chain/ladder compound La_3Sr_3Ca_8Cu_24O_41 is an insulator with hopping transport along the chains. In the temperature range 35 - 280 K, DC conductivity sigma_{DC}(T) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form sigma(ν, T) = σ_{DC}(T) + A(T)ν^{s}, where s \approx 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around T_{c} =300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

cond-mat.supr-con↗

arXiv2003

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)Sigma0 reactions at Egamma=1.5-2.4 GeV

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)sigma0 reactions are measured for the first time for Egamma=1.5-2.4 GeV and 0.6<cos(theta_cm(K+))<1.0 by using linearly polarized photons at the Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are positive and gradually increase with rising photon energy. The data are not consistent with theoretical predictions based on tree-level effective Lagrangian approaches. Including the new results in the development of the models is, therefore, crucial for understanding the reaction mechanism and to test the presence of baryon resonances which are predicted in quark models but are sofar undiscovered.

nucl-ex↗

arXiv2002

Shubnikov - de Haas effect in the quantum vortex liquid state of the organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$

We report the Shubnikov-de Haas (SdH) oscillations observed in the vortex liquid state of the quasi two dimensional organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$. The SdH oscillations can be observed down to about 5 T at 0.5 K, where the flux flow resistivity becomes as small as about 30 % of the normal state value. Below the upper critical field $H_{\rm c2}$ of about 7 T, the additional damping of the SdH oscillation amplitude appears, as well as that of the de Haas-van Alphen (dHvA) oscillations, with respect to the normal state one which is described with the standard Lifshitz-Kosevich formula. The magnitude of the additional damping near $H_{\rm c2}$ is the same with that observed in the dHvA oscillations and well explained by the theoretical predictions in consideration of fluctuations in the thermal vortex liquid state. In the quantum fluctuation region at lower temperature, however, only SdH effect shows the stronger damping than that of the dHvA oscillations. The different magnetic field dependence of the additional damping of the oscillation amplitude between the SdH and dHvA effects is discussed in connection with the effect of the transport current on the short-range order of vortices in the quantum vortex slush state reported at the same temperature and magnetic field region.

cond-mat.supr-con↗