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T. Sasaki

Publications and source records attributed to T. Sasaki.

142 records · Page 8Linked to original sources
arXiv2002

Optical Spectropolarimetry of SN 2002ap: High Velocity Asymmetric Explosion

We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis: the data were taken at two epochs, close to and one month later than the visual maximum (2002 February 8). In addition we present June 9 spectropolarimetry without analysis. The data show the development of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR triplet absorption trough at maximum light and in the Ca II IR triplet absorption trough a month later, with the latter showing a peak polarization as high as ~2 %. The intrinsic polarization shows three clear position angles: 80 degs for the February continuum, 120 degs for the February line feature, and 150 degs for the March data. We conclude that there are multiple asymmetric components in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the sky that is different from 1 by of order 10 %. Furthermore, we suggest very speculatively that a high velocity ejecta component moving faster than ~0.115c (e.g., a jet) contributes to polarization in the February epoch.

astro-ph↗
arXiv2002

Low temperature vortex liquid states induced by quantum fluctuations in the quasi two dimensional organic superconductor kappa-(BEDT-TTF)_{2} Cu(NCS)_{2}

We report the transport properties in the vortex liquid states induced by quantum fluctuations at low temperature in the layered organic superconductor kappa-(BEDT-TTF)_{2} Cu(NCS)_{2}. A steep drop of the resistivity observed below about 1 K separates the liquid state into two regions. In the low resistance state at lower temperature, a finite resistivity with weak temperature dependence persists down to 100 mK at least. The finite resistivity in the vortex state at T ~= 0 K indicates the realization of quantum vortex liquid assisted by the strong quantum fluctuations instead of the thermal one. A possible origin for separating these liquid states is a remnant vortex melting line at the original position, which is obscured and suppressed by the quantum fluctuations. A non-linear behavior of the in-plane resistivity appears at large current density in only the low resistance state, but not in another vortex liquid state at higher temperature, where the thermal fluctuations are dominant. The transport properties in the low resistance state are well understood in the vortex slush concept with a short-range order of vortices. Thus the low resistance state below 1 K is considered to be a novel quantum vortex slush state.

cond-mat.supr-con↗
arXiv2002

A New High-Redshift Lyman alpha Emitter: Possible Superwind Galaxy at z = 5.69

During the course of our deep optical imaging survey for Ly alpha emitters at z approximately 5.7 in the field around the z=5.74 quasar SDSSp J104433.04-012502.2, we have found a candidate strong emission-line source. Follow-up optical spectroscopy shows that the emission line profile of this object is asymmetric, showing excess red-wing emission. These properties are consistent with an identification of Ly alpha emission at a redshift of z=5.687 +/- 0.002. The observed broad line width, Delta V_{FWHM} ~= 340 km s^{-1} and excess red-wing emission also suggest that this object hosts a galactic superwind.

astro-ph↗
arXiv2002

Coherence-Incoherence and Dimensional Crossover in Layered Strongly Correlated Metals

Correlations between electrons and the effective dimensionality are crucial factors that shape the properties of an interacting electron system. For example, the onsite Coulomb repulsion, U, may inhibit, or completely block the intersite electron hopping, t, and depending on the ratio U/t, a material may be a metal or an insulator. The correlation effects increase as the number of allowed dimensions decreases. In 3D systems, the low energy electronic states behave as quasiparticles (QP), while in 1D systems, even weak interactions break the quasiparticles into collective excitations. Dimensionality is particularly important for a class of new exotic low-dimensional materials where 1D or 2D building blocks are loosely connected into a 3D whole. Small interactions between the blocks may induce a whole variety of unusual transitions. Here, we examine layered systems that in the direction perpendicular to the layers display a crossover from insulating-like, at high temperatures, to metallic-like character at low temperatures, while being metallic over the whole temperature range within the layers. We show that this change in effective dimensionality correlates with the existence or non-existence of coherent quasiparticles within the layers.

cond-mat.str-el↗
arXiv2002

High TC ferromagnetism in diluted magnetic semiconducting GaN:Mn films

Wurtzite GaN:Mn films on sapphire substrates were successfully grown by use of the molecular beam epitaxy (MBE) system. The film has an extremely high Curie temperature of around 940 K, although the Mn concentration is only about 3 ~ 5 %. Magnetization measurements were carried out in magnetic fields parallel to the film surface up to 7 T. The magnetization process shows the coexistence of ferromagnetic and paramagnetic contributions at low temperatures, while the typical ferromagnetic magnetization process is mainly observed at high temperatures because of the decrease of the paramagnetic contributions. The observed transport characteristics show a close relation between the magnetism and the impurity conduction. The double exchange mechanism of the Mn-impurity band is one of the possible models for the high-TC ferromagnetism in GaN:Mn.

cond-mat.mtrl-sci↗
arXiv2001

Magnetic and Electronic Phase Diagram and Superconductivity in the Organic Superconductors k-(BEDT-TTF)2X

The magnetic susceptibility of the organic superconductors $κ$-(h8 or d8-ET)$_{2}$$X$, $X =$ Cu(NCS)$_{2}$ and Cu[N(CN)$_{2}$]Br has been studied. A metallic phase below $T^{*} =$ 37 $\sim$ 38 K for $X =$ Cu[N(CN)$_{2}$]Br and 46 $\sim$ 50 K for $X =$ Cu(NCS)$_{2}$ has an anisotropic temperature dependence of the susceptibility and the charge transport. Partial charge-density-wave or charge fluctuation is expected to coexist with the metallic phase instead of the large antiferromagnetic fluctuation above $T^{*}$. The phase diagram and the superconductivity of $κ$-(ET)$_{2}$$X$ are discussed in connection with this phase.

cond-mat.supr-con↗
arXiv2001

High-resolution ac-calorimetry studies of the quasi-2D organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$

We report high-resolution specific heat measurements on the quasi-2D organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$, performed by using a most sensitive ac-modulation technique. The main observations are (i) a discontinuity at \tc\ much in excess of what is expected for a weak-coupling BCS superconductor and (ii) a quasiparticle contribution to the specific heat with an exponentially weak temperature dependence at $T \ll T_c$. The latter finding is incompatible with an order parameter that vanishes at certain parts of the Fermi surface. The data for $T \leq T_c$ can be well described by a strong-coupling extension of the BCS theory - the $α$-model - similar to what has been recently found for the $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br salt [Elsinger {\em et al.}, Phys. Rev. Lett. {\bf 84}, 6098 (2000)].

cond-mat.supr-con↗
arXiv2001

Molecular Beam Epitaxy of Wurtzite (Ga,Mn)N Films on Sapphire(0001) Showing the Ferromagnetic Behaviour at Room Temperature

Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0001) substrates by molecular beam epitaxy using ammonia as nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8K and 300K with magnetic field applied parallel to the film plane up to 7T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300K were ferromagnetic, while a GaN film showed Pauli paramagnetism like behaviour. The Curie temperatures of a (Ga,Mn)N film was estimated as 940K.

cond-mat.mtrl-sci↗
arXiv2001

Evidence for structural and electronic instabilities at intermediate temperatures in $κ$-(BEDT-TTF)$_{2}$X for X=Cu[N(CN)$_{2}$]Cl, Cu[N(CN)$_{2}$]Br and Cu(NCS)$_{2}$: Implications for the phase diagram of these quasi-2D organic superconductors

We present high-resolution measurements of the coefficient of thermal expansion $α(T)=\partial \ln l(T)/\partial T$ of the quasi-twodimensional (quasi-2D) salts $κ$-(BEDT-TTF)$_2$X with X = Cu(NCS)$_2$, Cu[N(CN)$_2$]Br and Cu[N(CN)$_2$]Cl. At intermediate temperatures (B), distinct anomalies reminiscent of second-order phase transitions have been found at $T^\ast = 38$ K and 45 K for the superconducting X = Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of $T^\ast$ are strictly anticorrelated with those of $T_c$. We propose that $T^\ast$ marks the transition to a spin-density-wave (SDW) state forming on minor, quasi-1D parts of the Fermi surface. Our results are compatible with two competing order parameters that form on disjunct portions of the Fermi surface. At elevated temperatures (C), all compounds show $α(T)$ anomalies that can be identified with a kinetic, glass-like transition where, below a characteristic temperature $T_g$, disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. We argue that the degree of disorder increases on going from the X = Cu(NCS)$_2$ to Cu[N(CN)$_2$]Br and the Cu[N(CN)$_2$]Cl salt. Our results provide a natural explanation for the unusual time- and cooling-rate dependencies of the ground-state properties in the hydrogenated and deuterated Cu[N(CN)$_2$]Br salts reported in the literature.

cond-mat.supr-con↗
arXiv2001

Spin-density-wave transition of (TMTSF)$_2$PF$_6$ at high magnetic fields

The transverse magnetoresistance of the Bechgaard salt (TMTSF)$_2$PF$_6$ has been measured for various pressures, with the field up to 24 T parallel to the lowest conductivity direction c$^{\ast}$. A quadratic behavior is observed in the magnetic field dependence of the spin-density-wave (SDW) transition temperature $T_{\rm {SDW}}$. With increasing pressure, $T_{\rm {SDW}}$ decreases and the coefficient of the quadratic term increases. These results are consistent with the prediction of the mean-field theory based on the nesting of the quasi one-dimensional Fermi surface. Using a mean field theory, $T_{\rm {SDW}}$ for the perfect nesting case is estimated as about 16 K. This means that even at ambient pressure where $T_{\rm {SDW}}$ is 12 K, the SDW phase of (TMTSF)$_2$PF$_6$ is substantially suppressed by the two-dimensionality of the system.

cond-mat.str-el↗
arXiv2001

Diffeomorphism on Horizon as an Asymptotic Isometry of Schwarzschild Black Hole

It is argued that the diffeomorphism on the horizontal sphere can be regarded as a nontrivial asymptotic isometry of the Schwarzschild black hole. We propose a new boundary condition of asymptotic metrics near the horizon and show that the condition admits the local time-shift and diffeomorphism on the horizon as the asymptotic symmetry.

gr-qc↗
arXiv2001

Superconducting Gap Structure of Spin-Triplet Superconductor Sr_2RuO_4 Studied by Thermal Conductivity

To clarify the superconducting gap structure of the spin-triplet superconductor Sr_2RuO_4, the in-plane thermal conductivity has been measured as a function of relative orientations of the thermal flow, the crystal axes, and a magnetic field rotating within the 2D RuO_2 planes. The in-plane variation of the thermal conductivity is incompatible with any model with line nodes vertical to the 2D planes and indicates the existence of horizontal nodes. These results place strong constraints on models that attempt to explain the mechanism of the triplet superconductivity.

cond-mat.supr-con↗
arXiv2000

Subaru Deep Survey I. Near-Infrared Observations

Deep near-infrared images of a blank 2'x2' section of sky near the Galactic north pole taken by Subaru Telescope are presented. The total integration times of the J and K' bands are 12.1 hours and 9.7 hours, resulting in 5-sigma limiting magnitudes of 25.1 and 23.5 mag, respectively. The numbers of sources within these limiting magnitudes found with an automated detection procedure are 385 in the J band and 350 in K'. Based on photometric measurements of these sources, we present number count vs. magnitude relations, color vs. magnitude diagrams, size vs. color relationships, etc. The slope of the galaxy number count plotted against the AB magnitude scale is about 0.23 in the 22 to 26 AB magnitude range of both bands. The spatial number density of galaxies as well as the slopes in the faint-end region given by the Subaru Deep Field (SDF) survey is consistent with those given by HST-NICMOS surveys as expressed on the AB magnitude diagram. Several sources having very large J-K' color are found including a few K' objects without detection at J. In addition, a number of faint Galactic stars are also detected, most of which are assigned to M-subdwarfs, together with a few brown dwarf candidates.

astro-ph↗
arXiv1995

Neutron Electric Dipole Moment in the Minimal Supersymmetric Standard Model

Neutron electric dipole moment (EDM) due to single quark EDM and to the transition EDM is calculated in the minimal supersymmetric standard model. Assuming that the Cabibbo-Kobayashi-Maskawa matrix at the grand unification scale is the only source of CP violation, complex phases are induced in parameters of soft supersymmetry breaking at low energies. Chargino one-loop diagram is found to give the dominant contribution of the order of $10^{-27}\sim 10^{-29}\:e\cdot$cm for quark EDM, assuming the light chargino mass and the universal scalar mass to be $50$ GeV and $100$ GeV, respectively. Therefore the neutron EDM in this class of model is difficult to measure experimentally. Gluino one-loop diagram also contributes due to the flavor changing gluino coupling. The transition EDM is found to give dominant contributions for certain parameter regions.

hep-ph↗
arXiv1993

Muonium Hyperfine Structure and the Decay $ μ^+\to e^+ +\overline ν_e +ν_μ$ in Models with Dilepton Gauge Bosons

We examine the muonium ($μ^+e^-$)-antimuonium ($μ^-e^+$) system in the models which accomodate the dilepton gauge bosons, and study their contributions to the ground state hyperfine splitting in ``muonium''. We also consider the exotic muon decay $μ^+\to e^+ +\overline ν_e +ν_μ$ mediated by the dilepton gauge boson, and obtain a lower bound $(M_{X^{\pm }}/g_{3l})>550 \rm GeV$ at 90\% confidence level for the singly-charged dilepton mass using the unitarity relation of the Kobayashi-Maskawa matrix for the 3-family case.

hep-ph↗