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S. Watanabe

Publications and source records attributed to S. Watanabe.

At least 73 records · Page 4Linked to original sources
arXiv2012

Heavy Fermion superconductor CeCu$_2$Si$_2$ under high pressure: multiprobing the valence crossover

The first heavy fermion superconductor CeCu$_2$Si$_2$ has not revealed all its striking mysteries yet. At high pressures, superconductivity is supposed to be mediated by valence fluctuations, in contrast to ambient pressure, where spin fluctuations most likely act as pairing glue. We have carried out a multiprobe (electric transport, thermopower, ac specific heat, Hall and Nernst effects) experiment up to $7 \text{GPa}$ on a high quality CeCu$_2$Si$_2$ single crystal. Reliable resistivity data reveal for the first time a scaling behavior close to the supposed valence transition, and allow to locate the critical end point at $4.5\pm0.2 \text{GPa}$ and a slightly negative temperature. In the same pressure region, remarkable features have also been detected in the other physical properties, acting as further signatures of the Ce valence crossover and the associated critical fluctuations.

cond-mat.str-el↗
arXiv2012

Abrupt change in the energy gap of superconducting Ba1-xKxFe2As2 single crystals with hole doping

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting (SC) gaps in this compound. The gap size of the outer hole Fermi surface (FS) sheet around the Brillioun zone (BZ) center shows an abrupt drop with overdoping (for x > 0.6) while the inner and middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results browse the different contributions of X2-Y2 and XZ/YZ orbitals to superconductivity in BaK and can be hardly completely reproduced by the available theories on iron-based superconductors.

cond-mat.supr-con↗
arXiv2012

Spin Accumulation in Nondegenerate and Heavily Doped p-Type Germanium

Spin accumulation induced in p-type germanium from Fe/MgO tunnel contacts is studied as a function of hole concentration p (10^16 - 10^19 cm-3). For all p, the contacts are free of rectification and Schottky barrier, guaranteeing spin injection into the Ge and preventing spin accumulation enhancement by two-step tunneling via interface states. The observed spin accumulation is smallest for nondegenerate doping (p ~ 10^16 cm-3) and increases for heavily doped Ge. This trend is opposite to what is expected from spin injection and diffusion theory. For heavily doped Ge, the observed spin accumulation is orders of magnitude larger than predicted.

cond-mat.mes-hall↗
arXiv2011

Common Origin of the Circular-dichroism Pattern in ARPES of SrTiO3 and CuxBi2Se3

Circular dichroism in the angular distribution (CDAD) of photoelectrons from SrTiO3:Nb and CuxBi2Se3 is investigated by 7-eV laser ARPES. In addition to the well-known node that occurs in CDAD when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two dimensional. The flower-shaped CDAD's occurring around the Fermi level of SrTiO3:Nb and around the Dirac point of CuxBi2Se3 are explained on equal footings. We point out that the penetration depth of the topological states of CuxBi2Se3 depends on momentum.

cond-mat.mtrl-sci↗
arXiv2010

Femtosecond core level photoemision spectroscopy on 1T-TaS2 using 60 eV laser source

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level (CL) trPES on 1T-TaS2, realized by developing a high-intensity 60 eV laser obtained by high-order harmonic (HH) generation. Ta4f CL-trPES offers the transient amplitude of the charge-density-wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e. collective amplitudon excitation) accompanying the transfer of 0.01 electrons among adjacent Ta atoms. CL-trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photo-related phenomena of interest.

cond-mat.str-el↗
arXiv2010

Orbital-dependent modifications of electronic structure across magneto-structural transition in BaFe2As2

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magneto-structural transition at TN ~ 140 K. A drastic transformation in Fermi surface (FS) shape across TN is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at kz ~ pi in the low-T antiferromagnetic (AF) state are dominated by the Fe3dzx orbital, leading to the two-fold electronic structure. These results indicate that magneto-structural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

cond-mat.supr-con↗
arXiv2009

Search for VHE gamma rays from SS433/W50 with the CANGAROO-II telescope

SS433, located at the center of the supernova remnant W50, is a close proximity binary system consisting of a compact star and a normal star. Jets of material are directed outwards from the vicinity of the compact star symmetrically to the east and west. Non-thermal hard X-ray emission is detected from lobes lying on both sides. Shock accelerated electrons are expected to generate sub-TeV gamma rays through the inverse-Compton process in the lobes. Observations of the western X-ray lobe region of SS433/W50 system have been performed to detect sub-TeV gamma-rays using the 10m CANGAROO-II telescope in August and September, 2001, and July and September, 2002. The total observation times are 85.2 hours for ON source, and 80.8 hours for OFF source data. No significant excess of sub-TeV gamma rays has been found at 3 regions of the western X-ray lobe of SS433/W50 system. We have derived 99% confidence level upper limits to the fluxes of gamma rays and have set constraints on the strengths of the magnetic fields assuming the synchrotron/inverse-Compton model for the wide energy range of photon spectrum from radio to TeV. The derived lower limits are 4.3 microgauss for the center of the brightest X-ray emission region and 6.3 microgauss for the far end from SS433 in the western X-ray lobe. In addition, we suggest that the spot-like X-ray emission may provide a major contribution to the hardest X-ray spectrum in the lobe.

astro-ph.HE↗
arXiv2009

CANGAROO-III search for TeV Gamma-rays from two clusters of galaxies

Because accretion and merger shocks in clusters of galaxies may accelerate particles to high energies, clusters are candidate sites for the origin of ultra-high-energy (UHE) cosmic-rays. A prediction was presented for gamma-ray emission from a cluster of galaxies at a detectable level with the current generation of imaging atmospheric Cherenkov telescopes. The gamma-ray emission was produced via inverse Compton upscattering of cosmic microwave background (CMB) photons by electron-positron pairs generated by collisions of UHE cosmic rays in the cluster. We observed two clusters of galaxies, Abell 3667 and Abell 4038, searching for very-high-energy gamma-ray emission with the CANGAROO-III atmospheric Cherenkov telescope system in 2006. The analysis showed no significant excess around these clusters, yielding upper limits on the gamma-ray emission. From a comparison of the upper limit for the north-west radio relic region of Abell 3667 with a model prediction, we derive a lower limit for the magnetic field of the region of ~0.1 micro G. This shows the potential of gamma-ray observations in studies of the cluster environment. We also discuss the flux upper limit from cluster center regions using a model of gamma-ray emission from neutral pions produced in hadronic collisions of cosmic-ray protons with the intra-cluster medium (ICM). The derived upper limit of the cosmic-ray energy density within this framework is an order of magnitude higher than that of our Galaxy.

astro-ph.HE↗
arXiv2009

CANGAROO-III Observation of TeV Gamma Rays from the vicinity of PSR B1 706-44

Observation by the CANGAROO-III stereoscopic system of the Imaging Cherenkov Telescope has detected extended emission of TeV gamma rays in the vicinity of the pulsar PSR B1706$-$44. The strength of the signal observed as gamma-ray-like events varies when we apply different ways of emulating background events. The reason for such uncertainties is argued in relevance to gamma-rays embedded in the "off-source data", that is, unknown sources and diffuse emission in the Galactic plane, namely, the existence of a complex structure of TeV gamma-ray emission around PSR B1706$-$44.

astro-ph.HE↗
arXiv2009

Superconducting electronic state in optimally doped YBa2Cu3O7-d observed with laser-excited angle-resolved photoemission spectroscopy

Low energy electronic structure of optimally doped YBa2Cu3O7-d is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the CuO2 plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is ~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger than Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the renormalization effect at ~60 meV shows up similarly as in other hole-doped cuprates. The momentum-dependence of the superconducting gap shows d_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV along the (0,0)-(pi,pi) direction. Possible origins of such an unexpected "nodeless" gap behavior are discussed.

cond-mat.supr-con↗
arXiv2009

Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases of Ti4O7

The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at Tc1=142 K and Tc2=154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p-3d resonant PES and HX-PES show a clear metallic Fermi-edge and mixed valency above Tc2. The low temperature phase below Tc1 shows a clear insulating gap of 100 meV. The intermediate phase between Tc1 and Tc2 indicates a pseudogap coexisting with remnant coherent states. HX-PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti4O7.

cond-mat.str-el↗
arXiv2009

Unusual Pseudogap-like Features Observed in Iron Oxypnictide Superconductors

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like features through comparison with the high-Tc cuprates.

cond-mat.supr-con↗
arXiv2008

The NeXT Mission

The NeXT (New exploration X-ray Telescope), the new Japanese X-ray Astronomy Satellite following Suzaku, is an international X-ray mission which is currently planed for launch in 2013. NeXT is a combination of wide band X-ray spectroscopy (3 - 80 keV) provided by multi-layer coating, focusing hard X-ray mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3 - 10 keV) provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD camera as a focal plane detector for a soft X-ray telescope and a non-focusing soft gamma-ray detector. With these instruments, NeXT covers very wide energy range from 0.3 keV to 600 keV. The micro-calorimeter system will be developed by international collaboration lead by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of Delta E ~ 7 eV by the micro-calorimeter will enable a wide variety of important science themes to be pursued.

astro-ph↗
arXiv2008

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi2B2C

We have performed temperature (T) - dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be well explained by the theoretical model and thus represents characteristic bulk electronic structure of the AF SC phase for the first time.

cond-mat.supr-con↗
arXiv2007

Integrability, stability, and adiabaticity in nonlinear stimulated Raman adiabatic passage

We study dynamics of a two-color photoassociation of atoms into diatomic molecules via nonlinear Stimulated Raman adiabatic passage (STIRAP) process. This system has a famous counterpart in (linear) quantum mechanics, and been discussed recently in the context of generalizing quantum adiabatic theorem to nonlinear systems. Here we use another approach to study adiabaticity and stability in the system: we apply methods of classical Hamiltonian dynamics. We found nonlinear dynamical instabilities, cases of complete integrability, and improved conditions of adiabaticity.

cond-mat.other↗
arXiv2007

Doping-dependence of nodal quasiparticle properties in high-$T_{\rm c}$ cuprates studied by laser-excited angle-resolved photoemission spectroscopy

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+δ}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($Γ$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $Γ\sim |ω|$, while $Γ$($ω=0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K $\lsim 0.07T_{\rm c}$), pronounced in the underdoped region.

cond-mat.supr-con↗
arXiv2007

Multiwavelength Observations of the Powerful Gamma-ray Quasar PKS 1510-089: Clues on the Jet Composition

We present the results from a multiwavelength campaign of the powerful Gamma-ray quasar PKS 1510-089. This campaign commenced with a deep Suzaku observation lasting three days for a total exposure time of 120 ks, and continued with Swift monitoring over 18 days. Besides Swift observations, the campaign included ground-based optical and radio data, and yielded a quasi-simultaneous broad-band spectral energy distribution from 10^9 Hz to 10^{19} Hz. The Suzaku observation provided a high S/N X-ray spectrum, which is well represented by an extremely hard power-law with photon index Gamma ~ 1.2, augmented by a soft component apparent below 1 keV, which is well described by a black-body model with temperature kT ~ 0.2 keV. Monitoring by Suzaku revealed temporal variability which is different between the low and high energy bands, again suggesting the presence of a second, variable component in addition to the primary power-law emission.We model the broadband spectrum of PKS 1510-089 assuming that the high energy spectral component results from Comptonization of infrared radiation produced by hot dust located in the surrounding molecular torus. In the adopted internal shock scenario, the derived model parameters imply that the power of the jet is dominated by protons but with a number of electrons/positrons exceeding a number of protons by a factor ~10. We also find that inhomogeneities responsible for the shock formation, prior to the collision may produce bulk-Compton radiation which can explain the observed soft X-ray excess and possible excess at ~18 keV. We note, however, that the bulk-Compton interpretation is not unique, and the observed soft excess could arise as well via some other processes discussed briefly in the text.

astro-ph↗