675 research outputs found
Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity
To elucidate the nature of the superconducting ground state of the
geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity
was measured down to low temperatures (~Tc/100). We found that the
quasiparticle mean free path is strikingly enhanced below a transition at
Tp=7.5K, indicating enormous electron inelastic scattering in the normal state.
In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2,
in contrast with the rapid growth expected for superconductors with an
anisotropic gap. This unambiguously indicates a fully gapped superconductivity,
in contrast to the previous studies. These results highlight that KOs2O6 is
unique among superconductors with strong electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting -Pyrochlore KOsO
Microwave penetration depth and surface resistance at 27 GHz are
measured in high quality crystals of KOsO. Firm evidence for
fully-gapped superconductivity is provided from . Below the second
transition at K, the superfluid density shows a step-like
change with a suppression of effective critical temperature .
Concurrently, the extracted quasiparticle scattering time shows a steep
enhancement, indicating a strong coupling between the anomalous rattling motion
of K ions and quasiparticles. The results imply that the rattling phonons help
to enhance superconductivity, and that K sites freeze to an ordered state with
long quasiparticle mean free path below .Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let
Ionization Source of a Minor-axis Cloud in the Outer Halo of M82
The M82 `cap' is a gas cloud at a projected radius of 11.6 kpc along the
minor axis of this well known superwind source. The cap has been detected in
optical line emission and X-ray emission and therefore provides an important
probe of the wind energetics. In order to investigate the ionization source of
the cap, we observed it with the Kyoto3DII Fabry-Perot instrument mounted on
the Subaru Telescope. Deep continuum, Ha, [NII]6583/Ha, and [SII]6716,6731/Ha
maps were obtained with sub-arcsecond resolution. The superior spatial
resolution compared to earlier studies reveals a number of bright Ha emitting
clouds within the cap. The emission line widths (< 100 km s^-1 FWHM) and line
ratios in the newly identified knots are most reasonably explained by slow to
moderate shocks velocities (v_shock = 40--80 km s^-1) driven by a fast wind
into dense clouds. The momentum input from the M82 nuclear starburst region is
enough to produce the observed shock. Consequently, earlier claims of
photoionization by the central starburst are ruled out because they cannot
explain the observed fluxes of the densest knots unless the UV escape fraction
is very high (f_esc > 60%), i.e., an order of magnitude higher than observed in
dwarf galaxies to date. Using these results, we discuss the evolutionary
history of the M82 superwind. Future UV/X-ray surveys are expected to confirm
that the temperature of the gas is consistent with our moderate shock model.Comment: 7 pages, 5 figures, 2 tables; Accepted for publication in Ap
Observation of thermodynamics originating from a mixed-spin ferromagnetic chain
We present a model compound that forms a mixed-spin ferromagnetic chain. Our
material design, based on the organic radicals, affords a verdazyl-based
complex (p-Py-V)2[Mn(hfac)2]. The molecular orbital calculations of the
compound indicate the formation of a mixed spin-(1/2, 1/2, 5/2) ferromagnetic
chain. The temperature dependence of magnetic susceptibility reveals its
ferromagnetic behavior. The magnetic specific heat exhibits a double-peak
structure and indicates a phase transition at the low-temperature peak. The
observed characteristics are explained using the quantum Monte Carlo
calculations. Furthermore, the modified spin-wave theory verifies that the
double-peak structure of the specific heat significantly reflects the relative
ration of the acoustic excitation band and the optical excitation gap
A Possible Phase Transition in beta-pyrochlore Compounds
We investigate a lattice of interacting anharmonic oscillators by using a
mean field theory and exact diagonalization. We construct an effective
five-state hopping model with intersite repulsions as a model for
beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase
transition line from large to small oscillation amplitude phases as temperature
decreases. We also discuss the possibility of a phase with local electric
polarizations. Our theory can explain the origin of the mysterious first order
transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6
We investigate with a microscopic model anharmonic K-cation oscillation
observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which
also shows a mysterious first-order structural transition at Tp=7.5 K. We have
identified a set of microscopic model parameters that successfully reproduce
the observed temperature dependence and the superconducting transition
temperature. Considering changes in the parameters at Tp, we can explain
puzzling experimental results about electron-phonon coupling and neutron data.
Our analysis demonstrates that the first-order transition is multipolar
transition driven by the octupolar component of K-cation oscillations. The
octupole moment does not change the symmetry and is characteristic to
noncentrosymmetric K-cation potential.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Metrology Camera System of Prime Focus Spectrograph for Subaru Telescope
The Prime Focus Spectrograph (PFS) is a new optical/near-infrared multi-fiber
spectrograph designed for the prime focus of the 8.2m Subaru telescope. PFS
will cover a 1.3 degree diameter field with 2394 fibers to complement the
imaging capabilities of Hyper SuprimeCam. To retain high throughput, the final
positioning accuracy between the fibers and observing targets of PFS is
required to be less than 10um. The metrology camera system (MCS) serves as the
optical encoder of the fiber motors for the configuring of fibers. MCS provides
the fiber positions within a 5um error over the 45 cm focal plane. The
information from MCS will be fed into the fiber positioner control system for
the closed loop control. MCS will be located at the Cassegrain focus of Subaru
telescope in order to to cover the whole focal plane with one 50M pixel Canon
CMOS camera. It is a 380mm Schmidt type telescope which generates a uniform
spot size with a 10 micron FWHM across the field for reasonable sampling of
PSF. Carbon fiber tubes are used to provide a stable structure over the
operating conditions without focus adjustments. The CMOS sensor can be read in
0.8s to reduce the overhead for the fiber configuration. The positions of all
fibers can be obtained within 0.5s after the readout of the frame. This enables
the overall fiber configuration to be less than 2 minutes. MCS will be
installed inside a standard Subaru Cassgrain Box. All components that generate
heat are located inside a glycol cooled cabinet to reduce the possible image
motion due to heat. The optics and camera for MCS have been delivered and
tested. The mechanical parts and supporting structure are ready as of spring
2016. The integration of MCS will start in the summer of 2016.Comment: 11 pages, 15 figures. SPIE proceeding. arXiv admin note: text overlap
with arXiv:1408.287
Metrology Camera System of Prime Focus Spectrograph for Subaru Telescope
The Prime Focus Spectrograph (PFS) is a new optical/near-infrared multi-fiber
spectrograph designed for the prime focus of the 8.2m Subaru telescope. The
metrology camera system of PFS serves as the optical encoder of the COBRA fiber
motors for the configuring of fibers. The 380mm diameter aperture metrology
camera will locate at the Cassegrain focus of Subaru telescope to cover the
whole focal plane with one 50M pixel Canon CMOS sensor. The metrology camera is
designed to provide the fiber position information within 5{\mu}m error over
the 45cm focal plane. The positions of all fibers can be obtained within 1s
after the exposure is finished. This enables the overall fiber configuration to
be less than 2 minutes.Comment: 10 pages, 12 figures, SPIE Astronomical Telescopes and
Instrumentation 201
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