239 research outputs found
Fermi surface with Dirac fermions in CaFeAsF determined via quantum oscillation measurements
Despite the fact that 1111-type iron arsenides hold the record transition
temperature of iron-based superconductors, their electronic structures have not
been studied much because of the lack of high-quality single crystals. In this
study, we completely determine the Fermi surface in the antiferromagnetic state
of CaFeAsF, a 1111 iron-arsenide parent compound, by performing quantum
oscillation measurements and band-structure calculations. The determined Fermi
surface consists of a symmetry-related pair of Dirac electron cylinders and a
normal hole cylinder. From analyses of quantum-oscillation phases, we
demonstrate that the electron cylinders carry a nontrivial Berry phase .
The carrier density is of the order of 10 per Fe. This unusual metallic
state with the extremely small carrier density is a consequence of the
previously discussed topological feature of the band structure which prevents
the antiferromagnetic gap from being a full gap. We also report a nearly
linear-in- magnetoresistance and an anomalous resistivity increase above
about 30 T for , the latter of which is likely related to the
quantum limit of the electron orbit. Intriguingly, the electrical resistivity
exhibits a nonmetallic temperature dependence in the paramagnetic tetragonal
phase ( 118 K), which may suggest an incoherent state. Our study provides
a detailed knowledge of the Fermi surface in the antiferromagnetic state of
1111 parent compounds and moreover opens up a new possibility to explore
Dirac-fermion physics in those compounds.Comment: 11 pages, 7 figures, 1 tabl
Oxygen Isotope Exchange Between Molten Silicate Spherules and Ambient Water Vapor with Nonzero Relative Velocity: Implication for Chondrule Formation Environment
Oxygen isotope compositions of chondrules reflect the environment of
chondrule formation and its spatial and temporal variations. Here, we present a
theoretical model of oxygen isotope exchange reaction between molten silicate
spherules and ambient water vapor with finite relative velocity. We found a new
phenomenon, that is, mass-dependent fractionation caused by isotope exchange
with ambient vapor moving with nonzero relative velocity. We also discussed the
plausible condition for chondrule formation from the point of view of oxygen
isotope compositions. Our findings indicate that the relative velocity between
chondrules and ambient vapor would be lower than several 100 m/s when
chondrules crystallized.Comment: 15 pages, 8 figures. Accepted for publication in Icaru
In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF
Both cuprates and iron-based superconductors demonstrate nematicity, defined
as the spontaneous breaking of rotational symmetry in electron systems. The
nematic state can play a role in the high-transition-temperature
superconductivity of these compounds. However, the microscopic mechanism
responsible for the transport anisotropy in iron-based compounds remains
debatable. Here, we investigate the electronic anisotropy of CaFeAsF by
measuring its interlayer resistivity under magnetic fields with varying field
directions. Counterintuitively, the interlayer resistivity was larger in the
longitudinal configuration () than in the transverse
one (). The interlayer resistivity exhibited a so-called
coherence peak under in-plane fields and was highly anisotropic with respect to
the in-plane field direction. At = 4 K and = 14 T, the
magnetoresistance was seven times larger in the than in the configuration. Our theoretical
calculations of the conductivity based on the first-principles electronic band
structure qualitatively reproduced the above observations but underestimated
the magnitudes of the observed features. The proposed methodology can be a
powerful tool for probing the nematic electronic state in various materials.Comment: 25 pages, 5 figure
Strain distribution analysis of sputter-formed strained Si by tip-enhanced Raman spectroscopy
Simultaneous nanometer-scale measurements of the strain and surface undulation distributions of strained Si (s-Si) layers on strain-relief quadruple-Si1-xGex-layer buffers, using a combined atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) system, clarify that an s-Si sample formed by our previously proposed sputter epitaxy method has a smoother and more uniformly strained surface than an s-Si sample formed by gas-source molecular beam epitaxy. The TERS analyses suggest that the compositional fluctuation of the underlying Si1-xGex buffer layer is largely related to the weak s-Si strain fluctuation of the sputtered sampl
Characterization of Core Optics in Gravitational-Wave Detectors: Case Study of KAGRA Sapphire Mirrors
We report the characterization of superlow-loss optics used in the second-generation gravitational-wave detectors currently in operation. The sapphire test-mass mirrors in the KAGRA detector are introduced as an example, but the techniques here are common to all detectors. In this work, we discuss mainly the surface topography obtained by interferometric techniques and the optical properties obtained with special setups
Sapphire mirror for the KAGRA gravitational wave detector
KAGRA, the Japanese interferometric gravitational wave detector currently under construction, will employ sapphire test masses for its cryogenic operation. Sapphire has an advantage in its higher thermal conductivity near the operating temperature 20 K compared to fused silica used in other gravitational wave detectors, but there are some uncertain properties for the application such as hardness, optical absorption, and birefringence. We introduce an optical design of the test masses and our recent R&D results to address the above properties. Test polish of sapphire substrate has especially proven that specifications on the surface are sufficiently met. Recent measurements of absorption and inhomogeneity of the refractive index of the sapphire substrate indicate that the other properties are also acceptable to use sapphire crystal as test masses
Approach for growth of high-quality and large protein crystals
Three crystallization methods, including crystallization in the presence of a semi-solid agarose gel, top-seeded solution growth (TSSG) and a large-scale hanging-drop method, have previously been presented. In this study, crystallization has been further evaluated in the presence of a semi-solid agarose gel by crystallizing additional proteins. A novel crystallization method combining TSSG and the large-scale hanging-drop method has also been developed
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