156 research outputs found
Pulsed UCN production using a Doppler shifter at J-PARC
We have constructed a Doppler-shifter-type pulsed ultra-cold neutron (UCN)
source at the Materials and Life Science Experiment Facility (MLF) of the Japan
Proton Accelerator Research Complex (J-PARC). Very-cold neutrons (VCNs) with
136- velocity in a neutron beam supplied by a pulsed neutron
source are decelerated by reflection on a m=10 wide-band multilayer mirror,
yielding pulsed UCN. The mirror is fixed to the tip of a 2,000-rpm rotating arm
moving with 68- velocity in the same direction as the VCN. The
repetition frequency of the pulsed UCN is and the time width
of the pulse at production is . In order to increase the UCN
flux, a supermirror guide, wide-band monochromatic mirrors, focus guides, and a
UCN extraction guide have been newly installed or improved. The
-equivalent count rate of the output neutrons with longitudinal
wavelengths longer than is ,
while that of the true UCNs is . The spatial density at
production is . This new UCN source enables us to
research and develop apparatuses necessary for the investigation of the neutron
electric dipole moment (nEDM).Comment: 32 pages, 15 fugures. A grammatical error was fixe
Molecular conductors from bis(ethylenedithio)tetrathiafulvalene with tris(oxalato)rhodate
This article reports a family of new radical-cation salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with tris(oxalato)rhodate: three salts with the formula β’’-(BEDT-TTF)4[(cation)Rh(C2O4)3]·solvent (solvent = fluorobenzene, chlorobenzene, or bromobenzene) and one with the formula pseudo-κ-(BEDT-TTF)4[(NH4)Rh(C2O4)3]·benzonitrile. We report here the syntheses, crystal structures, electrical properties and Raman spectroscopy of these new molecular conductors. The bromobenzene salt shows a decrease in resistivity below 2.5 K indicative of a superconducting transition and a Shubnikov-de Haas oscillation with a frequency of 232 T and effective mass m* of 1.27me
Effects of electron correlations and chemical pressures on superconductivity of β''-type organic compounds
We investigate low-temperature electronic states of the series of organic conductors β'' - [bis(ethylenedithio)tetrathiafulvalene] 4[(H3O)M(C2O4)3] G, where M and G represent trivalent metalions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of M and G give rise to systematic change in the cell parameters, especially in the b-axis length, which has a positive correlation with the superconducting transition temperature Tc. Analysis of temperature and magnetic field dependences of the electrical resistance including the Shubnikov–de Haas oscillations elucidates that the variation of charge disproportionation, the effective mass, and the number of itinerant carriers can be systematically explained by the change of the b-axis length. The changes of the transfer integrals induced by stretching/compressing the b axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom
Bulk Kosterlitz–Thouless type molecular superconductor β″-(BEDT-TTF)2[(H2O)(NH4)2Cr(C2O4)3]·18-crown-6
A new molecular superconductor, β”-(BEDTTTF)2[(H2O)(NH4)2Cr(C2O4)3].18-crown-6, has been synthesized from the organic donor molecule BEDT-TTF with the anion Cr(C2O4)3 3- . The crystal structure consists of conducting organic layers of BEDT-TTF molecules which adopt the β” packing motif (layer A), layers of NH4 + and Λ-Cr(C2O4)3 3- (layer B), layers of (H2O)(NH4)18- crown-6 (layer C), and layers of NH4 + and Δ-Cr(C2O4)3 3- (layer D) which produce a superstructure with a repeating pattern of ABCDABCDA. As a result of this packing arrangement this is the 2D superconductor with the widest gap between conducting layers where only a single donor packing motif is present (β”). Superconducting critical temperatures at ambient pressure observed by electrical transport and magnetic measurements are 4.0- 4.9 and 2.5 K, respectively. The strong 2D nature of this system, the broad transition to Tzero at 1.8K, and the transition of α of V ∝ Iα from 1 to 3 on I-V curves strongly suggests that the superconducting transition is very close to a Kosterlitz-Thouless transition. The magnetic field dependence of the superconducting critical temperature parallel to the conducting plane gives an upper critical field μ0Hc2// > 8 T, which is over the calculated Pauli-Clogston limit for this material
Quantum oscillations in a centrosymmetric skyrmion-hosting magnet GdRu2Si2
We have performed magnetic torque and resistivity measurements on a
centrosymmetric skyrmion-host GdRu2Si2, in which the dominant magnetic
interaction leading to skyrmion formation is under debate. We observe both the
de Haas-van Alphen and Shubnikov-de Haas oscillations in the forced
ferromagnetic phase. The angular dependence of the quantum oscillation
frequencies can be reproduced by the ab-initio calculation. The de Haas-van
Alphen oscillation is also observed in the double-Q phase with a different
frequency to that in the forced ferromagnetic phase, indicating a Fermi surface
reconstruction due to the coupling between localized spins and conduction
electrons. Based on these experimental findings, the magnetic interactions in
this system are discussed.Comment: 11 pages, 8 figure
First molecular superconductor with the tris(oxalato)aluminate anion, β″-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, and isostructural tris(oxalato)cobaltate and tris(oxalato)ruthenate radical cation salts
Peter Day’s research group reported the first molecular superconductor containing paramagnetic metal ions in 1995, β″-(BEDT-TTF)4(H3O)Fe(C2O4)3·C6H5CN. Subsequent research has produced a multitude of BEDT-TTF-tris(oxalato)metallate salts with a variety of structures and properties, including 32 superconductors to date. We present here the synthesis, crystal structure, and conducting properties of the newest additions to the Day series including the first superconductor incorporating the diamagnetic tris(oxalato)aluminate anion, β″-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, which has a superconducting Tc of ~2.5 K. β″-(BEDT-TTF)4(H3O)Co(C2O4)3·C6H5Br represents the first example of a β″ phase for the tris(oxalato)cobaltate anion, but this salt does not show superconductivity
Meridional Distribution of Middle-Energy Protons and Pressure-Driven Currents in the Nightside Inner Magnetosphere: Arase Observations
We examined the average meridional distribution of middle‐energy protons (10–180 keV) and pressure‐driven currents in the nightside (20–04 hr magnetic local time) ring current region during moderately disturbed times using the Arase satellite\u27s data. Because the Arase satellite has a large inclination orbit of 31°, it covers the magnetic latitude (MLAT) in the range of −40° to 40° and a radial distance of <6RE. We found that the plasma pressure decreased significantly with increasing MLAT. The plasma pressure on the same L* shell at 30° < MLAT < 40° was ∼10–60% of that at 0° < 4 MLAT < 10°, and the rate of decrease was larger on lower L* shells. The pressure anisotropy, derived as the perpendicular pressure divided by the parallel pressure minus 1, decreased with radial distance and showed a weak dependence on MLAT. The magnitude of the plasma beta at 30°<MLAT<40° was 1 or 2 orders smaller than that at 0°<MLAT<10°. The plasma pressure normalized by the value at 0°<MLAT<10° estimated from the magnetic strength and anisotropy was roughly consistent with the observed plasma pressure for L*=3.5–5.5. The azimuthal pressure‐gradient current derived from the plasma pressure was distributed over MLAT∼0–20°, while the curvature current was limited within MLAT∼0–10°. We suggest that the latitudinal dependence should be taken into account in interpretations of plasma parameters in successive orbits during magnetic storms
Arase Observation of the Source Region of Auroral Arcs and Diffuse Auroras in the Inner Magnetosphere
Auroral arcs and diffuse auroras are common phenomena at high latitudes, though characteristics of their source plasma and fields have not been well understood. We report the first observation of fields and particles including their pitch‐angle distributions in the source region of auroral arcs and diffuse auroras, using data from the Arase satellite at L ~ 6.0–6.5. The auroral arcs appeared and expanded both poleward and equatorward at local midnight from ~0308 UT on 11 September 2018 at Nain (magnetic latitude: 66°), Canada, during the expansion phase of a substorm, while diffuse auroras covered the whole sky after 0348 UT. The top part of auroral arcs was characterized by purple/blue emissions. Bidirectional field‐aligned electrons with structured energy‐time spectra were observed in the source region of auroral arcs, while source electrons became isotropic and less structured in the diffuse auroral region afterwards. We suggest that structured bidirectional electrons at energies below a few keV were caused by upward field‐aligned potential differences (upward electric field along geomagnetic field) reaching high altitudes (~30,000 km) above Arase. The bidirectional electrons above a few keV were probably caused by Fermi acceleration associated with the observed field dipolarization. Strong electric‐field fluctuations and earthward Poynting flux were observed at the arc crossing and are probably also caused by the field dipolarization. The ions showed time‐pitch‐angle dispersion caused by mirror reflection. These results indicate a clear contrast between auroral arcs and diffuse auroras in terms of source plasma and fields and generation mechanisms of auroral arcs in the inner magnetosphere
2D molecular superconductor to insulator transition in the β"-(BEDT-TTF)2[(H2O)(NH4)2M(C2O4)3]·18-crown-6 Series (M = Rh, Cr, Ru, Ir)
The series of salts β"-(BEDT-TTF)2[(H2O)(NH4)2M(C2O4)3]·18-crown-6 show ambient-pressure superconductivity when M = Cr, Rh. Evidence indicates that the previously reported Cr and Rh salts show a bulk Berezinski–Kosterlitz–Thouless superconducting transition. The isostructural ruthenium and iridium salts are reported here. The Ir salt represents the first radical-cation salt to contain a 5d tris(oxalato)metalate anion. The Ru and Ir salts do not show superconductivity but instead undergo a broad chemically induced metal to insulator transition at 155 K for ruthenium and at 100 K for iridium. The c axes of the Ru and Ir salts are much shorter than those of the Rh and Cr salts. Thus, the more stable metallic state of the Cr and Rh salts is associated with the more strongly 2D electronic systems. The different low-temperature behavior of the Ru and Ir salts, which exhibit a smaller interlayer spacing, could originate from a structural change in the anionic layer which thus can be easily transmitted to the donor layers and generate a localized state. However, another possibility is that it originates from Berezinski–Kosterlitz–Thouless effects
Off-equatorial Pi2 pulsations Inside and Outside the plasmapause observed by the Arase satellite
Using magnetic field and electron density data from the Arase satellite for the period from March 2017 to September 2019, we investigate the spatial properties of Pi2 pulsations in relation to the plasmapause over a wide latitudinal range (absolute magnetic latitude, |Mlat|, 0.7) with Pi2 pulsations in the north-south (H) component at low-latitude ground stations on the nightside, are dominantly identified from the magnetic fields in the radial (BR) and compressional (BP) components when the satellite is in the pre-midnight sector. In particular, high-coherence BP events are distributed over wide L-values and latitudinal ranges on the nightside in the pre-midnight sector. We identify the location of the plasmapause using the electron densities measured by Arase, and found that the BP-H power ratio and the cross phases of the high-coherence events show a gradual peak and a clear phase change from 0° to 180° in the vicinity of the plasmapause, respectively. These features indicate that mid- and low-latitude Pi2 pulsations on the nightside are excited by the plasmaspheric virtual resonance mode
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