Showing papers by "Japan Atomic Energy Research Institute published in 2000"

A first-order liquid–liquid phase transition in phosphorus

Abstract: First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 002 GPa During the transformation, the two forms of liquid coexist These features are strongly suggestive of a first-order liquid-liquid phase transition

Probing bulk states of correlated electron systems by high-resolution resonance photoemission

Abstract: Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref 6), it should become feasible to probe the bulk electronic states in these materials Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2 Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences

Hydrogel of biodegradable cellulose derivatives. I. Radiation‐induced crosslinking of CMC

Abstract: Radiation crosslinking of carboxymethylcellulose (CMC) with a degree of substitution (DS) from 0.7 to 2.2 was the subject of the current investigation. CMC was irradiated in solid-state and aqueous solutions at various irradiation doses. The DS and the concentration of the aqueous solution had a remarkable affect on the crosslinking of CMC. Irradiation of CMC, even with a high DS, 2.2 in solid state, and a low DS, 0.7 in 10% aqueous solution, resulted in degradation. However, it was found that irradiation of CMC with a relatively high DS, 1.32, led to crosslinking in a 5% aqueous solution, and 20% CMC gave the highest gel fraction. CMC with a DS of 2.2 induced higher crosslinking than that with a DS of 1.32 at lower doses with the same concentration. Hence, it was apparent that a high DS and a high concentration in an aqueous solution were favorable for high crosslinking of CMC. It is assumed that; high radiation crosslinking of CMC was induced by the increased mobility of its molecules in water and by the formation of CMC radicals from the abstraction of H atoms from macromolecules in the intermediate products of water radiolysis. A preliminary biodegradation study confirmed that crosslinked CMC hydrogel can be digested by a cellulase enzyme. (C) 2000 John Wiley & Sons, Inc.

Lesions in the sterol Δ7 reductase gene of Arabidopsis cause dwarfism due to a block in brassinosteroid biosynthesis

Abstract: Summary The brassinosteroid (BR) biosynthetic pathway, and the sterol pathway which is prerequisite to the BR pathway, are rapidly being characterized because of the availability of a large number of characteristic dwarf mutants in Arabidopsis. Here we show that the Arabidopsis dwarf5 mutants are disrupted in a sterol Δ7 reduction step. dwf5 plants display the characteristic dwarf phenotype typical of other BR mutants. This phenotype includes small, round, dark-green leaves, and short stems, pedicels, and petioles. Metabolite tracing with 13C-labeled precursors in dwf5 verified a deficiency in a sterol Δ7 reductase activity. All six independent alleles contain loss-of-function mutations in the sterol Δ7 reductase gene. These include a putative mRNA instability mutation in dwf5-1, 3′ and 5′ splice-site mutations in dwf5-2 and dwf5-6, respectively, premature stop codons in dwf5-3 (R400Z) and dwf5-5 (R409Z), and a mis-sense mutation in dwf5-4 (D257N). The dwf5 plant could be restored to wild type by ectopic overexpression of the wild-type copy of the gene. Both the Arabidopsis dwf5 phenotype and the human Smith–Lemli–Opitz syndrome are caused by loss-of-function mutations in a sterol Δ7 reductase gene, indicating that it is required for the proper growth and development of these two organisms.

Mechanism for Negative Poisson Ratios over the α - β Transition of Cristobalite, SiO 2 : A Molecular-Dynamics Study

Abstract: The adiabatic elastic constants ( ${C}_{\mathrm{ij}}$) of cristobalite have been evaluated successfully over the temperature range of 300--1800 K using the molecular-dynamics method with a fluctuation formula. Cristobalite shows a negative Poisson ratio over this temperature range. However, the mechanisms differ between the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases. In the cubic $\ensuremath{\beta}$ phase, ${C}_{44}$ exhibits a value extremely close to ${C}_{11}$ rather than ${C}_{12}$, in contrast to the Cauchy relation. This predicts a remarkable property that the longitudinal and transverse velocities coincide for the acoustic waves propagating along the $[100]$ direction.

Disappearance of giant ELMs and appearance of minute grassy ELMs in JT-60U high-triangularity discharges

Abstract: In JT-60U H-mode plasmas, giant (type I) ELMs disappear and minute grassy ELMs appear when triangularity δ, edge safety factor q95 and βp are high enough. Complete suppression of giant ELMs was observed at δ0.45, q956 and βp1.6. At higher δ (0.54), giant ELMs can disappear at a lower q95 (~4.0). In the grassy ELMy H-mode, edge temperature and pressure can be higher than those in giant ELMy H-mode and a favourable confinement can be sustained without an increase of the impurity concentration. An edge stability analysis suggests that the edge plasma is accessing the second stability regime of the high n ballooning mode in the grassy ELMy discharges.

Validation of a Continuous-Energy Monte Carlo Burn-up Code MVP-BURN and Its Application to Analysis of Post Irradiation Experiment

Abstract: In order to confirm the reliability of a continuous-energy Monte Carlo burn-up calculation code MVP- BURN, it was applied to the burn-up benchmark problems for a high conversion LWR lattice and a BWR lattice with burnable poison rods. The results of MVP-BURN have shown good agreements with those of a deterministic code SRAC95 for burn-up changes of infinite neutron multiplication factor, conversion ratio, power distribution, and number densities of major fuel nuclides. Serious propagation of statistical errors along burn-up was not observed even in a highly heterogeneous lattice. MVP-BURN was applied to the analysis of a post irradiation experiment for a sample fuel irradiated up to 34.1GWd/t, together with SRAC95 and SWAT. It was confirmed that the effect of statistical errors of MVP-BURN on a burned fuel composition was sufficiently small, and it could give a reference solution for other codes. In the analysis, the results of the three codes with JENDL-3.2 agreed with measured values within an error of ...

Optical guidance of terrawatt laser pulses by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary

Abstract: A new method of optical guidance by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary is proposed. An imploding plasma column has a concave electron-density profile in the radial direction, just before a stagnation phase driven by a converging current sheet and a shock wave. The feasibility of optical guidance of a high-intensity (>1 x 10(17) W/cm(2)) Ti:sapphire laser pulse by use of this method over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length, has been experimentally demonstrated. The guiding-channel formation process was directly probed with a He-Ne laser beam. The electron density in the fully ionized channel was estimated to be 2.0 x 10(17) cm(-3) on the axis and 7.0 x 10(17) cm(-3) on the peaks of the channel edge, with a diameter of 70 mum, as indicated by the experimental results, which were corroborated by a magnetohydrodynamics simulation.

Evidence for the existence of nitrogen-substituted graphite structure by polarization dependence of near-edge x-ray-absorption fine structure

Abstract: The electronic structure of carbon nitride $({\mathrm{CN}}_{x})$ film produced by low-energy ion implantation has been studied by N 1s near-edge x-ray-absorption fine structure (NEXAFS). The NEXAFS spectra of ${\mathrm{CN}}_{x}$ film have shown three sharp ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ and one broad ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ resonances whose intensities have clear polarization dependence. The polarization dependencies of the first and third ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ resonances have given a similar tendency to that of the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ resonance of graphite, while the second ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ resonance has given different behavior. From the results, we concluded that the first, second, and third ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ resonances originate from pyridinelike structure, cyanic structure, and graphitelike structure, respectively. The results show the clear experimental evidence that there exists a graphitelike structure in which some of the carbon atoms are substituted by nitrogen atoms.

Magnetic structure of the kagomé lattice antiferromagnet potassium jarosite KFe 3 ( OH ) 6 ( S O 4 ) 2

Abstract: We have examined the magnetic structure of the kagom\'e lattice antiferromagnet potassium jarosite (K jarosite: ${\mathrm{KFe}}_{3}{(\mathrm{OH})}_{6}{(\mathrm{S}\mathrm{O}}_{4}{)}_{2})$ by means of powder neutron diffraction. Extremely high degeneracy of the ground states prevents the long-range magnetic ordering at any temperature and the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structure is predicted theoretically to be favored rather than the $q=0$ structure at $T=0$ in a kagom\'e lattice Heisenberg antiferromagnet. Nevertheless, K jarosite shows long-range magnetic ordering at 65 K and the ordered magnetic structure was found to be the $q=0$ structure. In addition, although the $q=0$ structure has two degenerated states of ``positive'' and ``negative'' chirality, the observed magnetic structure contains only elemental triangles of positive chirality. We found that a weak single-ion-type anisotropy is crucial for selecting the observed magnetic structure. The long-range magnetic ordering at finite temperature in the jarosite family of compounds can be ascribed to this anisotropy.

The Fermi Surface in Filled Skutterudite RFe4P12 (R=La and Nd).

Abstract: We have investigated the de Haas-van Alphen (dHvA) effect and the transverse magnetoresistance in filled skutterudite LaFe 4 P 12 and NdFe 4 P 12 . The Fermi surface of LaFe 4 P 12 consists of a nearly spherical hole sheet and a multiply connected one. These features are explained reasonably well by a full potential linearized augmented-plane-wave (FLAPW) band structure calculation. The Fermi surface of NdFe 4 P 12 is similar to those in LaFe 4 P 12 except the exchange splitting in the dHvA frequencies. The cyclotron effective masses are enhanced roughly twice compared to those of LaFe 4 P 12 due to the electron-magnon interaction.

Collective dynamics of Josephson vortices in intrinsic Josephson junctions: exploration of in-phase locked superradiant vortex flow states

Abstract: In order to explore the “superradiant” condition for the moving Josephson vortices in intrinsic Josephson junctions (IJJs), we perform large scale numerical simulations for the IJJs with realistic dimensions under a layer-parallel magnetic field (H>1T). Three clear step-like structures, at which we find structural transitions in moving flux-line lattice, are observed in the I–V curve. The flow states of the Josephson vortices are classified into four regions, from I to IV, according to the applied current or voltage values. The superradiant state, in which moving Josephson vortices forms the in-phase rectangular lattice, is stabilized in region III. In the superradiant state, the power spectra of excited electric fields show very sharp peak structures and the power at the peak position is very large. From the eigen mode analysis, we show that the step-like structure in the I–V curve corresponding to structural transitions in moving lattices originates from the resonances between the transverse Josephson plasma modes and the motion of the Josephson vortices.

Ground-state magnetic structure of CeRh 2 Si 2 and the response to hydrostatic pressure as studied by neutron diffraction

Abstract: Neutron-diffraction experiments under high pressure have been done to study the magnetic structure of the ground state and the nature of the magnetic order of the pressure-induced superconductor ${\mathrm{CeRh}}_{2}{\mathrm{Si}}_{2}.$ Two models for the magnetic structure of the ground state, namely, a $4\ensuremath{-}\mathbf{q}$ superposed structure and a multidomain structure, have been tested and the former was found to be more realistic, though both of them have some inconsistency with the results of the previous NMR experiment. The pressure dependence of both the transition temperature and magnitude of the ordered magnetic moment indicate that the magnetism of this compound is basically itinerant despite its high transition temperature. Our studies also revealed that, when the pressure is close to the critical magnitude to destroy the antiferromagnetism, the compound can have a ``tiny ordered moment'' which is widely observed in the metallic Kondo-lattice compounds.