Susumu Sasaki

NQR and NMR of 139La in antiferromagnetic La2CuO4−δ

Rather complex Nuclear Quadrupole Resonance (NQR) spectrum of 139 La in La 2 CuO 4-δ , observed at low temperatures is successfully interpreted as a signal from a single La site which suffers a combined effect of a quadrupole coupling (coupling constant ν Q =6.38±0.02 MHz with asymmetry parameter η=0.01±0.01) and a small Zeeman term where the internal field ( H N =0.997±0.010 kOe) is nearly perpendicular to the direction of the maximum field gradient. The spin structure of Cu at low temperatures is suggested to be a simple antiferromagnet with sublattice moments parallel to the a -axis. The temperature dependences of ν Q and H N have been measured in detail. The fractional decrease of H N which is proportional to the Cu sublattice moments is found to be in general accord with the prediction of spin fluctuations theory for weak itinerant antiferromagnets.

Laboratory simulation of space weathering: Changes of optical properties and TEM/ESR confirmation of nanophase metallic iron

Abstract Reflectance spectra of S-type asteroids are different from those of ordinary chondrites. This spectral mismatch is explained by space weathering processes, where high-velocity dust particle impacts change the optical properties of the uppermost regolith surface of asteroids. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. Nanophase metallic iron particles, which are formed through vapor deposition from dust impact heating, are considered as the most essential cause of space weathering. In this study, we describe the spectral changes of olivine and pyroxene using nanosecond pulse laser irradiation and the presence of nanophase metallic iron particles in laser-irradiated materials by transmission electron microscopy (TEM) and electron spin resonance (ESR). The irradiated spectra of the samples show a reduction of the overall spectra (250–2600 nm) and a reddening with weakened absorption bands. Nanophase metallic iron particles were found not only in laser-irradiated olivine samples, but also in laser-irradiated pyroxene samples by TEM. Strong ESR signals, which derive from nanophase iron particles, are observed in the irradiated olivine samples. Moreover, ESR intensities increase with the space weathering degree simulated as laser irradiation time. One possible application of space weathering is the estimation of the relative age of asteroids using the relation between optical effects and quantities of produced nanophase iron particles.

NMR and NQR studies in high-T c oxides: YBa2Cu3O y (6.0≤y≤6.91)

The NQR and NMR techniques have been utilized to characterize the local oxygen coordination of inequivalent Cu sites and the electronic properties in both the normal and superconducting states of YBa2Cu3Oy (6.0<-y≤6.91). The distinct NQR lines associated with the different oxygen-coordinated Cu sites, hence the locally differentiated charged states, have been observed. The degree of charge differentiation at the Cu(2) plane sites was found to be increased with decreasingy from 6.91, which might be related with the decrease ofTc. An anomalous temperature dependence of Cu nuclear spin-lattice relaxation timeT1 has been observed for both the Cu(1) chain and Cu(2) plane sites fory=6.91 and it is discussed in connection with antiferromagnetic spin fluctuations in the normal state.

Cu chemistry of YBa2Cu3O6+x investigated by nuclear quadrupole resonance

Abstract The nuclear quadrupole resonance (NQR) technique has been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa2Cu3O6+x(0 ≤ x ≤ 0.91). Essentially, four distinct NQR lines which correspond to 2, 3,4 oxygen coordinated Cu sites in the Cu-O chains and 5 oxygen coordinated Cu sites in the Cu-O planes have been observed. The zero-field NQR frequencies of these are centered at about 30.1, 24.0, 22.0 and 31.5 MHz for 63Cu, respectively. For the antiferromagnetic ordered state (x ≤ 0.3), antiferromagnetic nuclear resonance (AFNR) has been observed at 90 MHz with quadrupole splittings associated with the moment-bearing Cu sites in the Cu-O planes. The relative intensities of these resonance lines depend on the oxygen content, and this gives us a microscopic understanding of the Cu chemistry of this system.

Fermi-Liquid Behavior and BCS s-Wave Pairing of K3C60 Observed by 13C-NMR

This Letter reports on NMR studies of 13 C in K 3 C 60 above and below T c with an applied field of 2.93 T. It is found that non-single-exponential recovery data can be fitted by two components and normalized to an identical shape. With T 1 defined in this way, Fermi-liquid behavior is identified above T c , and a broadened Hebel-Slichter peak observed below T c is well explained by the Eliashberg theory with 2Δ(0)/ k B T c =4.31. The possibility of strong electron correlation and the origin of the multi-exponential recovery are discussed.

Mass separation experiment with a partially ionized rotating plasma

An experiment on centrifugal separation of a He-Ar mixture gas has been performed in a weakly ionized rotating plasma. The concentration of Ar to He at the outer electrode has increased with increasing the axial magnetic field or radial current, and has reached the level 1.55 times larger than that before rotation when the magnetic field is 7.5 kG and the total electric current is 1.0 A. This result does not depend on the gas pressure from 0.6 to 20 Torr. A possible application of the device to uranium isotope separation is discussed.

Development of a lightweight space debris shield using high strength fibers

In order to protect space structure against space debris impacts, it is indispensable to develop a shield with high strength materials. A high strength fiber is one of potential materials from a viewpoint of strength, lightweight, and flexibility. The purpose of this study was to develop a new lightweight shield composed of high strength fibers against medium size debris impacts. We developed four kinds of shields using Vectran fibers, and hypervelocity impact tests were carried out by a railgun accelerator. The experimental results showed that the developed shield could stop the polycarbonate projectile with 13 mm in diameter, 1 gram in weight, and 6.9 km/sec in velocity. Adoption of the high strength fiber in the bumper materials may reinforce the protection capability and reduce the weight drastically.

Design of electrostatic accelerators for the development of microparticle detectors in Japan

Abstract Micrometeoroid detection in space usually involves high-velocity impact phenomena. In-situ dust detectors should be calibrated by a microparticle accelerator with a mass and velocity range comparable to micrometeoroids. A pilot model of an accelerator was constructed some years ago and fundamental research for an advanced facility has been performed. We are developing two new accelerators. A 3.75MV Van de Graaff electrostatic accelerator at The Research Center for Nuclear Science and Technology, University of Tokyo, has been modified to accelerate microparticles, and acceleration testing is now being carried out. The expected velocity range is 1–20 km/s for micron or sub-micron particles. Another small accelerator will be installed for easier handling, more frequent use, and lower-cost operation.

13C spin-lattice relaxation study on the electronic properties of K3C60

Abstract 13 C spin-lattice relaxation was measured for a superconducting fulleride K 3 C 60 . It is found that non-single exponential relaxation (NSER) data at low temperatures ( T T -independent shape above and below T c , are well reproduced by only the anisotropy parameter ( α spin ) for the hyperfine coupling that was derived from a line-shape analysis. From the simulation for various values of α spin , the shape of the NSER, as well as the asymmetric line shapes, is found to be caused by the anisotropy of an electronic single-site rather than electronic multi-sites. By extracting the isotropic part of relaxation time, ( T 1 ) iso , from the NSER, it is found that an extended Korringa relation holds up to 300 K with K ( α ) = 7.4, and that the T dependence of [( T 1 ) iso T ] −1 resultss from a decrease in the density of the states. Below T c , a broadened Hebel-Slichter coherence peak is observed, which means s-wave Cooper pairing. The T dependence of [( T 1 ) iso T ] −1 below T c is well reproduced by a theoretical prediction with the superconducting gap 2 Δ(0)/ k B T c = 4.3. The origin of the isotropic hyperfine coupling is also discussed.

39K-NMR observation of temperature-dependent spin susceptibility in K3C60

Abstract 39 K - NMR spectra of superconducting fulleride K3C60 are obtained in the normal state. The 39 K - NMR shifts of two symmetrical lines observed with no quadrupolar-split satellites, that are assigned to the potassium tetrahedral- and octahedral-sites, are found to show the same temperature (T) dependence as the 13 C - NMR shifts. From roughly estimated spin-lattice relaxation time, it is found that the conduction-electron density at the potassium site is nearly the same as that at the carbon site. These confirm that the uniform Pauli spin susceptibility, whose T dependence is considered to come from the predicted band effect, causes the reductions of both the 13 C - and 39 K - NMR shifts with decreasing T. Two experimental facts are pointed out for the T′ site observed.