Spin dynamics in a site diluted finite spin cluster system probed by positive muons

The site diluted finite spin cluster system Rb₂CO _x Mg_1–x F₄ (x=0.15, 0.4, 0.5) has been investigated by the ⁺SR technique. Muon spin relaxation was observed in these diluted magnetic systems in a weak longitudinal field of 100 G. By reducing temperature below 100 K, the experimental data show features of slowing down phenomena of the Co electron moments in the finite spin clusters.     

Ultra-slow muon facility for surface HFI studies

A concept, a design, a construction and an account of commissioning experiments are given for the recently completed ultra-slow muon facility at the pulsed muon facility of UT-MSL/KEK. The intense (more than 10³/s) slow ⁺ beam with an extremely narrow phase-space volume (0.2 eV×(3 cm)²) to be produced in this facility will open a new muon science including surface physics and chemistry and fundamental atomic physics.Post-doctoral fellow of Swiss National Science Foundation.     

High-Efficiency ZnCdSe/ZnSSe/ZnMgSSe Green Light-Emitting Diodes

Green light-emitting diodes (LEDs) were fabricated employing a ZnCdSe/ZnSSe triple quantum-well (TQW) active region surrounded by ZnMgSSe cladding layers grown on an n-type (100) GaAs substrate by molecular beam epitaxy (MBE). A 3.5 mW pure green emission was observed for the surface-emitting LED device at a peak wavelength of 513.3 nm (2.415 eV) with a spectral half-width of 11.7 nm (55 meV) under a 20 mA (4.6 V) direct current at room temperature (25°C). These correspond to an external quantum efficiency of 7.2%, a power conversion efficiency of 3.8%, a luminous current efficiency of 66 lm/A, and a luminous efficiency of 14 lm/W.     

Pulse laser ablation at water–air interface

We studied a new pulse laser ablation phenomenon on a liquid surface layer, which is caused by the difference between the refractive indices of the two materials involved. The present study was motivated by our previous study, which showed that laser ablation can occur at the interface between a transparent material and a gas or liquid medium when the laser pulse is focused through the transparent material. In this case, the ablation threshold fluence is reduced remarkably. In the present study, experiments were conducted in water and air in order to confirm this phenomenon for a combination of two fluid media with different refractive indices. This phenomenon was observed in detail by pulse laser shadowgraphy. A high-resolution film was used to record the phenomenon with a Nd:YAG pulse laser with 10-ns duration as a light source. The laser ablation phenomenon on the liquid surface layer caused by a focused Nd:YAG laser pulse with 1064-nm wavelength was found to be followed by the splashing of the liquid surface, inducing a liquid jet with many ligaments. The liquid jet extension velocity was around 1000 m/s in a typical case. The liquid jet decelerated drastically due to rapid atomization at the tips of the ligaments. The liquid jet phenomenon was found to depend on the pulse laser parameters such as the laser fluence on the liquid surface, laser energy, and laser beam pattern. The threshold laser fluence for the generation of a liquid jet was 20 J/cm². By increasing the incident laser energy with a fixed laser fluence, the laser focused area increased, which eventually led to an increase in the size of the plasma column. The larger the laser energy, the larger the jet size and the longer the temporal behavior. The laser beam pattern was found to have significant effects on the liquid jet’s velocity, shape, and history.     

New approaches inμ + SR studies on critical phenomena in Ni

The critical phenomena in Ni are probed by pulsedSR method under longitudinal- and zero external magnetic fields. The sample magnetization around the critical temperature is confirmed simultaneously by bulk magnetization measurement in situ, disappearance of transverseSR signal and recovery of asymmetry under longitudinal field. At the same time, the ratio of the ⁺ hyperfine field to the bulk magnetization in the ferromagnetic phase below the critical temperature is determined from the observables obtained only in the present experiment. The zero- and low-field longitudinal relaxation rate of muon does not diverge in approaching toT _c in the paramagnetic region, but seems to reach a saturation value.This work is supported by the Grand-in-Aid of the Japanese Ministry of Education, Culture and Science.     

NMR studies on polarized12B implanted in highly oriented pyrolytic graphite

NMR measurements on¹²B nuclei implanted in a thin sheet of highly oriented pyrolytic graphite (Grafoil) have been made to study the magnitude of the polarization of¹²B and its electric quadrupole coupling. The magnitude of the residual polarization of¹²B held in carbon material is of crucial importance to the experiment measuring the average polarization of¹²B produced in the muon capture by¹²C. It was found that the polarization of¹²B can be maintained at 101.0±2.2% in Grafoil under a magnetic field of 3 kG. The electric quadrupole coupling was determined to bee ² qQ/h=+1207.3±8.2 kHz. From the magnitude and the sign of the electric field gradienteq, the location of¹²B was found to be a substitutional site in the carbon lattice. The anomalous B-C bond involving thesp ₂ hybrid orbitals with partially filledp _z electrons is also discussed.     

The dynamic behavior of magnetic fluid adsorbed to small permanent magnet in alternating magnetic field

The dynamic behavior of a magnetic fluid adsorbed to a small NdFeB permanent magnet subjected to an alternating magnetic field was studied with a high speed video camera system. The directions of alternating magnetic field are parallel and opposite to that of the permanent magnet. It was found that the surface of magnetic fluid responds to the external alternating magnetic field in elongation and contraction with a lot of spikes. Generation of a capillary magnetic fluid jet was observed in the neighbourhood of a specific frequency of alternating field. The effect of gravitational force on surface phenomena of magnetic fluid adsorbed to the permanent magnet was revealed.     

Vocalization control in Mongolian gerbils (Meriones unguiculatus) during locomotion behavior

The vocalization behavior of Mongolian gerbils, a model animal of auditory physiology, was examined. A pair of gerbils was placed in a chamber, and their species-specific vocalizations and locomotive behaviors were recorded and analyzed. Two types of calls were predominantly produced: high-frequency upward frequency-modulated (HU-FM) calls and low-frequency multi-harmonic frequency-modulated (LM-FM) calls. Emission rates of HU-FM calls significantly decreased as the distance between the two gerbils increased, and playback of simulated HU-FM calls increased the emission rates. Acoustic analysis of HU-FM calls showed that the calls exhibited a stereotypic spectro-temporal structure including a fixed inter-onset interval (100-175 ms) and that individual differences in the frequency could convey the body size of the callers. The timing of HU-FM calls was highly synchronized with jump movements when an animal vocalized while jumping, suggesting the existence of tight locomotor-vocal coupling. Conversely, LM-FM calls were observed only when the gerbils tactilely contacted with each other while fighting over a food. These results suggest that Mongolian gerbils change the rates of call emissions and call types (e.g., LM-FM or HU-FM calls) in response to changes in visual and possibly tactile and auditory information. The functions of both calls are discussed in terms of their acoustic structures.     

Structure and mechanical properties of nylon 6.12 prepared by temperature slope crystallization. I. Crystallization of oriented spherulitic textures

Oriented spherulitic textures of a rod-shaped nylon 6.12 sample were crystallized by the temperature slope method. Crystallization conditions were compared by changing temperatures and growth rates. Three types of textures (negative spherulites, positive spherulites, and spherulitic aggregates) were observed by this method. The negative textures appeared when the growth rate was less than 0.1 mm/h. Crystalline orientation and mechanical properties of the textures were investigated by x-ray diffraction and micro-hardness measurements, respectively. The hydrogen-bonded (010) planes were perpendicular to the growth direction in the negative spherulite, while they were parallel to the growth direction in the positive spherulite. In the spherulitic aggregates, the b axis was parallel to the growth direction, while the (010) planes formed roughly an angle of 45° with the growth direction. Due to the orientation of the hydrogen-boned planes, the negative texture exhibited an anisotropy, with hardness values of 106 MPa and 137 Mpa when measured perpendicular or parallel to the growth direction, respectively.     

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method ( ⁺SR) is applied on La_2–x Sr _x CuO₄ around forx=0.12 at which the high-T _c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105x0.120. The magnetic phase boundary is very similar to the one of the La_2–x Ba _x CuO₄ in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO₂ plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T _c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.