Search Results (47 total)

We studied the spin accumulation and spin-transfer torque in a magnetic domain wall by solving the Boltzmann equation for spin accumulation with the diffusion approximation. We obtained analytical expressions of spin accumulation and spin-transfer torque. Both the adiabatic and the nonadiabatic components of the spin-transfer torque oscillate with the thickness of the domain wall. We showed that the oscillating component plays a dominant role in the nonadiabatic torque when the domain wall is thinner than the spin-flip length. We also showed that the magnitude of the nonadiabatic torque is inversely proportional to the thickness of the domain wall.

Optical properties near the band edge of hexagonal boron nitride were studied at 8 K. The photoluminescence spectrum shows two series of bands, namely, sharp (S) and diffuse (D), which are also distinguished by their fast (0.6 ns) for S and slow (5 ns) for D radiative decay time. Each series is composed of four bands with large Stokes shifts that are attributed to self-trapped excitons by the strong exciton-phonon interaction. The respective four luminescence bands of the two series originate from the four free-exciton levels in which the doubly degenerated dark and bright exciton levels theoretically predicted are resolved with the Jahn-Teller distortion in the excited states.

The critical current of the spin-transfer-torque-driven magnetization dynamics was studied by taking into account both spin pumping and the finite penetration depth of the transverse spin current. We successfully reproduced the recent experimental results obtained by Chen [Phys. Rev. B 74, 144408 (2006)] and found that the critical current remains finite even in the zero-thickness limit of the free layer. We showed that the remaining value of the critical current is determined mainly by spin pumping. We also showed that we could control the critical current by varying the spin-diffusion length of the nonmagnetic electrode adjacent to the free layer.

We report an experimental study on shape deformations of ternary vesicles undergoing phase separation under an osmotic pressure difference. The phase separation on various shape vesicles causes unique shape-deformation branches. In the domain coarsening stage, prolate, discocyte, and starfish vesicles show a shape convergence to discocytes, whereas a pearling instability is observed in tube vesicles. In late stages, the domains start to bud towards the inside or outside of the vesicle depending on the excess area. We discuss the deformation branches based on the membrane elasticity model.

The crystallographic and magnetic properties of CaRu_1−xMn_xO₃ (0≤x≤1.0) were investigated in detail by x-ray powder diffraction, magnetization, and magnetic Compton scattering measurements. The lattice parameters show considerable deviation from Vegard’s law. Ferromagnetism appears at a relatively large Mn concentration (x≥0.2), and the magnetization and the Curie temperature have a maximum at a Mn content near x=0.7. The magnetic Compton scattering measurement revealed that Mn makes a dominant contribution to the magnetization and the Mn moment is antiparallel to the Ru moment, which is induced by Mn doping. The anomalous change in the unit cell volume and the occurrence of ferromagnetism were discussed on the basis of the mixed-valence model of Mn³⁺, Mn⁴⁺, Ru⁴⁺, and Ru⁵⁺ ions. The Mn-composition dependence of the spontaneous magnetization was explained semiquantitatively assuming (1) ferromagnetic coupling between Mn³⁺ and Mn⁴⁺ ions, (2) antiferromagnetic coupling between Ru⁵⁺ and Mn ions, and (3) the theoretical spin moments of Mn³⁺, Mn⁴⁺, and Ru⁵⁺. The ferromagnetic interaction between Mn³⁺ and Mn⁴⁺ ions seems to make a dominant contribution to the Curie temperature. The CaRu_1−xMn_xO₃ system is considered to be a ferrimagnet induced through competition between the ferromagnetic interaction between Mn ions and the antiferromagnetic interaction between Ru⁵⁺ and Mn ions.

We analyzed the enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems. We show that the Gilbert damping constant depends both on the precession angle of the magnetization of the free layer and on the direction of the magnetization of the fixed layer. We find the condition to be satisfied to realize strong enhancement of the Gilbert damping constant.

The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering.

Neutron powder diffraction and magnetic Compton scattering measurements were conducted for ferromagnetic CaRu_0.85Fe_0.15O₃ at temperatures between 10 and 300 K. Anomalous volume expansion was observed in the neutron diffraction measurement below the Curie temperature (85 K), and Invar-like behavior was observed below 40 K. However, no structural phase transition was observed down to 10 K. The strong correlation between the volume expansion, ΔV, and the square magnetization, M², suggests that the anomalous volume expansion is due to the magnetovolume effect that is caused by the occurrence of ferromagnetism. The magnetic Compton scattering experiments revealed the existence of a magnetic moment on Ru and the antiferromagnetic configuration of Fe and Ru moments. The formation of a ferrimagnetic order through the induction of the magnetic moment on the Ru ion is a possible reason for the anomalous volume expansion observed for CaRu_0.85Fe_0.15O₃.

The magnetic Compton profile of CaRu_0.85Fe_0.15O₃ has been measured to elucidate the spin moment structure. The obtained profile is decomposed into two partial profiles of Ru 4d and Fe 3d, calculated on the basis of a Hartree-Fock molecular orbital calculation. This decomposition reveals that a spin moment of 0.38 μ_B per formula unit is induced on Ru and the moment aligns antiparallel to the spin moment (−0.18 μ_B per formula unit) of Fe. The observed antiferromagnetic spin coupling can be explained by the superexchange interaction between Ru and Fe ions via an O ion.

The dynamics of the localized region of the Lyapunov vector for the largest Lyapunov exponent is discussed in quasi-one-dimensional hard-disk systems at low density. We introduce a hopping rate to quantitatively describe the movement of the localized region of the Lyapunov vector, and show that it is a decreasing function of the hopping distance, implying a spatial correlation of the localized regions. This behavior is explained quantitatively by a brick accumulation model derived from hard-disk dynamics in the low density limit, in which hopping of the localized Lyapunov vector is represented as the movement of the highest brick position. We also give an analytical expression for the hopping rate, which is obtained as a sum of probability distributions for brick height configurations between two separated highest brick sites. The results of these simple models are in good agreement with the simulation results for hard-disk systems.

The five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C₁₁=811, C₁₂=169, C₁₃=0, C₃₃=27.0, and C₄₄=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.

We present a comparison between experimental soft x-ray spectra and density of states calculations of hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) single crystals. Cubic boron nitride films, grown on both mirror and scratched single crystal silicon wafers, have also been investigated using soft x-ray absorption spectroscopy (XAS) and soft x-ray emission spectroscopy (XES). Spectra measured at the 1s thresholds of boron and nitrogen give a complete picture of the occupied and unoccupied partial density of states for these materials. The films are shown to be a mixture of sp³ bonded nanocrystalline c-BN phase and sp² bonded h-BN phase. As the roughness of the deposition surface increases, a decrease in the amount of sp³ phase in the resulting film is observed. There are clear differences between the electronic structures of the nanocrystalline films and the single crystal samples. No differences between the spectra of the single crystals and previously reported measurements on powder samples of BN were observed.

The Lyapunov vectors corresponding to the steps of Lyapunov spectra for many-particle systems show time-oscillating behavior in two types of Lyapunov modes, one associated with time-translational invariance and the other with spatial translational invariance. Our result is that, for each coordinate direction, the longest period of the Lyapunov modes is twice as long as the period of the momentum autocorrelation function. A simple explanation for this relation is proposed and we argue that this result is generally true for many-particle systems. This gives the first quantitative connection between the Lyapunov modes and an experimentally accessible quantity.

The time-dependent mode structure of the Lyapunov vectors associated with the stepwise structure of the Lyapunov spectra and its relation to the momentum autocorrelation function are discussed in quasi-one-dimensional many-hard-disk systems. We obtain the complete mode structures (Lyapunov modes) for all components of the Lyapunov vectors, including the longitudinal and transverse components of both the spatial and momentum parts, and their phase relations. These mode structures are suggested by the form of the Lyapunov vectors for the zero-Lyapunov exponents. The spatial node structures of these modes are explained by the reflection properties of the hard walls used in the models. Our main result is that the largest time-oscillating period of the Lyapunov modes is twice as long as the time-oscillating period of the longitudinal momentum autocorrelation function. This relation is satisfied irrespective of the number of particles and the boundary conditions. A simple explanation for this relation is given based on the form of the time-dependent Lyapunov mode.

The extraordinary Hall resistivity ρ_xy and the magnetization M of a canonical spin glass AuFe (8   at.% Fe) were measured simultaneously as functions of temperature with the best care to the thermal and the magnetic field hysteresis. The data of ρ_xy show an anomaly at the spin glass transition temperature T_g and have different zero field cooling (ZFC) and field cooling (FC) measurements below T_g. Moreover, the value of ρ_xy/M, which represents the chiral susceptibility of the system in the present case, also shows the difference between ZFC and FC measurements. The results are consistent with the predictions of the chirality scenario of canonical spin glasses by Kawamura.

A nonequilibrium steady-state thermodynamics to describe shear flow is developed using a canonical distribution approach. We construct a canonical distribution for shear flow based on the energy in the moving frame using the Lagrangian formalism of the classical mechanics. From this distribution, we derive the Evans-Hanley shear flow thermodynamics, which is characterized by the first law of thermodynamics dE=TdS−Qdγ relating infinitesimal changes in energy E, entropy S, and shear rate γ with kinetic temperature T. Our central result is that the coefficient Q is given by Helfand’s moment for viscosity. This approach leads to thermodynamic stability conditions for shear flow, one of which is equivalent to the positivity of the correlation function for Q. We show the consistency of this approach with the Kawasaki distribution function for shear flow, from which a response formula for viscosity is derived in the form of a correlation function for the time-derivative of Q. We emphasize the role of the external work required to sustain the steady shear flow in this approach, and show theoretically that the ensemble average of its power Ẇ must be non-negative. A nonequilibrium entropy, increasing in time, is introduced, so that the amount of heat based on this entropy is equal to the average of Ẇ. Numerical results from nonequilibrium molecular-dynamics simulation of two-dimensional many-particle systems with soft-core interactions are presented which support our interpretation.

Low field ac-susceptibility experiments have been carried out to study the effect of “chemical” disorder and proximity to a magnetic quantum critical point (QCP) on the non-Fermi liquid (NFL) behavior in Ce(Ru_1−xRh_x)₂Si₂ for x=0.5 and 0.6 and CeCu_5.9Au_0.1. The susceptibility of strongly disordered NFL material Ce(Ru_1−xRh_x)₂Si₂ contains two components associated with different mechanisms; a disorder-driven component δχ and a mean-field (MF) quantum critical component χ_MF. δχ exhibits H∕T-scaling in the form of T^−γf(H∕T) with γ depending on x. In contrast, the disorder-driven component has not been observed in weakly disordered NFL material CeCu_5.9Au_0.1. The results of the scaling analysis strongly suggest that δχ is due to the quantum Griffiths singularity.

For high luminosity in electron-positron linear colliders, it is essential to generate low vertical emittance beams. We report on the smallest vertical emittance achieved in single-bunch-mode operation of the Accelerator Test Facility, which satisfies the requirement of the x-band linear collider. The emittances were measured with a laser-wire beam-profile monitor installed in the damping ring. The bunch length and the momentum spread of the beam were also recorded under the same conditions. The smallest vertical rms emittance measured at low intensity is 4 pm at a beam energy of 1.3 GeV, which corresponds to the normalized emittance of 1.0×1.0^-8   m. It increases by a factor of 1.5 for a bunch intensity of 10¹⁰ electrons. The measured data agreed to the calculation of intrabeam scattering within much better than a factor of 2.

A new operational scenario of advanced tokamak formation was demonstrated in the JT-60U tokamak. This was accomplished by electron cyclotron and lower hybrid waves, neutral beam injection, and the loop voltage supplied by the vertical field and shaping coils. The Ohmic heating (OH) solenoid was not used but a small inboard coil (part of the shaping coil), providing less than 20% of total poloidal flux, was used. The plasma thus obtained had both internal and edge transport barriers, with an energy confinement time of 1.6 times H-mode scaling, a poloidal beta of 3.6, and a normalized beta of 1.6, and a large bootstrap current fraction (>90%). This result opens up a possibility to reduce, and eventually eliminate, the OH solenoid from a tokamak reactor, which will greatly improve its economic competitiveness.

We introduce a definition of a “localization width” whose logarithm is given by the entropy of the distribution of particle component amplitudes in the Lyapunov vector. Different types of localization widths are observed, for example, a minimum localization width where the components of only two particles are dominant. We can distinguish a delocalization associated with a random distribution of particle contributions, a delocalization associated with a uniform distribution, and a delocalization associated with a wavelike structure in the Lyapunov vector. Using the localization width we show that in quasi-one-dimensional systems of many hard disks there are two kinds of dependence of the localization width on the Lyapunov exponent index for the larger exponents: one is exponential and the other is linear. Differences due to these kinds of localizations also appear in the shapes of the localized peaks of the Lyapunov vectors, the Lyapunov spectra, and the angle between the spatial and momentum parts of the Lyapunov vectors. We show that the Krylov relation for the largest Lyapunov exponent λ∼-ρ ln ρ as a function of the density ρ is satisfied (apart from a factor) in the same density region as the linear dependence of the localization widths is observed. It is also shown that there are asymmetries in the spatial and momentum parts of the Lyapunov vectors, as well as in their x and y components.

We present the measurement results of electron beam emittance in the Accelerator Test Facility damping ring operated in multibunch modes. The measurements were carried out with an upgraded laser wire beam profile monitor. The monitor has now a vertical wire as well as a horizontal one and is able to make much faster measurements thanks to an increased effective laser power inside the cavity. The measured emittance shows no large bunch-to-bunch dependence in either the horizontal or vertical directions. The values of the vertical emittance are similar to those obtained in the single-bunch operation. The present results are an important step toward the realization of a high-energy linear collider.

Boundary effects in the stepwise structure of the Lyapunov spectra and corresponding wavelike structure of the Lyapunov vectors are discussed numerically in quasi-one-dimensional systems of many hard disks. Four different types of boundary conditions are constructed by combinations of periodic boundary conditions and hard-wall boundary conditions, and each leads to different stepwise structures of the Lyapunov spectra. We show that for some Lyapunov exponents in the step region, the spatial y component of the corresponding Lyapunov vector δq_yj, divided by the y component of momentum p_yj, exhibits a wavelike structure as a function of position q_xj and time t. For the other Lyapunov exponents in the step region, the y component of the corresponding Lyapunov vector δq_yj exhibits a time-independent wavelike structure as a function of q_xj. These two types of wavelike structure are used to categorize the type and sequence of steps in the Lyapunov spectra for each different type of boundary condition.

We describe in this paper a measurement of vertical emittance in the Accelerator Test Facility (ATF) damping ring at KEK with a laser wire beam profile monitor. This monitor is based on the Compton scattering process of electrons with a laser light target which is produced by injecting a cw laser beam into a Fabry-Perot optical cavity. We installed the monitor at a straight section of the damping ring and measured the vertical emittance with three different ring conditions. In all cases, the ATF ring was operated at 1.28 GeV in a single bunch mode. When the ring was tuned for ultralow emittance, the vertical emittance of ε_y=(1.18±0.08)×10^-11   mrad was achieved. This shows that the ATF damping ring has realized its target value also vertically.

The master equation approach to Lyapunov spectra for many-particle systems is applied to nonequilibrium thermostated systems to discuss the conjugate pairing rule. We consider iso-kinetic thermostated systems with a shear flow sustained by an external restriction, in which particle interactions are expressed as a Gaussian white randomness. Positive Lyapunov exponents are calculated by using the Fokker-Planck equation to describe the tangent vector dynamics. We introduce another Fokker-Planck equation to describe the time-reversed tangent vector dynamics, which leads to the calculation of the negative Lyapunov exponents. Using the Lyapunov exponents provided by these two Fokker-Planck equations we show the conjugate pairing rule is satisfied for thermostated systems with a shear flow in the thermodynamic limit which allow us to replace the friction coefficient with a constant number. We also give an explicit form to connect the Lyapunov exponents with the time correlation of the interaction matrix in a thermostated system with a color field.

The structure of the Lyapunov spectra for the many-particle systems with a random interaction between the particles is discussed. The dynamics of the tangent space is expressed as a master equation, which leads to a formula that connects the positive Lyapunov exponents and the time correlations of the particle interaction matrix. Applying this formula to one- and two-dimensional models we investigate the stepwise structure of the Lyapunov spectra that appear in the region of small positive Lyapunov exponents. Long range interactions lead to a clear separation of the Lyapunov spectra into a part exhibiting stepwise structure and a part changing smoothly. The part of the Lyapunov spectrum containing the stepwise structure is clearly distinguished by a wavelike structure in the eigenstates of the particle interaction matrix. The two-dimensional model has the same step widths as found numerically in a deterministic chaotic system of many hard disks.