Search Results (39 total)

The quantum corrections to the energies of the Γ point optical phonon modes (Kohn anomalies) in graphene nanoribbons (NRs) are investigated. We show theoretically that the longitudinal optical (LO) modes undergo a Kohn anomaly effect, while the transverse optical (TO) modes do not. In relation to Raman spectroscopy, we show that the longitudinal optical modes are not Raman active near the zigzag edge, while the transverse optical modes are not Raman active near the armchair edge. These results are useful for identifying the orientation of the edge of graphene nanoribbons by G band Raman spectroscopy, as is demonstrated experimentally. The differences in the Kohn anomalies for nanoribbons and for metallic single-wall nanotubes are pointed out, and our results are explained in terms of pseudospin effects.

We measured the transverse acoustic impedance of superfluid ³He-B with a wall coated by several layers of ⁴He. The coating is known to enhance the specularity in quasiparticle scattering by the wall. We found a new anomaly, a bump and a peak, in the temperature dependence of the transverse acoustic impedance. This agrees with a theoretical calculation using a partially specular wall boundary condition. The new anomaly is shown to arise from a change in the surface density of states by coating and the scattering of thermally occupied surface bound states to other states. The change is towards the density of states of Majorana cone in the specular limit.

By using laser-photoemission (LPE) spectroscopy we have investigated inelastic interaction between vibrational modes and photoelectrons emitted from the Cu surfaces. Two plausible rules for vibrational excitation during LPE were obtained. One requires the wave vector matching between the vibrational mode and the electron in the photoexcited state on the surface. The other is that the polarization of the vibrational mode interacting inelastically must lie parallel to the surface plane. These results imply that this inelastic interaction arises dominantly from the resonance scattering and not from the dipole one.

A dissipative system with asymmetric interaction, as well as the optimal velocity model, generally shows a Hopf bifurcation concerned with the transition from homogeneous motion to the formation of nontrivial patterns. We reveal that the origin of Hopf bifurcation in macroscopic phenomena is strongly related to asymmetric interaction in a microscopic many-body system, using the continuum system derived from the original discrete system.

The 3-GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) was commissioned in October 2007, and successfully accomplished 3 GeV acceleration on October 31. Six run cycles through February 2008 were dedicated to commissioning the RCS, for which the initial machine parameter tuning and various underlying beam studies were completed. Then since May 2008 the RCS beam has been delivered to the downstream facilities for their beam commissioning. In this paper we describe beam tuning and study results following our beam commissioning scenario and a beam performance and operational experience obtained in the first commissioning phase through June 2008.

In the rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC), the longitudinal painting is important to alleviate the space-charge effects. It is known that the momentum offset injection and applying the second harmonic rf voltage improves the bunching factor, which is defined as the ratio of average and peak current. Our simulation studies show that the large-amplitude second harmonic, 80% to the fundamental, is optimum, and the second harmonic phase sweep improves the bunching factor at the beginning of the injection period. We performed the beam tests of longitudinal painting in the J-PARC RCS. We proved that the longitudinal painting with the 80% second harmonic, the momentum offset of -0.2%, and the second harmonic phase sweep improved bunching factors significantly.

We have investigated multiple ionization of Xe clusters by 61-nm 10¹¹–10¹² W/cm² extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan, using a dead-time-free three-dimensional momentum spectrometer. It was found that the average kinetic energy of atomic Xe⁺ ions increases when increasing the laser power density and the cluster size. For these experimental conditions significant frustration of the cluster photoionization occurs but no indication for heating mechanisms other than sequential photoabsorption by individual atoms in the cluster was found.

The low-energy spectrum of graphene nanoribbons with armchair edges (armchair nanoribbons) is described as the superposition of two nonequivalent Dirac points of graphene. In spite of the lack of well-separated two valley structures, the single-channel transport subjected to long-ranged impurities is nearly perfectly conducting, where the backward-scattering matrix elements in the lowest order vanish as a manifestation of internal phase structures of the wave function. For multichannel energy regime, however, the conventional exponential decay of the averaged conductance occurs. Since the intervalley scattering is not completely absent, armchair nanoribbons can be classified into orthogonal universality class irrespective of the range of impurities. The nearly perfect single-channel conduction dominates the low-energy electronic transport in rather narrow nanorribbons.

The excited-state atom densities in the negative glow of a direct-current glow discharge are derived from the spectral-line intensity of radiative atoms and the resonance-fluorescence photon flux of nonradiative atoms. The discharge is operated in a helium-argon gas mixture (molar fraction ratio 91:9; total gas pressure 5 Torr) at a dc current of 0.7–1.2 mA. The observations are made in the region of the maximum luminance in the cathode region, where high-energy electrons accelerated in the cathode fall are injected into the negative glow. The emission intensities of the He I, He II, Ar I, and Ar II spectral lines are measured with a calibrated tungsten ribbon lamp as an absolute spectral-radiance standard. Fluorescence photons scattered by helium and argon atoms in the metastable state and argon atoms in the resonance state are detected by the laser-induced fluorescence (LIF) method with the Rayleigh scattering of nitrogen molecules as an absolute standard of scattering cross section. The laser absorption method is incorporated to confirm the result of the LIF measurement. Excitation energies of the measured spectral lines range from 11.6 (Ar I) to 75.6 eV (He II), where the excitation energy is measured from the ground state of the neutral atom on the assumption that, in the plasma of this study, both the neutral and the ionic lines are excited by electron impact in a single-step process from the ground state of the corresponding neutral atoms. Experimental evidence is shown for the validity of this assumption.

The zero-field-cooled (ZFC) magnetization under the various field and temperature conditions has been investigated using the discontinuous Fe films. Among the various factors influencing the ZFC magnetization, it is found that either the thermal relaxation or the Langevin behavior dominates the ZFC magnetization, depending on the energy barrier distribution which is altered by the growth temperature. The peak temperature of ZFC magnetization follows the Néel-Brown model for the narrower energy barrier distribution. With broadening the energy barrier distribution for the higher growth temperature, the Langevin behavior of thermally fluctuated particles becomes dominant. The change of energy barrier distribution, namely, the dominant factor of ZFC magnetization, is explained by the broadening of size distribution and the degradation of crystallinity with increasing growth temperature. For both cases, we estimate the superparamagnetic blocking temperature T_B and obtain the effective magnetic anisotropy and the effective volume from the field dependence of T_B. From the obtained values, we show the presence of interparticle interaction for the Fe grown at 323  K, and discuss the effective magnetic anisotropy of randomly oriented particles grown above 573  K.

We observed the spin-half Kondo effect for a single uncapped large InAs self-assembled quantum dot at zero and at a high magnetic field. At zero field we observed the standard Kondo effect. The high-field Kondo effect appears only when the first and second spin-resolved Landau states become degenerate because of the large Zeeman effect, which we confirmed from the magnetic field dependence of the Kondo effect.

We report measurements of current noise auto- and cross correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite source-drain bias, the autocorrelation evolves from super- to sub-Poissonian in the two-lead case, and the cross correlation evolves from positive to negative in the three-lead case, consistent with transport through multiple levels. Cross correlations in the three-lead dot are found to be proportional to the noise in excess of the Poissonian value in the limit of weak output tunneling.

A quantum dot (QD) having two tunnel-coupled quantum wires (QWs) was fabricated to probe the QD’s spin configuration using the QWs as spin filters. The QWs were adjusted to be either spin-polarized or spin-depolarized under a magnetic field B. The spin filtering effect was only observed for spin-polarized QWs, and singlet and triplet states were distinguished for values of the filling factor 2<ν<4. The spin filtering rate increased due to Zeeman splitting in the QW as the B field was increased, but it decreased dramatically the region of ν<2 due to the cotunneling effect.

The Rapid Cycling Synchrotron (RCS) of the J-PARC complex in Tokai, Japan, is designed to accelerate a high intensity proton beam from 181 MeV, and later 400 MeV to 3 GeV in 20 ms within the 40 ms machine cycle. The beam power up to 1 MW demands a stable beam control to avoid excessive losses and activation of the accelerator chain. The fully digital control system is based on quadrature modulation and demodulation. In the amplitude control loops standard FIR filters separate the harmonics (h=2) and (h=4) after down conversion. For the phase loops at (h=2) and (h=4), intended to damp synchrotron oscillations, the delay in a FIR filter would limit the loop stability. Cascaded integrator comb filters, also called CIC filters, provide a shorter delay because they filter the longitudinal beam signal only where it is necessary. The notches are located at multiples of the revolution frequency of the proton beam. For fixed frequency accelerator applications, digital comb filters with fixed clock frequency are widely used to improve loop stability. For variable frequency accelerator applications, as in a proton synchrotron, where the frequency swing is larger than the notch width, usually the clock frequency of the comb filter is variable and chosen to be an integer multiple of the particle revolution frequency. At J-PARC RCS, the clock frequency has to be fixed. Tracking the frequency would require a variable noninteger number of filter taps. Here we present a filter, based on the weighted output of 2 CIC filters with variable length, and one tap difference. The filter function looks like a CIC with smoothly varying coefficients, where the notches follow the revolution frequency of the proton beam. The delay of this filter is approximately half of the corresponding FIR filter, so that the phase loops have a higher stability margin.

We investigate numerically the spin polarization of the current in the presence of Rashba spin-orbit interaction in a T-shaped conductor proposed by Kiselev and Kim [Appl. Phys. Lett. 78, 775 (2001)]. The recursive Green function method is used to calculate the three terminal spin-dependent transmission probabilities. We focus on single-channel transport and show that the spin polarization becomes nearly 100% with a conductance close to e²∕h for sufficiently strong spin-orbit coupling. This is interpreted by the fact that electrons with opposite spin states are deflected into an opposite terminal by the spin-dependent Lorentz force. The influence of the disorder on the predicted effect is also discussed. Cases for multichannel transport are studied in connection with experiments.

Complex transverse acoustic impedance of the superfluid ³He-B was measured at the frequencies of 10 to 80 MHz at 17.0 bar by a cw bridge method. The observed temperature dependence was well explained by the quasiclassical theory with random S-matrix model for a diffusive surface. The temperature dependence was influenced by pair breaking and by quasiparticle density of states at the surface, which was drastically modified from the bulk one by the presence of surface Andreev bound states.

A spin filtering in a two-dimensional electron system with nonuniform spin-orbit interactions (SOI) is theoretically studied. The strength of SOI is modulated perpendicular to the charge current. A spatial gradient of effective magnetic field due to the nonuniform SOI causes the Stern-Gerlach-type spin separation. The direction of the polarization is perpendicular to the current and parallel to the spatial gradient. Almost 100% spin polarization can be realized even without applying any external magnetic fields and without attaching ferromagnetic contacts. The spin polarization persists even in the presence of randomness.

We present a theory of sound propagation in superfluid ³He confined in aerogel, taking dragged aerogel motion into account. The superfluid dynamics coupled with the aerogel motion is formulated by use of the Keldysh Green’s function for weak-coupling superfluid Fermi liquid. We apply the theory to the hydrodynamic regime and calculate the attenuation of a hydrodynamic longitudinal sound mode, the so-called fast mode. The result is compared to the acoustic experiment reported by the Northwestern University group [R. Nomura, G. Gervais, T. M. Haard, Y. Lee, N. Mulders, and W. P. Halperin, Phys. Rev. Lett. 85, 4325 (2000); G. Gervais, R. Nomura, T. M. Haard, Y. Lee, N. Mulders, and W. P. Halperin, J. Low Temp. Phys. 122, 1 (2001)]. We find reasonable agreement between the theory and the experiment.

We derive a boundary action of N=2 super-Liouville theory which preserves both N=2 supersymmetry and conformal symmetry by imposing explicitly T=T̅ and G=G̅ . The resulting boundary action shows a new duality symmetry.

Electron transport through disordered systems that include spin scatterers is studied numerically. We consider three kinds of magnetic impurities: the Ising, the XY, and the Heisenberg. By extending the transfer matrix method to include the spin degree of freedom, the two terminal conductance is calculated. The variance of conductance is halved as the number of spin components of the magnetic impurities increases. Application of the Zeeman field in the lead causes a further halving of the variance under certain conditions.

Transverse sound in normal liquid ³He in aerogel is studied on the basis of the Landau transport equation taking into account the simultaneous oscillation of the aerogel. We show that the nature of transverse sound is strongly modified if aerogel is immersed in the liquid. Scattering of ³He quasiparticles by the aerogel molecules causes friction between the liquid and the aerogel, giving rise to coupled motion of the two systems. There exists a low-attenuation transverse sound mode in this coupled system. The high-temperature behavior of its attenuation α is given by α∝const+T^-2, which is in contrast to α∝T in pure liquid ³He in the hydrodynamic regime.

The Ginzburg-Landau model for the anisotropic XY spin system in an oscillating magnetic field below the critical temperature T_c, ψ̇(r,t)=(T_c-T)ψ-|ψ|²ψ+γψ^*+∇²ψ+h cos(Ωt) is both theoretically and numerically studied. Here ψ is the complex order parameter and γ stands for the real anisotropy parameter. It is numerically shown that the spatially uniform system shows various characteristic oscillations (dynamical phases), depending on the amplitude h and the frequency Ω of the external field. As the control parameter, either h or Ω, is changed, there exist dynamical phase transitions (DPT), separating them. By making use of the mode expansion analysis, we obtain the phase diagrams, which turn out to be in a qualitative agreement with the numerically obtained ones. By carrying out the Landau expansion, the reduced equations of motion near the DPT are derived. Furthermore, taking into account the spatial variation of order parameters, we will derive the analytic expressions for domain walls, which are represented by the Néel and Bloch type walls, depending on the difference of the coexistence of phases.

Fusion performance of reversed shear discharges with an L-mode edge has been significantly improved in a thermonuclear dominant regime with up to 2.8 MA of plasma current in the JT-60U tokamak. The core plasma energy is efficiently confined due to the existence of persistent internal transport barriers formed for both ions and electrons at a large minor radius of r/a∼0.7 near the boundary of the reversed shear region. In an assumed deuterium-tritium fuel, the peak fusion amplification factor defined for transient conditions involving the dW/dt term would be in excess of unity.