Search Results (16 total)

The evolution of the electronic properties of thin anthracene films on highly oriented pyrolytic graphite [0001] substrates as a function of temperature has been investigated using ultraviolet photoelectron spectroscopy. The change in the valence line shape with increasing substrate temperature has been univocally associated with the occurrence of different molecular orientations and structural phases (e.g., “flat-lying” mono- and multilayer films and a multilayer phase with a “standing-up” orientation). These thin-film phases are characterized by ionization energies varying up to 0.9 eV; the surface dipole and possibly to a minor extent polarization energy contributions are shown to be related to the specific molecular packing/molecular orientation and to the interplay between the molecule-molecule vs molecule-substrate interactions. Furthermore, for the monolayer system, a vibrational fine structure of the highest occupied molecular orbital (HOMO) is clearly revealed, thus allowing a detailed study of the HOMO hole-vibration coupling and the temperature-dependent broadening. Showing a large number of recently discussed contributions to the valence line shape, anthracene thin films emerge as “benchmark” systems to study the behavior of holes relevant for the charge transport in organic electronic devices.

We have developed a high-resolution cavity-beam position monitor (BPM) to be used at the focal point of the ATF2, which is a test beam line that is now being built to demonstrate stable orbit control at ∼nanometer resolution. The design of the cavity structure was optimized for the Accelerator Test Facility (ATF) beam in various ways. For example, the cavity has a rectangular shape in order to isolate two dipole modes in orthogonal directions, and a relatively thin gap that is less sensitive to trajectory inclination. A two stage homodyne mixer with highly sensitive electronics and phase-sensitive detection was also developed. Two BPM blocks, each containing two cavity BPMs, were installed in the existing ATF beam line using a rigid support frame. After testing the basic characteristics, we measured the resolution using three BPMs. The system demonstrated 8.7 nm position resolution over a dynamic range of 5  μm.

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.

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.

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.

We examine the first order Fermi acceleration on the presumption that supernova remnant shocks cross ambient magnetic fields with various angles. These oblique shocks accelerate particles more efficiently than the parallel shocks and elevate the maximum energies achievable by the particles. The primary cosmic ray spectrum is strongly dependent upon these energies. We also consider the dependence of the injection efficiency and of spectral indices on obliquity. When indices and absolute fluxes at 10¹² eV are given for six nuclear groups from balloon-borne data, each energy spectrum develops a smooth rigidity dependent knee structure. The resultant total spectrum also behaves similarly and fits well with ground-based experimental data up to several 10¹⁷ eV. It is shown as well that the chemical composition changes significantly from lighter to heavier nuclei as the energies of particles exceed the knee region. Other predicted curves are compared with the experimental data which they reproduce rather well.

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8×10^-8 rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.

We describe the first measurement of an electron beam size in the accelerator test facility damping ring at KEK with a laser wire beam profile monitor. This monitor is based upon the Compton scattering process of electrons with a laser light target, which is produced by injecting a cw laser beam into a Fabry-Pérot optical cavity. We have observed clear signals of the Compton scattered photons and confirmed that the observed energy spectrum as well as the count rate agree with the expected ones. From the measurement, we have deduced the vertical beam size σ_b to be 9.8±1.1±0.4 μm, where the first (second) error represents statistical (systematic) uncertainty. Various improvements are in progress to enhance the signal-to-noise ratio, which is essential for the detailed study of the beam dynamics.

We apply the singular value decomposition method to matrices made from eigenvectors of the transfer matrix of a 19-vertex model on a square lattice with full frustration. The conformal anomaly c is estimated to be close to 3/2. This result suggests that the 19-vertex model with full frustration has a single transition due to two kinds of degrees of freedom, each of which has U(1) symmetry and Z₂ symmetry. Moreover, we find that the value of index η is smaller than that at the Berezinskii-Kosterlitz-Thouless (BKT) transition by considering a logarithmic correction to the finite size analysis. This result for η suggests that there is another universality class which is different from that of either the Onsager type or the BKT type.

We embody the density-matrix renormalization-group (DMRG) method for the 19-vertex model on a square lattice in order to investigate the Berezinskii-Kosterlitz-Thouless transition. Elements of the transfer matrix of the 19-vertex model are classified in terms of the total value of arrows in one layer of the square lattice. By using this classification, we succeed in reducing enormously the dimension of the matrix that has to be diagonalized in the DMRG method. We apply our method to the 19-vertex model with the interaction K=1.0866 and obtain c=1.006(1) for the conformal anomaly.

We introduce a magnetic hard hexagon model with two-body restrictions for configurations of hard hexagons and investigate its critical behavior by using Monte Carlo simulations and a finite size scaling method for discrete values of activity. It turns out that the restrictions bring about a critical phase which the usual hard hexagon model does not have. An upper and a lower critical value of the discrete activity for the critical phase of the proposed model are estimated as 4 and 6, respectively.

We have studied a ground-state phase transition in an S space of a spin-S antiferromagnetic Ising model on a triangular lattice, by using Monte Carlo simulations with a pseudotemperature. The critical index η decreases from the value 1/2 for the S=1/2 system as S increases and becomes zero for S≳S_c. We introduce an occupation number of hard hexagons to the system with the pseudotemperature. From the behavior of the sublattice magnetization and the occupation number of hard hexagon, the critical value of spin S_c is estimated to be 3.

We find that the ordinary universality does not hold for a frustrated spin-1 Ising model with nearest-neighbor bilinear interaction J₁ and next-nearest-neighbor bilinear interaction J₂ on the square lattice. We estimated critical indices ν, η, and y_s by applying the Monte Carlo renormalization-group method to the model. The critical index ν varies with J₁/J₂ in a region where the ground state is of the super-antiferromagnetic configuration. Although the behavior of ν is analogous to that of the spin-1/2 Ising model as a function of J₁/J₂, the value of ν appears to be smaller than that of the corresponding spin-1/2 Ising model. The value of η agrees with the value predicted by the Onsager universality class. The crossover exponent y_s agrees with the value estimated for the spin-1/2 Ising model.

The buildup of the population of self-trapped excitons, at various depths beneath the surface of an alkali halide crystal irradiated with high-energy pulsed electrons of 20-ps half width, represents sequential energy transfer from one layer to another, owing to secondary electrons released by the primary ones. Hence it is possible to observe sequentially the spatial profile of a quantity proportional to the differential energy transfer from the secondary electron shower at each depth of a sample.

For the CuO₂ square lattice, as a parent system of high-T_c superconductors, a numerical study has suggested that the effective-spin model contains a large cyclic four-spin exchange interaction J_c. This paper investigates the effects of J_c upon magnetic Raman scattering and upon properties of the ground state by an exact numerical method mainly for a 16-site cluster. It is found that the main Raman peak with the B_1g symmetry is shifted by 20% to the lower-energy side with a realistic magnitude for J_c, which is about one fourth the size of the nearest-neighbor exchange J. Accordingly a value of J extracted from analyses of experimental data becomes larger by 10% than that estimated with the use of the Heisenberg model. Furthermore the four-spin exchange enhances a shoulder due to multimagnon states at an energy of about 4J, and this result compares favorably with the experimental line shape. Detailed discussion is given on the effect of the four-spin exchange upon the ground-state properties such as the staggered magnetization, the ground-state energy, weight of various spin configurations, and the spin-spin correlation function.

The relativistic configuration-interaction method with analytical relativistic Hartree-Fock-Roothaan (RHFR) basis functions for atomic systems is presented. One-electron functions used for constructing configuration state functions (CSF’s) are obtained with the RHFR method in which the large and small components of the radial part of a four-component wave function are expanded in terms of an analytical basis set consisting of Slater-type orbitals. Numerical application of the method to neonlike atomic systems is carried out. It is shown that calculated excitation energies with the method are in good agreement with experiment. The Z-dependent behavior of the optical oscillator strengths for various electric-dipole transitions from the ground state in the systems is also given.