Modern Physics Letters B

The recent progress of the high-order harmonic generation in laser-produced ions is presented. The experimental results obtained at RIKEN using a 500-fs KrF laser (248 nm) are summarized. Rare gas-like ions produced much higher-order harmonics than rare gases did when the laser intensity was increased to 10¹⁷W/cm². In addition to rare gas-like ions, various metal ions were used as nonlinear media. The maximum harmonic orders are found to be simply proportional to the ionization potentials of the nonlinear media, including neutral rare gases. The maximum harmonic order observed was the 21st (11.8 nm) in Pb²⁺. The advantages of using ions in laser-produced plasmas as nonlinear media for the high-order harmonic generation are discussed.

The morphology of monatomic step edges on vicinal Si(001) surfaces is briefly reviewed. With increasing miscut angle, the Si(001) surface exhibits an interesting sequence of phase transitions. For a miscut angle smaller than about 0.03°, a hilly structure with step loops is found, whereas a phase of wavy steps coexisting with a phase of straight steps is observed in the range of 0.03°–0.1°. If the miscut angle lies in the range of 0.1° up to about 3°, two types, one much straighter than the other, of monatomic step edges develop. For even larger miscut angles, a phase of straight biatomic step edges is found.

The fundamental energetic parameters, like kink formation energies, step edge energies and step-step interactions, which govern the thermodynamic behavior are determined using Scanning Tunneling Microscopy (STM) images of 0.5° misoriented Si(001). Despite the weak strength of energetic and entropic step-step interactions as compared to the interactions along the step edge, i.e. the kink formation and step edge energies, they have a profound effect on the meandering of the step edges. Both entropic and energetic step-step interactions are proportional to L⁻² (where L is the average terrace length). The behavior of the step edges at temperatures above room temperature will be addressed with the aid of a high-temperature STM. The freeze-out temperature, T_f, of the monatomic step edges is estimated to be higher than 600–700 K. The temperature T_f is significantly above the roughening temperature, T_r, of the step edges. This means that at room temperature, the monatomic step edges on vicinal Si(001) are always rough.

The DX center in n-type Al_xGa_1−xAs has been studied over the past two decades because of technological importance and physical interest. We review from the theoretical side the development of studies on the microscopic origin of the DX center. Recent studies based on the state-of-the-art technique calculations especially focus on the neutral onsite and negatively charged broken-bond models among many models proposed so far. It has not been confirmed from total energy calculations which model is energetically stable. Many theoretical efforts, however, reveal that the broken-bond model reproduces a variety of experimental results for the DX center and is hence the best candidate.

Thins films of Sr_1−xNd_xCuO₂ (infinite CuO₂ layer) compounds with x=0.10–0.15 have been prepared by the rf-magnetron sputtering method. X-ray diffraction analysis showed that for the parent compound of SrCuO₂, there is a solubility limit x~0.11 for Nd. Corresponding to this limit, a single infinite-CuO₂-layer phase (Nd, Sr)CuO₂ thin film with smallest c-axis length was obtained. Exceeding this limit, infinite-layer phases with longer c-axis and long c-axis tetragonal phase (the second phase) coexisted. Further, the superconducting indication presented by both resistive transition (at ~28 K) and Meissner effect was also found for a thin-film sample with x~0.11.

The screening properties of fullerene molecules are described by means of a continuum model which uses the electronic wave functions of planar graphite as a starting point. The long-distance behavior of the system gives rise to a renormalizable theory, which flows towards a nontrivial fixed point. Its existence implies an anomalous dielectric constant. The screening properties are neither metallic nor insulating. Alternatively, the intramolecular screening is obtained from a simple approximation to the electronic wave functions. Intermolecular effects are also calculated. As a consistency check, it is shown that the observed polarizability of C₆₀ is well-reproduced.

A study of a minimal anomalous occupancy is carried out when a small gap occurs near the Fermi surface. A limit which tends softly to the normal occupancy can be introduced. This framework is applied to a quasi-bidimensional fermion gas which can mimick the copper oxide planes in cuprate superconductors. The energy scale implied by this minimally anomalous occupancy (the soft normal limit) for high-T_c cuprate superconductors is of the order of the Debye energy. Yet this anomalous occupancy framework overcomes the phonon barrier and a good description for the critical temperature of a representative set of high-T_c copper oxide superconductors is obtained.

The first principle discrete variational method in the spin polarized local density approximation is used to calculate the local properties of 15 atom clusters representing variable crystal size bcc vanadium. Four distinct magnetic configurations are recognized as the lattice constant varies from 5.4 to 8.4 (a.u). At the lowest end, the clusters are paramagnetic (PM) whereas at the upper end, clusters are ferromagnetic (FM). In between, antiferromagnetic couplings prevail. The local magnetic moment increases, in a fashion not unlike second order transitions, from zero in the PM range to nonzero in the AFM region. Transitions between other phases are first order. The systematics of these transitions are ascribed to the general shape of the density of states. The contact magnetic hyperfine field, charge density, and 3d partial occupations at the central sites are also calculated.

In this paper, we have rigorously shown that, in a thermodynamic limit, there always exists a one-spin-flipped state degenerate with the Nagaoka state in one-band Hubbard model on a two and three-dimensional simple cubic lattice when the number of holes (with 0≤α<1, N_∧ and N_e are the numbers of sites and electrons respectively) for all U (including U=∞). It is also shown that there does not exist a finite critical value U_c for which, when U>U_c, the Nagaoka state has the ground-state energy of the system for a two-dimensional square lattice when ρ→0.

In estimating the nonresonance nuclear reaction cross sections σ(E) at low energies (≲20 keV) needed for astrophysical calculations, it is customary to extrapolate higher energy (≳20 keV) data for σ(E) to low energies using the Gamow transmission coefficient representing the probability of bringing two charged particles to zero separation distance, which is unphysical and unrealistic since the Coulomb barrier does not exist inside the nuclear surface. We present a general extrapolation method based on a more realistic barrier transmission coefficient, which can accommodate simultaneously both nonresonance and resonance contributions.

An exactly solvable scalar microscopic model describing a radiation field and its interaction to a matter degrees of freedom in one-dimensional inhomogeneous dielectric with dispersion is proposed. A consistent canonical quantization scheme for the model is developed. Two modes of quantum collective excitations of polariton type are found in the general case. The case of an interface between two semi-infinite media is considered explicitly.