In a first step we use an ab initio rigid-ion model (RIM) to calculate the lattice parameters and the phonon dispersion of the infinite-layer compounds CaCuO2, SrCuO2, and BaCuO2. We find an increase of both the planar and the axial lattice constant when going from CaCuO2 through SrCuO2 to BaCuO2. The rate of increase of the planar lattice constant with respect to the alkaline-earth ionic radius is calculated to be smaller for the replacement of Sr by Ba than for the replacement of Ca by Sr. Both results are in accordance with experimental studies. The phonon dispersion in the RIM exhibits several unstable branches mainly related to axial displacements of the oxygens, indicating the tendency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO2; simultaneously, the maximum phonon frequency decreases. AnA2uzone-center mode with very large LO-TO-splitting exists in all three compounds (ferroelectric mode). In a second step charge fluctuations (CF) are taken into account at the copper- and oxygen ions, using SrCuO2 as an example. Due to the vanishing of the ferroelectric split a branch with very steep dispersion forms in the [001] direction in the metallic phase whereas the zone-centerA2umodes are unchanged in the insulating phase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are associated with theZ-point Sr-axial-breathing mode: CF of uniform sign within the CuO planes but alternating sign in consecutive planes do occur in the metalic phase. This interplane charge transfer is, on the other hand, suppressed in the insulating phase due to the 2D electronic structure assumed. Instead, large induced site-potential changes emerge in this case. 相似文献
We investigate the ground-state properties of the two-dimensional Hubbard model with an additional Holstein-type electron-phonon coupling on a square lattice. The effects of quantum lattice vibrations on the strongly correlated electronic system are treated by means of a variational squeezed-polaron wave function proposed by Zheng, where the possibility of static (frozen) phonon-staggered ordering is taken into account. Adapting the Kotliar-Ruckenstein slave boson approach to the effective electronic Hamiltonian, which is obtained in the vacuum state of the transformed phonon subsystem, our theory is evaluated within a two-sublattice saddle-point approximation at arbitrary band-filling over a wide range of electron-electron and electron-phonon interaction strengths. We determine the order parameters for long-range charge and/or spin ordered states from the self-consistency conditions for the auxilary boson fields, including an optimization procedure with respect to the variational displacement, polaron and squeezing parameters. In order to characterize the crossover from the adiabatic (=0) to the nonadiabatic (=) regime, the frequency dependencies of these quantities are studied in detail. In the predominant charge (spin) ordered phases the static Peierls dimerization (magnetic order) is strongly reduced with increasing . As the central result we present the slave boson ground-state phase diagram of the Holstein-Hubbard model for finite phonon frequencies. 相似文献
An one-pot approach was developed for the synthesis of substituted 5,7-dihydro-1,6-naphthyridines and 5,6,7,8-tetrahydroquinolines with moderate to good yields. This pathway is a modified two-step synthesis of Kröhnke pyridine and involves a four-component tandem reaction of N-phenacylpyridinium bromide, aromatic aldehydes, substituted or nitrogen-containing cyclic ketones and a nitrogen source. This multi-component reaction is performed using microwave irradiation heating of the reaction substrates under an environment of NH4OAc/HOAc.
We have studied the energy spectra and the electronic states of a soliton in the weakly coupled electron-phonon systems using an extension of SSH model that includes non-nearest neighbor hopping interactions. The results show that: (1) the electron-hole symmetry of the energy band structure implied by SSH model is broken, and the energy gap 2 increases. (2) for a negative charged soliton, only two bound states have been found, one of them is the midgap state, another is a new shallow state near the bottom of the conduction band; for a neutral soliton, all three bound states exist as in the SSH model, but their localizations are strengthened; for a positive charged soliton, four bound states have been found, one of which is an additional state near the top of the conduction band. 相似文献
Intense magnetic dipole scattering that has been observed recently in several transparent insulators arises nonlinearly and saturates as predicted by classical theory, but also varies with electronic structure of the medium. 相似文献
In 2D optical patterns obtained in a Liquid Crystal Light Valve with optical feedback, we show a new kind of geometrical frustration
which comes from the imposed form of the boundaries. The circular section of the incoming laser beam presents a symmetry which
belongs to the O(2) group, whereas the optical feedback selects patterns with a symmetry restrained to a dihedral subgroup
of O(2). By imposing boundaries which respect the symmetry of the dihedral group, we lift the frustration and obtain perfectly
ordered patterns.
Received 19 January 2001 and Received in final form 2 June 2001 相似文献
For the optimal laser display, it is crucial to select and control color signals of proper wavelengths in order to construct a wide range of laser display colors. In traditional laser display schemes, color control has been achieved through the mechanical manipulation of red, green, and blue (RGB) laser beam intensities using color filters. To maximize the effect of a laser display and its color contents, it is desirable to generate laser beams with wide selection of wavelengths. We present an innovative laser display control technique, which generates six channel laser wavelengths from a white-light laser using a RF-controlled polychromatic acousto optical modulator (PCAOM). This technique enables us not only to control the intensity of individual channels, but also to achieve true color signals for the laser beam display including RGB, yellow, cyan, and violet (YCV), and other intermediate colors. For the optimal control of the PCAOM and galvano-mirror, we designed and fabricated a PC interface board. Using this PC control, we separated the white-light from an Ar–Kr mixed gas laser into various wavelengths and reconstructed them into different color schemes. Also we demonstrated the effective control and simultaneous display of reconstructed true color laser beams on a flat screen. 相似文献
Existing methods for equalization of enclosures are traditionally carried out in an empty room due to the annoying effect of the signals employed (pink noise, 1/3 octave tones, etc.) over the audience. Under these conditions, the frequency response and the subsequent equalization are determined from results that do not take into account the presence of people nor any other change that may occur in the room while the musical event takes place. In addition, this kind of equalization cannot be objectively readjusted during the event. In this contribution, a method to determine the frequency response of enclosures in an imperceptible way for the audience is presented. It involves including a test signal within a musical track that, thanks to the behaviour of human hearing, can be masked and successfully recovered using reference microphones. Hence, it is possible to obtain the room transfer function in the presence of public and perform the equalization adjustments while the music track is being played. The results agree completely with traditional methods, the great advantage being the possibility of performing the equalization at any moment during the musical event and in a transparent fashion with respect to the audience. 相似文献