Physica B+C最新文献

This paper shows that matter-wave interferometers employing low-velocity neutral atoms can be used as inertial sensors with sensitivities that exceed those of conventional mechanical sensors and multiple circuit optical interferometers by many powers of ten. The energy and mass dependence of the phase shifts that are due to rotation and acceleration are different. Thus a pair of interferometers with different energies and/or masses can perform simultaneous independent measurements of rotation and acceleration. A proposed configuration is one formed by a sequence of planar diffraction gratings operating in high order. Gratings consist of near-resonant standing-wave laser beams. Laser decelerated and cooled atomic beams provide a suitable source. Path curvature due to acceleration and rotation is canceled by magnetic field gradients that produce an effective magnetic levitation of the atoms in a feedback arrangement that maintains null phase shift.

Our contribution presents results of investigations concerning neutron diffraction by vibrating perfect crystals of Si and SiO₂. They were excited in longitudinal, flexural, thickness and thickness-shear modes of vibration with resonance frequencies ranging from 1 to 1500 kHz. In this case, when the acoustic wavelength is much larger than the extinction distance the following phenomena caused by the deformation and Doppler shift were observed: broadening of the perfect crystal rocking curve, enhancement of the integrated reflectivity by up to two orders of magnitude and time modulation of the diffracted beam. When neutron time-off-flight through the vibrating crystal is comparable to the vibration period, multiple reflections having influence on the reflectivity take place. While investigating the forbidden reflections in the crystal excited into vibrations, a strong multiple diffraction phenomena resulting in “umweganregung” effect was observed. Also the vibrating crystals as neutron choppers with 1 μs pulses and a repetition rate of up to 100 kHz were treated.

Perfect crystal interferometry is becoming a standard tool for precise measurements of coherent scattering lengths. So far accuracy was limited by the resolution achieved with the determination of the wavelength and the wavelength distribution. The newly tested nondispersive sample arrangement avoids this shortcoming because the phase shift becomes independent of the wavelength. Arbitrarily shaped samples can be measured by an adapted Christiansen filter method, where the scattering length density of the powdered sample is matched to the scattering length density of a surrounding liquid. The following values for the bound coherent scattering lengths were determined: b_c(Bi) = 8.5165(62) fm by the standard method, b_c(Bi) = 8.521(4) fm by the nondispersive method and b_c(Si) = 4.157(3) fm by the Christiansen filter method. A phase sensitivity of 2.2 × 10⁻⁵ was achieved, which could easily be increased to a level of 10⁻⁶.

A number of neutron optical experiments carried out over the past decade have searched for possible violations of the Schrödinger equation. Among the hypothetical effects considered were: non-linearities, the effects of quantum gravity and quanternionic forms. The relevant experiments are reviewed along with some new proposals.

It is usually assumed that neutron optical phenomena are adequately described by a one-body Schrödinger equation containing a complex optical potential that is given by the average value of the Fermi pseudopotential. The main problem with this elementary approach is that it only includes the attenuation of the neutron wave function in the medium resulting from absorption, and neglects the often more important contribution to the attenuation from diffuse scattering which is also present in reality. The purpose of this paper is to show how the rigorous theory of dispersion overcomes the above problem by taking properly into account the local field effects and a correction to the scattering amplitude that are neglected in the elementary theory. The importance of the local field correction in some current measurements of neutron scattering lengths by neutron optical experiments is also indicated.

The ³E_2g(e³_2ga¹_1g) ground state of chromocene is treated as a system consisting of two holes, one in the e_2g and the other in the a_1g orbital. By a suitable choice of the basis vectors the vibronic eigenvalue equations of this case are transformed into isomorphic forms to the corresponding equation for the one electron case. The method of canonical transformation and variational approach developed in part I therefore is applied to chromocene also. The variational wavefunctions and energies are used to explain the EPR and Raman spectroscopy results on the complex.

The generation of squeezed states of the cavity radiation field in the two-atom one-mode model with multi-photon transitions is investigated. The time-dependent squeezing factors are calculated. The conditions for optimum squeezing are derived.

The solitonic mechanism for the protonic transfer is used to calculate a mobility of domain walls in hydrogen-bonded ferroelectrics. A one-dimensional model is considered. It is assumed that the sequence of proton jumps in double-minimum wells of hydrogen bonds brings about a domain wall motion. Calculations carried out for KH₂PO₄ crystals can explain the high mobility of domain walls in these substances.

In an analysis of the influence of the Jahn-Teller (JT) distortion in KCuF₃ on the exchange-coupling constants (ecc) along (J_S ≈ -190K), and perpendicular to (J_AS ≈ 0) the c-axis, two models are considered. First, a three-center, four-electron model is adopted with d(e_g)- and p_σ-orbitals at the cations and anion, respectively. Experimental values of ionization potentials and electron affinities are employed. The model accounts for the JT-distortion by using the induced admixtures of the e_g-orbitals on the copper cations. The computations involve CI. Secondly, taking s-orbitals at all centers, we perform three-center, four-electron model calculations supplemented with two-anion non-additive exchange contributions obtained from four-center, six-electron model calculations. The latter contributions, strongly geometry dependent, account for the effect of JT-distortion on the ecc's. First-order exchange-perturbation theory is used. In the applications to KCuF₃ we adopt in the two models the same orbital parameterization, based on atomic properties, with which the ecc's in the regular perovskites KMF₃ : M = Mn, Co and Ni, are reproduced. The resulting values for J_S and J_AS agree with experiment. The vanishing value of J_AS is structural in the first (pd-) model and accidental in the second (s-) model. This discrepancy is discussed.

The liquidus curves of twenty five simple-eutectic binary alloys are constructed using the semi-empirical theory of heats of formation developed by Miedema and coworkers. Overall the predictions of this theory are quite correct. In cases where discrepancies exist, it is possible to improve the results by retaining the formal expression of the heat of formation proposed by Miedema, but modifying the prescribed values of the heats of solution.