Search

The EPR spin Hamiltonian parameters of transition ion active centres in frozen aqueous solutions of proteins are distributed as a reflection of distributions in spacial conformations. This phenomenon is generally referred to with the generic term ‘g-strain’, however, its manifestation is not limited to a distribution in the g-values. The equivalent name ‘D-strain’ applies to the situation common for biological half-integer high-spin systems whose powder EPR shape is predominantly modified through the distributive effects in the second-order terms of the spin–spin interaction Hamiltonian. A simple, one-parameter model is developed to account for D-strain, and this forms the basis for an efficient and practical numerical analysis procedure for D-strained spectra in the weak field limit. Analysis of S = 5/2, 7/2, and 9/2 protein examples is used to illustrate the drastic modification of relative intensities and widths and the occurrence of extra turning points in these spectra as a consequence of D-strain.

Temperature hysteresis of the cholesteric pitch in planar cholesteric layers was observed in [1,2]. A similar hysteresis of the cholesteric pitch in planar layers has to take place in the applied electric (magnetic) field. The restriction on the sample characteristics and strength of the surface anchoring potential are found which ensure defectless mechanism of the pitch field and temperature variations (jumps in the pitch value). The equations describing temperature and field pitch variations are presented and analysed. For weak field an universal character of the pitch variations, similar to those for the temperature variations and dependent on the value of some parameter only, is revealed. In the frame of Rapini model anchoring potential the field and temperature dependencies of the pitch variations and the hysteresis in the pitch jumps for specific values of the mentioned parameter are calculated. Transmission optical spectra of the layer for normal incidence of light are calculated at the points of p...

The effect of pressure on the luminescence spectrum of the weak field system Cr3+ : LiTaO3 is presented. By using pressure to induce a low to high crystal field strength transition, we demonstrate the ability of pressure to identify distinct dopant bonding environments in luminescent materials.

A domain wall model is proposed for relaxor ferroelectrics typified by Pb(Mg1/3Nb2/3. Under a large applied field or a large internal field, domain walls move over a long distance by overcoming localized barriers. This movement is responsible for the hysteresis loop and depolarization at temperatures below T max. The activation volumes obtained by analyzing the coercive field dependence on temperature and frequency suggest that the barriers encountered by the domain walls are of the size of 1000 unit cells, and they increase as the relaxor becomes more like a normal ferroelectric. Since polarization is usually incomplete in relaxors even under a large polarizing field, depolarization readily occurs without the need of nucleating new domains. Domain walls are also responsible for permittivity peaks at high temperature under a weak field. However, without a strong field, domain walls are partially pinned by barriers and can only oscillate within the unpolarized regions between barriers. The mobilit...

The electric field dependence of the mobility is surveyed by time-of-flight (TOF) experiments on a-Ge: H, a-SiGe: H, compensated a-Si: H, and amorphous multilayer Si/SiGe/Si specimens. For all samples, the mobility was found to have very weak field enhancement by the electric field, less than 60% at 2 × 105 V cm−1. This result is inconsistent with predictions of a long-range potential fluctuation (LRPF) model and questions the validity of the LRPF model in explaining the low mobility in these materials. The rnultilayer specimens are constructed with an a-SiGe: H layer (layers) embedded in an a-Si:H matrix in order to create band edge fluctuations. The multilayer structure is confirmed by a Rutherford backscattering (RBS) experiment. TOF measurements on these samples reveal no change in electron and hole mobilities and the mobility activation energy from that of undoped a-Si: H. This result indicates that the carrier transport behaviour in the presence of long range potential fluctuations may not ...

Previous studies of the phase properties of optical linear amplifiers and attenuators have been restricted to situations where the effect on the phase of single-mode light is found to be diffusive. In this paper we give an analysis of phase-sensitive linear amplification and attenuation in the weak-field regime that clearly reveals a non-diffusive effect on the phase. Indeed we show that a weak field which has initially a uniform phase probability density can inherit non-random phase properties from both phase-sensitive attenuators and amplifiers. In contrast, we find that phase-insensitive amplifiers and attenuators have merely a diffusive effect on the phase of the light.

A model of dispersive hopping transport is presented. Carrier transport under non-equilibrium conditions is shown to have characteristic features of an adiabatic process. The time dependences of transport characteristics are governed by the time dependence of the density of ‘deep’ localized states. An adiabatic equation of dispersive transport is derived. The general results are applied to systems having an exponential distribution of localized states.

We report observations on nematic droplets of a dual-frequency addressable nematic mixture using polarization microscopy. When low frequency (f = 100 Hz) electric fields are applied, the dielectric anisotropy is positive and the droplets show the usual behaviour, i.e. dark polarization rings which move outward with increasing field strength. When high frequency (f = 50 kHz) fields are applied, however, the dielectric anisotropy is negative and the dark polarization rings move inward with increasing field strength. The appearance of the droplets in transmission is compared to computer-simulated pictures using a simple model which describes the change of the director field within a weak field limit.

To obtain information about the mechanism of high Tc superconductivity in YBa2Cu3O7-δ (YBCO) the dependence of Hall coefficient RH and electrical resistivity ρ on the temperature were studied in range from 77 to 300K. An inflexion point θ was found to exist around 130K in RH(T) and ρ(T) curves. Both the mobility μ and the concentration of the holes were evaluated above and below θ, separately. The μ of the holes dominant below θ was ten times greater than that above θ. The concentration of the heavy holes above θ was, however, one hundred times larger than that below θ. A discussion will be given in terms of the transport process of the holes drifting through P[sgrave]- and Pσ-orbits on the CuO2 planes in YBCO.

The switching of polarization in ferroelectrics is considered as the first-order phase transition. By the analogy with the theory of crystal growth it is assumed that the switching process is determined by the oversaturation degree which correspond in ferroelectrics to the magnitude of electric field on the domain boundary. It is supposed that the mechanism of domain wall motion in strong field is due to the twodimensional nucleation and in weak field is due to the one-dimensional nucleation. The criterion of strong field IS defined. From this point of view it have been explained the peculiarities of domain structure dynamics in two uniaxial ferroelectrics: lead germanate and gadolinium molybdate.

In my abstract (Runcorn, l993) I have summarized the impact that palaeomagnetism has had on studies of the Earth and the Moon and may be expected to have on Mars, when rocks are returned from this planet and detailed satellite magnetometer surveys are made from moderate heights above its surface. It is surely one of the most remarkable discoveries of science that such a weak field as that of the Earth or early Moon is faithfully recorded in rocks, though today geophysicists accept it as commonplace. It is equally remarkable that palaeomagnetism turned out to be the key technique for settling such fundamental questions of geodynamics as continental drift and sea floor spreading and the constitution and early dynamics of the Moon. Further developments in palaeomagnetism are difficult to predict but in my abstract I have produced a list of priorities with which I expect most palaeomagnetists would agree.

Light-scattering measurements on tobacco mosaic virus (TMV) solutions subjected to an electric field have been carried out to check some original features of the author's theory. The experimental changes of Hv, Hh, and Vv under weak field strengths are in agreement with theoretical predictions and show that the results are consistent with positive values of both electrical and optical anisotropies of TMV. A large part of this paper is concerned with a new theoretical and experimental study on the (ΔVv)z variations which allow the determination of the ratio L/Λ, if γ is known.

We propose a model to calculate multiphoton transition rates in rare earth ions based on the Green's function formalism. Analytical results are obtained by considering a truncated set of solutions of the time dependent Schrodinger equation which approximates the exact Green's function by one which we consider to be appropriated to describe multiphoton processes in rare earth ions for arbitrary field intensities. In the weak field limit the results of perturbation theory are recovered.

The IDRI effect in a crystal of LiIO3 is well pronounced when an intense laser beam is used (E ­ 105 V/cm). The effect causes sharp bending of the fundamental ray due to the refraction at the exit face of the crystal whereas the SH does not show such a behaviour, due to the weak field associated with it. This effect is shown experimentally using a laser beam of energy 1·79 eV which is focused near the exit face of the crystal (0·191 cm) of thickness 0·364 cm. The focused laser beam creates opaque sharp edges of size 6 w which produce the fine structure of the ruby radiations due to diffraction. Simultaneously, SHG yields a Fraunhoffer diffraction pattern due to the focusing. The effect of focusing increases the rate of various processes considerably (degradation of SH power, SH structure formation, radiation-induced damaging of the crystal, broadening of the central maximum and the ratio of the first maximum to the last maximum of SHG). The previous results are explained and some new results are verified ...

The fortieth anniversary of Ralph Brown's Historical Geography of the United States provides an appropriate occasion to review some of the accomplishments and status of North American historical geography. This small but expanding field has produced a very substantial literature, including several new landmark publications during the past year. Historical geographers have earned high respect in the large world of historical scholarship and are participating in the general reassessment of national histories. They have also been successfully reaching out to a broader public. However, despite such accomplishments and recognition outside geography, historical geography is seriously underrepresented in academic departments. Overcoming that incongruity is one of the obvious and essential ways of strengthening the dangerously weak field of American geography in general.

The system of hard rods in an external, uniform gravitational field is studied and exact expressions obtained for the partition functions, and the one- and two-particle distributions. In principle all higher order distributions can be exactly expressed as finite sums and the Laplace transforms of the one-particle density and pressure for a semi-infinite system are reducible to single integrals. Limiting cases of weak field and small rod diameters are examined. In the former case, these results agree with Percus and Lebowitz's local density expansion. In the latter case, corrections to the barometric pressure and density laws are obtained. Finally some mathematical difficulties involved in the calculation of the virial expansion and distribution of roots of the grand partition function are mentioned.