摘要:

The elementary interaction forcefpfor a small normal conducting precipitate particle is usually estimated assuming that the entire condensation energy of the superconductor 1/2  &mgr;0H2cis lost in the volume of the particle, i.e., that the particle is equivalent to a void. It is demonstrated that this assumption, which neglects the proximity effect, leads to a serious over estimate offp, by as much as three orders of magnitude. The ratio offpof a normal precipitate tofpof a void of the same volume is shown to be of the order (t/&xgr;0)2, wheretis the smallest dimension of the particle and &xgr;0is the BCS coherence length. In addition, the ratio is temperature dependent, becoming smaller (larger correction) asTapproachesTc. These predictions are compared with recent experimental measurements of pinning by voids and precipitates and are shown to rationalize some hitherto puzzling experimental discrepancies between pinning by the two types of defects. Application of these ideas to flux pinning by other defects, such as grain boundaries and dislocations, is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.328368

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

We have investigated the dependence of the growth‐induced anisotropy on the orientation of the crystalline growth face in a series of (YLaEu)3 (FeGa)5 O12films epitaxially grown so that the growth face normal deviates from the [111] direction in a (11¯0) plane by angles varying from 0° to 14.6°. We have determined the direction of the respective net easy axis tilt angles and have found them all to lie in the (11¯0) tilt planes at angles ranging up to ∼60°. We have also measured the azimuthal dependence of the ferromagnetic resonance field in the film plane for the various samples. The tilt angle data were analyzed using an expansion for the growth induced anisotropy carried to fourth order in &bgr;, the direction of the growth face normal. The results of this analysis were used to compute the angular variation of the ferromagnetic resonance field; the results are qualitatively correct in shape although deviating quantitatively by ∼20%. In contrast, the conventional fit to second order in &bgr; is qualitatively incorrect.

ISSN:0021-8979    

DOI:10.1063/1.328369

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

A statistical theory is given to derive the effective (or macroscopic) permittivity in the random anisotropic material from the standpoint of statistical continuum mechanics. The random medium discussed has a degree of orientational order, but does not have translational order. A equation governing the macroscopic electrostatic field can be obtained using formal perturbation methods. It is shown that the expressions for the effective permittivity are described in terms of the order parameterSof ordering in the orientation of the constituent crystals, the volume concentration of the constituent &fgr;, and the principal values of the permittivity tensor of the crystals.

ISSN:0021-8979    

DOI:10.1063/1.328370

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Even during the reading process, the repetitive application of an incident pulse gives rise to an increase and a saturation in the echo height in piezoelectric powder. This phenomenon was experimentally measured in detail as a function of the repetition period, the applied field strength, and the driving frequency. An attempt was made to explain the amplified and saturated readout echo theoretically by introducing an elementary model based on the particle rotational mechanism. In addition to the efficient enhancement of the deviation of the particle orientation, a fluctuation component was macroscopically investigated assuming that the increase and decrease is in unequilibrium during the repetitive application of the readout pulse sequence. The fluctuation was considered especially in the reading process to be caused by the deviation of the dipolar field associated with the statice deviation of the particle orientation caused by the two‐pulse sequence. Calculated results derived from the fluctuation mechanism showed satisfactory agreement with experimental results.

ISSN:0021-8979    

DOI:10.1063/1.328371

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The domain structures in BaTiO3ceramics differ from those of single crystals. The spatial configuration of the domains in the ceramics has been derived from etch patterns. All observed etch patterns can be described by two types of domain configurations. Thin layered domains separated by 90 ° walls are piled up to form sloped stacks. Neighboring stacks are bounded by two types of boundaries. One of them is made up of 90 ° and 180 ° domain walls. The other type has a more complex structure which is not free of polarization charge. In this case the pattern accounts for a head to side relation of polarization vectors. Both structures allow the grain, merely by wall motion, to be easily squeezed to arbitrary deformations up to strains of the order of the spontaneous strain. The elastic energy stored in a mixed wall has been estimated. It shows order of magnitude agreement with the energy of the 90 ° walls if the observed domain size is taken into account.

ISSN:0021-8979    

DOI:10.1063/1.328372

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Experimental results relating mechanical strain and visible‐light scattering to ferroelectric domain switching as the electric displacement of lead lanthanum zirconate titanate (PLZT) 7/65/35 is switched through the hysteresis loop are presented. Both strain and light scattering measurements were made before and after storing a photoinduced space‐charge fieldESCin first a radially constrained and then an unconstrained PLZT disc. The results of these measurements tend to clarify the role ofESCin photoferroelectric information storage. Specifically,ESCis nonvolatile and cannot be reoriented by an externally applied switching field alone. The thresholds for domain switching in the bulk of the PLZT are lowered in the direction ofESC, but they are increased to the extent of preventing some domain switching antiparallel toESC.

ISSN:0021-8979    

DOI:10.1063/1.328373

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Numerical calculations of thermally stimulated depolarization currents (TSDC) and thermally stimulated polarization currents (TSPC) have been carried out by using the bistable model of Fro¨hlich and taking into account the temperature dependence of equilibrium polarization according to a Langevin function. For dipolar processes characterized by a single relaxation time as well as by a spectrum of relaxation times obeying classical distribution functions (Wagner, Cole‐Cole, Fuoss‐Kirkwood, Davidson‐Cole, Havriliak‐Negami), the following predictions have been obtained: (1) A TSDC peak must be always of larger magnitude than the corresponding TSPC peak as a result of an increase in polarization taking place during the cooling step; (2) For the same reason, the magnitude and area of a TSDC peak must be dependent on the cooling rate adopted in the poling process; it also follows that, unless an instantaneous cooling is postulated, such a peak can no more be considered as depicting the relaxation properties of the material at the temperature of polarization; (3) A TSPC peak must be followed by a significant current reversal as a necessary result of the tangential convergence of the transient polarization with its decreasing saturation limit in the high‐temperature range. These predictions have been tested in a polymeric system selected for its well resolved relaxation spectrum (styrene‐butadiene block copolymer). Full qualitative agreement between experiment and theory has been found as far as the behavior of the &agr; relaxation (glass transition) is concerned. These results show that the variations of equilibrium polarization occuring during the nonisothermal steps of TSDC and TSPC experiments cannot beapriorineglected, as is still commonly done for the determination of dipolar relaxation parameters.

ISSN:0021-8979    

DOI:10.1063/1.328374

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The absolute efficiency of ZnCdS:Ag screens under electron beam excitation is studied. This experimental study concerns the dependence of efficiency on the excitation voltage, on the screen’s thickness, and on the phosphor’s grain size. Measurements have shown that the appropriate excitation voltage for maximum efficiency of each screen is not the maximum possible, but depends on the grain size of the phosphor. All experimental results are discussed and explained, taking into account the arrangement of grains on the screen, the internal construction of each grain, and the penetration depth of electrons in the screen.

ISSN:0021-8979    

DOI:10.1063/1.328375

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The edge luminescence of CuGaSe2is studied on single crystals, from liquid helium temperature to room temperature. Time resolved spectroscopy measurements permit us to describe one of the emission bands in terms of donor‐acceptor pair transitions. Three other emission bands are observed. We attribute them to free‐to‐bound transitions. We determine three impurity levels: 110, 80, and 50 meV. Annealings in different atmospheres permit us to think that the 50‐meV level is due to a cation vacancy acting as an acceptor, and the 80‐meV level to a Se vacancy, which would act as a donor [Miglioratoetal., J. Appl. Phys. 46, 1777 (1975)].

ISSN:0021-8979    

DOI:10.1063/1.328376

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

All theories of the photoacoustic effect to date have assumed that heat is deposited in the sample only where light is absorbed, i.e., that no spatial migration of the energy occurs. We present here an extension of the standard Rosencwaig‐Gersho photoacoustic theory, taking into account the possibility of spatial energy migration. Also included in the theory are the effects of surface quenching, i.e., the possibility that excitations near the surface of the sample decay more efficiently than excitations in the bulk. By comparing the experimentally determined chopping frequency dependence of the photoacoustic signal to that predicted by the theory, it should be possible to determine the diffusion constant for energy migration.

ISSN:0021-8979    

DOI:10.1063/1.328377

出版商:AIP    

年代:1980

数据来源: AIP