摘要:

A theory for amplification of acoustic waves in the presence of dc and oscillating electric fields in a piezoelectric semiconductor is developed. Acoustic gain constants for bothksl≪1 andksl≫1 are derived, whereksis the wave vector for the acoustic wave, andlis the mean free path for the electron in the piezoelectric medium. As expected, the expressions for the gain constants reduce to those for the ordinary acoustic amplifier case when the applied oscillating electromagnetic signal is zero. The theory indicates that it is possible to transfer modulation from the electromagnetic wave to the acoustic wave.

ISSN:0021-8979    

DOI:10.1063/1.1658693

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Appreciable enhancement of the acoustoelectric gain by longitudinal magnetic fields is observed in the classical regime (ℏ&ohgr;c<kT). The enhancement increases monotonically with magnetic field, reaching saturation at fields corresponding to &ohgr;c&tgr; being equal to a few units. The dependence of the magnetoacoustic enhancement on carrier concentration is analogous to that of the longitudinal magnetoresistance, both are more pronounced the lower the carrier concentration. The observed effect of the longitudinal magnetic field on the acoustic gain is similar to that reported for the acoustic attenuation in the quantum limit (ℏ&ohgr;c≫kT), but for the regime studied (ℏ&ohgr;c<kT) no theory has as yet been developed.

ISSN:0021-8979    

DOI:10.1063/1.1658694

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The optical modulation has been observed in semiconductive CdS single crystals at room temperature by means of the optical probe method using a monochromatic light. The light transmitted through materials with a wavelength corresponding to the intrinsic absorption edge was strongly modulated. The optical modulation observed here is caused primarily by the acoustic flux rather than by the high electric field in the domain. The acoustic domain arrives at its full growth near the point 2 mm from the cathode and travels towards the anode. The aspect of the acoustic domain is subjected to the variation by the strength of the applied electric field, and the domain velocity varies from 2.2×105cm/sec to 3.0×105cm/sec according to the increasing applied electric field up to 3040 V/cm.

ISSN:0021-8979    

DOI:10.1063/1.1658695

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The interaction of acoustic surface waves in a piezoelectric with carrier waves in a semiconducting film on the surface is considered. Particular attention is paid to the limitation of the small signal theory due to nonlinear effects. It is found that large signal effects, which occur at moderate acoustic power levels, are important when the maximum rf charge density exceeds the available background charge density. Measurements of acoustic attenuation and acoustoelectric current in a CdS film on the surface ofY‐cut quartz are reported. A fair agreement between theory and experiment is found.

ISSN:0021-8979    

DOI:10.1063/1.1658696

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The attenuation of longitudinal ultrasonic waves in single‐crystal germanium has been measured at several frequencies between 0.15 and 2.84 GHz in the temperature range 10°–60°K. The measured attenuation was the same for propagation in the⟨100⟩,⟨110⟩, and⟨111⟩crystallographic directions, and varied with temperature asT7at the highest frequencies and lowest temperatures, and asT3at the lowest frequencies near 60°K. The frequency dependence was less than linear at the lowest temperatures and between linear and quadratic at 60°K.

ISSN:0021-8979    

DOI:10.1063/1.1658697

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

For generating the numbers 1, 2, …Min random sequence, an electronic modulo‐Mcounter is used, driven by the pulses from a high‐frequency pulse generator. If the pulse train is interrupted at a random time, then the counter stops at random in one of itsMpossible states, producing thus a random‐number moduloM. The random time is the time at which a Geiger‐Mueller tube registers an electron from a90Sr source. The electronic circuitry is designed such that variations in the characteristics of the components do not impair the randomness. Due to the simplicity of the circuit, the degree of randomness obtainable can be discussed in detail. The randomness of the primarily generated ``basic sequence'' is limited by the finite value of the quotient (pulse‐generator frequency) / (number‐generation frequency). An extremely high degree of randomness can be realized by ``contracting'' the basic sequence, i.e., by adding (moduloM) strings ofk(k=2, 3, 4, or even higher) successive numbers of the basic sequence, to form one number of the final sequence. This contraction can be achieved very easily by interrupting the pulse train only after exactlykelectrons have been recorded by the Geiger‐Mueller tube. The performance of a generator was tested by recording a basic sequence of generated numbers on paper tape. For probing the long‐time reliability of the generator these recordings were made over an 18 months period, and the randomness tests were designed to discover also temporary malfunctions. In accord with the theoretical expectation, these tests did not indicate any deviation from randomness.

ISSN:0021-8979    

DOI:10.1063/1.1658698

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

An approximate method for describing radiation transport is derived. Photoelectric effect and Compton effect are assumed to be the only mechanisms affecting the transport. Thus the method is restricted to the x‐ray and low‐energy gamma‐ray region of the electromagnetic spectrum. Further, the radiation is assumed to be a collimated beam incident normal to a homogeneous semi‐infinite slab. The method described goes one step beyond the classical ``straight‐ahead'' approximation used in the theory of gamma‐ray attenuation. Because of the assumptions used in obtaining the solution, the expressions derived can most appropriately be applied for the case where Compton scattering is near isotropic (relatively low energies) and where the photoelectric cross section is larger than the Compton cross section.

ISSN:0021-8979    

DOI:10.1063/1.1658699

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Initial dissociation rates of CO2in pure CO2glow discharges and CO2mixtures with He, N2, H2have been determined in the 1‐Torr pressure range using a mass spectrometric sampling technique combined with plasma diagnostic methods. The observed results for the pure CO2case are discussed in terms of dissociative attachment and collisional dissociation of vibrationally excited CO2molecules both of which contribute to the dissociation process; most influential in mixtures is H2which reduces the dissociative process. At the same time, the quasistationary values of the CO2&sngbnd;CO&sngbnd;O2mixture as well as that of CO2&sngbnd;CO&sngbnd;O2with He, N2, H2have been measured: [CO2]/[CO][O2]1/2is approximately constant at pressures >500 mTorr and linearly dependent upon H2concentration.

ISSN:0021-8979    

DOI:10.1063/1.1658700

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The generalized Holstein‐Biberman transport theory developed in Part I is used to calculate the effect of reflecting walls in increasing the decay time of Doppler broadened resonance radiation in cylinders and slabs. The results are compared with the corresponding results obtained from the Cayless diffusion theory for optical thicknesses between about 20 and 3000. It is shown that the Cayless theory grossly underestimates the effect over the entire range, giving values for the fractional increase in decay time which are too small by about a factor of 3 at the smaller optical thicknesses, and too small by about a factor of 30 at the larger thicknesses. The discrepancy is shown to arise partly from the inaccurate boundary condition used in the Cayless theory and partly from the inapplicability of the mean free path concept to processes involving the transport of resonance radiation. The consequences for the behavior of low‐pressure mercury rare‐gas discharges are discussed, and it is estimated that wall reflectance must begin to have a significant effect on the characteristics of such discharges at reflectances as low as 20% rather than at the 50% reflectance predicted by the Cayless theory.

ISSN:0021-8979    

DOI:10.1063/1.1658701

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A method for growing single‐crystal Mg2Pb is described. A Bridgman method is used, starting with anonstoichiometricmelt (<31.3 at.% Pb). This requirement follows from the recent discovery that Mg2Pb is not congruently melting but forms from a peritectic reaction. The usefulness of an x‐ray microprobe for the analysis of crystal growing problems is demonstrated by a study of eutectic inclusions observed in some Mg2Pb ingots. Measurements are reported of the angular dependence of the magnetoresistance and the magnetic field dependence of the Hall coefficient in fields up to 5200 G and at temperatures of 4.2° and 77°K. The phenomenological coefficients,b, c, andd, determined from the angular dependence of the magnetoresistance show strong field dependence at 4.2°K and obey the symmetry condition for [100] oriented many valley ellipsoids at all fields and temperatures. The Hall coefficient exhibits a strong field dependence, similar top‐type Ge, which has been fitted by a simple model consisting of two spherical bands. The mobilities and carrier densities obtained by fitting the data from several well‐annealed samples to the two‐band model are &mgr;1≥7×104cm2/V·sec,n1≃7×1017holes/cm3, &mgr;2≃3×103cm2/V·sec,n2≃4×1020holes/cm3at 4.2° K.

ISSN:0021-8979    

DOI:10.1063/1.1658702

出版商:AIP    

年代:1970

数据来源: AIP