摘要:

Measurements of the transmission amplitude and phase of 20 GHz microwaves through superconducting granular aluminum films were made as a function of temperature from 4.2° to 1.75°K. The films were fabricated by vacuum deposition in the presence of controlled amounts of oxygen, and exhibited small grain size (∼40 Å) and enhanced transition temperatures of up to 2.86°K. The temperature coefficients of resistance (TCR) for the films ranged from slightly positive to almost zero, and for several films the TCR was negative. For all films measured, the microwave transmission data agreed very well with that computed by using the Mattis and Bardeen conductivity functions.

ISSN:0021-8979    

DOI:10.1063/1.1657942

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The advantages of using superconducting windings and cryogenic temperatures in transmission electron microscopes have been recognized for some time but not fully achieved; they are: improved resolution because of higher fields and shorter focal lengths, reduced size, better vacuum, improved mechanical stability, and greater electrical stability with persistent mode of operation of lens field. At ORNL, a 500‐kV microscope is being developed with the goal of 1‐Å point‐to‐point resolution in biological specimens. All lenses will use superconducting windings operated in the persistent mode. A quadrupole‐octupole lens system will correct the primary spherical aberration of the objective. Over‐all in‐instrument magnification will be 5 ×106X. An investigation of round‐lens fields shaped primarily by currents showed that low aberrations could not be attained. Lenses using pole pieces of 1010 steel, Armco iron, and Hiperco 50 were evaluated in magnetic field tests. Typical figures are an uncorrected resolution limited by spherical aberration of 1.21‐Å in an Armco iron lens with a 7.6‐mm bore and 32‐kG field. Further work with Hiperco 50 and dysprosium pole pieces at fields above 30 kG is in progress.

ISSN:0021-8979    

DOI:10.1063/1.1657943

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A magnetic suspension and guidance system for a very high‐velocity (MACH 10) ground‐based rocket sled facility has been studied.1These results have been extended and applied to the study of a high speed (300 mph or greater) passenger train using this system. Suspension and guidance forces are obtained through the interaction of the magnetic field of an on‐board superconducting magnet with eddy currents induced in a stationary metallic guideway. It was found that at high velocities the levitation force can be calculated as if the guideway had infinite conductivity. Thus, magnetic image methods can be used resulting in an image forceFiFl=&mgr;0(ni)2&pgr;Z2Z−(L2+Z2)1/2−(W2+Z2)1/2−(W2+Z2)1/2−(L2+W2+Z2)1/2W2+Z2(Z2)−(L2+Z2)1/2−(L2+W2+Z2)1/2L2+Z2(Z2),wherenis the number of turns, andithe current in a rectangular magnet of length 2land width 2wat a suspension heightz. At finite velocities, the resistivity of the guideway reduces the actual lift forceFlbelowFiand produces a drag forceFd. Semiphenomenological expressions for these forces as a function of velocity and suspension height have been found for a magnet traversing a guideway of semi‐infinite thickness and infinite lateral extentFl/Fi=exp{−&zgr;[1+2(z/2l)3/2]−1}Fd/Fi=12(1−exp{−&zgr;[1+3(z/2l)5/2]−1])exp{−&zgr;[1+2(z/2l)3/2]−1},where &zgr; = (4&pgr;/&mgr;&sgr;vz)1/2,vis the velocity, and &sgr; the conductivity of the guideway.Flis approximatelyFiwhen &zgr; < 1. These results are in semiquantitative agreement with experiments on normal magnets suspended over rotating metallic plates.2Six superconducting magnets 0.5 m square, conducting 3×105A‐tare sufficient to levitate a 100‐passenger vehicle weighing 60 000 pounds at velocities of 50–300 mph. These magnets would provide silent and smooth suspension and guidance, would require little maintenance, and would not require a high‐precision guideway. The important results of this work are: (1) the lift force approachesFiasymptotically; at speeds of ∼50 mph for magnets 0.5 m square,Fl≈0.8Fi. (2) The drag force at high speeds (>50 mph) decreases asv−1/2. (3) The suspension height is adjustable, 0.15 m seems reasonable. (4) Superconducting magnets of modest dimensions are practical and necessary for this application.

ISSN:0021-8979    

DOI:10.1063/1.1657944

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Direct measurements have been made of the effect of strain on the critical temperature of tin, aluminum, and indium films. The films (<1000 Å) were vacuum deposited onto glass and mylar substrates at room temperature and were then strained by mechanically bending the substrate in liquid helium.Tcdepended linearly on the strain over the range of observation, ∼10−3, for both tensile and compressive bending. The slopes of theTcvs strain curves were 8.7° and 9.1°K per unit strain, respectively for tin and aluminum films on glass substrates and 4.9° and 3.8°K per unit strain for tin and indium films on mylar substrates. Earlier measurements1,2have indicated considerable variation in theTcdependence on strain from film to film, but our data are quite reproducible and do not seem to depend markedly on the evaporation conditions. For an isotropic material or an anisotropic material for which the relative orientation of the strain axis and the axes of crystal symmetry are known it is possible to relate the change inTcto a corresponding volume change. On this basis one can compare the results on thin films with strain and pressure measurements on bulk samples. ComparingdlnTc/dlnVfor thin films and bulk samples3we find that the values for aluminum agree within five percent, those for tin films on glass are lower by thirty percent, and those for indium films are higher by a factor of three. The change inTcdue solely to a change in sample volume can be calculated as a function of the Debye temperature on the basis of the McMillan theory1by using the Gruneisen relation and taking &lgr;, the phonon‐mediated electron interaction, to be inversely proportional to the square of the Debye temperature. Comparing experimental results with these calculations we find that the experimental results fordTc/d&thgr;Dlie within the limits imposed by &mgr;* = 0.1 to 0.2 for bulk aluminum, tin, and indium and thin‐film aluminum and tin. (Materials for which the Coulomb pseudopotential, &mgr;*, has been measured lie within these limits.)

ISSN:0021-8979    

DOI:10.1063/1.1657947

出版商:AIP    

年代:1969

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1657950

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The instability threshold of magnetoelastic waves has been calculated for arbitrary angle &thgr; between the dc magnetic field and the direction of propagation for the case in which the frequency of the potentially unstable vibrations is either considerably smaller or considerably larger than the crossover frequency of the magnon‐phonon dispersion diagram. Under these conditions, the vibrations are substantially pure phonons. When magnetic losses are neglected, the lowest critical field is of the order ofhcrit≃&ohgr;/&ggr;&sgr;&tgr;e, where &sgr;=&ggr;b22/C44M0, &ohgr; is the pump frequency, &ggr; the gyromagnetic ratio, &tgr;ethe elastic relaxation time,b2the magnetoelastic constant,C44the shear modulus, andM0the saturation magnetization. If half the pump frequency is less than the crossover frequency, the lowest threshold occurs for transverse waves that propagate at an intermediate (and typically rather small) angle to the dc field direction; if it is larger than the crossover frequency, the lowest threshold occurs for transverse waves having &thgr;=&pgr;/2. The influence of magnetic loss upon the threshold is small for low‐loss materials such as yttrium‐iron garnet as long as the frequency of the unstable vibrations is not too close to the crossover frequency.

ISSN:0021-8979    

DOI:10.1063/1.1657951

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The electrical resistance‐vs‐temperature curve of an amorphous Fe80P13C7alloy, obtained by rapid quenching from the liquid state, has a minimum at low temperature. Because the quenching conditions cannot be accurately controlled, the temperature at which the minimum occurs varies from specimen to specimen and covers a range from 10° to 50°K. A study of 14 different specimens shows a correlation between this temperature and the ratio of the resistances at helium temperature to that at room temperature. This correlation is interpreted in terms of the existence of regions of short range order, or small amounts of crystalline phases in imperfectly quenched specimens.

ISSN:0021-8979    

DOI:10.1063/1.1657952

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The ordinary Hall coefficient of the amorphous ferromagnetic alloy Fe80P13C7is 3.0×10−11V·cm/A·G. Its spontaneous Hall coefficient is 0.6×10−9V·cm/A·G and is independent of temperature from 4.2°K to its Curie temperature at 586°K if the ordinary Hall coefficient does not change much over the temperature range. The free electron assumption calculated by Samoilovich and Kon'Kov explains the temperature independent behavior of the spontaneous Hall coefficient.

ISSN:0021-8979    

DOI:10.1063/1.1657953

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The methods of Eshelby, Read, and Shockley [Acta Met.1, 251 (1953)] and of Stroh [Phil. Mag.3, 625 (1958)] for determining the anisotropic elastic field of a dislocation are reviewed briefly and several problems in their application are discussed. The Stroh method is extended to the dislocation displacement field. The resulting equations are applied to the⟨111⟩screw dislocation in cubic crystals.

ISSN:0021-8979    

DOI:10.1063/1.1657954

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Regular substructures were observed in grain boundaries of an annealed Fe‐0.75% Mn alloy by transmission electron microscopy. A detailed analysis was made on one of the grain boundaries. The result indicates that the analyzed grain boundary may be considered as a coincidence site grain boundary.

ISSN:0021-8979    

DOI:10.1063/1.1657955

出版商:AIP    

年代:1969

数据来源: AIP