Vol. 38, No. 2 (1989)
1989-01-20
CONTENT
1989, 38 (2): 175-182.
doi: 10.7498/aps.38.175
Abstract +
As the discrete lattice structure of crystal is considered, it is easy to see that slip can only take place between atoms, thus no atom is situated at the centre of dislocation exactly, the discrete elastic energy belongs to each atom near the centre of dislocation is limited everywhere. Under the assumption of rigid dislocation, using the analytical results of the elastic theory for dislocations directly, the inner cut radius of a singular crystalline dislocation and its periodic variation along with the position of dislocation centre are obtained. For the case of singular screw dislocation in simple orthogonal crystal, the first order approximation coincides with the result of Peierls model. The self energies and effective inner cut radii for two kinds of crystalline system (fcc and bcc) are calculated, and the effect of anisotropic elasticity is taken into account primarily.It is shown that the slip plane for a dislocation is not a geometric plane. The slip plane for the screw dislocation in bcc crystal looks like a bee nest.The possibility to further estimate some second ary effects by this discrete elasticity approach is pointed out.
1989, 38 (2): 183-192.
doi: 10.7498/aps.38.183
Abstract +
The effect of the distribution of dislocation density in the core region is taken into account in the discrete elastic approach. The self-energies, effective inner cut radii, as functions of the position of dislocation center are calculated for screw dislocations with different core structures in isotropic elastic fcc and bcc lattices. The P-N forces for different slip planes are estimated.The effect of the thermal motion of atoms on P-N force is discussed. This discrete elastic approach is compared with other methods. We believe that the discrete elastic approach is an effective method for comprehensive investigation
1989, 38 (2): 193-201.
doi: 10.7498/aps.38.193
Abstract +
Ionic conductivity and activation volume of bulk sample and comprssed powder disc of amorphous lithium ionic conductor B2O3-0.7Li2O-0.7LiCl-xAl2O3 (x= 0.05, 0.15) in the hydros-tatie pressure range of 0.0001-1.23 GPa were investigated. We found that the peak of conductivity of compressed powder disc consisted of contact conductivity and bulk conductivity. For hydrostatic pressure dependence of ionic conductivity of the bulk sample, a preliminary microscopic explanation was obtained by considering the physical picture of ionic transport passages. In addition, we observed that decrease of the content of alumina lowers the pressure of conductivity maximum. Variation of ionic conductivity with pressure was measured after heating samples at different temperature or at 300℃ for 4-20 hours. The behaviors were due to the amorphous phase separation and successive crystallization process of the two amorphous phases.
1989, 38 (2): 202-208.
doi: 10.7498/aps.38.202
Abstract +
We present a coordinate transformation approach to study the effect of anisotropic diffusion-on the growth of DLA clusters. For two-dimensional square lattice, we calculate analytically the Hausdorff dimension of the anisotropic diffusive DLA clusters, and show that the Hausdorff dimension D varies continuously with the anisotropic diffusion probability p, with Dmax = 5/3 and Dmin = 3/2. We also compare our results with the numerical results of Jullien et al., and find a very good coincidence. Finally, we discuss analytically the generalized dimensions Dq for DLA clusters with anisotropic diffusion.
1989, 38 (2): 209-217.
doi: 10.7498/aps.38.209
Abstract +
In this paper, the rf-amplitude threshold of chaos in a Josephson junction with quadratic damping is studied. Theoretical predictions based on the method of Melnikov is compared with numerical simulations. It is shown that the chaotic region predicted by the method of Melnikov spans only a narrow region of rf-amplitude and consists of phase locked chaos. The experimentally observed threshold of chaos is shown to coincide with the onset of unlocked chaotic behavior at higher rf-amplitude.
1989, 38 (2): 218-227.
doi: 10.7498/aps.38.218
Abstract +
We have studied the size effect in the film of n-vector model with periodic boundary by using field theoretical method. It is found that when the system dimension satisfies the condition d>, the size effect takes the Fisher's scaling form, in other cases, no-scaling can be defined. Further more we have also discussed the dimensional crossover phenomena and state equation.
1989, 38 (2): 228-235.
doi: 10.7498/aps.38.228
Abstract +
The ECRH experiment has been carried out on a simple mirror-MM-2 device by use of a 15 GHz high power gyrotron. The experimental results showed that the preionization time will be rapidly shorten following raising the pressure of puffed gas. Duing the "C-mode" operation where the gas pressure is higher, the radial profile of plasma density exhibits a saddle shape. The gas "pressure window" for building a hot electron ring is (0.4-1.2)×10-5 Torr, as the gyrotron output is about 30 kW. Simply by using a movable Langmuire probe accompanied with diamagnetic measurment. it has been determined that the hot electron ring radius is 7 cm, the thickness is about 4 cm and the axial boundary of the ring extends from Z= ±10 cm to Z=±20cm, when the central field is 2.95kg. The hot electron temperature is 140 to 170 keV. The average beta value of hot electron ring is (4-5)%. The radial bursts of electrons due to ring instabilities accompanied with the drops of the diamagnetic signal have also been observed.
1989, 38 (2): 236-246.
doi: 10.7498/aps.38.236
Abstract +
A measuring system is developed for diagnostics of the electron energy distribution function (EEDF) in a electrodeless low pressure rf discharge with a triple-probe system by using the probe-current modulation method. The EEDF of a nitrogen plasma in the pressure range of 10-3-10-1 Torr are obtained. The effect of rf voltage penetrating the probe sheath on the determination of the EEDF is studied both theoretically and experimentally. A new approach used to estimate the value of rf voltage in the probe sheath and to calculate the actual EED from distorted distribution recorded during experiments is presented and the experimental results are corrected by using the proposed method to obtain the true EEDF. The difference between average electron energy calculated from corrected EED and that calculated from the V-I characteristics of the probe is slight, being not greater than 5%, this result seems to give indirect evidence in favor of the procedure used to determine the actual EED and the reliability of the measuring system.
1989, 38 (2): 247-255.
doi: 10.7498/aps.38.247
Abstract +
The hollow cylinder is a geometry to provide high average power outputs for solid state lasers. The pumping induced temperature distribution and thermal stresses in such a glass eylinder are calculated for continuously and repetitively pumping operations. The analytical fxpression of maximum pumping power is given, from which the results for rod and slab lasers can be derived.
1989, 38 (2): 256-263.
doi: 10.7498/aps.38.256
Abstract +
We presented a method for calculating the chemisorption energy of simple gases on transition metals series. The simplified d-band model of Allan has been applied to describe the electronic structure of transition metal surface, and the chemisorption energy is calculated by making use of the generalized phase shifts. The method is employed to study the adsorption of hydrogen and oxygen on 3d and 4d transition metals. The calculated results of the chemisorption energies show remarkable systematic trends, in good agreement with experiment. The calculated adsorbate induced electronic energy level [also being referred to as the split-off energy] is at approximately 6 eV below Fermi energy, that is comparable to UPS experiments. The conclusion deduced from the calculation is as follows: the difference between the Fermi level and effective adsorbed energy level (EF-εa.) plays the crucial role in determining the trends in the chemisorption energy, the variation in the bandwidth across the series and the adatom-metal atom transfer matrix element ν are of secondary importance.
1989, 38 (2): 264-272.
doi: 10.7498/aps.38.264
Abstract +
We synthesized new superconducting phases in TlBa2Can-1CunO2n+2.5 by solid state reaction. The crystal structures of TlBa2CaCu2O6.5 and TlBa2Ca2Cu3O8.5 were determined by means of X-ray powder diffraction method. The two compounds belong to primary lattice with space group D4h1-P4/mmm. The lattice constants of TlBa2CaCu2O6.5 are α= 3.847 ?, c = 12.73 ?. The lattice constants of TlBa2Ca2Cu3O8.5 are α = 3.847?, c= 15.89?. Each cell contains one chemical formula unit. The cations are distributed alternately at positions of (0,0,z) and (1/2, 1/2, z) along Z axis. TlBa2CaCu2O6.5 shows zero resistance at 101.4K. TlBa2Ca2Cu3O8.5) shows zero resistance at 112K. The oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n+1 is separated by single Tl-O layer in the crystal structure of TlBa2Can-1CunO2n+2.5, while the oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n is separated by bilayer of Tl-O in the crystal structure of Tl2Ba2Can-1CunO2n+2. At the same n value, Tc of Tl2Ba2Can-1CunO2n+4 is about 10K higher than Tc of TlBa2Can-1CunO2n+2.5. This indicates that Tc depends on the number of Tl-O layer as well as the number of [CuO5] and [CuO4] in oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n.
1989, 38 (2): 273-279.
doi: 10.7498/aps.38.273
Abstract +
An order-disorder phase diagram of fcc FexCu1-x binary alloy system was obtained by using tetrahedron cluster variation method with nearest-neighbour interaction. A calculation of lattice parameters as functions of composition and temperature in the phase transition region was carried out. The results showed that the formation of an amorphous phasl in the FE-CU system is probably attributed to the lattice distortion.
1989, 38 (2): 280-284.
doi: 10.7498/aps.38.280
Abstract +
We calculate the ground-state properties of hcp-WC material by local density functional theory and force theorem with LMTO-ASA approximation. Using the electronic pressure as a function of the lattice parameters, the equilibrium lattice constant, the bulk moduli and sound velocity have been calculated. It is found that the error of lattice constant is less than 10%. The effects of different ways for choosing; the atomic sphere radii on the calculated results are discussed. The bond characters of hcp-WC are also analyzed briefly in terms of the electronic partial pressures.
1989, 38 (2): 285-289.
doi: 10.7498/aps.38.285
Abstract +
Starting from 3-dimensional model, we investigate the effect of dc space-charge fields on the equilibrium orbits of electrons in free-electron lasers, and find the appearance of a new unstable range of the electron equilibrium orbits.
1989, 38 (2): 290-295.
doi: 10.7498/aps.38.290
Abstract +
The phonon spectra of substitutional impurities in zincblende structure are calculated. The form law and characteristics for imparity induced local modes, gap modes and resonant modes are analyzed systematically. Besides the quasi-local mode above TA acoustic band, we also find a new low frequncy resonant mode just below TA acoustic band.
1989, 38 (2): 296-300.
doi: 10.7498/aps.38.296
Abstract +
The chemisorption of water vapor on Si(111)7×7 surface at low temperatures and the effect of annealing were studied, with X-ray photoelectron spectroscopy and energy loss spectroscopy. It was found that the water molecules taken up on the Si(111)7×7 surface were dissociative at 150 K.
1989, 38 (2): 301-306.
doi: 10.7498/aps.38.301
Abstract +
Systematical studies on the adsorption of pyridine molecules on variously prepared Ag surfaces reveals that the micro-scale cavity-like active sites play an important role in the adsorption of molecules either under ultra-high vacuum or atmospheric environments.
1989, 38 (2): 307-312.
doi: 10.7498/aps.38.307
Abstract +
Measurements of magnetoresistance of upper critical magnetic field have been made on superconducting metallic glass Zr78Co22. The results correspond qualitatively with the weak localization theory including spin-orbit scattering for three demensional system. Quantitatively, there exists an obvious deviation, which originates mainly from superconducting fluctuations. In the measuring temperature range, the value of upper critical field Mc2 varies linearly with temperature, and we obtain the inelastic scattering rate τi-1=1.3×1O10T2, which can be atributed to the electron-phonon processes in disordered metals This value is slightly lower than that predicted by Bergmann theory.
1989, 38 (2): 313-316.
doi: 10.7498/aps.38.313
Abstract +
We report the experimental results on anisotropy of the normal state resistivity, transition temperature and magnetic transition in single crystal GdBa2Cu3O7-δ. We find that both ρ∥ and ρ⊥ are fitted very well to the Anderson-Zou relationship ρ = A/T + BT, except near Tc where fluctuation enhanced conductance becomes prominent. The striking difference in A, B of two directions suggests that charge transport proceeds by different mechanisms in the two directions, that the system exhibit remarkable two-dimensional character in the normal stete. The superconductivity in the c direction is weaker than that in ab plane: Tce⊥ is 2K lower than Tce∥. a. c. magnetic shielding effect when H∥ab plane is only 76% of that as H⊥ab plane.
1989, 38 (2): 317-322.
doi: 10.7498/aps.38.317
Abstract +
The quantization axis, which is one of the principal axes of the zero-field splitting tensor, can be determined by means of the EPR two-circle goniometer. The sets of two dimensional angles of the orientations on the principal plane perpendicular to the quantization axis have been calculated according to the orthogonality of vectors. One can measure the EPR spectrum of different orientations based on the calculated sets of two dimensional angles, and specify the two other principal axes. In this way, one can determine the Eulerian angles by which the laboratory axis system (x, y, z) are related to the principal axis system (X, Y. Z). The procedure has been applied to the measurement of X band EPR data for Cr3+-doped Na3(AlMo6O24H6)·8H2O single crystal as an illustrative example.
1989, 38 (2): 323-325.
doi: 10.7498/aps.38.323
Abstract +
We report here the M?ssbauer study on Al86-xFe14+x( x= 0, 2, 4, 6, 8.5) quasicrystal alloys. The result shows that there is mostly decagonal phase for x = 0, it can be fitted by two sub-spectra which have area ratio of 1.69, close to the value of the golden mean. It implies that quasicrystal structure includes two kinds of "penrose polyhedrons". The alloys we studied here contain more crystalline phases when x increasing. The alloy with x = 8.5 contains almost only crystalline Al3Fe phase.
1989, 38 (2): 326-332.
doi: 10.7498/aps.38.326
Abstract +
31P nuclei are used as probes in the NMR studies of xLi2O·P2O5·yCdO glass samples with different molar ratio (x, y). The static NMR spectra are obtained through employing the Bruker MSL-300 spectrometer with a operating frequency of 121.487 MHz. Three basic types of phosphorous structure units join together randomly to form networks in the glasses, each of which has its own NMR characteristic. The relative portions of those basic structure units depend on the molar contents of Li2O and CdO, i,e x and y. For the description of the unit portions, we have postulated a simple model, the rationality of which has been justified by the coincidence between the theoretical prediction based on the model and the conclusions obtained from the simulation of the observed NMR powder patterns.
1989, 38 (2): 333-337.
doi: 10.7498/aps.38.333
Abstract +
Domain structure variation of Nd2Fe14B single crystal with its thickness is studied. A critical thickness Lo is found, below which the maze domain pattern appears in the basal plane. When L>Lo, a set of dot-like domains appears inside the basal domains, they are the bottom of cone-shaped reversal domain. The width of basal domain and both size and number of the cone-shaped reversal domain increase with increasing sample thickness. The multiple layered reversal domains can even form with further thickness increasing. The formation of coneshaped domain is discussed, the critical thickness value for cone-shaped domain formation is estimated as well. Based on the experimental results, the domain wall energy is also calculated.
1989, 38 (2): 338-342.
doi: 10.7498/aps.38.338
Abstract +
Spin-locking o f central transition in half-integer quadrupolar systems was studied. The particles which occupy the states of |-1/2> and |l/2> may form an isolated sub-system, and it obeys the spin temperature theory in rotating frame. We calculated the intensity of spin-locking signal by density operator approach. The result was verified by experimental observations on 23Na-NMR spectra of NaNo2 and NaOH powders.
1989, 38 (2): 343-348.
doi: 10.7498/aps.38.343
Abstract +
The ground-state structures and ground-state energies of small Sin (N = 2-7) clusters are investegated using the Stillinger-Weber potential. The vibrational spectra of these structures are calculated with the elastic energy discribed by Keating model. Form the equation of state, the anelting temperatures are estimated. The melting temperatures of Si4 and Si6 are relatively high.
