摘要:

The technique of electron‐beam‐induced current (EBIC) has been used to study the recombination activity of misfit dislocations in Si/SiGe epilayers. EBIC contrast measurements recorded as a function of temperature (T=80–300 K),c(T), were found to show a completely altered character following copper contamination of SiGe epilayers. In as‐grown ‘‘clean’’ material the dislocations were found to exhibit a very small contrast at low temperature only. For a Cu contamination of about 1 ppb the contrast increased markedly at low temperatures but remained invisible near room temperature. Thisc(T) behavior can be attributed to shallow trap levels at the dislocations. For a Cu contamination around 15 ppb the majority of dislocations exhibited contrast in the whole temperature range, being a consequence of near‐midgap centers at the dislocation. Hydrogen plasma treatment of these dislocations was observed to passivate the contrast near room temperature but did not show a pronounced effect on the contrast at low temperatures, so that the very small dislocation contrast found for clean material was not restored by hydrogen. A Cu contamination treatment in the ppm range resulted in a dramatic increase of the contrasts in the whole temperature range. Investigations by transmission electron microscopy (TEM) revealed in that material copper precipitates connected with the misfit dislocations. In contrast to the low‐contaminated material no direct decoration of the dislocations could be observed. TEM images revealed that the EBIC dislocation line contrast corresponded to bundles of up to 15 individual dislocations. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359802

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Alternating current (ac) losses in silver sheathed mono‐ and multicored (Bi,Pb)2Sr2Ca2Cu3O10superconducting tapes were investigated at liquid neon temperature (27 K) under various direct current (dc) background magnetic fields. The loss behavior was well accounted for by the Bean critical state model with the inclusion of eddy current loss in the silver sheath. The interfilamentary coupling loss was found to be negligibly small for the present orientation of the tape. In addition, a severe distortion of the loss signal and theB‐Hhysteresis curve was observed at 27 K when a high dc magnetic field was applied simultaneously with ac fields. The losses of the tapes were also measured at 77 K and compared with the 27 K values. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359803

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Epitaxial trilayer heterostructures of YBa2Cu3O7−&dgr;/BaxSr1−xTiO3/YBa2Cu3O7−&dgr;were grown on silicon‐on‐sapphire buffered by a double layer of CeO2/Y–ZrO2. Such structures may be considered for tunable microwave filters. The top and bottom YBa2Cu3O7−&dgr;films were wellc‐axis oriented, free from microcracks and had superconducting transitionsTc’s in the range 86–90 K. A thin antidiffusion layer of SrTiO3(d≊70 A˚) between YBa2Cu3O7−&dgr;and BaxSr1−xTiO3(x=0.25–0.9) promoted better crystallinity and higherTcof the top superconducting film. An Ag/BaxSr1−xTiO3/YBa2Cu3O7−&dgr;capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan &dgr; of the BaxSr1−xTiO3layer. Maximum values of the permittivity of the BaxSr1−xTiO3layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1−xTiO3(x=0.25–0.75) layers depended strongly (≊20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan &dgr; was much higher in films than in bulk crystals. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359804

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Bi2Sr2Ca0.64Cu1.64Ox(nominally Bi2212) powders were fabricated into powder‐in‐tube Ag‐ and Ag(7 at. % Cu)‐sheathed tapes by cold and hot rolling to investigate the effects of sheath composition and rolling conditions on their microstructural development and superconducting properties. Bi2212 tapes with Ag(Cu) sheaths exhibited improved grain alignment and interfacial uniformity, as well as enhanced formation of the Bi‐free phase (≊Sr7.5Ca6.5Cu14Ox), relative to the Ag‐sheathed specimens. The hot‐rolled Ag(Cu)‐sheathed tapes displayed superior critical current densities (Jc), where magnetizationJcm=1.5×106(H∥c) and 4.6×105A/cm2(H⊥c) atT=5 K,H=1 T. Correspondingly, these specimens had transport critical current densities (Jct) of 6.7×104A/cm2(H∥c) and 5.4×104A/cm2(H⊥c) atT=4.2 K,H=0 T and 2.2×104A/cm2(H∥c) and 3.0×104A/cm2(H⊥c) atT=4.2 K,H=14 T. The chemical stability of the Ag(Cu) sheath regions during the partial melting process was also studied. Rapid oxidation of copper produced Cu2O precipitates in the sheath at 885 °C, and subsequently a Cu2O‐free zone developed near the core/sheath interface. A theoretical analysis of Cu2O precipitate formation and decomposition during thermal processing is presented. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359805

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have studied the magnetic properties of the Cu lattice of Ho2CuO4. This cuprate can only be synthesized at high pressure. It crystallizes in a structure derived from the Nd2CuO4‐type structure (T′ phase), modified by local distortions in the CuO2planes. We have measured dc magnetization versus temperature and isothermal dc magnetization versus magnetic field. These measurements were made in the range of temperature from 5 to 540 K applying magnetic fields up to ±50 kOe. ac susceptibility has also been measured in the range of temperatures from 80 to 320 K, using different frequencies between 33 and 1000 Hz and excitation fields between 1 and 10 Oe. We have found a three‐dimensional antiferromagnetic (AF) ordering of the Cu lattice of Ho2CuO4belowTN=271(1) K. This AF order is accompanied by a weak ferromagnetic (WF) component, due to a small canting of the Cu spins away from perfect AF alignment. BelowTN, at zero field, the interlayer AF coupling makes the WF components in neighboring CuO2planes to be oriented antiferromagnetically and, thus, to remain almost compensated. Only above a field‐induced metamagnetic transition is the weak ferromagnetism evidenced. The WF component persists aboveTN, in a large temperature interval, induced by the magnetic field. The magnetization of the paramagnetic (PM) Ho lattice, which follows a Curie–Weiss law with &mgr;eff=10.16(1)&mgr;B/Ho atom and &thgr;=7.6(5) K, is superimposed to the magnetism of the Cu lattice. There exists an exchange coupling between both lattices and the PM Ho ions are polarized through an internal magnetic field due to the WF component of the Cu moments. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359806

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The magnetovolume effects and magnetic anisotropy of the new Ce3(FeTi)29and Ce3(FeCr)29compounds have been studied by measuring their linear thermal expansion (LTE), pressure dependence of the Curie temperatureTc, compressibility, and anisotropy field. The value ofTc, its pressure dependencedTc/dp, and anisotropy fieldHAfor the Ce(FeCr)12phase are also reported. The linear thermal expansion of these compounds exhibits Invar‐like behavior with large positive values of the spontaneous magnetostriction atTc[e.g., &ohgr;s(0)≊1.2% in Ce3(FeTi)29]. Different values of the compressibility have been observed in the ferromagnetic and paramagnetic states of Ce3(FeTi)29(&kgr;f=1.25×10−3kbar−1and &kgr;p=1.12×10−3kbar−1) in qualitative agreement with the large magnetovolume effect observed in the LTE.Tcfor all the phases studied decreases under pressure with initial slopesdTc/dp=−3.75, −3.6, and −2.7 K/kbar for Ce3(FeTi)29, Ce3(FeCr)29, and Ce(FeCr)12, respectively. Two anisotropy fields have been detected in Ce3(FeTi)29associated with saturation along theb(HA) andc(H′A) axes of the monoclinic structure. The temperature dependence ofH′Afor Ce3(FeCr)29andHAfor Ce(FeCr)12is also reported. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359807

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This article presents a comparison of one‐, two‐, and three‐dimensional models for domain wall motion coercivity. The effect of wall bending in two‐dimensional models as compared to one‐dimensional models and the additional effect of wall flexing in three‐dimensional models is studied. The models are implemented numerically and calculations are carried out for soft magnetic materials. A discussion of how parameters must be chosen in lower dimensional models to obtain the correct magnitude of coercivity is included. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359808

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A common practice in the study of wave propagation in stratified structures is to use the Floquet (or Bloch) condition to derive the dispersion relation, leading to the passband and stopband structures. However, the Floquet condition is valid only for an infinite system while a real system always has finite dimensions. We report a study on wave propagation in a finite 2‐2 composite by using the transfer (T) matrix technique. Through introducing a new definition for the dispersion relation using theTmatrix, the passbands and stopbands are calculated for a finite system without the Floquet condition. The formation of stopbands and passbands with the increase of composite size can now be clearly seen. The spatial profile of the vibration pattern inside a finite composite can also be calculated using this technique, which reveals strong edge effects. The effects of randomization on the wave localization in a 2‐2 composite are also studied. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360701

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The characteristics of the spectra emitted from the species ejected by excimer laser were systematically analyzed using optical emission spectroscopy and Boltzmann plot approach. These characteristics were correlated with the growth behavior of Pb0.95La0.05(Zr1−yTiy)0.9875O3films withy=1(PLT), ory=0.3(PLZT), synthesized by pulsed laser deposition technique. The increase in laser energy density moderately enhances the intensity of the spectral lines and insignificantly modifies the plasma temperature (Tp) of the Ti‐species in the plume. The increase in oxygen pressure PO2, on the other hand, only slightly reduces the intensity of the spectral lines but tremendously lowers the Tp‐values of the Ti‐species. The plasma temperature is around TPLZTp=20 300 K and TPLTp=12 500 K for PO2<0.01 mbar and abruptly decreases to around TPLZTp=13 300 K and TPLTp=8000 K for PO2=0.1 mbar or 1 mbar. High plasma temperature of the PLZT plume, as compared with that of the PLT plume, is presumably the source which induces the formation of secondary phase in the PLZT films. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359809

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A stratified biphase structure can have many mechanical resonance modes due to the existence of several length scales in the system. The resonance effect is greatly enhanced if there are periodic synchronized multiple driving sources in the structure. For example, a single beam or a linear array composite transducers used in medical ultrasonic imaging. Such resonance behavior can be studied using an extended transfer matrix technique which we name: multisource T‐matrix technique. Using this technique we have studied the effects of randomization in a 2‐2 composite. It is found that for dispersing the pitch resonance the randomization of ceramic spacing is more effective for low ceramic content, while randomization of ceramic width is more effective for high ceramic content. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359810

出版商:AIP    

年代:1995

数据来源: AIP