摘要:

Superconducting artificially superstructured films (ASFs) composed of MoN and TiN have been prepared by an alternate reactive deposition method. Alternate layers of MoN and TiN were epitaxially grown on MgO(100) single‐crystal substrates. Those ASFs with the periodicities of [MoN(0.4 nm)/TiN(0.4nm)], [MoN(0.2 nm)/TiN(3.4 nm)], and [MoN(2.4 nm)/TiN(0.2 nm)] were successfully synthesized. It was confirmed that the precise thickness of ASFs could be controlled on the atomic level. Average lattice constants of ASFs were calculated from the positions of satellite peaks observed in x‐ray diffraction. Structural analyses indicate the formation of a singleB1‐MoN phase when conditions are optimized. ASFs containingB1‐MoN layers showed lower superconducting transition temperatures (Tc’s) than those of ASFs with &ggr;‐Mo2N. A few probable reasons for the lowerTcare discussed.

ISSN:0021-8979    

DOI:10.1063/1.345726

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Microfilamentary composites of continuous Nb fibers in Cu matrices were tested for superconducting transport properties after heat treatments at 200–900 °C. The critical current density at applied magnetic field was reduced dramatically as the heat treatment temperature was increased. The scaling relations describing the critical current as a function of applied field shifted from conventional (1−h)2scaling to higher‐order relations as the heat treatment history was varied. Critical temperature transitions were abnormally broad in the fine‐filament composites examined. The upper critical fields and transition temperatures approached bulk values after severe heat treatment.

ISSN:0021-8979    

DOI:10.1063/1.345727

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Photoacoustic measurements for the polycrystalline samples of orthorhombic GdBa2Cu3O7−xand DyBa2Cu3O7−xhave been carried out in the temperature region around the superconducting transition. The relative values of the specific heat for the superconducting samples are obtained from the photoacoustic signal by using the thermal conductivity data measured by the longitudinal heat flow method and show a good agreement with the specific heat data measured using a calorimetric method by others. The usefulness of the photoacoustic technique for the specific heat measurement is confirmed by the photoacoustic measurement of KDP(KH2PO4) sample near the phase transition.

ISSN:0021-8979    

DOI:10.1063/1.345728

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Granular alloys of iron with other transition metals have been fabricated in both Al2O3and SiO2matrices. Both stable (Fe‐Ni, Fe‐Co) and metastable (Fe‐Cu) alloys have been achieved. Under appropriate deposition conditions, the grains are single‐phase alloys ranging in size from 15 to 50 A˚. Superparamagnetism and enhanced coercivity have been observed, and the effect of grain size on these phenomena has been investigated. We have also studied the effects of the different insulating matrices.

ISSN:0021-8979    

DOI:10.1063/1.346100

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Magnetic behavior for the three series of alloys U1−xYxCu2±ySi2∓y, U1−xYxCr2Si2, and U(Cu1−xCrx)2Si2has been studied in the temperature range from 2 to 550 K. In all cases a complete solid solution with the ThCr2Si2type of structure was observed from x‐ray powder analysis with no ordering among the substituting atom species. Whereas YCu2Si2did not reveal any homogeneous region on Si/Cu substitution, uranium‐rich alloys show significant changes in the unit cell dimensions when comparing U1−xYxCu2Si2and U1−xYxCu2+ySi2−y. In both latter alloy series a negative deviation of thecparameter from Vegard’s rule is found almost compensated for by a positive deviation ofawhen Y is substituted for U. Substitution of Cu by Cr resulted in a linear dependence of the lattice dimensions.YCu2Si2and YCr2Si2are temperature independent Pauli paramagnets above 2 K, whereas the UCu2±ySi2∓yand the U(Cu1−xCrx)2Si2systems are characterized by the onset of ferromagnetic ordering atTM=105 K. AboveTMall samples are in the paramagnetic state in which the uranium atoms carry an effective moment larger than 2 &mgr;B. Saturation moments reveal a maximum for a copper deficiency ofy∼0.1, whereas excess copper correlates with a strong magnetocrystalline anisotropy indicating the presence of narrow domain walls. Substitution of U by Y in U1−xYxCu2±ySi2∓yleads to a linear decrease ofTMand for concentrations of 0.4≤x≤0.8 metamagnetic behavior is observed. The antiferromagnetic transition is followed by a ferromagnetic alignment of the U atoms below the Ne´el temperature. The maximum of the anisotropy energy coincides with the minimum in thecparameter atx∼0.65. The Y‐rich alloys U1−xYxCu2∓ySi2±ydisplay antiferromagnetism with a rather complex structure as indicated by the change of sign of the paramagnetic Curie–Weiss temperature &thgr;p. In the U1−xYxCr2Si2alloys the antiferromagnetic coupling is destroyed by rather small amounts of Y.

ISSN:0021-8979    

DOI:10.1063/1.345729

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effects of the heat treatment of Bi‐substituted magnetic garnets in a flowing oxygen containing ozone with less than a few vol % (O3annealing) on the optical absorption &agr; in a near‐infrared spectral region and on the ferromagnetic resonance linewidth &Dgr;Hhave been investigated in detail. By O3annealing for typically 3 h at a temperature of 700 °C and subsequent O2annealing around 500 °C, the &agr; is reduced to less than 1 dB/cm at a wavelength of 1.3 &mgr;m, and also the &Dgr;His reduced to 0.8 Oe at 9 GHz in a very reproducible manner. The reduction of both &agr; and &Dgr;His thought to be caused by the decrease of Fe2+ions originally formed mainly by oxygen vacancies. The only negative point of O3annealing is that it inevitably accompanies some surface damage probably caused by the attack of oxygen atom radicals produced by the thermal dissociation of ozone which affects magnetostatic surface waves so as to increase the insertion loss. However, this problem has been successfully solved by protecting the surface with a thin oxide layer such as a Ta2O5on. O3annealing with a protective oxide layer on is extremely valuable because &agr; is still reduced effectively and magnetostatic surface waves can propagate with a smaller insertion loss.

ISSN:0021-8979    

DOI:10.1063/1.345704

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A solution technique is presented that calculates the static electric field at any point in a concentrated random dispersion of spherical particles when a uniform external electric field is applied across the dispersion. The particles may be of arbitrary size, position, and material composition. Using a rapidly converging iterative technique, the three electric field components may be calculated at any point in the medium or inside any particle. We assume all component materials are electrically isotropic and homogeneous and that the net charge on each particle is zero. This work is a first step in gaining a more fundamental understanding of electrostatic discharge phenomena through particulate‐filled systems. An important key is the use of the reduced dimension particle packing algorithms which simulate the microstructure of the particle pack.

ISSN:0021-8979    

DOI:10.1063/1.345705

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Field‐induced phase transformation and thermally induced phase transition in vinylidene fluoride (VDF) and trifluoroethylene copolymers with VDF content higher than 82 mol% and their ferroelectric and piezoelectric properties have been studied by means of x‐ray diffraction, DSC, D‐E hysteresis, and piezoelectric resonance. It is found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of &agr;, &bgr;, and possibly &ggr; phases. Application of a strong ac electric field transforms these mixed phase crystals into &bgr;‐phase crystals completely. The &bgr;‐phase crystals are ferroelectric and exhibit strong piezoelectric activity stable up to the melting temperature (160–180  °C depending on VDF content); the Curie point coincides with the melting point. Unpoled, mixed phase film has a paraelectric or rotational phase below the melting temperature, in which thick lamellar single crystals are grown extensively, whereas &bgr;‐phase film prepared by mechanical drawing has no paraelectric phase below the melting point. Definite ferroelectric‐to‐paraelectric phase transition in P(VDF‐TrFE) is inferred to occur only in thick lamellar crystals grown in the paraelectric phase. A phase diagram of P(VDF‐TrFE) is described.

ISSN:0021-8979    

DOI:10.1063/1.345706

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Photoluminescence (PL) and thermally stimulated conductivity (TSC) data on high‐resistivity,p‐type CdTe single crystals are presented. The PL emission in these samples consists of two closely overlapping components peaking at approximately 1.47 and 1.49 eV. Thermal quenching of these signals reveals activation energies of ∼0.02 and ∼0.13 eV for the former component, and ∼0.11 eV for the latter. TSC signals at temperatures corresponding to those over which thermal quenching occurs are observed. The TSC peaks are due to hole release with activation energies which agree with those obtained from the thermal quenching studies. Etching of the samples removes surface damage caused by mechanical polishing. The surface damage produces nonradiative pathways by which electron‐hole recombination can take place without luminescence. A model based on free‐electron to trapped‐hole recombination is presented to account for the data. It is shown, from numerical solutions of the rate equations describing the model, that by explicitly incorporating into the model more than one hole state at which radiative recombination can occur, shifts in the emission energy during time‐resolved and intensity‐dependence studies can be expected.

ISSN:0021-8979    

DOI:10.1063/1.345707

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Visible luminescence between 2.0–3.5 eV of undoped and boron‐doped diamond formed by plasma‐assisted chemical vapor deposition has been investigated by cathodoluminescence. Electroluminescence from Schottky diode of boron‐doped semiconducting diamond has been observed for the first time and found to be due to the same luminescent center as that of cathodoluminescence. In the particles or films where the content of nitrogen and boron was greatly reduced, the cathodoluminescence peaks occurred at 2.8–2.9 eV. The characteristics of these emission spectra are very similar to those obtained in type‐IIa diamond where dislocations are luminescent. The doping of boron during the deposition form another luminescent center at 2.3–2.4 eV. From the monochromatic cathodoluminescence imaging, the luminescent regions differ in the two peaks. {100} sectors are much more luminescent than {111} sectors at the signal of 2.8 eV. This phenomenon has been discussed based on the difference in defect or impurity concentration of each sector.

ISSN:0021-8979    

DOI:10.1063/1.345708

出版商:AIP    

年代:1990

数据来源: AIP