摘要:

Experimental magnetic field dependent Hall and resistivity data is presented for two modulation-doped AlGaN/GaN heterostructures in the temperature range from 6 to 300 K and for a magnetic field up to 12 T. The mobility and concentration of electrons within the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface and within the underlying GaN layer are readily separated and characterized using quantitative mobility spectrum analysis. The observed transport parameters of the 2DEG are explained using the classical band theory for a degenerate electron gas. Analysis of the temperature dependencies of mobility and electron concentration in the GaN layer and 2DEG indicates that electron transport in the GaN layer is dominated by carriers in the conduction band for the case of low-doping(<1017cm−3), and by conduction via an impurity band for highly doped material(>1018cm−3). The simultaneous analysis of the multilayer AlGaN/GaN structure applied in this work renders the results applicable directly to modulation-doped field-effect transistors based on similar structures. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366137

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Light emitting devices with a sandwich structure: metal/EC6T/indium-tin-oxide, using end-capped sexithiophene (EC6T) as active organic material were fabricated by vapor deposition. Current and intensity of electroluminescence (EL) of the EC6T layers were measured as a function of voltage for various metals (Ca, Mg, Al, In, Ag) in a wide range of temperatures (4–300 K) and thicknesses of the EC6T layers (40–350 nm). External quantum efficiencies(10−6–10−3)and rectification ratios significantly depend on the metal contact which is compatible with a Schottky barrier. Electron injection from the metal at higher voltages correlates with the onset of significant EL. Current–voltage(I–V)curves exhibit a strong temperature and thickness dependence, mainly due to the charge transport across the EC6T layer. At low temperaturesI–Vcurves show space charge limited currents. Modeling including double injection and trap states is performed. Results are discussed under the aspect of further device optimization. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366138

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effects of proton-induced linear defects on the dissipationless flow of current in multifilamentary Ag-sheathed tapes of(Bi,Pb)2Sr2Ca2Cu3O10superconductors in the temperature range of 55–77 K are reported. Randomly oriented columnar defects of estimated areal density5×1010/cm2produced by 800 MeV proton-induced fission fragments provide substantial enhancement of critical currents of these tapes. Furthermore, these defects are more isotropic flux pinning centers than high-energy heavy ion irradiation-induced columnar defects. However, there is a tradeoff between enhanced pinning and the deleterious effects of radiation damage at higher proton fluences. It is shown that some of the global damage caused by the products of fission can be repaired by a suitable heat treatment of the samples. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366139

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

YBa2Cu3O7(YBCO) thin films were grown onSrTiO3substrates using a pulsed laser deposition system. The laser beam was scanned over the target in such a way to obtain a reproducible method of composition control at the substrate. In total, 17 films were grown under optimal conditions; they exhibited mean transition temperatures of 89.7±0.4 K and mean (005) rocking curve widths of 0.07°±0.02°. The narrowest (005) rocking curve width was measured to be 0.037°. Both the rocking curve line shapes and the dependence of transition temperature on rocking curve width suggest the films have single-crystal-like structure. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366140

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1&mgr;m2Nb/Al–Al2O3/Nbjunctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366160

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

When a pair of coils is positioned on opposite sides of a superconducting thin film, measurement of their mutual inductance may in principle be used to infer the penetration depth &lgr; in the superconductor. We have studied how to optimize this measurement with respect to coil design, and have found that the approach that has been generally used is far from the optimum. Useful simplifications to the expression relating mutual inductance to penetration depth are derived. An analysis of the sources of uncertainty in determining &lgr; is presented. For an optimized coil set, the major source of uncertainty often is uncertainty of the thickness of the film. The sensitivity to changes in &lgr; is also studied; it is shown that this can approach 1 pm for a typical high temperature superconductor sample. Finally, it is shown that the analysis may be extended to normal metal films, with the skin depth playing a role similar to that of the penetration depth in superconductors. Measurement of a high conductivity normal metal foil can be used to check calculated calibration factors for a coil set or to determine the skin depth. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366287

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Based on the Bean model with a constant critical current densityjc,novel and very precise, analytical approximations for the axial induction componentBz(r) inside and outside of a finite, axially magnetized, circular cylinder with arbitrary aspect ratio in the fully penetrated, remanent critical state are presented. Starting from any mapped field profile the possibility to determine the material parameterjcby means of aglobaloptimum fit procedure is demonstrated. Some characteristic results for melt-textured yttrium barium copper oxide material are outlined. We found thatjcdetermined by vibrating sample magnetometry (VSM) exceeds that from mapping at least by about 50&percent;. This systematic discrepancy is discussed on the basis of two independent effects resulting in a local, space-dependent critical current density: theB-dependence ofjcand material inhomogeneities. The first effect becomes important for sample radii in the1-cm-range, although theBz-profile remains axially symmetric and well fitted by the Bean model. The second one may lead to asymmetricBz-profile deformations connected with a decreasing fitting significance. For the first and second effect we estimate the considerable ratio of the absolute maximum to absolute minimum of the local, critical current density of 2 and 4, respectively, which explains the observed discrepancies between theeffectivejcfrom mapping and VSM measurements. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366141

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A study of Fe–Mn (32&percent; and 34.65&percent;) and Fe3Pt Invar alloys during the transition from bulk to ultrafine (3–20 nm) particles is done at room temperature by combined x-ray diffraction and Mo¨ssbauer spectroscopy techniques. The particles obtained by evaporation of foil or filings of bulk material in an Ar atmosphere at pressures from 0.16 to 50 Torr were rapidly quenched during their production. Mo¨ssbauer measurements confirmed the availability of two spin states in both Fe–Mn and Fe3Pt fcc particles like it has been observed in previous studies of fine Fe and Fe–Ni (30&percent;–35&percent;) particles. Various heat treatments of the Fe3Pt foil with the initial bcc structure made it possible to obtain its fcc modification in either the entirely ordered or the entirely disordered state. It was shown that the disordered fcc structure in a bulk sample could exist in two spin states (ferromagnetic and paramagnetic) whereas the ordered structure was only in a ferromagnetic state. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366142

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co10Cu90granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters (particle size and density, interparticle distance) and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366164

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The dc magnetic field, frequency, and temperature dependence of electromagnetic absorption at rf frequencies of 335 kHz and 10 MHz are measured in the as sintered samples of colossal magneto resistive materialsLa0.7Sr0.3MnO3andLa0.7Ca0.3MnO3by measuring the penetration depth using the method of Schawlow and Devlin. Very large magneto-electromagnetic absorption effects, much larger than the magneto resistive effects, are observed. A dc magnetic field of 1000 G changes the penetration depth at 350 kHz by 53&percent; at room temperature inLa0.7Sr0.3MnO3,whereas the resistance of the sample only decreases by 0.75&percent; for the same field. The effects are primarily a result of the decrease of the permeability of the sample with dc magnetic field in the ferromagnetic state. A simple dc magnetometer employing the as sinteredLa0.7Sr0.3MnO3contained in the coil of anLCoscillator as the sensing element is described.

ISSN:0021-8979    

DOI:10.1063/1.366143

出版商:AIP    

年代:1997

数据来源: AIP