摘要:

Photoluminescence spectra in the near‐band‐gap region of Si1−xGexalloys (x=0.04 and 0.15) grown on Si(100) substrates by molecular beam epitaxy have been measured at 4.2 and 12 K. Radiative recombinations of free and bound excitons in thin layers of Si1−xGexalloys have been clearly observed for the first time. No‐phonon transitions and transverse‐optical (TO) phonon‐assisted transitions have been identified.The luminescence lines become broader with an increase in excitation intensity; the broadening is interpreted to be due to the generation of the bound multiexciton complexes (BMECs). The position of the band‐edge luminescence lines is determined by the strain in the epitaxial layer as well as the alloy composition. The defect‐relatedLband appears in the case ofx=0.15.

ISSN:0003-6951    

DOI:10.1063/1.104014

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Critical currents of superconducting silver‐sheathed tapes of Bi1.8Pb0.4Sr2Ca2.2Cu3Oxhave been measured in the magnetic flux density range 0–19.75 T at 4.2, 15, and 20 K. One tape achieved a critical current of 60.6 A at 19.75 T and 20 K; the corresponding critical current density is 551 A/mm2. In the same field, the tape has critical currents of 72.3 A (657 A/mm2) at 15 K and 94.2 A (856 A/mm2) at 4.2 K. At 77 K and in zero field, the tape carries 32.9 A (299 A/mm2). These results indicate that ‘‘highTc’’ superconducting magnets of engineering interest may soon be feasible.

ISSN:0003-6951    

DOI:10.1063/1.104146

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Using a three‐layer YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−xinsitulaser deposition process, we have constructed thin‐film flux transformers which have pickup loops 10 mm on a side and input coils with ten turns. Each layer was patterned with a shadow mask or photolithography. The flux transformers were inductively coupled to a dc superconducting quantum interference device (SQUID) at 4.2 K to determine their transition temperature, and the temperature dependence of their critical current and coupling efficiency. The best flux transformer had a transition temperature of 77 K, and enhanced the magnetic field response of the SQUID by a factor of 9.5.

ISSN:0003-6951    

DOI:10.1063/1.104015

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Single‐crystal Nb films coated with a thin polycrystalline Nb film were adopted as the bottom layer of Nb/Al‐AlOx/Nb superconducting tunnel junctions. The junctions were applied to detection of x rays from55Fe. The signal charge induced by each radiation was one order of magnitude larger than that from usual junctions and was two orders of magnitude larger than that from semiconductor Si detectors.

ISSN:0003-6951    

DOI:10.1063/1.104016

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Insituresistivity measurements have been performed to monitor the effect of activated oxygen (atomic oxygen, oxygen ions) on films subjected to an electron cyclotron resonance (ECR) oxygen plasma. The resistivity ofc‐axis and mixeda+coriented YBa2Cu3O7−xfilms, which were depositedinsituby 90° off‐axis magnetron sputtering, was measured as a function of time in an ECR oxygen plasma. The resistivity of a film in the plasma corresponds to that found in the same film subjected to higher oxygen pressure when no plasma is present. The resistivity is shown to be determined by the flux of atomic oxygen. The steady‐state concentration in the film is found to depend on the kinetics of oxygen dissociation at the film surface. From the temperature dependence of the activity of atomic oxygen, we determined the activation energy for the decomposition of O2at the surface of films with mixeda+caxis orientation andc‐axis orientation to be ∼1.3 eV and ∼2.1 eV, respectively. It is proposed that in the presence of atomic oxygen, the phase diagram of oxygen content versus temperature of YBa2Cu3O7−xis changed in a way that depends on the atomic oxygen flux.

ISSN:0003-6951    

DOI:10.1063/1.104147

出版商:AIP    

年代:1990

数据来源: AIP