摘要:

Grating-free quantum well infrared photodetectors (QWIPs) for 8–10 &mgr;m applications have been fabricated. The QWIPs fabricated with InGaAs/GaAs quantum wells show excellent performance with normal-incident infrared radiation. A detectivity of2×1010 cm Hz1/2/Wand a responsivity of 0.23 A/W have been obtained for these devices. The devices were compared with QWIPs with 1D and 2D surface gratings. No significant improvement in responsivity is observed for devices with 1D gratings. Devices with 2D gratings have significantly higher peak responsivity but only in a very limited spectral range. The GaAs/AlGaAs QWIPs in the similar spectral range have also been fabricated and, however, shown much weaker TE absorption. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119446

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Strain effects on critical current densities have been examined for conductors containing nearly stoichiometricNb3Alfilaments with fine grains. TheNb3Alphase in these multifilamentary conductors are prepared by phase transformation from supersaturated Nb(Al) bcc solid solution and show high-field critical current densities much larger than those for conventionally preparedNb3Alconductors, where theNb3Alphase is known to be off-stoichiometric. The degradation of critical current densities with−0.7&percent;intrinsic strain is ca. 20&percent; at 12 T, comparable with those for conventionalNb3Alconductors of high strain tolerance. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119447

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The properties of theY1Ba2Cu3O7−xbiepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositingY1Ba2Cu3O7−xthin film by cylindrical magnetron sputtering technique on the (110)SrTiO3substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctionsICand increased the normal state resistance times area to values of the order of1(&mgr; &OHgr; cm2).Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119448

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A clock recovery circuit has been successfully tested at frequencies up to 20 GHz. This cell is designed for a rapid-single-flux-quantum (RSFQ) telecommunication data switch. It serves to set the receiver clock in phase with the incoming digital signal. The circuit consists of a dc-to-SFQ converter, ring oscillator [(RO) is a closed-loop RSFQ Josephson transmission line], confluence buffer, and an 8-bit binary counter. The input signal transforms to SFQ pulses, and each pulse resets the phase of the ring oscillator, giving a locking time of 1 bit. Thus, the pull-in (capture) range and hold-in (tracking) range are the same, and strictly depend on the encoding of the input signal. This range is estimated to be about 1 GHz at frequency 20 GHz, if the sequence of consecutive ONEs or ZEROs does not exceed 20 bits. The quality factorQROof ring oscillator is about 2000, which gives a jitter of 50 fs for a 35-junction RO. A sampling technique was used to demonstrate phase recovery (phase locking) with only one incoming pulse per 512 clock periods. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119449

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Novel nondestructive evaluation measurements made using niobium dc superconducting quantum interference devices with integrated asymmetric first-order gradiometers are described. Comparative theoretical and experimental studies of their spatial response have been described, and it is shown that the gradiometric response makes operation possible in an unshielded and electromagnetically noisy environment. As a demonstration of their capabilities, subsurface defects in a multilayer aluminum structure have been located and mapped using induced eddy currents at 70 Hz, with no magnetic shielding around the specimen or cryostat. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119450

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Using a high-resolution magneto-optical imaging technique, we study the flow of transport currents around cracks in monofilament(BiPb)2Sr2Ca2Cu2Oxcomposite conductors. Shunting through the Ag clad is directly observed. The current in the silver decays exponentially with distance from the crack as expected in a one dimensional distributed resistance model. At 77 K, the transfer length, &lgr;, is approximately 0.56 mm, implying an interface resistivity of7.5×10−8 &OHgr; cm2.This transfer length is found to increase slightly with increasing temperature. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119451

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A large increase(∼8×)in critical current density,Jc,was achieved forin situlaser ablatedYBa2Cu3O7−&dgr;–Agfilms. High-resolution Auger electron spectroscopic investigation indicates that the Ag-doped films are stoichiometric with a relatively low grain-boundary extension (8 nm) in contrast to undopedYBa2Cu3O7−&dgr;films (32 nm). Further analysis suggests that the doped film contains a much lower silver content(<0.15 wt &percent;)than in the target material (5 wt &percent;). These observations are in excellent agreement with the temperature dependence ofJc,the room-temperature resistivity, and the surface resistance results. Thus,Jcenhancement in Ag-doped films can be attributed to their superior properties, viz., improved microstructure characteristics and the reduced resistive grain boundaries. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119452

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We show the importance of the role of strain inLa0.7Sr0.3MnO3films by revealing the dominance of stress anisotropy effects over magnetocrystalline anisotropy effects in the magnetic anisotropy of these films. Magnetic anisotropy measurements of (001) and (110)La0.7Sr0.3MnO3thin films onSrTiO3andLaGaO3substrates, with excellent structural quality, reveal twofold symmetry on (110)La0.7Sr0.3MnO3films and fourfold symmetry on (001) films. Such symmetries can only be explained by stress anisotropy contributions in the plane of the film. In conjunction with the thickness dependence of the magnetic properties, the results indicate the dominant role of strain in the magnetic properties of these doped manganites. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119454

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

GaMn icosahedral particles with quasicrystalline order have been found in Mn implanted and rapidly annealed GaAs by means of selected-area electron diffraction, high-resolution, and dark-field electron microscopy. The orientation relationship between the submicron quasicrystals with the icosahedral(2/m 3¯5¯)symmetry and the face-centered cubic (fcc) GaAs matrix is determined to be:i5¯(the fivefold inversion axis in(2/m 3¯5¯))//[110]GaAs,i3¯(the threefold inversion axis)//[111¯]GaAs,andi2(the twofold axis)//[121¯]GaAs.The statistics of these structural studies, combined with magnetic force microscopy, indicate that the submicron quasicrystals are ferromagnetic. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119486

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Ferroelectric domains inLiNbO3have been investigated by means of electrostatic force microscopy. Polarization-inverted gratings with 4 &mgr;m periodicity were fabricated by titanium diffusion into both+cand−cfaces of single-domainLiNbO3crystals. The distribution of the electric field in the vicinity of the sample surface was measured using scanning probe microscopy. The electrostatic force image was found to correlate with the shape of the domain-inverted profile observed by scanning electron and optical microscopies. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119455

出版商:AIP    

年代:1997

数据来源: AIP