摘要:

The valence‐band offsets at (&bgr;‐GaN)/(&bgr;‐AlN) (001), (110), (111) interfaces and strained (&agr;‐GaN)/(&agr;‐AlN) (0001) interface are determined systematically byabinitiocalculations using supercells of up to (6+6) layers, and by an average bond energy model. The results are in very good agreement with experimental data. It is found that the variation of the band gap in (&agr;‐GaN)n/(&agr;‐AlN)n(0001) system withnis completely different from that in the (&bgr;‐GaN)/(&bgr;‐AlN) system. These different band‐gap behaviors are shown to be related to the internal electric fields in &agr;‐GaN and &agr;‐AlN slabs, which are induced by the difference of the spontaneous polarizations in these slabs. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363146

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Josephson junctions based on Nb/AlOx/Nb trilayer technology have demonstrated excellent quality, exhibiting very low dissipation in the subgap region. This property is very important for all those experiments which have to deal with the quantum behavior of macroscopic variables in Josephson devices. In view of performing an experiment of macroscopic quantum coherence with a Josephson device, we measured the escape temperature and the return current of several junctions fabricated with different processes and having different characteristics, by cooling them from 4.2 to 0.3 K. We show that, for what concerns the process of escape from the zero‐voltage state, no difference can be found among different junctions. On the other hand, the dissipation properties are strongly influenced by the current density, with lower current density resulting in lower intrinsic dissipation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363147

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Patterns on YBa2Cu3O6.87films, laser written in O2or N2atmospheres, and with various writing power densities and scanning speeds, have been studied by Raman scattering. For laser writing in O2at atmospheric pressure, with low power density and fast scanning speed, it is found systematically that the oxygen content of YBa2Cu3Oxcan be reduced. Only when the writing power density is high and the scanning speed is low (e.g., 2.3 mW/&mgr;m2and 1 &mgr;m/s), do the laser‐irradiated lines in an O2atmosphere approach full oxygenation (x→7). For laser writing in N2at atmospheric pressure, the oxygen stoichiometry always decreases, rendering the lines nonsuperconducting. The structural phase transition from orthorhombic to tetragonal can be identified with laser writing in a N2atmosphere with a writing power density of 1.9 mW/&mgr;m2and a scanning speed of 20 &mgr;m/s. The oxygen O(4) line (∼500 cm−1) and the Ba line (115 cm−1) disappear, and the intensity of the Cu(2) line (144 cm−1) rises sharply when the phase transition occurs. This study shows how Raman scattering can be used as a routine technique, which is fast, sensitive, and nondestructive, and is able to measure oxygen content on a fine scale (a few &mgr;m) during high‐Tcsuperconductor device processing. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363148

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thin films of YBa2Cu3O7−&dgr;(YBCO) on LaSrAlO4(001) substrates with various thicknesses have been investigated by Raman spectroscopy. The Raman spectra of the films have been obtained by properly subtracting the spectrum of the substrate from the recorded spectra. This data anaysis enables us to determine the Raman spectra of YBCO films with thicknesses down to 12 nm (about 10 unit cells). The evolution of the peak positions and the linewidths of theB1gmode with thickness allow us to conclude that the residual strains in these films are negligibly small within the measured thickness range. The influence of the birefringence on measurement of the degree of epitaxy by Raman has been studied. With the reduction of film thickness the degree of epitaxy is slightly increased in these films. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363149

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The relation between random telegraph noise and 1/f‐type background voltage fluctuations in current biased high‐Tcfilms has been investigated. The experiments have revealed that the telegraph noise in high‐Tcsuperconducting films is due to a combined action of a macroscopic two‐level fluctuator and a detector. The macroscopic telegraph signal does not constitute an elementary contribution to 1/fnoise. However, the behavior of telegraph and 1/f‐like voltage fluctuations have shown astonishing similarities. Telegraph voltage amplitudes and 1/fpower spectrum magnitude scale linearly with changing current flow above almost identical threshold currents. The telegraph signal symmetry characteristics have been found to be linear also but with distinctively different offset currents. Analysis of the measured characteristics indicates that the telegraph and 1/ffluctuations are due to different flux fluctuators coupled to observable voltages by the same detector mechanism. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363150

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied the staticI–Vcharacteristics and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions. In junction stacks consisting of two junctions with identical critical currentsIc, theIcvsHcharacteristics have been observed to deviate from the usual Fraunhofer pattern in the small junction limit; the data are consistent with a model involving a structural phase transformation to a triangular vortex lattice with increasingH. In a finite voltage state interlayer coupling leads to splitting of the Swihart mode, which manifests itself as Fiske steps with different voltage spacings. The results provide clear evidence that the two junctions in the stack do phase lock. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363151

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The structure of Nd3Fe29−xTixNyhas been determined by neutron diffraction and compared with the unnitrided parent. Nitrogen is found in the two large octahedral interstitial sites with four Fe and two rare‐earth neighbors, leading to a concentration ofy=4. The lattice expands by 6.4% (&Dgr;V/V) but the expansion is anisotropic in the axes. Although most of the Fe–Fe bond lengths expand, a few show significant contraction, most notably the Fe3–Fe3 dumbbell, which has an extremely short 2.18 A˚ length. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363152

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Molecular‐beam‐epitaxial GaN layers change from strongly conductive (&rgr;&bartil;10−2&OHgr; cm at 300 K) to semi‐insulating (&rgr;&bartil;106&OHgr; cm) as the N flux is increased. Layers grown at low fluxes show strongn‐type conduction, with transport in the conduction band at high temperatures and in a shallow donor band at low temperatures. For layers grown at high N fluxes, the Hall coefficients become too small to measure, suggesting hopping conduction among deep centers. The temperature‐dependent resistivity data are most consistent with multiphonon, rather than single‐phonon, hopping. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363128

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A disadvantage of rare‐earth–transition‐metal (TM) ‐based magneto‐optical (MO) disks is the weak intrinsic chemical stability of the MO layer. This property can be improved by the addition of several elements, but elements with partly occupieddshells show a large influence on the magnetic and recording properties of the MO layer. Indium, as a 5pmetal, is known to improve the long‐term stability of TbFeCo layers. We investigated the magnetic and recording properties of TbFeCo–In layers as a function of In content. Only a small decrease of Curie temperature, compensation temperature, and anisotropy was observed. The carrier‐to‐noise ratio (CNR) of TbFeCo–In disks was determined as a function of In and TM content. A CNR≳53 dB was obtained for In contents up to 14 at. %. With 27 at. % In, the CNR is still larger than 50 dB. The aging stability of MO disks showed a significant improvement for In contents larger than 10 at. %. For MO recording, both requirements, high CNR and long‐term stability, can be obtained with TbFeCo–In layers with an In content between 10 and 14 at. %. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363153

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present a quantitative and comparative study of three different photothermal techniques based on the signal produced on vertical/slanted thermal barriers under a variety of barrier/sample conditions. Models based on integral methods are developed to calculate the amplitude and phase of the sample surface temperature and mirage deflection in surrounding air. The geometries studied include: vertical, tilted, buried, and finite size barriers separating identical or different media. The models incorporate the probe and pump beam sizes, the thermal resistance of the barrier and the optothermal characteristics of the sample. Experimental measurements are performed on a variety of fabricated barriers with three modulated photothermal techniques: the thermoreflectance, infrared radiometry, and mirage detection. We discuss in a comparative way the limits, drawbacks and the applicability of each technique. Model fits to the experimental results allow characterization of the thermal barrier (spatial localization, geometry, orientation, and size) and provide an accurate determination of its thermal resistance. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363154

出版商:AIP    

年代:1996

数据来源: AIP