摘要:

Photoluminescence (PL) spectroscopy has been performed on a set of self‐assembled InSb, GaSb, and AlSb quantum dot (QD) heterostructures grown on GaAs. Strong emission bands with peak energies near 1.15 eV and linewidths of ∼80 meV are observed at 1.6 K from 3 monolayer (ML) InSb and GaSb QDs capped with GaAs. The PL from a capped 4 ML AlSb QD sample is weaker with peak energy at 1.26 eV. The PL bands from these Sb‐based QD samples shift to lower energy by 20–50 meV with decreasing excitation power density. This behavior suggests a type II band lineup. Support for this assignment, with electrons in the GaAs and holes in the (In,Ga,Al)Sb QDs, is found from the observed shift of GaSb QD emission to higher energies when the GaAs barrier layers are replaced by Al0.1Ga0.9As.

ISSN:0003-6951    

DOI:10.1063/1.115747

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report the preparation of high quality GaN films using dimethylhydrazine as a nitrogen source. GaN films were deposited on (0001) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. The film has excellent morphology in flat surface free from cracks. The full width at half maximum of the x‐ray diffraction rocking curve measured from (0002) plane of GaN has exhibited as narrow as 147 arcsec. The optical quality of the film is concluded to be excellent from the photoluminescence property measured at room temperature, where the intensity of the band edge emission is much stronger than that of deep level emission. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115748

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A scanning near‐field microscope design using the reflected light intensity as the feedback mechanism is described. Multiple fibers with high numerical apertures provide a high collection efficiency in a reflection geometry. The performance with regard to its response to large spatial variations has been tested by using a Si‐grating sample and with regard to variations of local indices of refraction by using GaAs/AlGaAs heterostructure samples. In addition, spatially resolved spectroscopy on GaAs/AlGaAs heterostructures has been obtained. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115749

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

YBa2Cu3O7step‐edge junctions with widths down to 0.5 &mgr;m are fabricated on SrTiO3substrates by Ar ion‐beam milling of the steps, high‐pressure on‐axis magnetron sputtering, electron beam patterning and ion‐beam etching of the microbridge. For ratios of film thickness to step height of ∼1/2 the current‐voltage characteristics show Shapiro steps under microwave irradiation and resistively shunted junction like behavior. The periodic dependence of the critical current upon the magnetic field resembles a Fraunhofer pattern. The period of the current variation &Dgr;B0depends upon the widthwof the junction according to the theoretical prediction for planar thin Josephson junctions: &Dgr;B0=1.84&fgr;0/w2. Junctions with widths of 0.7 &mgr;m possess a large magnetic field stability with &Dgr;B0≊100 G. Small junctions (w<1 &mgr;m) exhibit voltage jumps in the Fraunhofer pattern, which are explained by flux penetration of single vortices into the electrodes. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115750

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We use scanning micro‐Raman spectroscopy (SMRS) to characterize the epitaxial film quality as well as low‐temperature scanning laser microscopy (LTSLM) to image electrical properties of YBa2Cu3O7devices locally in both lateral dimensions. Investigations have been performed on crossovers as used in flux transformers of magnetometers, patterned in a multilayer process on SrTiO3(100) substrates. We find a correlation between an inhomogeneous superconducting performance and a disturbance of epitaxy on locations where the crossovers pass edges of the underlying bridge. The distribution of the oxygen is not affected at these points. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115751

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Bi2Sr2CaCu2Ox(Bi‐2212) superconducting thick films incorporating Bi‐2212 superconducting whiskers have been prepared using a partial melting method. Hysteresis behavior of critical current density (Jc) for magnetic field is observed in a film without whiskers (monolithic film), but not noticeably in composite film (whisker content: 20 wt %). It is found from ac susceptibility measurement that a peak in &khgr;″, resulting from weak links, appears at a higher temperature for the composite film than for the monolithic film.Jcdecreases with increasing magnetic field more rapidly for the monolithic film than for the composite film less than 1 T. These results indicate that the incorporation of the whiskers is effective for overcoming weak links. In consequenceJcfor the composite film is about three times higher than for the monolithic film. It is clear from SEM observation that a microstructural improvement leads to an increase inJc. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115752

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Systematic modifications on the nanometer scale of YBa2Cu3O7−&dgr;(YBCO) epitaxial, thin films have been achieved by using a scanning tunneling microscope in air at room temperature. Working with tunneling parameters slightly above those used for imaging results in irreversible, nanometer‐sized surface modifications. The surface topography of our YBCO films showed characteristic growth spirals, of which one revealed a remarkable ‘‘S’’‐shaped top end. This unusual growth behavior indicates possibly a distortion caused by a line defect in the second to last turn of the spiral. We succeeded in cutting through this structure by producing a 2.5 nm wide groove across it. This corresponds to the controlled removal of only a few unit cells of YBCO. As an alternative modification technique, bias voltage pulses were applied, leading to the formation of 3 nm wide craters. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115753

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross‐tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross‐tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross‐tie wall. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115754

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Bulk samples of La2−2xCa1+2xMn2O7with the layered Sr3Ti2O7‐type perovskite structure have been successfully synthesized and investigated with respect to their magnetic and electrical properties. It is found that La2−2xCa1+2xMn2O7(x=0.25) is a metallic ferromagnet with a magnetic transition temperatureTcof 215 K. The large magnetoresistance (MR) effect with &Dgr;&rgr;/&rgr;0of ∼60% at 1.8 T was observed in a wide temperature range below a cusp temperature in resistivity of 96 K, which is well below the magneticTc. This behavior is quite different from that of the well‐known double‐exchange ferromagnets such as La1−xCaxMnO3, where large MR effects are restricted to a narrow temperature range around the ferromagnetic transition. The present result could be interpreted by using the double‐exchange theory incorporating the anisotropy resulting from the two‐dimensional Mn‐O‐Mn networks in La2−2xCa1+2xMn2O7. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115755

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We discuss the coercive behaviour of systems with fourfold symmetry. In agreement with recent experiments, in simulation studies we found a two step process with two coercive fields for thexand theymagnetizations. A new physical picture for the spin reversal is obtained. Dipolar effects are found to be very important. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115756

出版商:AIP    

年代:1996

数据来源: AIP