ALBERT

Notes: This article describes three planar layouts of superconducting multiturn flux transformers integrated with a coplanar resonator for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. The best magnetic field noise values of 22 and 11.5 fT/Hz1/2 in the white noise regime were obtained for the layout with two input coils and the layout with the labyrinth resonator, respectively. Excess low-frequency noise (about 200 fT/Hz1/2 at 10 Hz) was present. Computer simulation showed that the loss in this trilayer system was dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconducting films. The rf coupling coefficient krf between the resonator and the flip-chip-coupled SQUID was also estimated. The values krf2(approximate)14×10−3 obtained for the layout with two input coils, and krf2(approximate)45×10−3 for the layout with the labyrinth resonator were considerably higher than the typical value of krf2(approximate)7×10−3 for the single-layer coplanar resonator. These high coupling coefficients have compensated the somewhat degraded unloaded quality factor of the resonator, thus securing the optimum operation of the rf SQUID. © 2000 American Institute of Physics.

Notes: We investigated the quantum-size effects of quantum well (QW) on gain and threshold current density for InGaAsP/GaAs (λ=808 nm) laser diodes. In this work, a comparison is made of lasers with different QW thickness while keeping the optical confinement factors constant. We found that the threshold current density and differential efficiency were not affected by narrowing the QW thickness. The theoretical model taking into account the mixing of the valence bands and momentum relaxation for InGaAsP/GaAs lasers with spontaneous emission (optically pumped) measurement shows that the absence of difference between these structures can be attributed to the high relaxation rate. © 1996 American Institute of Physics.

Notes: Special double- and separate-confinement InGaAsP/GaAs heterostructures intended for photoluminescence measurements have been grown by low-pressure metal-organic chemical-vapor deposition. The band gap of the active region quaternary material was close to 1.5 eV, and the waveguide of the separate-confinement structures was near 1.8 eV. Measurement of the integrated luminescence efficiency at 300 K has shown that over a wide range of excitation level (10–103 W/cm2) radiative transitions are the dominant mechanism for excess carrier recombination in the active region of the structures studied. As determined by spectral measurements, the excess carrier concentration in the waveguide of the separate-confinement heterostructures and the intensity of the waveguide emission band correspond to a condition of thermal equilibrium of the excess carrier populations in the active region and the waveguide. The ratio of the intensity of the waveguide emission to the active region emission fits a model which assumes that the barrier height for minority carriers (holes) is equal to the difference in band gaps between the active region and the waveguide region.

Notes: Sharp and straight step edges on (001) LaAlO3 (LAO) substrates were ion milled by using an electron beam defined amorphous carbon thin film mask. YBa2Cu3O7 (YBCO) thin films patterned to narrow strips across the step edges gave high quality Josephson junctions. Their current–voltage (I–V) curves could be well described by the resistively shunted junction model with or without excess current. By varying the YBCO film thickness over a fixed step height, the critical current density (jc) of the junction could be changed by several orders of magnitude. For junctions with high jc, typical IcRn (product of critical current and normal resistance) values of around 100 μV at 77 K and more than 1 mV at 4.2 K were obtained. Some excess current was observed. For junctions with low jc, the dependence of Ic on an applied magnetic field was strong even at low temperatures. The Ic showed a main peak in the center and well-defined periods as a function of applied magnetic field. The minimum Ic value suppressed by the magnetic field was about 20% of its maximum value at 4.2 K. Junctions with low jc usually showed hysteretic I–V curves at low temperatures. The McCumber constant βc fell in the range of 0.8–2. Fiske and flux-flow resonances were observed for some junctions. The shunting capacitances of the junctions were estimated from the McCumber constant βc, Fiske resonances, and flux-flow resonances. A shunting capacitance value per unit area of 12–35 fF/μm2 was obtained. High resolution cross-sectional transmission electron microscopy was used to study YBCO films grown across straight and wavy step edges. Two 90° tilt boundaries were formed at the edge of a step. The top and bottom YBCO films had their c axis oriented normal to the (001) plane of LAO. In the edge region, the c axis of the YBCO film was tilted by 90°, the a axis was normal to the (001) plane of LAO, and the b axis was lying along the step edge. For wavy step edges, second phase particles usually appeared in the YBCO film along the step edge region. Defects were found on the surface of the LAO substrate along the wavy step edge region. These defects might act as nucleation centers for the second phase particles in the YBCO film. © 1996 American Institute of Physics.

Notes: We have simulated the superconducting coplanar resonators of different designs that we have fabricated and tested as the tank circuit of radio frequency superconducting quantum interference devices. The coplanar resonator is formed by two microstrip lines surrounding a flux concentrator with each line having a slit. The simulated resonant frequencies agreed well with the experimental values, where frequencies decreased with an increasing mutual angular position between the two microstrip slits, and with a short circuit between the two microstrip lines. The simulation also showed that the loss in the system is mainly contributed by the dielectric loss of the substrate materials. © 1998 American Institute of Physics.

Notes: A compressively strained InAsSb/InAs multiple quantum-well (MQW) structure was grown by low-pressure metal-organic chemical vapor deposition. Maximum output power (from two facets) up to 1 W with differential efficiency about 70% was obtained from a MQW laser with stripe width of 100 μm and cavity length of 700 μm for emitting wavelength of 3.65 μm at 90 K in pulse mode operation. About 2 times lower threshold current density was obtained from the MQW lasers for a temperature range of 90 to 140 K compared to the double heterostructure lasers grown on the same growth conditions. © 1997 American Institute of Physics.

Notes: Far fields in perpendicular direction to the junction are investigated in double heterostructure (DH) and multiple quantum well (MQW) midwave-infrared InAsSb/InPAsSb/InAs lasers (λ=3.2–3.6 μm). Strong broadening of the far fields in the DH lasers was observed with increases in temperature and/or current. On the contrary, MQW lasers with otherwise identical structure exhibit very stable far fields as narrow as 23° for all the operating conditions investigated. Our experiment and theoretical modeling suggest that these different behaviors of far fields in DH and MQW lasers are attributed to the refractive index fluctuation in the InAsSb laser active region. © 1997 American Institute of Physics.

Notes: In this work, we study the origin of the optical losses in Al-free InGaAsP/GaAs (λ=0.808 μm) and InGaAs/GaAs/InGaP (λ=0.980 μm) lasers. Theoretical modeling and the experimental results indicate that the scattering of the laser beam by refractive index fluctuation in the alloys is the dominant loss in our lasers, and the loss due to the free-carrier absorption and scattering by interface roughness are negligible. © 1996 American Institute of Physics.

Notes: High-temperature superconductor GdBa2Cu3O7 thin films were grown in situ on LaAlO3 single-crystal substrates by dc magnetron sputtering using a single planar target. 93% samples had a zero resistance transition temperature Tc0 more than 90 K and a transition width less than 1 K. The best films had a Tc0 of 92.5 K, a transition width of 0.57 K, and a critical current density of 3.6×106 A/cm2 at 77 K. The reproducibility is very good. X-ray diffraction analysis showed c axis normal to the film's surface.

Notes: Strong Fiske type resonance peaks have been observed in current–voltage curves of mtsp;YBa2Cu3O7 step edge junctions on mtsp;LaAlO3 substrates. We determined effective shunting capacitances from the voltages of the resonances. These data compare well with those obtained from the McCumber constant mtsp;βc. Typical values of the shunting capacitance per unit area for these junctions were estimated to be in the range of 20–30 mtsp;fF/μm2. The surface resistance normalized to 1 GHz at the conjugating junction interfaces was estimated to be in the range of 1–3 μΩ. © 1995 American Institute of Physics.