摘要:

We show that energy position and line shape of donor–acceptor‐pair luminescence bands in ZnSe:N/GaAs epilayers depend very sensitively on excitation density and compensation. A continuous development from structureless red‐shifted broad to well structured donor–acceptor‐pair (DAP) bands is observed for increasing excitation density. The red shift is explained by the fluctuating potential affecting the bands and impurity levels and is caused by random distribution of charged impurities in highly compensated samples. The shift is reduced when these charge fluctuations are diminished due to an increasing number of impurities being neutralized via light‐induced carrier excitation. These effects have not been taken into account in previous work concerning doped II‐VI materials; however, they have to be considered when evaluating the frequently used hypothesis of a deep donor in ZnSe:N as an explanation of low‐energy broadband DAP emission. The influence of band fluctuations on the behavior of the DAP luminescence and excitation spectra is qualitatively discussed. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114566

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The bolometric performance of a high‐Tctransition edge bolometer has been evaluated within the temperature range 80 K<T<300 K. The detectivityD* of the device is peaking at transition midpoint and remains at moderate levels up to room temperature. The bolometric time constant of the device increases from 0.33 ms at transition midpoint to 1.55 ms at ambient temperature. The noise pattern displays 1/f behavior at low frequency and is scaling with bias current and the thermal resistance coefficient &bgr; of the superconducting film. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114567

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

High quality epitaxial YBa2Cu3O7−x(YBCO) thin films have been deposited on 12 &mgr;m thick (11¯02) oriented sapphire substrates using a CeO2buffer layer. Both layers were deposited by pulsed laser deposition (PLD) using a blackbody‐type heater. The YBCO films showed superconductive transition temperatures of 88–89.5 K and transition widths ≤0.5 K. Structural evaluation of the films indicate that the YBCO films arec‐axis oriented, with the YBCO (005) peak showing a rocking angle full width half‐maximum (FWHM) of about 0.7°. The &fgr;‐scans of the (103) YBCO peak indicate a high degree of in‐plane epitaxy with no signs of high angle grain boundaries. The high quality of these films combined with the low heat capacity of the substrates, make these structures ideal for infrared detector applications. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114568

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The 4.2 K and 27 K current‐carrying performance of a high‐temperature superconducting (HTS) coil was measured in background fields up to 22.54 T generated by a hybrid magnet (Hybrid III) at the MIT Francis Bitter National Magnet Laboratory. The coil, 40 mm winding i.d., 108 mm winding o.d., and 113 mm high, consists of 17 double pancakes, each wound with silver‐sheathed BSCCO‐2223 tapes. Each pancake is the product of a react‐and‐wind method. In total, the test coil contains ∼1200 m of BSCCO‐2223 conductor weighing ∼7 kg. Prior to the measurements in Hybrid III, the coil was tested in zero background field in the temperature range from 4.2 to 77 K. It was coupled to a Gifford–McMahon type cryocooler and at 15 K generated a peak field of 2.1 T; at 18 K, it generated 1.9 T, operating continuously for ∼50 h. In a 22.54 T background field of Hybrid III, the coil reached critical currents of 116.5 A ([Jc]sc, critical current density based on the BSCCO cross‐sectional area only, of 261 A/mm) at 4.2 K and 67 A ([Jc]sc=150 A/mm) at 27 K, establishing record net fields at respective temperatures of 24.0 and 23.4 T for HTS magnets. These currents correspond to overall winding current densities of 47 and 27 A/mm. High‐field critical current data for short samples of the tape of the same formulation at 4.2 and 27 K are also presented. Although a [J]scof 261 A/mm at 24 T and 4.2 K for the test coil is significantly less than ∼600 A/mm for the short samples at the same operating point, if factors such as length, bending, and even differences in defining critical current are considered, the coil and short samples have nearly the same critical current performance. Electromagnetic stresses do not seem to have any negative effects on coil performance. Record fields of 24.0 and 23.4 T were achieved after the test coil had experienced, over a period of 15 months, 20 thermal cycles between room temperature and cryogenic temperatures. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114569

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have fabricated high‐TcSQUID magnetometers with a pickup coil and a multiturn input coil integrated onto the same substrate. A four‐channel system operating in liquid nitrogen has been developed and used to measure biomagnetic signals emanating from both the heart and the brain. The magnetic field sensitivity of the integrated SQUID magnetometer reaches 70 fT/&sqrt;Hz at 1 kHz and 280 fT/&sqrt;Hz at 1 Hz. The use of superconductor‐normal metal‐superconductor (SNS) step‐edge junctions with YBa2Cu3O7−&dgr;as the superconductor and Ag–Au alloy as the normal metal allows the measurements to be performed with standard dc bias electronics. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114570

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have performed a detailed analysis of the low frequency 1/fnoise in  YBa2Cu3O7−&dgr;   and  Bi2Sr2CaCu2O8+xgrain boundary Josephson junctions (GBJs) fabricated on SrTiO3bicrystal substrates. The normalized fluctuation of the critical current, &dgr;Ic/Ic, and the normal resistance, &dgr;Rn/Rnwere found to be almost independent of temperature and the misorientation angle. Furthermore, the magnitude of the fluctuations is very similar for both high‐Tccuprates. Correlation experiments showed that the fluctuations ofIcandRnare anti‐correlated. Our analysis strongly suggests that the source of 1/fnoise in high‐Tcbicrystal GBJs are localized defect states in an insulating grain boundary barrier with fluctuating electron occupation. The effective charge trapping time within single traps was found to decay exponentially with increasing bias voltage. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114571

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A method is presented to determine the Labusch parameter &agr; from the measurement of magnetic stiffness &eegr;. Quantitative results on various YBa2Cu3O7−&dgr;samples are reported. For all samples &eegr; is a function of the external magnetic field and does not depend on magnetic history. For small magnetic fields &eegr; obeys aB1.5behavior. At fields above 120 mT the melt grown material and the thin film turn over to aB2dependence. The results can be understood in terms of a crossover from nearly single vortex characteristics to the elastic interaction of vortices in the flux line lattice. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114572

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We describe the effects of disorder on the coherence properties of a 2×2 array of Josephson junctions (a ‘‘plaquette’’). The disorder is introduced through variations in the junction characteristics. We show that the array will remain one‐to‐one frequency locked despite large amounts of the disorder, and determine analytically the maximum disorder that can be tolerated before a transition to a desynchronized state occurs. Connections with largerN×Marrays are also drawn. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114573

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Resistance noise measurements of several types reveal the field history dependence of domain structure in sputtered Co/Cu multilayers. We find many smaller domains as the field is decreased from saturation towards zero, but as the field changes sign and is increased in the opposite direction we observe a smaller number of larger domains. Cycling the field without changing its sign preserves the smaller domains, strongly reducing the Barkhausen noise. Discrete fluctuations in resistance due to individual domains yield domain size estimates. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114574

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The origin of diversity in the reported ferroelectric phase transition (Curie) temperaturesTc, of KTiOPO4(KTP) crystals grown from self‐fluxes has been studied. A series of crystals has been grown by the top‐seeded solution method from fluxes of different K/P molar ratios and solute concentrations, covering most of the KTP crystallization field in the K2O–P2O5–TiO2ternary system. Curie temperatures of as‐grown and high‐temperature annealed crystals have been measured using the standard dielectric technique and found to spread from 883 to 960 °C. We show that the actualTcvalues depend on the solution chemical composition, and a functional representation of this dependence is given. Inherent gradients of theTcin as‐grown KTP crystals are discussed in terms of the nonuniformity of their physical properties. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114575

出版商:AIP    

年代:1995

数据来源: AIP