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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 1129-1142 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One can use a direct current (dc) superconducting quantum interference device (SQUID) as a current amplifier by injecting a signal current Is into part of the superconducting loop and detecting the resultant change in critical current Ic. The current gain max ||dIc/dIs|| can be increased by making the inductances of the two arms of the SQUID asymmetric, thereby skewing the transfer function Ic vs Is. Detailed simulations of the device performance include an analysis of the impact of inductance and junction asymmetry on the transfer function. At 77 K a maximum current gain of 5 should be achievable albeit for frequencies below 1 GHz. An increase in the operating speed to 2 GHz can be achieved by reducing the gain to 2. We have fabricated asymmetric YBa2Cu3O7−x dc SQUIDs on SrTiO3 bicrystal substrates and operated them at 77 K. Using a second, readout SQUID to monitor changes in the critical current, we have achieved a low-frequency current gain up to 2.3. By coupling two SQUIDs together, we have increased the gain to 8.5. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 3809-3813 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A prototype three-axis magnetometer has been developed, intended for geophysical applications, involving high-transition temperature dc superconducting quantum interference devices (SQUIDs). Each SQUID was fabricated from a thin film of YBa2Cu3O7−x deposited on a SrTiO3 bicrystal to form two Josephson junctions, and was directly coupled to a single-turn pickup loop patterned in the same film. The three orthogonally mounted sensors were immersed in liquid nitrogen in a fiberglass Dewar and operated in separate flux-locked loops. In nominally zero ambient magnetic field, the best magnetic field resolution was 170 fT Hz−1/2 in the white noise, increasing to 225 fT Hz−1/2 at 1 Hz. The dynamic range was ±1.6×106 Hz1/2 in the white noise, and the highest slew rate achieved was 1.6 mT s−1, at 900 Hz. The 3 dB point in the frequency response was about 90 kHz. The three channels were mutually orthogonal to about 1%. The three-axis magnetometer was also operated outside the laboratory in the presence of 60 Hz and radio-frequency noise. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 1095-1097 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We performed numerical simulations and experiments on Josephson vortex flow transistors based on parallel arrays of YBa2Cu3O7–δ grain boundary junctions with a cross gate line allowing us to operate the same devices in two different modes named the Josephson fluxon transistor (JFT) and Josephson fluxon–antifluxon transistor (JFAT). The simulations yield a general expression for the current gain versus number of junctions and normalized loop inductance and predict higher current gain for the JFAT. The experiments are in good agreement with simulations and show improved coupling between gate line and junctions for the JFAT as compared to the JFT. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 1027-1029 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated Josephson field-effect transistors based on YBa2Cu3O7−δ bicrystal grain-boundary junctions (GBJs) and epitaxial SrTiO3 films as gate insulators. The SrTiO3 gate insulator shows high products of the breakdown field Ebd and the dielectric constant εr up to Ebdεr=1.3×1010 V/m allowing measurements over a wide range of applied gate electric-field Eg. The critical current Ic of the GBJs is found to depend highly nonlinear on Eg. Remarkably, the measured Ic(Eg) are very similar to the εr(Eg) curves. This strongly suggests that the observed electric-field effect is not due to a field-induced change in carrier concentration but is related to the dielectric properties of the SrTiO3 gate insulator. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 1555-1557 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low temperature scanning electron microscopy (LTSEM) has been used to analyze the superconducting properties of integrated magnetometers involving a dc superconducting quantum interference device. The study yielded spatially resolved information on the critical temperature Tc and critical current density Jc in the upper and lower superconducting layers of the YBa2Cu3O7−δ/SrTiO3/YBa2Cu3O7−δ structure. The Tc of the lower film was depressed by several kelvin, and the Jc of the upper film was lower where it crossed the edge of a lower YBa2Cu3O7−δ film covered with SrTiO3. A gradient in Tc was observed in the lower film near the edge of a via, arising presumably from oxygen diffusion during fabrication. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 1010-1012 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated Josephson vortex-flow transistors (JVFTs) based on asymmetric parallel arrays of YBa2Cu3O7−δ bicrystal grain boundary junctions. The critical current Ic and the voltage V at fixed bias current Ib were measured as a function of the control current Ictrl through a control line inductively coupled to the array. For JVFTs with an asymmetric in-line geometry a high current gain g=∂Ic/∂Ictrll ranging between about 20 at 40 K and 14 at 70 K was achieved. This current gain is much higher than that obtained for symmetric devices and results from the self-field effect of the electrode currents. In contrast to the current gain the transresistance rm=∂V/∂Ictrl of the JVFTs could not be enhanced by an asymmetric device structure. The current-voltage characteristics of the asymmetric JVFTs show pronounced step-like structures caused by the self-field of the electrode currents. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 2209-2211 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct current superconducting quantum interference devices (SQUIDs) made from high transition temperature superconductors typically exhibit a still unexplained higher white noise and smaller transfer function than numerical simulations based on the coupled Langevin equations predict. We present a systematic study that compares experimental results with both numerical simulations and a recently developed analytical approach based on a Fokker–Planck equation. To cover a wide range of SQUID parameters, two samples of different inductances were measured at variable temperature. While several predictions of the analytical theory could be verified and a good qualitative agreement with both theories is observed, only one sample showed good quantitative agreement. Furthermore, our simulations performed in the limit of large thermal fluctuations show excellent agreement with the analytic theory. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 3341-3343 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have investigated La1.85Sr0.15CuO4−δ grain boundary Josephson junctions (GBJs) fabricated on [001] tilt SrTiO3 bicrystals with misorientation angles of 24° and 36.8°. The resistive transition and the current–voltage characteristics of the GBJs are found to be close to those predicted by the resistively shunted junction (RSJ) model. Their product of the critical current density, Jc, and the normal resistance times area ρn, scales proportional to about (Jc)q with q=0.5–0.6 similar to what is found for YBa2Cu3O7−δ GBJs. By analyzing Fiske resonances the Swihart velocity of the GBJs and the ab-plane London penetration depth of La1.85Sr0.15CuO4−δ are obtained to c¯=2.5×106 m/s and λL=200 nm at 4.2 K, respectively. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 373-375 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x (YBCO–STO–YBCO) trilayers, in which each layer is patterned photolithographically, capping the first YBCO film with an in situ STO film. Atomic force microscopy demonstrates that the capping process dramatically improves the quality of the surface of the second layer, allowing the growth of an upper YBCO film with a substantially reduced level of low-frequency flux noise. A magnetometer with a multiturn flux transformer coupled to a dc superconducting quantum interference device achieved a magnetic field noise of 74 fT Hz−1/2 at 1 Hz, improving to 31 fT Hz−1/2 at 1 kHz. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 230-232 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated and tested picovoltmeters in which the voltage source is connected in series with a calibrated resistor and a coil inductively coupled to a high-transition temperature superconducting quantum interference device operating at 77 K. The coil consists of either seven or ten turns of copper wire or of two turns patterned in a film of YBa2Cu3O7−x(YBCO). The lowest voltage noise achieved at 1 Hz was 30 pV Hz−1/2 and 2.3 pV Hz−1/2 for input coils made of copper and YBCO, respectively. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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