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Publications (13)28.03 Total impact

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    ABSTRACT: The scanning SQUID (Superconducting Quantum Interference Device) microscope is an extremely sensitive instrument for imaging local magnetic fields. We describe one such instrument which combines a novel pivoting lever mechanism for coarse-scale imaging with a piezoelectric tube scanner for fine-scale scans. The magnetic field sensor is an integrated miniature SQUID magnetometer. This instrument has a demonstrated magnetic field sensitivity of <10<sup>−6</sup> gauss/√Hz at a spatial resolution of ∼10 µm. The design and operation of this scanning SQUID microscope are described, and several illustrations of the capabilities of this technique are presented. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, is shown. The use of this instrument for the first observation of half-integer flux quanta, in tricrystal thin-film rings of YBa 2 Cu 3 O 7−δ , is described. The half-integer flux quantum effect is a general test of the symmetry of the superconducting order parameter. One such test rules out symmetry-independent mechanisms for the half-integer flux quantum effect, and proves that the order parameter in YBa 2 Cu 3 O 7−δ has lobes and nodes consistent with d-wave symmetry.
    Ibm Journal of Research and Development 12/1995; · 0.69 Impact Factor
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    ABSTRACT: The ballistic transport of picosecond digital signals along terminated microstrip lines is a key feature of single flux quantum (SFQ) logic. In this work, we have used fast photoconductors driven by a femtosecond laser to measure directly for the first time the propagation and termination of picosecond pulses along Nb microstrip lines. Electrical pulses as short as 0.8 ps full-width-at-half-maximum (FWHM) have been measured. After propagating distances of 200 μm, 1 mm, and 6.5 mm, the pulses broadened to 1.0, 1.2, and 1.8 ps (FWHM), respectively. In the frequency domain, attenuation is found to be negligible up to the gap frequency of Nb(0.7 THz), beyond which attenuation increases rapidly. We have also measured the reflections of the picosecond pulses off different terminations: open circuit, short circuit, and various resistors. The effective suppression of reflections with matched resistive loads is demonstrated up to frequencies approaching 1 THz.
    IEEE Transactions on Applied Superconductivity 07/1995; · 1.20 Impact Factor
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    ABSTRACT: We have combined a novel low temperature positioning mechanism with a single‐chip miniature superconducting quantum interference device (SQUID) magnetometer to form an extremely sensitive new magnetic microscope, with a demonstrated spatial resolution of ∼10 μm. The design and operation of this scanning SQUID microscope will be described. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, will be presented, and a representative application will be discussed. © 1995 American Institute of Physics.
    Applied Physics Letters 02/1995; 66(9):1138-1140. · 3.79 Impact Factor
  • K. G. Stawiasz, M. B. Ketchen, L. R. Narasimhan
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    ABSTRACT: We have characterized the white noise performance of integrated DC SQUID susceptometers in magnetic fields up to 400 G. The field is applied in the plane of the tunnel junctions and pickup loops. For 1 μm diameter junctions, the noise does not increase significantly as the field is increased to 125 G. Further increase of the applied field results in higher noise until approximately 200 G where the critical current of the junctions is at a minimum. The noise then improves to approximately 4 times the zero field value at the maximum of the first lobe of the single junction interference pattern. We have also measured the noise of a SQUID with 0.6 μm junctions and found no increase up to 250 G.
    IEEE Transactions on Applied Superconductivity 01/1995; 5(2):3230-3232. · 1.20 Impact Factor
  • K.G. Stawiasz, M.B. Ketchen, A.W. Kleinsasser
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    ABSTRACT: A PtRh resistor has been developed as an integral part of an advanced planarized low-T<sub>c</sub> Josephson technology that is compatible with Si integrated circuit processing. Electron-beam evaporated from a single source, the PtRh films have a sheet resistance well controlled in the range of 3-20 Ω/square that is independent of temperature from 4.2 K down to at least 10 mK. The contact resistivity between PtRh, with a sheet resistance of 5 Ω/square and Nb interconnects is typically 0.7 Ω·μm
    IEEE Transactions on Applied Superconductivity 10/1994; · 1.20 Impact Factor
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    ABSTRACT: The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.
    Applied Physics Letters 08/1994; · 3.79 Impact Factor
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    ABSTRACT: The operation of a SQUID array with 100 DC SQUIDs has been demonstrated using a single flux-locked loop. The SQUID array had a maximum dynamic range of ±1.3×10<sup>8</sup>/√(Hz) in the low frequency region, a high slewing rate over a wide frequency range, and an extrinsic white noise energy sensitivity of 6×10<sup>-31</sup>J/Hz. These data were obtained with a very simple feedback circuit made from three inexpensive operational amplifiers that operated in the DC-feedback mode. The feedback loop did not have any impedance matching circuit between the SQUID array and the room temperature electronics. Our results show that a SQUID array can have a significant impact on those applications that demand good noise performance and a very high dynamic range
    IEEE Transactions on Applied Superconductivity 01/1994; · 1.20 Impact Factor
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    ABSTRACT: The authors have designed, fabricated, and measured a number of Nb-AlO<sub>x</sub>-Nb octagonal washer DC superconducting quantum interference devices (SQUIDs) and miniature DC SQUID susceptometers having minimum feature size down to 0.5 μm. With SQUID inductance values on the order of 100 pH, typical noise performance is better than 1 μΦ<sub>0</sub>/√Hz. The small minimum feature size of input coils and pickup loop structures will facilitate tight coupling to a wide variety of systems ranging from submicron particles and structures to conventional ~μH input circuits. A single-washer SQUID with an 80-turn 0.5-μm-linewidth, 630-nH input coil has user-friendly V -Φ (voltage-flux) curves and a coupled energy sensitivity of 20 h at 4.2 K. Susceptometers with pickup loops ranging from 20-μm to 0.8-μm across have very user-friendly V -Φ curves and a resolution of order 100 μ<sub>B</sub>/√Hz at 4.2 K for devices with the smallest loops
    IEEE Transactions on Applied Superconductivity 04/1993; · 1.20 Impact Factor
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    ABSTRACT: Using a completely in-situ process, the authors have investigated Ag metal contacts to thin films of Y<sub>1</sub>Ba<sub>2</sub>Cu<sub>3 </sub>O<sub>7</sub>, in order to study the properties of the Ag/YBCO interface as well as the Josephson effect in SNS (superconductor-normal-superconductor) bridges made with this technique. Measurements of the temperature dependence of the Josephson current in these devices have been made and are compared to recent theoretical predictions. SNS devices which exhibited Josephson effects had the critical-current-resistance products of the junctions limited by the high specific contact resistance of the SN interfaces. The lowest values of the specific contact resistance obtained were on the order of 10<sup>-8</sup> Ω-cm<sup>2</sup> for contacts in the c -axis direction. The influence of the specific contact resistance on the magnitude of the Josephson current in SNS bridges is discussed
    IEEE Transactions on Applied Superconductivity 04/1993; · 1.20 Impact Factor
  • K.G. Stawiasz, M.B. Ketchen
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    ABSTRACT: Series arrays of 50 gradiometer superconducting quantum interference devices (SQUIDS) and 100 magnetometer SQUIDs have been fabricated using Nb/AlO<sub>x</sub>/Nb junctions with a planarized all-refractory technology for superconductivity (PARTS), the white noise of these arrays has been measured. The individual devices are 50-pH, 2-hole and 100-pH, 1-hole SQUIDs with integrated single turn input coils, 1-μm<sup>2</sup> and 0.5-μm<sup>2</sup> junctions, and PtRh shunt resistors. The input coil inductance of 10 nH will effectively match with a wide bandwidth to miniature pick-up loop structures for various experiments. Ideally, the coupled energy sensitivity should remain constant as devices are added in series, while the output impedance rises to a level practical for direct coupling to room-temperature electronics. The output impedance of the arrays is 250-1000 Ω. The white noise was measured directly with an ultra low noise preamplifier at room temperature. The best result was Φ <sub>N</sub>=0.12 μΦ<sub>0</sub>/√Hz with a corresponding coupled energy sensitivity of 56 h
    IEEE Transactions on Applied Superconductivity 04/1993; · 1.20 Impact Factor
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    ABSTRACT: By use of a completely in situ process, we have investigated Ag metal contacts to thin films of YBa2Cu3O7 (YBCO), in order to study the properties of the Ag/YBCO interface as well as the Josephson effect in SNS bridges made with this technique. Measurements of the temperature dependence of the Josephson current in these devices have been made, and are compared to recent theoretical predictions. SNS devices which exhibited Josephson effects had the critical current–resistance (IcRn) products of the junctions limited by the high specific contact resistance of the SN interfaces. The lowest values of the specific contact resistance obtained were on the order of 10-8 Ω cm2 for contacts in the c-axis direction. The specific contact resistance was measured as a function of deposition conditions for the Ag as well as annealing temperature and time. The influence of the specific contact resistance on the magnitude of the Josephson current in SNS bridges is discussed, and a review of the contact literature for noble-metal/YBCO contacts is presented.
    Physical review. B, Condensed matter 09/1992; 46(13). · 3.77 Impact Factor
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    ABSTRACT: We have, for the first time, demonstrated integrated dc superconducting quantum interference devices (SQUIDs) with input coils of linewidth down to 0.5 μm. The SQUID inductance L consists of a single octagonal washer or two or four such washers configured in parallel. The input coil of inductance L i , which couples to L with a mutual inductance of M i , consists of fine‐line octagonal spiral(s) fabricated in close proximity to the washer(s). For a two‐washer SQUID with twin 80‐turn, 0.5 μm spirals, L i =2.5 μH, M i =12.7 nH, L=109 pH, and the coupling constant k<sup>2</sup>=0.85. This entire device occupies an area of less than 0.2 mm<sup>2</sup>.
    Applied Physics Letters 08/1992; · 3.79 Impact Factor
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    ABSTRACT: We have demonstrated a new planarized all‐refractory technology for low T c superconductivity (PARTS). With the exception of the Nb‐AlO x ‐Nb trilayer preparation, the processing is done almost exclusively within an advanced Si technology fabrication facility. This approach has allowed us to leverage highly off of existing state‐of‐the‐art lithography, metal etching, materials deposition, and planarization capabilities. Using chemical‐mechanical polish as the planarization technique we have fabricated Josephson junctions ranging in size from 0.5–100 μm<sup>2</sup>. Junction quality is excellent with the figure of merit V m typically exceeding 70 mV. PARTS has yielded fully functional integrated Josephson devices including magnetometers, gradiometers, and soliton oscillators.
    Applied Physics Letters 12/1991; · 3.79 Impact Factor