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Applied Superconductivity, IEEE Transactions on

Issue 1 • Date March 1993

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Displaying Results 1 - 25 of 712
  • 11 channel magnetometer for biomagnetic measurements in unshielded environments

    Publication Year: 1993 , Page(s): 1894 - 1897
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    The authors describe a novel 11-channel magnetometer designed for operation in unshielded environments. The planar geometry of the system is particularly suitable for recording biomagnetic signals over a flat surface, as required for cardiac and gastrointestinal applications. The specific configuration of the sensor array also makes it possible to use the system for reliable and fast cardiac clinical measurements. Ambient noise rejection was achieved by the use of second-order gradiometers, optimized by means of additional fine balancing, whereas a sensitivity of less than 10 fT/Hz/sup 1/2/ was obtained by low-noise DC-SQUIDs (superconducting quantum interference devices), using Nb/Nb/sub 2/O/sub 5//PbAuIn technology for the Josephson junctions. The system is presently utilized for a systematic investigation of the localization properties of cardiac sources from data obtained during single-shot measurements, which are believed to be suitable for clinical studies. The results of localization are compared with anatomical information from magnetic resonance imaging data.<> View full abstract»

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  • Operation of a hybrid 28-channel neuromagnetometer

    Publication Year: 1993 , Page(s): 1890 - 1893
    Cited by:  Papers (1)
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    A 28-channel system for neuromagnetic measurements operative at CNR-IESS (Instituto di Elettronica dello Stato Solido) is described. The instrument uses 28 Nb/PbAuIn DC-SQUID (superconducting quantum interference device) sensors coupled with a flux transformer configuration. The detection coils are arranged on concentric circumferences. Sixteen axial first-order gradiometers are arranged in the internal circles, while the external circle consists of 12 first-order planar gradiometers. The vertical gradiometers show a white noise level of approximately=5 fT/ square root Hz, while the planar ones have a noise approximately=6 fT/cm square root Hz, with a 1/f corner at a few hertz. The evoked potentials of normal subjects and patients affected by various kinds of vascular disturbances have been studied. The system has succeeded at one-shot localization of some cortical activity. The instrument has also been used to record the spontaneous brain activity in patients affected by focal epilepsy.<> View full abstract»

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  • Design and fabrication of multichannel DC SQUIDs for biomagnetic applications

    Publication Year: 1993 , Page(s): 1887 - 1889
    Cited by:  Papers (7)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (307 KB)  

    A hybrid DC superconducting quantum interference device (SQUID) system is developed for biomagnetic applications. It consists of a high-performance square-shaped double-washer DC SQUID and a highly accurate planar pick-up coil. The DC SQUID device and the pick-up coil are designed and fabricated separately. Coupling this device and the separately fabricated pick-up coil allows flexibility in the construction of multichannel systems as well as sensitivity and stability.<> View full abstract»

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  • A modular 31-channel SQUID system for biomagnetic measurements

    Publication Year: 1993 , Page(s): 1883 - 1886
    Cited by:  Papers (3)
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    A modular multichannel superconducting quantum interference device (SQUID) system, in which every channel can be optimized or replaced individually, was further improved. The number of channels was increased to 31. The noise level is better than 10 fT/ square root Hz. A novel way of RF shielding using conductive paint avoids degradation of the SQUID characteristics due to RF interference without introducing significant extra noise, so that the system works without any Faraday cage. A simplified way of coupling the modulation and feedback signal directly to the SQUID was developed and tested successfully. The SQUID module with superconducting connections to the gradiometer and its superconducting shield was miniaturized to an outer diameter of 5 mm, so that it can be placed near the gradiometer without introducing significant unbalance. Tests have demonstrated that the accuracy of the system with respect to the localization of a single current dipole is better than 2 mm.<> View full abstract»

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  • Whole cortex, 64 channel SQUID biomagnetometer system

    Publication Year: 1993 , Page(s): 1878 - 1882
    Cited by:  Papers (7)  |  Patents (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (649 KB)  

    The authors report on the development and testing of a novel, whole cortex, 64-channel superconducting quantum interference device (SQUID) biomagnetometer system operating in an unshielded environment. The essential features of this instrument, including the cryogenics system, the room-temperature digital electronics, and signal processing capabilities, are described. A noise cancellation scheme is incorporated which allows extraction of biomagnetic signals over the surface of the cortex while operating in the presence of large background magnetic interference. A system description and the results of preliminary magneto-encephalographic measurements are presented. Measurements of noise performance and human auditory evoked response in an unshielded industrial environment have largely validated the noise cancellation methods incorporated in the instrument.<> View full abstract»

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  • Accuracy comparisons of Josephson array systems (voltage standards)

    Publication Year: 1993 , Page(s): 1874 - 1877
    Cited by:  Papers (1)
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    Five Josephson-array voltage standard systems were compared using several different methods. All of the tests were performed on a site at a 1.018-V level, either by direct connection or through successive measurements of independent voltage sources. The resulting agreement between different systems measuring the same source was generally better than 10.0 parts in 10/sup -9/, limited by source noise and detector resolution. Direct array-to-array comparisons for independent systems achieved agreement to within random uncertainties of 0.2 parts in 10/sup -9/. The basic conclusion is that Josephson-array voltage standard systems can be readily transported and tested to assure one-site equivalence. Also, these tests can be done quickly and with high precision, limited by the detector noise if directly compared, or by the transfer reference noise if done indirectly.<> View full abstract»

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  • Josephson voltage standard at low drive frequencies

    Publication Year: 1993 , Page(s): 1870 - 1873
    Cited by:  Papers (7)
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    The authors report on the design and operation of a superconducting microstrip resonator containing four series-connected highly hysteretic Josephson junctions in the top layer. This arrangement generates constant-voltage steps at X-band drive frequencies useful for a Josephson voltage standard. At 12 GHz, steps up to 5 mV have been obtained with step amplitudes of about 100 mu A. The resonator principle can be used to save microwave power and to simplify the microwave circuit. To obtain sufficiently large and stable steps the critical current density and the junction length must be matched carefully with the order of maximum step used for the operation of the Josephson voltage standard. The X-band microwave drive frequencies offer substantial advantages in terms of costs and handling of microwave equipment.<> View full abstract»

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  • Characteristics of the relaxation oscillating SQUID with tunnel junctions

    Publication Year: 1993 , Page(s): 1866 - 1869
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (372 KB)  

    The authors describe a relaxation oscillating superconducting quantum interference device (SQUID) fabricated using the Nb/AlO/sub x//Nb Josephson tunnel junction technique. It is demonstrated that this SQUID transfers flux changes into frequency changes. The relative linewidth of the frequency spectrum is on the order of 10/sup -5/. The spectral density of frequency fluctuations is 2*10/sup 4/ Hz/sup 2//Hz at both 10 kHz and 100 kHz. A method to evaluate the flux resolution of relaxation oscillating SQUIDs without a flux-locked loop is also proposed; the result is S/sub phi /(f)=2.1*10/sup -14/ phi /sub 0//sup 2//Hz at both 10 kHz and 100 kHz. The advantage of the relaxation oscillating SQUID is that it does not suffer from amplifier noise as long as the signal is treated in the frequency domain.<> View full abstract»

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  • Relaxation oscillation SQUIDs with high delta V/ delta Phi

    Publication Year: 1993 , Page(s): 1862 - 1865
    Cited by:  Papers (9)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (407 KB)  

    Relaxation oscillation SQUIDs (ROSs) based on Nb/Al, AlO/sub x//Al/Nb Josephson tunnel junctions have been designed and fabricated. The hysteretic SQUIDs (superconducting quantum interference devices) have a maximum critical current of about 130 mu A and an inductance of 20 pH. A voltage modulation of 40 mu V and a flux to voltage transfer delta V/ delta Phi of 4 MV/ Phi /sub 0/ have been measured in these SQUIDs. Double relaxation oscillation SQUIDs (DROSs), which are based on two hysteretic SQUIDs, showed transfer coefficients up to 77 mV/ Phi /sub 0/. The intrinsic white flux noise of the DROSs is smaller than 3-5 mu Phi /sub 0// square root Hz. It is noted that the results are very promising for a next generation of SQUID systems with simplified read-out.<> View full abstract»

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  • Resonant properties of a DC SQUID coupled to a multiturn input coil

    Publication Year: 1993 , Page(s): 1858 - 1861
    Cited by:  Papers (1)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (406 KB)  

    In order to investigate the characteristics of a DC superconducting quantum interference device (SQUID) coupled to a multiturn input coil, resonances occurring in this configuration, such as input coil resonances and washer resonances, are studied using an expanded model of the coupled SQUID. These resonances are shown to be suppressed well using proper damping resistors. The expressions for the optimum damping resistors have been obtained as a function of SQUID parameters. It is also shown that the optimum values of the damping resistors depend on the configuration of the SQUID washer. Based on the results obtained for the resonant properties, the authors discuss the design of highly sensitive DC SQUIDs coupled to the multiturn input coil.<> View full abstract»

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  • Study of current peaks in DC SQUID with integrated coupling coil

    Publication Year: 1993 , Page(s): 1853 - 1857
    Cited by:  Papers (2)
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    The mechanism and the damping condition are investigated for the resonant peak in the current-voltage characteristics of a DC superconducting quantum interference device (SQUID) coupled to an integrated multiturn input coil. It is experimentally confirmed that the resonant voltage, V/sub r/, depends on parameters of the coupling structure. The V/sub r/ and peak height are calculated based on an RF admittance and a balance equation for incident and dissipated powers in them SQUID. The analytical results agree qualitatively with experimental ones. A simple analytical formula for the damping condition is derived.<> View full abstract»

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  • Nonlinear response of low temperature superconducting thin film gradiometer sense loops

    Publication Year: 1993 , Page(s): 1848 - 1852
    Cited by:  Papers (4)
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    Experimental measurements of the nonlinear response of superconducting thin-film gradiometer sense loops to changes in magnetic field at 4.2 K are described. The sense loops were fabricated from niobium and niobium nitride. The basic experiment involved the ramp of a uniform magnetic field from zero up to a predetermined value. The performance of the loops was measured by means of a superconducting-quantum-interference-device (SQUID)-based readout system inductively coupled to the samples. Threshold values of external field were observed to characterize the onset of nonlinear behavior for all samples measured. The performance of the loops as measured by this onset of nonlinearity is correlated with important parameters including material, aspect ratio, edge quality, and the effect of crossovers. The empirical results are compared with predictions from an ideal diagmagnet model to describe demagnetization effects.<> View full abstract»

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  • Interactive graphics simulation program for the RF-SQUID

    Publication Year: 1993 , Page(s): 1845 - 1847
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    An interactive graphics simulation program for the RF-SQUID (superconducting quantum interference device) has been developed. The program allows the user to graphically observe fluxoid quantization, the amplitude-limiting effect of the tank circuit voltage due to the SQUID coupling, the periodic voltage response due to an external DC flux, the tank circuit and SQUID voltage versus time, and the tank circuit voltage versus current. Having a graphical user interface, the program also provides a means for changing circuit and SQUID parameters, namely, RF drive, DC bias, coupling, tuning, Q, and critical current.<> View full abstract»

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  • Analysis of the 4-junction SQUID

    Publication Year: 1993 , Page(s): 1841 - 1844
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    The circuit of the 4-junction superconducting quantum interference device (SQUID), which has two series of Josephson junctions at each side of the SQUID coil, is analyzed. The 4-junction SQUID is considered to have the advantage of a larger voltage modulation depth than that of the normal DC SQUID. The authors have tried to make its behavior clear by computer simulations. It has been found that the 4-junction SQUID has a different mode from that of the normal DC SQUID in its dynamic behavior, which causes hysteresis in the I-V characteristics. It has also been found that a large voltage modulation depth can be realized in spite of large SQUID coil inductance, so that the area of the primary coil can be more than 10 times larger. The noise performance of the 4-junction SQUID is also analyzed, and it is possible that this SQUID has higher sensitivity by virtue of circuit-parameter optimization.<> View full abstract»

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  • A method for simulating a flux-locked DC SQUID

    Publication Year: 1993 , Page(s): 1837 - 1840
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    The authors describe a computationally efficient and accurate method for simulating a DC superconducting quantum interference device's (SQUID's) V- phi (voltage-flux) and I-V characteristics which has proven valuable in evaluating and improving various SQUID readout methods. The simulation of the SQUID is based on fitting of previously acquired data from either a real or a modeled device using the Fourier transform of the V- Phi curve. This method does not predict SQUID behavior, but rather is a way of replicating a known behavior efficiently with portability into various simulation programs such as SPICE. The authors discuss the methods used to simulate the SQUID and the flux-locking control electronics, and present specific examples of this approach. Results include an estimate of the slew rate and linearity of a simple flux-locked loop using a characterized DC SQUID.<> View full abstract»

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  • High slew rate 'channel equalized' DC SQUID flux-locked loop: concept and simulation

    Publication Year: 1993 , Page(s): 1832 - 1836
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (616 KB)  

    The concept of improving conventional DC superconducting quantum interference device (SQUID) flux-locked loop (FLL) performance by applying a channel equalization circuit after the pre-amp, but prior to the demodulation process in order to compensate for bandwidth limitations imposed by conventional DC SQUID impedance matching networks is discussed. The equalization circuit is a bandlimited inverse filter which corrects for the phase and amplitude distortion caused primarily by the DC SQUID impedance matching network. Improvements in the FLL performance were verified with analog circuit simulations in both the time and frequency domains. Using an analog circuit simulator the various subcircuits of the FLL were modeled, and a comparison between a conventional FLL and an equalized one was performed. Computer simulations for the open and closed loop cases were used to quantify the increase in slew rate for the equalized FLL system.<> View full abstract»

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  • Performance of DC SQUIDs fabricated on 4-inch silicon wafer

    Publication Year: 1993 , Page(s): 1828 - 1831
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (788 KB)  

    A Nb/AlO/sub x//Nb Josephson process has been developed to fabricate DC superconducting quantum interference devices (SQUIDs) for biomagnetic measurements on 4-in silicon wafers. The purpose of the process is to fabricate high-quality junctions and to keep the quality uniform over a silicon wafer. All the films were deposited using magnetron sputtering. The components of the SQUID, except the junctions, were formed by taper etching to improve the step coverage. The uniformity of the SQUID characteristics (critical current, modulation depth, and mutual inductance between SQUID and modulation coil) was measured, and an excellent value of less than +or-5% was obtained. Three different configurations of SQUIDs, single washer (SW), double washer series (DWS), double washer parallel (DWP) have been fabricated with the same characteristics. The SW (single washer) had a white noise level of 6.3*10/sup -6/ Phi /sub 0// square root Hz, lower than those of DWS and DWP. 1/f crossover frequencies were below 1 Hz.<> View full abstract»

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  • Magnetic field sensors with digital feedback read-out

    Publication Year: 1993 , Page(s): 1824 - 1827
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (499 KB)  

    Magnetic field sensors consisting of a DC superconducting quantum interference device (SQUID) and a close-by superconducting digitizer, both realized in NbN-MgO-NbN technology, are presented. The superconducting circuits have been specifically designed to suppress the influence of external fields on the operation of the sensor and to assure good balancing for gradiometer input coils. It is shown how the different parts of the sensor can be adapted for different modes of operation. The superconducting sensors are operated with digital feedback electronics mounted in the dewar. The operation of digital circuitry in the vicinity of the sensors affects their operation slightly. Most functions, particularly the adjustment and optimization of bias points, can be controlled from a remotely positioned data acquisition system. In view of multisensor applications several precautions have been taken to decouple the data acquisition from the electronics units.<> View full abstract»

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  • Improved sensitivity of planar microwave based RF-SQUIDs using a cryogenic HEMT preamplifier

    Publication Year: 1993 , Page(s): 1820 - 1823
    Cited by:  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (418 KB)  

    The design and performance of a planar microwave RF-SQUID (superconducting quantum interference device) read out by a cryogenic high-electron-mobility-transistor (HEMT) preamplifier are described. The SQUID sensor consists of a planar half-wavelength microstrip resonator, into which the SQUID loop is integrated. The SQUID is operated at a pump frequency of 1.7 GHz. The cryogenic preamplifier uses a microwave HEMT, located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 K and 1.5 K. In an open loop, a flux noise of 2*10/sup -6/ Phi /sub 0// square root Hz was measured at 4.2 K, resulting in an energy resolution of about 100 h. The design of a 3-GHz magnetometer is also described, and the preliminary results of the measurements are reported.<> View full abstract»

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  • A coupled DC SQUID with low 1/f noise

    Publication Year: 1993 , Page(s): 1816 - 1819
    Cited by:  Papers (5)
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    A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.<> View full abstract»

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  • Even harmonic Josephson magnetometry

    Publication Year: 1993 , Page(s): 1812 - 1815
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    A novel Josephson junction magnetometry scheme is proposed in which magnetic flux is applied to Josephson junctions with a modulation flux +or-z( Phi /sub 0//2 pi ) cos omega t. Two Josephson junctions are used in a push-pull mode to perform amplification through two mechanisms: negative inductance amplification of DC (low frequency) components and parametric amplification of the second harmonic. The second harmonic component of the Josephson currents, which is directly proportional to the strength of the input flux, is then selected and amplified by a phase-sensitive detector. Computer simulations show that the device behaves very well as a magnetic flux sensor with a DC offset free feature. The device's parameters and noise problems are also discussed. A preliminary analysis indicates that it is likely to have a better sensitivity than superconducting quantum interference devices.<> View full abstract»

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  • Noise measurements of series SQUID arrays

    Publication Year: 1993 , Page(s): 1808 - 1811
    Cited by:  Papers (12)  |  Patents (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (410 KB)  

    Series arrays of 50 gradiometer superconducting quantum interference devices (SQUIDS) and 100 magnetometer SQUIDs have been fabricated using Nb/AlO/sub x//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- mu m/sup 2/ and 0.5- mu m/sup 2/ 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 Omega . The white noise was measured directly with an ultra low noise preamplifier at room temperature. The best result was Phi /sub N/=0.12 mu Phi /sub 0// square root Hz with a corresponding coupled energy sensitivity of 56 h.<> View full abstract»

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  • Detection of magnetic flux with superconducting quantum interference gratings

    Publication Year: 1993 , Page(s): 1804 - 1807
    Cited by:  Papers (3)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (369 KB)  

    The authors have carried out finite-inductance calculations of the critical current vs. flux (I/sub c/- Phi ) and voltage vs. flux (V- Phi ) characteristics of superconducting interferometers with many Josephson junctions in parallel. At least two features of the calculations suggest that many junction interferometers, called superconducting quantum interference gratings, might be advantageous for the detection of magnetic flux. First, the voltage noise can be reduced significantly for a given flux-to-voltage transfer coefficient, a feature which is likely to improve the magnetic flux sensitivity of both low- and high-T/sub c/ superconducting devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction grating such as enhancement and narrowing of the peaks in the I/sub c/- Phi characteristic. Specific schemes for efficiently coupling flux into the device are proposed.<> View full abstract»

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  • A high performance integrated DC SQUID magnetometer

    Publication Year: 1993 , Page(s): 1800 - 1803
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (603 KB)  

    The authors have carried out extensive mappings of the DC characteristics and flux noise of an integrated, DC superconducting quantum interference device (SQUID) magnetometer as functions of bias current and applied flux. The open-loop white flux noise at 1 kHz measured without flux modulation is Phi /sub n/<7*10/sup -7/ Phi /sub 0// square root Hz throughout a broad region of operation where the current-voltage and voltage-flux characteristics are smooth. The corresponding flux density noise B/sub n/<4 fT/ square root Hz. In addition, a recent model of the coupled DC SQUID was used to calculate the frequency-dependent impedance Z(f) seen by the Josephson junctions. The peaks observed in Z(f) are shown to be consistent with features in the current-voltage characteristics at high bias currents.<> View full abstract»

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  • Octagonal washer DC SQUIDs and integrated susceptometers fabricated in a planarized sub- mu m Nb-AlO/sub x/-Nb technology

    Publication Year: 1993 , Page(s): 1795 - 1799
    Cited by:  Papers (3)  |  Patents (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (614 KB)  

    The authors have designed, fabricated, and measured a number of Nb-AlO/sub x/-Nb octagonal washer DC superconducting quantum interference devices (SQUIDs) and miniature DC SQUID susceptometers having minimum feature size down to 0.5 mu m. With SQUID inductance values on the order of 100 pH, typical noise performance is better than 1 mu Phi /sub 0// square root 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 approximately mu H input circuits. A single-washer SQUID with an 80-turn 0.5- mu m-linewidth, 630-nH input coil has user-friendly V- Phi (voltage-flux) curves and a coupled energy sensitivity of 20 h at 4.2 K. Susceptometers with pickup loops ranging from 20- mu m to 0.8- mu m across have very user-friendly V- Phi curves and a resolution of order 100 mu /sub B// square root Hz at 4.2 K for devices with the smallest loops.<> View full abstract»

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Syracuse University
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