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I was a graduate student in the Royal Society Mond Laboratory, University of Cambridge, from 1964 to 1967. I describe how my listening to a seminar by Brian Josephson followed by a crucial suggestion by our thesis advisor, Brian Pippard, the very next day launched my career for the next one-half century.

In 1985, experiments revealed the quantum behaviour of a macroscopic degree of freedom: the phase difference across a Josephson junction. The authors recount the history of this milestone for the development of superconducting quantum circuits.

We examine the behavior of low frequency flux noise measured in ten Nb-NbOx-Pbln dc SQUIDs in the temperature range 1 K to 20 mK. As previously reported, the flux noise power spectrum SΦ(f) typically scales with frequency as A/fα. Remark ably, the excess noise power A at 1 Hz increases as the temperature is decreased below about 0.7 K, saturating to a value around (10 μΦα)2/Hz below about 0.2 K. Here we report on the dependence of both the magnitude A and the slope a on the size and linewidth of the SQUID loops. In particular, at the lowest temperatures we find that a declines to values as low as 0.5 in the smallest devices, while a is in the range 0.9-1 for the largest devices.

Following Brian Josephson's prediction in 1962, Anderson and Rowell observed Josephson tunneling in 1963. The following year, Jaklevic, Lambe, Silver and Mercereau demonstrated quantum interference in a superconducting ring containing two Josephson tunnel junctions. Subsequently, the first practical devices emerged, including the point-contact dc and rf SQUIDs (Superconducting QUantum Interference Devices) of Zimmerman and Silver and Clarke's SLUG (Superconducting Low-inductance Undulatory Galvanometer) — a blob of solder frozen around a length of niobium wire. The return to the tunnel junction as the Josephson element was heralded by the cylindrical SQUID in 1976. The square washer dc SQUID developed by Ketchen and Jaycox in 1982 remains the workhorse design for most applications. Theories for the dc and rf SQUIDs were worked out in the 1970s. Today, SQUIDs (mostly dc) are used in a variety of configurations — for example, as magnetometers, gradiometers, cryogenic current comporators, low-frequency and microwave amplifiers, and susceptometers — in applications including magnetoencephalography, magnetocardiography, geophysics, nondestructive evaluation, precision gyroscopes, standards, cosmology, nuclear magnetic resonance, reading out superconducting quantum bits, and a myriad of one-of-a-kind experiments in basic science. Experiments are described to search for galaxy clusters, hunt for the axion, and perform magnetic resonance imaging in microtesla fields.

MRI is a powerful technique for clinical diagnosis and materials characterization. Images are acquired in a homogeneous static magnetic field much higher than the fields generated across the field of view by the spatially encoding field gradients. Without such a high field, the concomitant components of the field gradient dictated by Maxwell's equations lead to severe distortions that make imaging impossible with conventional MRI encoding. In this paper, we present a distortion-free image of a phantom acquired with a fundamentally different methodology in which the applied static field approaches zero. Our technique involves encoding with pulses of uniform and gradient field, and acquiring the magnetic field signals with a SQUID. The method can be extended to weak ambient fields, potentially enabling imaging in the Earth's field without cancellation coils or shielding. Other potential applications include quantum information processing and fundamental studies of long-range ferromagnetic interactions.

Very large scale integration of Josephson junctions in a two-dimensional series-parallel array has been achieved by ion irradiating a YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} film through slits in a nano-fabricated mask created with electron beam lithography and reactive ion etching. The mask consisted of 15,820 high-aspect ratio (20:1), 35-nm wide slits that restricted the irradiation in the film below to form Josephson junctions. Characterizing each parallel segment k, containing 28 junctions, with a single critical current I{sub ck} we found a standard deviation in I{sub ck} of about 16%.

We investigate rf SQUIDs (Superconducting QUantum Interference Devices), coupled to a resonant input circuit, a readout tank circuit and a preamplifier, by numerically solving the corresponding Langevin equations and optimizing model parameters with respect to noise temperature. We also give approximate analytic solutions for the noise temperature, which we reduce to parameters of the SQUID and the tank circuit in the absence of the input circuit. The analytic solutions agree with numerical simulations of the full circuit to within 10%, and are similar to expressions used to calculate the noise temperature of dc SQUIDs. The best device performance is obtained when \beta_L'\equiv 2\pi L I_0\Phi_0 is 0.6 - 0.8; L is the SQUID inductance, I_0 the junction critical current and \Phi_0 the flux quantum. For a tuned input circuit we find an optimal noise temperature T_{N,opt}\approx 3Tf/f_c, where T, f and f_c denote temperature, signal frequency and junction characteristic frequency, respectively. This value is only a factor of 2 larger than the optimal noise temperatures obtained by approximate analytic theories carried out previously in the limit \beta_L'<, Comment: submitted to J. Low Temp. Phys

In magnetic resonance imaging (MRI) performed at fields of 1 T and above, the presence of a metal insert can distort the image because of susceptibility differences within the sample and modification of the radiofrequency fields by screening currents. Furthermore, it is not feasible to perform nuclear magnetic resonance (NMR) spectroscopy or acquire a magnetic resonance image if the sample is enclosed in a metal container. Both problems can be overcome by substantially lowering the NMR frequency. Using a microtesla imaging system operating at 2.8 kHz, with a superconducting quantum interference device (SQUID) as the signal detector, we have obtained distortion-free images of a phantom containing a titanium bar and three-dimensional images of an object enclosed in an aluminum can; in both cases high-field images are inaccessible.

Magnetic resonance imaging (MRI) suffers from artifacts caused by concomitant gradients when the product of the magnetic field gradient and the dimension of the sample becomes comparable to the static magnetic field. To investigate and correct for these artifacts at very low magnetic fields, we have acquired MR images of a 165-mm phantom in a 66-μT field using gradients up to 350 μT/m. We prepolarize the protons in a field of about 100 mT, apply a spin-echo pulse sequence, and detect the precessing spins using a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). Distortion and blurring are readily apparent at the edges of the images; by comparing the experimental images to computer simulations, we show that concomitant gradients cause these artifacts. We develop a non-perturbative, post-acquisition phase correction algorithm that eliminates the effects of concomitant gradients in both the simulated and the experimental images. This algorithm assumes that the switching time of the phase-encoding gradient is long compared to the spin precession period. In a second technique, we demonstrate that raising the precession field during phase encoding can also eliminate blurring caused by concomitant phase-encoding gradients; this technique enables one to correct concomitant gradient artifacts even when the detector has a restricted bandwidth that sets an upper limit on the precession frequency. In particular, the combination of phase correction and precession field cycling should allow one to add MRI capabilities to existing 300-channel SQUID systems used to detect neuronal currents in the brain because frequency encoding could be performed within the 1–2 kHz bandwidth of the readout system.

We describe our frequency-domain readout multiplexer for transition-edge sensor (TES) bolometers and present measurements of an eight-channel multiplexer. Each sensor is biased with a sinusoidal bias at a distinct frequency. As the sensor absorbs power, it amplitude-modulates its sinusoidal bias. Sensor currents are summed and measured with a single superconducting quantum interference device (SQUID) array. The SQUID array consists of 100 dc-SQUID's in series and is operated with shunt feedback electronics which have a slew rate of 1.210/sup 7/ /spl Phi//sub 0//s. A tuned filter consisting of an inductor and capacitor are placed in series with each sensor to both limit the bandwidth of the Nyquist noise from each sensor and to allow us to bias all multiplexed sensors with a common wire. We place an upper limit on crosstalk between adjacent channels of 0.004, well below our design requirements. Demodulated noise spectra from multiplexed sensors show the expected white noise levels at frequencies above 200 mHz.

We use a low transition temperature (T/sub c/) Super-conducting Quantum Interference Device (SQUID) to perform in vivo magnetic resonance imaging (MRI) at magnetic fields around 100 microtesla, corresponding to proton Larmor frequencies of about 5 kHz. In such low fields, broadening of the nuclear magnetic resonance lines due to inhomogeneous magnetic fields and susceptibility variations of the sample are minimized, enabling us to obtain high quality images. To reduce environmental noise the signal is detected by a second-order gradiometer, coupled to the SQUID, and the experiment is surrounded by a 3-mm thick Al shield. To increase the signal-to-noise ratio (SNR), we prepolarize the samples in a field up to 100 mT. Three-dimensional images are acquired in less than 6 minutes with a standard spin-echo phase-encoding sequence. Using encoding gradients of /spl sim/100 /spl mu/T/m we obtain three-dimensional images of bell peppers with a resolution of 2/spl times/2/spl times/8 mm/sup 3/. Our system is ideally suited to acquiring images of small, peripheral parts of the human body such as hands and arms. In vivo images of an arm, acquired at 132 /spl mu/T, show 24-mm sections of the forearm with a resolution of 3/spl times/3 mm/sup 2/ and a SNR of 10. We discuss possible applications of MRI at these low magnetic fields.

Superconducting quantum interference devices (SQUIDs) are sensitive detectors of magnetic flux. A SQUID consists of a superconducting loop interrupted by either one or two Josephson junctions for the RF or dc SQUID, respectively. Low transition temperature (T/sub c/) SQUIDs are fabricated from thin films of niobium. Immersed in liquid helium at 4.2 K, their flux noise is typically 10/sup -6//spl Phi//sub 0/ Hz/sup -1/2/, where /spl Phi//sub 0//spl equiv/h/2e is the flux quantum. High-T/sub c/ SQUIDs are fabricated from thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/, and are generally operated in liquid nitrogen at 77 K. Inductively coupled to an appropriate input circuit, SQUIDs measure a variety of physical quantities, including magnetic field, magnetic field gradient, voltage, and magnetic susceptibility. Systems are available for detecting magnetic signals from the brain, measuring the magnetic susceptibility of materials and geophysical core samples, magnetocardiography and nondestructive evaluation. SQUID "microscopes" detect magnetic nanoparticles attached to pathogens in an immunoassay technique and locate faults in semiconductor packages. A SQUID amplifier with an integrated resonant microstrip is within a factor of two of the quantum limit at 0.5 GHz and will be used in a search for axions. High-resolution magnetic resonance images are obtained at frequencies of a few kilohertz with a SQUID-based detector.

We describe studies of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) of liquid samples at room temperature in microtesla magnetic fields. The nuclear spins are prepolarized in a strong transient field. The magnetic signals generated by the precessing spins, which range in frequency from tens of Hz to several kHz, are detected by a low-transition temperature dc SQUID (Superconducting QUantum Interference Device) coupled to an untuned, superconducting flux transformer configured as an axial gradiometer. The combination of prepolarization and frequency-independent detector sensitivity results in a high signal-to-noise ratio and high spectral resolution (∼1 Hz) even in grossly inhomogeneous magnetic fields. In the NMR experiments, the high spectral resolution enables us to detect the 10-Hz splitting of the spectrum of protons due to their scalar coupling to a 31P nucleus. Furthermore, the broadband detection scheme combined with a non-resonant field-reversal spin echo allows the simultaneous observation of signals from protons and 31P nuclei, even though their NMR resonance frequencies differ by a factor of 2.5. We extend our methodology to MRI in microtesla fields, where the high spectral resolution translates into high spatial resolution. We demonstrate two-dimensional images of a mineral oil phantom and slices of peppers, with a spatial resolution of about 1 mm. We also image an intact pepper using slice selection, again with 1-mm resolution. In further experiments we demonstrate T1-contrast imaging of a water phantom, some parts of which were doped with a paramagnetic salt to reduce the longitudinal relaxation time T1. Possible applications of this MRI technique include screening for tumors and integration with existing multichannel SQUID systems for brain imaging.

A single vortex of flux, formed inside a superconducting Josephson junction, has been detected undergoing quantum tunnelling — a feature that could be developed into a quantum bit.

We describe the development of a frequency-domain multiplexer (MUX) to read out arrays of superconducting transition-edge sensors (TES). Fabrication of large-format arrays of these sensors is becoming practical; however, reading out each sensor in the array is a major instrumental challenge. Frequency-domain multiplexing can greatly simplify the instrumentation of large arrays by reducing the number of SQUID's (superconducting quantum interference devices) and wires to the low temperature stages. Each sensor is AC biased at a different frequency, ranging from 380 kHz to 1 MHz. Each sensor signal amplitude-modulates its respective AC bias frequency. An LC filter associated with each sensor suppresses Johnson noise from the other sensors. The signals are combined at a current summing node and measured by a single SQUID. The individual signals from each sensor are then lock-in detected by room temperature electronics. Test chips with fully lithographed LC filters for up to 32 channels have been designed and fabricated. The capacitance and inductance values have been measured and are close to the design goals. We discuss the basic principles of frequency-domain multiplexing, the design and testing of the test chips, and the implementation of a practical system.

Cryogenic sensors composed of transition-biased superconducting films have demonstrated remarkable sensitivity at /spl gamma/-ray, x-ray, optical, and far-infrared to millimeter wavelengths. However, for these sensors to find widespread application in astronomy and materials analysis, technologies for building and reading out large arrays are required. We are currently developing a frequency-domain multiplexing scheme for the read-out of large numbers of microcalorimeters using a much smaller number of amplifiers. In this scheme, each sensor is biased at an identifying frequency and operated in a series LC circuit to suppress out-of-band noise. Here, we present results demonstrating the undegraded operation of two /spl gamma/-ray sensors multiplexed using this technique. In addition, we provide a series of design rules which relate the minimum bias frequency and the values of the reactive elements in the system to a small number of sensor properties. Finally, we discuss the ultimate limits on the number of sensors that can be measured with a single amplifier.

We study the dynamics of a two-dimensional ensemble of randomly distributed classical Heisenberg spins with isotropic RKKY and weaker anisotropic dipole-dipole couplings. Such ensembles may give rise to the flux noise observed in SQUIDs with a $1/{f}^{\ensuremath{\alpha}}$ power spectrum with $\ensuremath{\alpha}\ensuremath{\lesssim}1$. We solve numerically the Landau-Lifshitz-Gilbert equations of motion in the dissipationless limit. We find that Ising-type fluctuators, which arise from spin clustering close to a spin-glass critical behavior with ${T}_{c}=0$, give rise to $1/{f}^{\ensuremath{\alpha}}$ noise. Even weak anisotropic interactions lead to a crossover from the Heisenberg-type criticality to the much stronger Ising-type criticality. The temperature-dependent exponent $\ensuremath{\alpha}(T)\ensuremath{\lesssim}1$ increases and approaches unity when the temperature is lowered. This mechanism acts in parallel to the spin diffusion mechanism. Whereas the latter is sensitive to the device geometry, the spin-clustering mechanism is largely geometry independent.

We demonstrate that a voltage-biased superconducting bolometer (VSB), read out with superconducting quantum interference device (SQUID), can be biased with a sinusoidal voltage. We show that the load curves taken with AC- and DC-bias are nearly identical indicating that there is no identifiable degradation in the performance of the bolometer due to the AC bias. We discuss the use of a frequency-domain SQUID multiplexer with an array of VSB's that are AC-biased.

Scanning SQUID microscopy is shown to be capable of detecting deformation gradients when accompanied by significant changes in hardness. Deformation which does not increase the dislocation density (as measured by the increase in hardness) has little effect. The effect can be produced by different techniques including cold rolling and tensile deformation. The effect is shown to occur in at least two different alloys with very different microstructures indicating the possibility of wide applicability.

We describe a high transition temperature superconducting, first-order gradiometer intended for biomagnetic measurements in an unshielded environment. The gradiometer involves a single-layer, planar flux transformer with two loops of unequal size, the smaller of which is inductively coupled to the pickup loop of a directly coupled magnetometer. In this configuration, the presence of the flux transformer reduces the sensitivity of the magnetometer by only about 5%. The flux transformer is patterned in a thin film of YBa2Cu3O7−δ deposited on a 100 mm diam wafer, and has a baseline of 48 mm. The flux transformer and magnetometer substrates are permanently bonded together in a flipchip arrangement. The common mode rejection of uniform magnetic field fluctuations in any direction is better than 1 part per 100. The outputs of two such gradiometers are subtracted to form a second-order gradiometer, which rejects first-order gradient fluctuations to about 1 part in 100. With the aid of three orthogonally mounted ...

The remanent magnetization of ferromagnetic steel with five different microstructures has been studied using a microscope based on a high-transition-temperature d.c. Superconducting QUantum Interference Device (SQUID). The samples were prepared using different heat treatments, beginning with the same material composition. Each sample was magnetized in turn in a magnetic field of 50 mT, and the remanent magnetization was studied by scanning the sample over the SQUID using a two-dimensional translation stage with a range of 50×50 mm 2 . With a sample-to-SQUID separation of 400–500 μm, this procedure yielded a two-dimensional magnetic field image produced by the local remanent magnetization of the sample. In addition, microstructural information has been obtained using optical imaging, allowing the magnetic images to be correlated with the underlying microstructure.

We describe a new technique to control in situ charging energy of systems of coupled metallic or superconducting islands. To illustrate the technique, we have fabricated two-dimensional arrays of Al islands on GaAs/AlAs heterostructures. Each island is coupled to its nearest-neighbor by a submicron Al/AlO x /Al tunnel junction and to the three-dimensional electron gas (3DEG) located below the surface of the heterostructure by a capacitance C g . We vary C g , which dominates the charging energy of the array, by depleting the electrons in the 3DEG by means of a negative voltage applied to the array. With the array driven normal by a magnetic field, a decrease in C g increases in both the offset voltage and the period of the Coulomb blockade oscillations.

We fabricated superconducting single-electron transistors (SETs) on a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) located 1000 A below the surface. The superconducting island is coupled to the leads by submicron tunnel junctions with a tunnel junction capacitance of 0.3 fF, and to the 2DEG by a gate capacitance of 1.5 fF. The tunnel resistances of the junctions are comparable to the quantum resistance and therefore the Coulomb blockade is suppressed by the strong Josephson coupling in these devices. In a device with a tunnel resistance of 9 kΩ , we observe large Coulomb blockade-like oscillations as a function of gate voltage with a peak-to-peak amplitude of 18e2/h. The dissipation of the SET is varied by changing the resistance of the 2DEG. Temperature and dissipation dependence of these large Coulomb blockade-like oscillations is explained by co-tunneling of the Cooper pairs across the SET.

We have studied the low-frequency noise properties of high-T/sub c/ directly coupled magnetometers involving dc Superconducting Quantum Interference Devices (SQUIDs) connected to four different types of pickup loop. Two of the pickup loops have a solid-washer design, while the other two comprise structures with narrow (4 /spl mu/m) linewidths. The devices were fabricated from YBCO thin films laser deposited on bicrystal substrate. The noise properties of the magnetometers cooled and maintained in static magnetic fields were measured at 77 K. The pickup loop contributes to the 1/f noise of field-cooled devices solely through vortex motion at tapered outer edges; this noise was eliminated by repatterning the edges to make them sharp. We demonstrate that solid-film washers can be replaced with narrow-linewidth structures without any loss in effective area.

We have constructed a high-T/sub c/ SQUID spectrometer to detect NMR signals from samples at room temperature in magnetic fields up to 3 mT. The multiloop SQUID magnetometer has a system noise of about 30 fT/Hz/sup 1/2/ at the relevant frequencies of 2 to 100 kHz. The magnetometer is operated in vacuum at 77 K, and is separated from the sample, which Is less than 1.5 mm away, by a sapphire window. In a magnetic field of 2 mT we can detect the proton spin echo at 86 kHz without signal averaging. This sensitivity enables us to obtain one-dimensional images. In addition, we present data on hyperpolarized /sup 129/Xe, which has an optically pumped polarization of several percent.

The recently developed “microscope” based on a high-Tc dc SQUID (superconducting quantum interference device) is used to detect the magnetic fields produced by the motion of magnetotactic bacteria, which have permanent dipole moments. The bacteria, in growth medium at room temperature, can be brought to within 15μm of a SQUID at liquid nitrogen temperature. Measurements are performed on both motile and nonmotile bacteria. In the nonmotile case, we obtain the power spectrum of the magnetic field noise produced by the rotational Brownian motion of the ensemble of bacteria. Furthermore, we measure the time-dependent field produced by the ensemble in response to an applied uniform magnetic field. In the motile case, we obtain the magnetic field power spectra produced by the swimming bacteria. Combined, these measurements determine the average rotational drag coefficient, magnetic moment, and the frequency and amplitude of the vibrational and rotational modes of the bacteria in a unified set of measurements. In addition, the microscope can easily resolve the motion of a single bacterium. This technique can be extended to any cell to which a magnetic tag can be attached.

We have studied the effects of heat treatment and mechanical stress on the remanent magnetization of ferromagnetic steels using a high-transition-temperature (HTC) Superconducting QUantum Interference Device (SQUID) microscope. Samples were prepared by different heat treatments, which produced varied microstructures, and different rolling treatments, which produced varied levels of deformation. The samples were subsequently magnetized in fields of 50 mT, and the remanent magnetization was measured by rastering the sample over the SQUID using a two-dimensional (2D) translation stage with a scanning range of 50 mm/spl times/50 mm. With a separation between the SQUID and sample of approximately 0.5 mm, this produced a 2D magnetic field image due to the local remanent magnetization of the sample. In addition, microstructural information was determined using optical imaging, allowing us to correlate the effects of heat treatment and mechanical stress on local remanent magnetisation with detailed microstructural information. Since the strength and integrity of steels can be well predicted from microstructural information, correlation of this information with the 2D remanent magnetization images could lead to an effective method for the non-destructive evaluation of ferromagnetic steels through a simple measurement of remanent magnetization.

We have investigated the transport properties of two-dimensional arrays of normal islands interconnected by tunnel junctions fabricated close to a conducting ground plane. For bias voltages $V$ below a threshold voltage ${V}_{\mathrm{th}},$ the current $I$ is thermally activated with an activation energy proportional to ${V}_{\mathrm{th}}\ensuremath{-}V.$ For $Vg{V}_{\mathrm{th}},$ $I$ scales as $(V\ensuremath{-}{V}_{\mathrm{th}}{)}^{\ensuremath{\zeta}},$ where $\ensuremath{\zeta}=2.01\ifmmode\pm\else\textpm\fi{}0.04$ and $1.58\ifmmode\pm\else\textpm\fi{}0.04$ for two samples. For constant $V,$ $I$ exhibits Coulomb-blockade oscillations as a function of the voltage of the ground plane. The amplitude of these oscillations scales as $(V\ensuremath{-}{V}_{\mathrm{th}}{)}^{\ensuremath{\chi}},$ where $\ensuremath{\chi}=0.68\ifmmode\pm\else\textpm\fi{}0.18$ and $0.31\ifmmode\pm\else\textpm\fi{}0.10$ for the same samples, respectively. These values are consistent with our theoretical prediction $\ensuremath{\chi}=(\ensuremath{\zeta}\ensuremath{-}1)/2.$

We report a new class of $c$-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a $\mathrm{YBa}{}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) crystal. We measure the critical current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed $d$- and $s$-wave pairing in YBCO, with a reversal in the sign of the $s$-wave component across the twin boundary.

A high-T/sub c/ SQUID microscope has been constructed for imaging samples at room temperature and atmospheric pressure. The SQUID is mounted on the upper end of a sapphire rod, cooled to liquid nitrogen temperature, below a 3-/spl mu/m thick silicon nitride vacuum window. This arrangement yields a SQUID-to-sample separation as low as 15 /spl mu/m. A sample can be placed on the window or scanned over the SQUID to produce magnetic images.

We have fabricated a two-dimensional array of Josephson junctions within 100nm of a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The screening provided by the 2DEG causes the array to show superconducting behavior despite a large junction resistance. Varying the resistance per square of the 2DEG changes the dissipation in the electrodynamic environment of the array independently of any other parameters in the system. As the resistance increases, the current-voltage characteristics of the array change from superconducting to insulating in character. {copyright} {ital 1997} {ital The American Physical Society}

Advanced composites of glass, carbon, or aramid fibers combined with suitable resins have been used for a number of years in the aerospace and automotive industries. A number of projects are currently studying possible applications in the construction and civil engineering industries. The advantages and disadvantages of these new materials in the context of the requirements of the construction industry are reviewed, and ways in which they could be introduced are considered. The construction industry is highly conservative, the complete opposite of the aerospace and automotive industries, which are continually seeking to improve their products. It is also a relatively low-tech industry, in which much of the actual construction work is carried out by relatively unskilled labor. On the other hand, it is a high-volume industry that consumes materials in tonnages unknown elsewhere. Finally, most structures are singular hence may be thought of as prototypes. These, and other aspects of the problem of in...

We have used a dc superconducting quantum interference device to measure the spectral density of magnetic flux noise, {ital S}{sub {Phi}}({ital f}), generated by vortex motion in crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) both before and after irradiation with 3.1-MeV protons and 1-GeV Au ions. In addition, we have studied the effects of irradiation on the critical current {ital J}{sub {ital c}} of the same samples. Both types of irradiation reduced {ital S}{sub {Phi}}({ital f}) at most temperatures and magnetic fields and increased {ital J}{sub {ital c}} at all temperatures and magnetic fields. By measuring {ital S}{sub {Phi}}({ital f}) versus temperature, we extract the distribution of vortex pinning energies, {ital D}({ital U}{sub 0}). Both crystals have peaks in {ital D}({ital U}{sub 0}) near 0.1 eV and 0.2 eV before irradiation, and a much reduced peak near 0.1 eV after irradiation. The noise level after either kind of irradiation was substantially higher than in an unirradiated YBCO film. {copyright} {ital 1996 The American Physical Society.}

A microscope has been constructed in which a high‐transition temperature dc superconducting quantum interference device (SQUID) in vacuum is brought to within 140 μm of samples at room temperature and pressure. The SQUID is mounted on the upper end of a sapphire rod, cooled to liquid nitrogen temperature, below a 75‐μm thick sapphire window. Samples can either be placed directly on the window or scanned over the SQUID to produce magnetic images. A square‐washer SQUID with inner and outer dimensions of 30 and 50 μm can resolve 130 μm magnetic features. The instrument is intended primarily for biological applications.

In heavily twinned crystals or films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO), ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing symmetry is expected to cause the cancellation of first-order Josephson tunneling through a YBCO-Pb tunnel junction grown on an a-b face; any residual tunneling is thus second order. As a result, microwaves at frequency f are predicted to induce steps on the current-voltage characteristic at voltages that are multiples of $\frac{1}{2}(hf/2e)$. Experimentally, steps are observed only at multiples of $hf/2e$, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.

We review the progress towards an understanding of the electrodynamics in the vortex-state which arises in response to a magnetic field. B. Experimentally, the goal is to characterize the conductivity of cuprate superconductors in the presence of an applied magnetic field over a broad frequency range from microwave to infrared wavelengths. The theoretical challenge is to understand these measurements with microscopic, as well as phenomenological, models. In describing the experiments to date we will focus on a relatively new technique, coherent terahertz spectroscopy, which is uniquely capable of measuring the complete complex conductivity tensor.

We consider the effect of gap nodes on the electrodynamic response of superconductors in the vortex state. We show that for a gap with line nodes it is not possible to ignore the effect of field-induced pairbreaking on the electrodynamic response. We describe measurements of both the diagonal and the Hall components of the resistivity which support this conclusion.

We have used a liquid-nitrogen-cooled scanning SQUID microscope to obtain magnetic images at frequencies ranging from zero to 200 GHz, that is, from audio frequencies through the radio frequency band and into the millimeter microwave band. We present images of thin film and bulk metallic samples, with a spatial resolution of about 30 /spl mu/m, taken using various readout schemes we have developed. >

We have fabricated three types of high T/sub c/ magnetometers using a YBa/sub 2/Cu/sub 3/O/sub 7-x/-SrTiO/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7-x/, multilayer process: multiturn flux transformers coupled to single layer dc SQUIDs in a flip-chip arrangement, multiturn flux transformers integrated with dc SQUIDs, and multiloop magnetometers (fractional turn SQUIDs). The magnetic field noise of the best flip-chip magnetometer was 74 fTHz/sup -1/2/ at 1 Hz and 31 fTHz/sup -1/2/ at 1 kHz. The magnetic field noise of the multiloop magnetometer, measured in a YBCO tube, was 37 fTHz/sup -1/2/ at 1 Hz and 18 fTHz/sup -1/2/ at 1 kHz. >

Phase-sensitive spectroscopy is used to characterize fully the complex resistivity tensor of YBa{sub 2}Cu{sub 3}O{sub {ital x}} films in the mixed state, in the frequency range 100 to 500 GHz and temperature range 10 K to {ital T}{sub {ital c}}. We compare our results with the predictions of a vortex model containing three parameters: viscosity, pinning, and Magnus force. The viscosity and Magnus parameter we measure are in disagreement with theoretically predicted values. We show that the discrepancies can be explained by incorporating the effects of a highly anisotropic or {ital d}-wave gap.

The magnetic flux noise generated by films and crystals of B${\mathrm{i}}_{2}$S${\mathrm{r}}_{2}$CaC${\mathrm{u}}_{2}$ ${\mathrm{O}}_{8+y}$ and YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$ ${\mathrm{O}}_{7\ensuremath{-}x}$, up to 30 \ensuremath{\mu}m thick and cooled in nominally zero magnetic field, has been measured at opposing surfaces by two dc superconducting quantum interference devices. For both materials, the noise sources at the two surfaces were highly correlated at specific temperatures in a given cooldown. This result suggests that the observed vortices moved as rigid rods. At other temperatures, the noise was mostly uncorrelated, suggesting that the relevant vortices were pinned at more than one point along their length.

A study was carried out to observe a heat-affected zone (HAZ) in a deformed Fe-Cr-Ni system containing α′ martensite using high-Tc superconducting quantum interference device (SQUID) microscopy. Microstructure and remanent magnetization images were studied by an optical microscope and a SQUID microscope, respectively. The HAZ, in which only the face-centered-cubic austenite phase exists, could be visualized by the SQUID microscope. It was also found that the SQUID images were consistent with the results from the microstructural analysis. It was concluded that a simple SQUID measurement may serve as an effective method for a nondestructive evaluation of ferromagnetic steel phases by correlating remanent magnetization images to microstructural characteristics.

A series of five dc superconducting quantum interference devices (SQUIDs) have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12±1 to 6±1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2–4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor–insulator–superconductor and hot-electron bolometer mixers.

The spectral density S(Φ)(f) = A(2)/(f/1 Hz)(α) of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically varied geometries shows that α increases as the temperature is lowered; in so doing, each spectrum pivots about a nearly constant frequency. The mean-square flux noise, inferred by integrating the power spectra, grows rapidly with temperature and at a given temperature is approximately independent of the outer dimension of a given SQUID. These results are incompatible with a model based on the random reversal of independent, surface spins.

We present a new theoretical framework to analyze microwave amplifiers based on the dc SQUID. Our analysis applies input-output theory generalized for Josephson junction devices biased in the running state. Using this approach we express the high frequency dynamics of the SQUID as a scattering between the participating modes. This enables us to elucidate the inherently nonreciprocal nature of gain as a function of bias current and input frequency. This method can, in principle, accommodate an arbitrary number of Josephson harmonics generated in the running state of the junction. We report detailed calculations taking into account the first few harmonics that provide simple semi-quantitative results showing a degradation of gain, directionality and noise of the device as a function of increasing signal frequency. We also discuss the fundamental limits on device performance and applications of this formalism to real devices., 12+ pages, 12 figures, 2 appendices

Measurements of the temperature and bias dependence of single-electron transistors (SETs) in a dilution refrigerator show that charge noise increases linearly with refrigerator temperature above a voltage-dependent threshold temperature, and that its low-temperature saturation is due to SET self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with the electrons in the SET, which can be at a much higher temperature than the substrate. We suggest that the noise is caused by electrons tunneling between the SET metal and nearby potential wells.

Coherent time-domain spectroscopy was used to measure the complex transmission tensor of several $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films in magnetic fields up to 6 T at temperatures from 10 to 200 K. The complex conductivity tensor was determined from transmission measurements in the frequency range 150-800 GHz without the need for Kramers-Kronig analysis. Both the real and imaginary parts of ${\ensuremath{\sigma}}_{\mathrm{xy}}$ were found to peak near 40 K, exceeding their normal state values by more than a factor of 10. This behavior is ascribed to the Hall effect of quasiparticles in the superconducting state.

Using scanning tunneling microscopy (STM), we studied 1[ital T]-TaS[sub 2] in all four of its charge-density-wave (CDW) supporting phases over the temperature range 360--143 K. Special attention was given to the search for discommensurate structures and the distinguishing of true CDW discommensurations and domains from apparent discommensurations and domains formed by interference between the CDW and the atomic lattice. In the lowest-temperature commensurate ([ital C]) phase, we find that the CDW is in the commensurate configuration as expected. We use the [ital C] phase to investigate the effects of multiple tips and find that shifts in the apparent registration of the CDW relative to the atomic lattice result. In the nearly commensurate ([ital NC]) phase, the CDW is in a true domain structure as evidenced by satellite spots in the Fourier transforms of the STM images. In the triclinic ([ital T]) phase, the STM data indicate that the CDW is in a striped domain phase, which is significantly different from the domain model previously proposed. Finally, in the high-temperature incommensurate ([ital I]) phase, we find unexpected satellite spots in the STM Fourier transforms, suggesting that a CDW modulation is also present in this phase.