The advent of high-$T_c$ superconductors gave great impetus to the development of thin-film superconducting quantum interference devices (SQUIDs) for operation at temperatures up to the boiling point of liquid nitrogen, 77 K. The spectral density of the white flux noise can be calculated analytically for rf SQUIDs and by computer simulation for dc SQUIDs; however, observed noise spectral densities are typically an order of magnitude higher. Low-frequency $1/f$ noise from thermally activated vortex motion is a much bigger issue in high-$T_c$ SQUIDs at 77 K than in low-$T_c$ SQUIDs because of the low flux-pinning energies in high-$T_c$ superconductors. The magnitude of the noise depends strongly on the quality of the thin films, and much effort has been expended to improve techniques for depositing ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ (YBCO) on lattice-matched single-crystal substrates. Substantial effort has also been invested in the development of new types of Josephson junctions, of which grain-boundary junctions are the most widely used in SQUIDs. Appropriate electronic read-out schemes largely eliminate $1/f$ noise from fluctuations in the junction critical current in both rf and dc SQUIDs. Typical levels of white flux noise are a few $\mu {\Phi }_0{\mathrm{Hz}}^{\mathrm{-}1/2}$ (${\Phi }_0$ is the flux quantum). Magnetometers—consisting of a superconducting flux transformer coupled to a SQUID—achieve a white magnetic-field noise as low as 10 fT ${\mathrm{Hz}}^{\mathrm{-}1/2}$, increasing to typically 30 fT ${\mathrm{Hz}}^{\mathrm{-}1/2}$ at 1 Hz. When these devices are operated in an unshielded environment, it is important to minimize the motion of trapped vortices and induced supercurrents, which can greatly increase the $1/f$ noise. The ambient noise is far greater than the intrinsic noise of the devices, but can be substantially reduced by various gradiometer configurations. There is now considerable effort to apply high-$T_c$ SQUIDs in magnetocardiology, nondestructive evaluation, microscopy, and geophysics.

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 report on THz emission measurements and low temperature scanning laser imaging of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the “cold” part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions (JJs) based on, e.g., $d$-wave superconductors or JJs with ferromagnetic barrier. In this situation, the role of the second harmonic can become dominant; in this case, it determines the scenario of a $0\text{−}\pi$ transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is positive and the first harmonic changes sign as a function of any control parameter, e.g., temperature, the ground state phase changes abruptly between 0 and $\pi$. If the second harmonic is negative, the ground state phase changes continuously passing all values between 0 and $\pi$ and the realization of a so-called $\phi$-junction is possible in the vicinity of the point where the first harmonic vanishes. We study the unusual properties of this kind of Josephson junction such as critical currents, magnetic field penetration, plasma gap, microwave response, and zero field steps. We also analyze the possible experimental techniques for the observation of predicted effects.

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase $\psi =\pm \phi$. The value of $\phi$ can be chosen by design in the interval $0<\phi <\pi$. The junctions used in our experiments are fabricated as 0-$\pi$ Josephson junctions of moderate normalized length with asymmetric 0 and $\pi$ regions. We show that (a) these $\phi$ Josephson junctions have two critical currents, corresponding to the escape of the phase $\psi$ from $-\phi$ and $+\phi$ states, (b) the phase $\psi$ can be set to a particular state by tuning an external magnetic field, or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions.