Nonsinusoidal Current-phase Relation Research Articles

Voltage modulation of high-T c SQUIDs with step-edge junctions

We studied the V-I characteristics and the voltage modulation of washer-type step-edge YBa 2C 3uO y dc SQUIDs. It was found that the banks of the SQUID were composed of two junctions in series. The V-I curve can be fitted to the RSJ model with a sinusoidal current phase relation for the first grain-boundary junction while the second junction show a non-sinusoidal current phase relation. The V-Φ curves of SQUIDs can be characterized by the parameters of the first junctions if the SQUID is biased at I ∼ I c1 where I c1 is the critical current of the first junction. The implications of the data are discussed.

Properties of superconductor-antiferromagnet-superconductor Josephson junctions in SO(5) theory

We derive the qualitative behavior of superconductor-antiferromagnet-superconductor (SAS) Josephson junctions described by Zhang's SO(5) theory. The main differences between these junctions and conventional superconductor-insulator-superconductor junctions arise from the nonsinusoidal current-phase relation derived by Demler et al. for thin SAS junctions. Using a simple approximation to this nonsinusoidal function, the current voltage relation, Shapiro steps, thermal fluctuation effects, and the diffraction pattern in a magnetic field are obtained.

Subharmonic Shapiro steps and noise in high-T c superconductor Josephson junctions

An experimental investigation is made of the subharmonic Shapiro steps observed on the I-V curves of high-T c superconductor Josephson junctions and on the bias-voltage dependences of the rf noise and detector response when the junctions are subjected to external submillimeter radiation. Structures of this type are ordinarily described by a nonsinusoidal current-phase relation, which is why subharmonic steps appear. Numerical modeling of the processes occurring in a Josephson junction by means of a simple current-phase relation, as in the case of an SNS junction, gives good agreement with experiment. The width of the characteristic Josephson generation line of the junction was estimated on the basis of the noise dependences and the selective detector response. The width can be explained by taking into account the shot noise of the tunneling component of the conductivity. A model of the conductivity of a high-T c superconductor Josephson junction, consisting of a tunnel junction with microshorts possessing metallic conductivity, is discussed.

Investigation of YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions on MgO bicrystal substrates

We report on the characterization of the dynamic (ac) properties of YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions on MgO bicrystal substrates with 24/spl deg/ angle. Shapiro steps have been measured in the frequency range 80-95 GHz. Measured current-voltage characteristics, as well as the dependence of Shapiro step height on microwave power, were compared to numerical simulations within the RSJ model including thermal noise. At 77 K, the junctions showed ideal RSJ behavior. The occurrence of half-integer Shapiro steps at lower temperatures could be explained by an inhomogeneous critical current density of the junctions yielding a dc SQUID-like behavior. A nonsinusoidal current-phase relation could be ruled out because of the periodicity of the half-integer steps.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fabrication and measurement of cylindrical Nb3Ge rf SQUIDs

Nb3Ge thin-film microbridges are fabricated on a cylindrical substrate to form rf SQUIDs. High temperature operations well above 10 K (up to 17.5 K) are obtained. An intrinsic flux noise of the order of 10−4 Φ0/(Hz)1/2 is estimated from the step rise parameter. Comparison with a theoretical noise indicates a nonsinusoidal current phase relation of the microbridge.

Theory of the stationary macroscopic quantum behavior of inductively coupled superconducting weak links

A theory of the stationary response of a flux-controlled superconducting weak link is presented in the framework of reversible thermodynamics. The superconducting loop containing the weak link (SQUID)—considered as a “black box”—can be characterized by a nonlinear inductance ℒ s which uniquely describes the constitutive properties of the macroscopic quantum system. The influence of the temperature-dependent (nonsinusoidal) current-phase relation (CPR) on − s and on the stability of the SQUID when operated in the nonhysteretic mode is analyzed taking into account the full nonlinearity of the CPR. The model presented enables a rigorous circuit analysis, even in the case of a nonlinear flux transformer.

Electronic analogs of double-junction and single-junction SQUIDs

We describe electronic analogs of a double-junction (dc) SQUID and a single-junction (rf) SQUID that are simple to construct and operate, and are considerably more flexible than those previously reported. The Josephson junction analog is based on the Resistively-Shunted Junction (RSJ) model, and uses a novel method based on a sample-and-hold circuit to generate the sin ϑ ’’supercurrent.’’ Examples are shown of the single-junction I–V characteristics, threshold curves of critical current vs applied flux and I–V curves for the double-junction SQUID analog, and rf SQUID characteristics as a function of applied flux and rf drive current. Other applications of the analogs, including the incorporation of a nonsinusoidal current-phase relation, are discussed.

Phase-locking and microwave impedance of Josephson junctions with linear-periodic current-phase relations

Some types of Josephson junctions such as long microbridges, S-N-S junctions, and junctions with a series palastic inductance are known to have nonsinusoidal current-phase relations which can be approximated by linear-periodic functions. In this paper the equivalent circuit equation of the resistively shunted junction model with linear-periodic current-phase relation is solved. The heights of zeroth and first constant-voltage steps in the I-V characteristics and the impedance at these steps are obtained analytically. This analysis shows that the impedance at the first step lies on a semicircle in the impedance plane with its center at the impedance of the zeroth step. The calculated impedance is compared with the experimental results by Claridge et al. The theory presented here can explain some features of experimental results which differ considerably from predictions based on a sinusoidal current-phase relation. Among these are the value of bias current corresponding to the center of the first step, and the appearance of ’’shoulders’’ in the imaginary part of the impedance at the first step.

Origin of the Nonsinusoidal current-phase relation of a superconducting bridge

The current-phase relation of a long superconducting bridge is investigated with the use of the Aslamazov-Larkin model and the Ginzburg-Landau equation. The feedback effect of the supercurrent to the phase difference in the weak link is taken into consideration. The derived nonsinusoidal current-phase relation explains the experiments of Jackel et al. very well.

Current-phase relations and noise in rf biased squids

An investigation has been made of the effect of the weak link current-phase relation on noise in rf biased SQUIDs. Non-sinusoidal current-phase relations have been observed in various weak links and these non-sinusoidal relations have been correlated with significantly increased intrinsic noise in the SQUID ring. The current-phase relation is also found to affect the amplitude of the rf SQUID ring dissipation. The result of a rf SQUID system noise analysis shows that, due to increased intrinsic noise and reduced ring dissipation, the minimum attainable noise for a SQUID ring having a very non-sinusoidal current-phase relation is considably greater than for a ring with a sinusoidal relation.

Subharmonic steps in the I-V characteristic of short microbridges due to non-sinusoidal current-phase relations

Subharmonic steps in the I-V characteristic of short microbridges due to non-sinusoidal current-phase relations