N. Schopohl

N o n − Φ 0 − p e r i o d i c macroscopic quantum interference in one-dimensional parallel Josephson junction arrays with unconventional grating structure

A theoretical study is presented on a number N of resistively shunted Josephson junctions connected in parallel as a disordered 1D array by superconducting wiring in such a manner that there are N-1 individual SQUID loops with arbitrary shape formed.

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2.

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.

Nonperiodic flux to voltage conversion of series arrays of dc superconducting quantum interference devices

A theoretical study on the voltage response functionof a series array of dc SQUIDs is presented in which the elementary dc SQUID loops vary in size and, possibly, in orientation. Such series arrays of two-junction SQUIDs possess voltage response functions vs. external magnetic field B that differ substantially from those of corresponding regular series arrays with identical loop-areas, while maintaining a large voltage swing as well as a low noise level. Applications include the design of current amplifiers and quantum interference filters.We present a theoretical study on the voltage response function 〈V〉 of series arrays of dc superconducting quantum interference devices (SQUIDs) for which the elementary dc SQUID loops vary in size and, possibly, in orientation. If the distribution of the array loop sizes is chosen according to an arithmetic relation, 〈V〉 is not a Φ0-periodic function of the strength of external magnetic field B. For arithmetic arrays the periodicity of 〈V〉 is controlled by the geometry of the array alone and does not depend on spreads in the array junction parameters. If small fluctuations are added to the loop size distribution, 〈V〉 becomes a unique function around a global minimum at B=0 without possessing significant additional minima for finite B. This filter property does not apply for conventional SQUIDs and series arrays of identical or nearly identical SQUID loops. Applications of arithmetic series arrays include the design of current amplifiers and novel quantum interference filters, which possess large voltage ...

Gap parameters and collective modes for 3He-B in the presence of a strong magnetic field

First we determine the 4 × 4 matrix Greens function for a p-wave pairing superfluid in a magnetic field where the order parameter is given by a real 3 × 3 matrix. For the B phase we take an order parameter which is equal to δ1 times a 3 × 3 matrix, yielding a rotation of angle θ about the z-axis and a dilatation δ2/δ1 along the z-axis. Then the self-consistency equation for the 4 × 4 matrix self-energy reduces to three equations for δ1, δ2, and cos θ, and a fourth equation for the renormalized Larmor frequency. We find that, for increasing field, δ1 increases and δ2 decreases with respect to the zero-field gap δ00. Above a (temperature-dependent) critical field we find δ2 = 0 and α2 = 90° corresponding to the planar state of lowest dipole energy. The correlation functions for the order parameter collective modes are calculated with the help of a previous theory. The results can be expressed in terms of six universal functions describing internal magnetization and virtual excitations of pairs of quasiparticles with all spin orientations. The complete set of eigenfrequencies as functions of the field is calculated for T = 0 and q = 0. The longitudinal NMR frequency is found to be almost independent of the field. We find a splitting of the pair-vibration frequencies (8/5) 1/2δ00 and (12/5)1/2δ00 which is linear in the field. This splitting is caused by fluctuations involving the spin-singlet component of the anomalous propagator. The splitting of the pair-vibration frequency (12/5)1/2δ00 (of the order 6H MHz, H in kG) should be observable by sound absorption experiments in strong magnetic fields.

Magnetic field dependence of the Leggett angle, the susceptibility, and the longitudinal nuclear magnetic resonance of3He-B

The magnetic field dependences of the Leggett angle, the susceptibility, the free energy, and the longitudinal NMR frequency of3He-B are calculated from a generalized weak coupling theory. Employing the experimental values for the Landau parametersF0a andF2a, reasonable agreement with the susceptibility data measured recently by Hoyt, Scholz, and Edwards is obtained. Substantial field dependence of the Leggett angle and the longitudinal NMR frequency is predicted.

High-T/sub c/ superconducting quantum interference filters for sensitive magnetometers

We present several kinds of Superconducting Quantum Interference Filters (SQIFs) which are all realized with high-T/sub c/ superconductors. All SQIFs use the same configuration of 30 loops of different size. The properties of these SQIF types - serial arrays, parallel arrays, and a combination of both - are discussed concerning their usefulness for magnetometry. These properties are the formation of the desired single voltage peak, its peak voltage and full width at half maximum, and the magnetic field noise. Concerning all parameters an improvement can be achieved with SQIFs of all types compared to a single SQUID.

Superconducting multiple loop quantum interferometers

We present experimental results on the magnetic field B dependent voltage response V(B) of a number of N resistively shunted Josephson junctions connected in parallel by a multiple loop network. For an appropriate distribution of the array loop sizes, such networks constitute superconducting quantum interference filters (SQIFs) with voltage response functions V(B) that are not /spl Phi//sub 0/-periodic. The voltage response of SQIFs is a unique function around a global minimum at B=0 that has a high sensitivity and a low noise level. In contrast to conventional superconducting quantum interference devices, SQIFs can be directly employed as highly sensitive magnetometers allowing the absolute measurement of magnetic fields. The experimental results are in very good agreement with the theoretical predictions on which basis the SQIF has been fabricated, All findings suggest that superconducting quantum interference filters may allow the design of novel superconducting devices.

High-performance magnetic field sensor based on superconducting quantum interference filters

We have developed an absolute magnetic field sensor using a superconducting quantum interference filter (SQIF) made of high-Tc grain-boundary Josephson junctions. The device shows the typical magnetic-field-dependent voltage response V(B), which is a sharp deltalike dip in the vicinity of zero-magnetic field. When the SQIF is cooled with magnetic shield, and then the shield is removed, the presence of the ambient magnetic field induces a shift of the dip position from B0≈0 to a value B≈B1, which is about the average value of the Earth’s magnetic field, at our latitude. When the SQIF is cooled in the ambient field without shielding, the dip is first found at B≈B1, and the further shielding of the SQIF results in a shift of the dip towards B0≈0. The low hysteresis observed in the sequence of experiments (less than 5% of B1) makes SQIFs suitable for high precision measurements of the absolute magnetic field. The experimental results are discussed in view of potential applications of high-Tc SQIFs in magnetom...

Highly sensitive magnetometers for absolute magnetic field measurements based on quantum interference filters

We present an experimental study on absolute field magnetometers based on superconducting quantum interference filters (SQIFs). Two different prototype SQIF circuits have been designed and fabricated. The first type comprises a one dimensional Josephson junction array possessing unconventional grating structure. The second type is a series array of loops with different areas each loop having two Josephson junctions. The flux to voltage transfer function of both SQIFs has a unique voltage signal around zero applied flux. Our results show that SQIFs can be applied as highly sensitive magnetometers for absolute magnetic field measurements.

Superconducting quantum interference filters operated in commercial miniature cryocoolers

Superconducting Quantum Interference Filters (SQIFs) are multi-loop arrays of Josephson junctions possessing unconventional grating structures. The flux-to-voltage transfer function of SQIFs is unique and has a single delta-peak like characteristics at zero applied magnetic field. The fault tolerance of SQIFs allows them to be realized in standard high-temperature grain boundary Josephson junction technology. We have successfully designed high-temperature YBCO SQIFs with voltage swings of more than 3 mV and a rather large dynamical range. This large dynamical range allows high-Tc SQIFs to be operated in commercial miniature cryocoolers at 50 K-80 K without any significant problems. Typical parasitic oscillations which are induced by the 55 Hz driven compressor of the cryocooler do not degrade the performance of SQIFs. Even if SQIFs are operated fully unshielded no significant degradation of the performance has been observed.