Jörg Tomes

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...

Superconducting quantum interference filters as absolute magnetic field sensors

We propose Superconducting Quantum Interference Filters (SQIFs) as high sensitive magnetic field detectors. The SQIF is made of high critical temperature grain boundary Josephson junctions, and it is surrounded by an on chip superconducting pick-up loop which enhances the magnetic field sensitivity of about 10 times with respect to the same SQIF without pick-up loop. The devices are operated in Stirling microcoolers, at a temperature of about 70 K. In the presence of an applied magnetic field B, SQIFs show the typical magnetic field dependent voltage response V(B), which is sharp delta-like 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 B/sub 0/ /spl ap/ 0 to a value B /spl ap/ B/sub 1/, which is about the average value of the Earth magnetic field, at our latitude. The low hysteresis observed in the sequence of experiments makes SQIFs suitable for high precision measurements of the absolute magnetic field. Typical magnetic flux noise spectra of SQIFs show a white noise level of about 0.6 /spl mu/T//spl radic/Hz. Comparative measurements of the direct spectra with the spectra measured by using noise reduction techniques reveal a significant decrease of the 1/f noise levels. The experimental results are discussed in view of potential applications of high critical temperature SQIFs in magnetometry.

Two dimensional superconducting quantum interference filters

We have successfully developed a novel superconducting quantum interferometer based on Josephson junction networks with unconventional loop size distribution. For distinct theoretically determined distributions, the magnetic field B dependent dc voltage V(B) of the interferometer possesses a unique delta-peak like characteristics around B=0. Such devices are called Superconducting Quantum Interference Filters (SQIFs). The unique voltage response of SQIFs allows novel applications, e.g., absolute magnetic field sensors, high speed logical switches and non hysteretic low noise amplifiers which can be directly connected to standard room temperature electronics. In this paper we present new experimental and theoretical results on high performance two dimensional Superconducting Quantum Interference Filters (2D SQIFs). Such 2D SQIFs can be used as absolute magnetic field sensors. Our results indicate that due to the scaling behavior of the flux to voltage transfer function and the scaling of the white output noise, a highly sensitive absolute field sensor based on 2D SQIFs can be very small in size.

Effects of magnetic field on two-dimensional superconducting quantum interference filters

We present an experimental study of two-dimensional (2D) superconducting quantum interference filters (SQIFs) in the presence of a magnetic field B. Although the nonlinear dynamics of the 2D SQIF are much more complex than those of previously studied one-dimensional SQIFs, we found for the 2D SQIF a similar dependence of the voltage V on the magnetic field applied, which is characterized by a unique delta-like dip at B=0. The voltage span of the dip depends on the distribution of areas of the individual loops, and on the bias current, and it scales proportionally to the number of rows simultaneously operating at the same working point. In addition, the voltage response of individual rows of the 2D SQIF is sensitive to the field gradient generated by a control line superimposed on the homogeneous field of a coil. This feature suggests the use of these devices as highly sensitive absolute detectors of spatial gradients of the magnetic field.

Quadratic mixing of radio frequency signals using superconducting quantum interference filters

The authors demonstrate quadratic mixing of weak time harmonic electromagnetic fields applied to superconducting quantum interference filters (SQIFs), manufactured from high-Tc grain boundary Josephson junctions and operated in active microcooler. The authors use the parabolic shape of the dip in the dc voltage output around B=0 to mix quadratically two external rf signals, at frequencies f1 and f2 well below the Josephson frequency fJ, and detect the corresponding mixing signal at ∣f1−f2∣. Quadratic mixing also takes place when the SQIF is operated without magnetic shield. The experimental results are well described by a simple analytical model based on the adiabatic approximation.

Two-Tone Response of Radiofrequency Signals Using the Voltage Output of a Superconducting Quantum Interference Filter

In the presence of weak time-harmonic electromagnetic fields, superconducting quantum interference filters (SQIFs) show the typical behavior of nonlinear mixers. The SQIFs are manufactured from high-Tc grain boundary Josephson junctions and operated in active microcooler. The dependence of dc voltage output Vdc versus static external magnetic field B is nonperiodic and consists of a well pronounced unique dip at zero field, with marginal side modulations at higher fields. We have successfully exploited the parabolic shape of the voltage dip around B = 0 to mix quadratically two external time-harmonic rf-signals, at frequencies f1 and f2 below the Josephson frequency fJ, and detect the corresponding mixing signal at |f1− f2|. When the mixing takes place on the SQIF current–voltage characteristics, the component at 2f2−f1 is present. The experiments suggest potential applications of a SQIF as a nonlinear mixing device, capable to operate at frequencies from dc to few gigahertz with a large dynamic range.

Synchronized patterns in hierarchical networks of neuronal oscillators with D 3 × D 3 symmetry

Abstract The spatiotemporal patterns generated by systems of nine coupled nonlinear oscillators which are equivariant under the permutation symmetry group D 3 × D 3 are determined. This system can be interpreted as a hierarchically organized network composed of three interacting systems each of which consists of three coupled oscillators. We determine generic synchronized oscillation patterns and transitions between these analytically, by numerical simulations, and experimentally with an electronic analog-network. In the theoretical analysis the representative nonlinear ordinary differential equations are reduced to the normal form equations for coupled Hopf bifurcations in an eight-dimensional center eigenspace, whose generic states have been classified previously. The results are applied to a specific model system in which the network is formed by a class of oscillators, each composed of two asymmetrically coupled Hopfield neurons. Experiments performed on an analog-electronic network of such nonlinear oscillators show that most of the states predicted by the theory of the Hopf bifurcation with D 3 × D 3 - symmetry appear in a stable way. We find a great variety of periodic and quasiperiodic oscillation patterns of maximal and submaximal symmetry which can be classified in a two-level pattern hierarchy. In addition to these states we find in simulations homoclinic cycles within the same isotropy class as well as heteroclinic switchings between such cycles.

Two-Tone Response in Superconducting Quantum Interference Filters

We successfully exploit the parabolic shape of the dc voltage output dip around B = 0 of a superconducting quantum interference filter (SQIF) to mix weak external r f signals. The two-tone response of weak time harmonic electromagnetic fields has been detected on the spectral voltage output of the SQIF at frequency f₀ = f₁ - f₂, for various frequencies f₁ and f₂ ranging from few MHz up to 20 GHz. The two-tone response is a characteristic function of static magnetic field B and of bias current I_b, related to the second derivative of the dc voltage output.

Detection of symmetry of attractors from observations II: an experiment with S 4 symmetry

Abstract In part I of this paper we adapt the detectives of Barany et al. (1993) for symmetries of attractors of symmetric dynamical systems to apply to experiments. As an illustration of this theory we consider the dynamics of a system of four forced coupled electronic oscillators with full permutation symmetry (S4). On varying parameters in the circuit we obtain attractors with many different symmetry types and detect these using the methods described in part I.