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Peter Grünberg Institute / Institute of Complex Systems

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SQUIDs and Readout Electronics

Ultra-sensitive superconducting quantum interference devices (SQUID) and suitable readout electronics are developed. SQUIDs are especially useful for the detection of the weak low frequency magnetic fields, e.g. those generated by biological currents from heart or brain activities.

SQUID principle

SQUID principle

The high sensitivity is achieved by the macroscopic coherence of the electronic wavefunction of the superconducting state. Therefore the SQUID sensor has to be cooled below the critical temperature of the superconducting material. Our sensors are based on radio-frequency (rf) SQUIDs. These devices consist of one Josephson junction in a superconducting ring.

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SQUID fabrication

SQUID fabrication

A well-defined grain boundary constitutes a weak link within an epitaxial YBaCuO layer. These grain boundary junctions are prepared by ion beam etching of a steep ditch into single crystal LaAlO3 or SrTiO3 substrates. Epitaxial YBCO thin films are grown by laser ablation, forming a step-edge grain boundary Josephson junction at the edge of the ditch. The SQUID structure is patterned photolithographically using chemical etching.

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SQUID bootstrap circuit

SQUID bootstrap circuit

Recently, we developed a novel direct readout scheme for dc superconducting quantum interference device (SQUID), called SQUID Bootstrap Circuit (SBC). The SBC consists of two branches: one makes the current-flux characteristics asymmetric and the other enhances the dynamic resistance of the SQUID. SBC constitutes a simple and convenient direct readout electronics which has a wider dc SQUID parameter tolerance than other readout schemes.

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