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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 principlePrinciple of an rf SQUID

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.

The laws of quantum physics demand that the magnetic flux enclosed by a superconducting ring is quantized. Changes of the external magnetic field are compensated for by a variation of the superconducting current around the ring. When a weak link (a so-called Josephson junction) is inserted into the ring, a Superconducting QUantum Interference Device (SQUID) is formed. It may be used as an extremely sensitive magnetometer. The Josephson contact limits the maximum shielding current. The fundamental parameters governing the behavior of the SQUID are the critical current of the Josephson junction, its Ohmic resistance in the normalconducting state, and the inductance of the ring.

SQUID fabricationSEM of the step edge junction of a SQUID

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. On a monocrystalline LaAlO3 substrate, a ditch of approximately 300 µm depth is milled by argon ion beam etching. Then, Yttrium-Barium-Copper-Oxide (YBCO) of about 200 µm thickness is epitaxially deposited onto the substrate by laser ablation, forming a step-edge grain boundary Josephson junction at the edge of the ditch. The SQUID structure is patterned using photolithography and wet-chemical etching. It consists of a 100 µm × 100 µm loop, a 3.5 mm diameter washer, and a 3 µm wide bridge across the ditch. Here, the superconducting film forms grain boundaries at the edges of the ditch. Thus, a grain boundary Josephson junction is realized.

Substrate resonator schemerf SQUID readout using a substrate resonator

The SQUID is read out via an inductively coupled tank circuit based on a substrate resonator. With this design, the tank circuit noise is minimized and the quality factor of the tank circuit is maximized. The SQUID is read out inductively via the coaxial transmission line connecting to a coupling coil. In the flux-locked loop, the SQUID is kept at a constant flux state by generating a magnetic feedback field compensating all measured external flux variations.

Additional Information


Prof. Dr. Hans-Joachim Krause
Tel.:  +49-2461-61-2955

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Research Group

Magnetic Field Sensors






 T. Hong, H. Wang, Y. Zhang, H.-J. Krause, A.I. Braginski, X. Xie, A. Offenhäusser, M. Jiang, Flux modulation scheme for direct current SQUID readout revisited, Appl. Phys. Lett. 108, 062601 (2016).


J. Xu, C. Benden, Y. Zhang, J. Li, H.-J. Krause, Harmonic analysis for finding the optimum working point of high-Tc rf SQUID, IEEE Trans. Appl. Supercond. 26, 1600504 (2016).


J. Zeng, Y. Zhang, M. Schmelz, M. Mück, H.-J. Krause, A.I. Braginski, Y.-H. Lee, R. Stolz, X. Kong, X. Xie, H.-G. Meyer, A. Offenhäusser, M. Jiang, Analysis of a dc SQUID readout scheme with voltage feedback circuit and low-noise preamplifier, Supercond. Sci. Technol. 27, 085011 (2014).