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STSM of Celia Gonzalez at at the University of Konstanz (Germany)

The aim of this short-term scientific mission (STSM) was to advance the fabrication of high-quality van der Waals heterostructure (vdW) devices for the investigation of the electronic properties of novel hybrid superconducting systems based on 2D materials. Specifically, we are interested in studying Josephson junctions comprised of 2D superconductors, e.g., NbSe2, and 2D magnetic materials, such as CrGeTe3, CrBr3, etc. With this STSM, we intended to benefit from the infrastructure for the preparation of vdW heterostructures in inert atmospheres of the host research group of Dr. Angelo di Bernardo at the University of Konstanz in Germany, as well as from their know-how related to magnetic materials, to fabricate 2D Josephson junctions based on NbSe2.

During that period of time, we fabricated 2D Josephson junction devices, being NbSe2 the superconducting material. Regarding the weak link, we used a ferromagnetic material, Cr2Ge2Te6 (flakes of different thicknesses) and an antiferromagnetic material, NiPS3. Besides, one sample BSCCO/NbSe2/BSCCO was prepared, in collaboration with a PhD student from the group of Dr. Angelo di Bernardo. For each of them, we performed the following operations: exfoliation of the flakes and fabrication of the 2D heterostructures.

These heterostructures were fabricated by two different methods, in order to compare them and optimize the performance of the junctions. The first fabrication method was a dry-transfer technique combining PDMS and PC within a glove box in an inert N2 gas atmosphere, which involves the exfoliation of the flakes on a Si/SiO2 wafers substrate and picking them up with the PDMS/PC stamp to get the desired geometry. For this method, it is necessary to heat the sample up to 90°, so we expected some degradation of the materials due to the high temperature. The second method was the one we use in our group at Universidad Autónoma de Madrid, which consists of the consecutive stamping of the flakes with a PDMS stamp on the prepatterned electrodes. This method does not require any heat although the interface between the layers may be not atomically clean due to polymer residues. The preliminary electrical transport measurements showed an excellent performance of the devices. This characterization was primarily employed for verifying that the sample preparation process was suitable for these materials. Furthermore, low-temperature measurements will be realized in Spain, using either a 3He refrigerator with a base temperature of 250 mK and a 8T magnet or a dilution refrigerator with a base temperature of 10 mK and a three-axis vector magnet (6T–1T–1T). We are interested in studying the effects that can arise from the interplay between superconductivity and magnetism in these systems.

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