Joachim Wosnitza

The interplay of alternative materials, large-scale integration technologies, and innovative devices plays a pivotal role on the way to fault-tolerant quantum computing. In this context, gate-controlled superconductivity (GCS) emerges as a key enabling technology, promising to advance superconducting logic and quantum electronics, complementing conventional approaches based on Josephson effects and quantum interference. Here, we demonstrate all-metallic superconducting ZrN nanostructures on silicon, fabricated entirely using a subtractive, monolithic dry-etch approach with a 300 mm CMOS-compatible process flow. For Dayem-bridge weak links with decreasing width down to 27 nm, we observe Tc of 7.3 K, width-dependent switching characteristics, and a progressive approach of the tunneling limit. In laterally gated Dayem-bridge and nanowire devices, we observe GCS with full suppression of the critical current, where required gate voltages <5 V promise compatibility with CMOS interfaces. Asymmetries in polarity-dependent power injection via field emission support GCS models that attribute the effect to quasiparticle relaxation through phonon emission. Additional asymmetries in the GCS effect suggest an influence of the substrate condition after dry etching. Field emission features in the gated nanowire device imply superimposed current paths along the gate edge and corners. Finally, we analyze magnetic interference in Dayem-bridge weak links and superconducting quantum interference devices, where quantum interference is governed by Josephson effects and inductance loop asymmetries. Our results contribute to the scaling efforts in hybrid superconducting electronics and quantum-processing units.

Cryogenic magnetic refrigeration becomes more and more important nowadays, especially for the liquefaction of gases such as hydrogen. In this study, we have synthesized La1−zCez(Fe0.88−yMnySi0.12)13 samples and investigated their magnetic and magnetocaloric properties in order to assess their potential for cryogenic applications. By adjusting the Mn and Ce content and adding excess rare-earth elements, the first-order ferromagnetic transition was lowered from 200 to 40 K and the adiabatic temperature change of the samples was measured directly using pulsed magnetic fields. The sample with the lowest transition temperature still showed a significant adiabatic temperature change in magnetic fields up to 10 T, with an increasingly stronger first-order transition observed in samples with higher Ce substitution. In addition, we synthesized spherical powder with diameters between 20 and 120 μm using ultrasonic atomization while maintaining the magnetic transition, which is a promising starting material for future additive manufacturing of magnetocaloric materials.

The rising interest in increased manufacturing maturity of quantum processing units is pushing the development of alternative superconducting materials for semiconductor fab process technology. However, these are often facing CMOS process incompatibility. In contrast to common CMOS materials, such as Al, TiN, and TaN, reports on the superconductivity of other suitable transition-metal nitrides are scarce, despite potential superiority. Here, we demonstrate fully CMOS-compatible fabrication of HfN and ZrN thin films on state-of-the-art 300 mm semiconductor process equipment, utilizing reactive DC magnetron sputtering on silicon wafers. Measurement of mechanical stress and surface roughness of the thin films demonstrates process compatibility. We investigated the materials phase and stoichiometry by structural analysis. The HfN and ZrN samples exhibit superconducting phase transitions with critical temperatures up to 5.84 and 7.32 K, critical fields of 1.73 and 6.40 T, and coherence lengths of 14 and 7 nm, respectively. A decrease in the critical temperature with decreasing film thickness indicates mesoscopic behavior due to geometric and grain-size limitations. The results promise a scalable application of HfN and ZrN in quantum computing and related fields.

The performance of iron-based superconductors in high magnetic fields plays an important role for their practical application. In this work, we measured the magnetostriction and magnetization of BaFe1.908Ni0.092As2 single crystals using pulsed magnetic fields up to 60 T and static magnetic fields up to 33 T, respectively. A huge longitudinal magnetostriction (of the order of 10–4) was observed in the direction of twin boundaries. The magnetization measurements evidence a high critical-current density due to strong bulk pinning. By using magnetization data with an exponential flux-pinning model, we can reproduce the magnetostriction curves qualitatively. This result shows that the magnetostriction of BaFe1.908Ni0.092As2 can be well explained by a flux-pinning-induced mechanism.