Dieter Koelle

Focused helium ion beam (He-FIB) irradiation with 30-keV ions is a key tool for nanoscale patterning and defect engineering in high-transition temperature ( T c ) cuprate superconducting devices, yet its usable fluence window is constrained by the competing requirements of reliable superconductivity suppression and minimal structural degradation. In this work, we provide a comprehensive dataset on the effects of large-area 30-keV He + ion exposure on the electric transport and superconducting properties of epitaxial YBa 2 Cu 3 O 7 − δ (YBCO) thin films. X-ray diffraction shows a fluence-driven loss of crystalline order accompanied by an out-of-plane lattice expansion and an orthorhombic-to-tetragonal transition, culminating in predominant amorphization at the highest fluence of 1 × 10 16 cm − 2 . Raman spectra exhibit increasing disorder while lacking signatures of oxygen depletion, indicating that irradiation mainly generates oxygen-related Frenkel defects rather than changing the carrier concentration. Consistently, with increasing fluence, the normal-state resistivity ρ N ( T ) at temperature T above T c increases strongly, while ∂ ρ N / ∂ T remains nearly unchanged at moderate fluence. The suppression of T c is accurately described by Abrikosov–Gor’kov pair breaking and reaches complete quenching of superconductivity at 4.5 × 10 15 cm − 2 . The anisotropic upper critical fields decrease approximately exponentially with increasing fluence; the vortex activation energy is reduced, and the anisotropy drops, in contrast to oxygen-depleted YBCO. Hall-angle analysis confirms a nearly constant carrier density but a systematic increase in defect scattering and reduced mobility, consistent with a crossover toward the dirty limit at high fluence. These results establish quantitative fluence thresholds and a practical operational window for He-FIB nanopatterning of YBCO quantum circuits.

Irradiation of a material with ions can cause various defects that can lead to structural phase transitions and the modification of the material's properties. Here we study the irradiation of epitaxially grown thin films of the high‐temperature superconductor YBa ₂ Cu ₃ O ₇ with 30 keV ions which leads to the expansion of the crystal lattice, degradation of superconducting properties, and eventual transition to an insulator. Fabrication of such insulating regions with a focused He‐Ion beam with a spot size of nm is a powerful technique for fabrication of superconducting nano‐devices. Using low‐temperature resistivity measurements, diffraction with a nanofocused X‐ray beam and atomic force microscopy, we investigated how the structure and the electric transport properties of YBa ₂ Cu ₃ O ₇ depend on the irradiation dose in a range 10–100 ions/nm ² and on the lateral size of the irradiated area in a range 30–5000 nm.

Electrothermal memristors operate by a resistive switching mechanism based on Joule heating. During the switching process, metallic filaments form between the electrical contacts in a highly reversible and reproducible manner. Our experiments on planar VO2-based devices show that multiple filaments can form when several electronic terminals are involved. The configuration, position, and size of the filaments depend sensitively on the applied inputs and on their history. Because of this history dependence and the interchangeability of the terminals, these devices are promising building blocks for programmable networks with built-in local memory.

This paper presents a compact, single-ended high-temperature superconducting (HTS) helium-focused ion beam (He-FIB) Josephson junction (JJ) mixer. The planar circuit is fabricated from a thin film layer of YBa₂Cu₃O_7-x deposited on a 10 mm × 10 mm Lanthanum Strontium Aluminium Tantalate (LSAT) substrate. A comprehensive six-step microstrip design methodology is outlined, detailing the JJ's physical geometries, high frequency and DC grounding, impedance matching, and filtering networks. The complete microstrip topology is designed to achieve an approximate radio frequency (RF) input passband between 9.9 GHz to 10.6 GHz with high out-of-band isolation, a single local oscillator (LO) resonance at 11.54 GHz and an intermediate frequency (IF) lowpass cut-off at 2.3 GHz. The proposed RF impedance matching network is uniquely designed to mitigate the significant performance limitation associated with normal resistances ($\rm{R}_\rm{n}$) scatter observed in HTS JJs. By implementing the proposed design methodology, a stable percentage bandwidth and $\rm{S}_{11}$ $\leq$ -10 dB is maintained between ${\mathit{R}}_\rm{n} =$ 5 $\Omega$ to 10 $\Omega$, which coincides with the fabrication variance for He-FIB JJs. The HTS mixer is manufactured and its DC and high frequency behaviours are characterised at 6 K, 10 K, 20 K, 30 K, and 40 K. The measured ${\mathit{R}}_\rm{n}$ is 9.77 $\Omega$, which significantly deviated from the predicted value of 5 $\Omega$, emphasising the importance of the adaptability and versatility of the implemented impedance matching network. Exposed to the lowest measured temperature, the HTS mixer exhibited a maximum conversion gain of -4.93 dB which demonstrates the potential capabilities in He-FIB induced barrier JJs on LSAT substrates as frequency mixers.

In 2025, the Year of Quantum Science and Technology ( https://quantum2025.org/ ), we celebrate a century of quantum mechanics, witnessing a surge in activities that illuminate its inherent strangeness and drive technological innovation. Superconductivity, discovered 114 years ago, stands as a prime example, offering direct and compelling evidence of macroscopic quantum phenomena. Beyond its ability to conduct immense currents without loss, superconductivity reveals the quantum realm operating on a scale we can directly observe and manipulate. The macroscopic quantum coherence, where an ensemble of particles is described by a single wave function, leads to remarkable consequences: dissipation-less current and flux quantization—the basic properties exploited in superconducting quantum circuit fabrication. This Roadmap has been inspired by intensive discussions and collaborations emerging from the European Cooperation in Science & Technology COST-Action CA21144 (SuperQuMap—Superconducting Nanodevices and Quantum Materials for Coherent Manipulation). The aim of the COST Action SuperQuMap is to establish a strong European network centered on macroscopic quantum behavior in superconductors, bringing together groups of different backgrounds and more than 30 countries. The roadmap outlines the network’s concrete activities, driving advancements in superconductor-based quantum technologies and charting future directions. Spanning fundamental research to practical applications, the roadmap incorporates insights from industry partners developing quantum computation. It begins by exploring quantum materials, highlighting how topology and electronic correlations could catalyze a quantum leap in technology. We then delve into manipulating the superconducting phase, leveraging advancements in magnetism, 3D fabrication, and tunable correlations. Further, we showcase the advanced microscopy techniques—such as angle-resolved photoemission spectroscopy and scanning probes—used to visualize quantum behavior. Finally, and crucially, we detail the quantum devices developed within the network, and their transformative impact on modern quantum computing approaches.

Recent results with shunted YBa₂Cu₃O_7-x Josephson junctions (JJs) fabricated using direct-writing by helium-focused ion beam (He-FIB) demonstrate that this technology is useful to achieve a YBa₂Cu₃O_7-x voltage standard. By adding a shunt resistor via a subsequent lift-off step, the $\rm{{\mathit{R}}}_\rm{n}$ value and thereby the spread of the JJs can be reduced. These results were achieved with one large shunt across multiple JJs. This study presents different designs of shunt resistors, which allow measuring the JJs individually. The different shunt resistor designs are described and evaluated by comparing measurement results without and with the shunt resistor. These show the desired effect of controlling $\rm{{\mathit{R}}}_\rm{n}$, but also a change in $\rm{{\mathit{I}}}_\rm{c}$ is visible. The spread in parameters is investigated both without and with shunt resistors. Shapiro steps at gigahertz frequencies are presented for single JJs. The study concludes with a comparative measurement between a YBa₂Cu₃O_7-x He-FIB JJ and a Josephson Arbitrary Waveform Synthesizer from PTB Braunschweig, showing a deviation of 0.05% and thus demonstrating its potential applicability.

Fabrication of YBa₂Cu₃O_7-x Josephson junctions (JJs) using a direct-write helium-focused helium ion beam (He-FIB) technique has great potential for various applications due to tunable JJ parameters and arbitrary positioning. This technique allows placing multiple JJs in series to create JJ arrays. They could be used as a voltage standard that is operated at higher temperatures and driving frequencies than the currently used niobium-based JJ arrays. Small YBa₂Cu₃O_7-x He-FIB JJ arrays with superposed Shapiro steps for up to three JJs have already been demonstrated. The number of JJs in the array was limited by the parameter spread of the individual JJs. This study presents WRspice simulations of JJ arrays, which indicate that the normal state resistance $R$_n is the most crucial parameter to achieve flat Shapiro steps in JJ arrays. The idea of adding a shunt resistor to the JJ array to deal with the spread in $R$_n values is investigated. A new design for GHz frequencies is presented, and the fabrication process is described. The properties of the shunt resistor are evaluated by van der Pauw measurements. Comparison of measurements with and without shunt resistor shows the desired effect of changing the total resistance of the JJ array while maintaining the critical current value. Finally, measurements of JJ arrays demonstrating superposed Shapiro steps at 20 k at GHz frequencies are presented.