Thilo Bauch

Enhancing superconductivity through material design is a central goal in quantum materials research. Moiré engineering, where twisting stacked layers creates long-wavelength modulations and flat bands, amplifies electronic correlations and raises the superconducting critical temperature T _c , but is largely confined to van der Waals materials with limited tunability. Here we explore an alternative: imposing artificial superpotentials on homogeneous systems to engineer flat minibands. We show that a periodic superpotential in a two-dimensional system enhances superconductivity by reconstructing electronic bands and creating regions of large density of states, substantially increasing T _c . Unlike conventional flat-band systems, where superfluid stiffness arises solely from quantum geometry, a modulated system also inherits kinetic energy from filled minibands below the Fermi level, coexisting with a positive quantum geometric contribution to yield finite, robust stiffness. The resulting state remains resilient against weak to moderate disorder. These findings establish superlattice engineering as a tunable route to enhanced superconductivity beyond twist-based moiré systems, and a promising platform for next-generation superconductors.

We investigate phase-coherent quantum transport in nanoscale Al–Bi₂Se₃–Al nanoribbon Josephson junctions by combining normal-state conductance spectroscopy with a junction-length-dependent study of Josephson transport. Differential conductance maps versus bias and gate voltage reveal Fabry–Pérot interference, whose periodicity matches the nanoribbon width, consistent with transverse quantization and quasi-ballistic surface-state trajectories in 430 nm wide devices. A systematic investigation of the characteristic voltage I _c R _n as a function of junction length L exhibits a clear plateau for L ≤ 500 nm, indicative of a short ballistic contribution to the Josephson transport, and decreases for longer junctions as diffusive transport dominates. Together, Fabry–Pérot interference and Josephson transport measurements provide complementary, channel-selective evidence for quasi-ballistic surface-state transport persisting over several hundred nanometers in hybrid topological insulator nanostructures. These results demonstrate the potential of Bi₂Se₃ nanoribbon Josephson junctions as a nanoscale platform for phase-coherent superconducting electronics, topological quantum computing architectures, and topological spintronic devices.

Thermomigration (TM) is an ex-situ technique that enables tuning of the transport properties in YBa2Cu3O7−δ -based nanostructures by driving oxygen migration through local Joule heating. We apply an AC current bias to conduct TM on Grooved Dayem nanoBridge (GDB)-based SQUIDs and investigate its impact on transport characteristics via current-voltage and voltage modulation measurements. By adjusting both the timing and amplitude of the applied AC bias, we identified two distinct TM modes: a ”hard” regime, characterized by high-bias, rapidly repeated AC-current bursts that reduce the critical current, and a ”soft” regime, in which lower-bias bursts separated by longer waiting times selectively tune the differential resistance at the operating point. Both tuning methods significantly enhance device performance, with voltage modulation improvements higher than a factor 5. Our findings confirm TM as a powerful post-fabrication tool for optimization of the transport properties of GDB-based HTS SQUIDs.

Superconducting materials are key for technologies enabling a large number of devices, including THz wave mixers and single-photon detectors, though they are limited at very low temperatures for conventional superconductors. High-temperature operation could, in principle, be offered using cuprate superconductors. However, the complexity of the material in thin film form, the extremely short coherence length, and material stability have hindered the realization of THz devices working at liquid nitrogen temperatures. YBa2Cu3O7−δ (YBCO) nanodevices have demonstrated nonlinear properties typical of Josephson-like behavior, which have the potential for mixing of AC signals in the THz range due to the large superconducting energy gap. Here, we present AC Josephson functionalities for terahertz waves utilizing Abrikosov vortex motion in nanoscale-confined, fully planar YBCO thin film bridges. We observe Shapiro step-like features in the current–voltage characteristics when irradiating the device with monochromatic subterahertz waves (100–215 GHz) at 77 K. We further explore these nonlinear effects by detecting THz high-order harmonic mixing for signals from 200 GHz up to 1.4 THz using a local oscillator at 100 GHz. Our results open a path to an easily fabricated HTS nonlinear nanodevice based on dimensional confinement, suitable for terahertz applications.

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.

We study quantum interference effects in hybrid devices consisting of topological insulator Bi2Se3 nanoribbons contacted by two Al electrodes. Conductance oscillations as a function of bias and gate voltage reveal a distinct crossover in transport behavior. For junctions narrower than 500 nm, the oscillation periodicity matches the junction width, consistent with Fabry-Perot interference and quasi-ballistic transport over nearly micrometer scales. In contrast, wider devices exhibit irregular, quasi-periodic oscillations that we attribute to universal conductance fluctuations, signaling a transition to diffusive transport as the junction width becomes too large for transverse quantization to govern the interference pattern. Complementary Josephson current measurements confirm that ballistic modes persist over long distances, far beyond what magnetotransport suggests. These findings highlight the sensitivity of Fabry-Perot interference and Josephson current probes to ballistic transport channels.

Enhancing superconductivity through material design is a central goal in quantum materials research. Moire engineering, where twisting stacked layers creates long-wavelength modulations and flat bands, has shown how electronic correlations can be amplified and eventually used to raise the superconducting critical temperature Tc. Yet this approach is largely confined to van der Waals materials and offers limited tunability. Here we explore a moire-inspired alternative: imposing artificial superpotentials on otherwise homogeneous systems to engineer flat electronic minibands. Whether such superlattice potentials can truly enhance superconductivity and sustain a finite superfluid stiffness remains, however, an open question. Our calculations show that a periodic superpotential imposed to a 2D system can indeed enhance superconductivity by reconstructing the electronic bands and creating regions of large density of states, leading to a substantial increase of Tc. In contrast to conventional flat band systems, where the superfluid stiffness arises solely from quantum geometry through the quantum metric, a modulated system inherits kinetic energy from the filled minibands below the Fermi level. This inherited component coexists with a positive quantum geometric contribution, yielding a finite and robust stiffness even when the upper band becomes nearly flat. The resulting superconducting state remains coherent and resilient against weak to moderate disorder. Our findings demonstrate that engineered superpotentials offer a tunable route to enhance superconductivity beyond twist based moiré systems, unifying flat band amplification of pairing with preserved phase stiffness. They further highlight the central role of quantum geometry in shaping collective electronic phenomena and point to superlattice design as a promising platform for next-generation superconductors.