Wulf Wulfhekel

High quality superconducting tantalum resonators with beta phase defects

  1. Ritika Dhundhwal,
  2. Haoran Duan,
  3. Lucas Brauch,
  4. Soroush Arabi,
  5. Dirk Fuchs,
  6. Amir-Abbas Haghighirad,
  7. Alexander Welle,
  8. Florentine Scharwaechter,
  9. Sudip Pal,
  10. Marc Scheffler,
  11. José Palomo,
  12. Zaki Leghtas,
  13. Anil Murani,
  14. Horst Hahn,
  15. Jasmin Aghassi-Hagmann,
  16. Christian Kübel,
  17. Wulf Wulfhekel,
  18. Ioan M. Pop,
  19. and Thomas Reisinger
For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials.
Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from α-tantalum, avoiding the meta-stable β-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing β-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors (Qi) up to (5.0±2.5)×106 in the single photon regime. They outperform resonators with no sign of the β-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of β-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.
arxiv pdf

Microscopic charging and in-gap states in superconducting granular aluminum

  1. Fang Yang,
  2. Tim Storbeck,
  3. Thomas Gozlinski,
  4. Lukas Grünhaupt,
  5. Ioan M. Pop,
  6. and Wulf Wulfhekel
Following the emergence of superconducting granular aluminum (grAl) as a material for high-impedance quantum circuits, future development hinges on a microscopic understanding of its
phase diagram, and whether the superconductor-to-insulator transition (SIT) is driven by disorder or charging effects. Beyond fundamental relevance, these mechanisms govern noise and dissipation in microwave circuits. Although the enhancement of the critical temperature, and the SIT in granular superconductors have been studied for more than fifty years, experimental studies have so far provided incomplete information on the microscopic phenomena. Here we present scanning tunneling microscope measurements of the local electronic structure of superconducting grAl. We confirm an increased superconducting gap in individual grains both near and above the Mott resistivity ρM≈400 μΩcm. Above ρM we find Coulomb charging effects, a first indication for decoupling, and in-gap states on individual grains, which could contribute to flux noise and dielectric loss in quantum devices. We also observe multiple low-energy states outside the gap, which may indicate bosonic excitations of the superconducting order parameter.
arxiv pdf