Filip Krizek

The effect of niobium thin film structure on losses in superconducting circuits

  1. Maxwell Drimmer,
  2. Sjoerd Telkamp,
  3. Felix L. Fischer,
  4. Ines C. Rodrigues,
  5. Clemens Todt,
  6. Filip Krizek,
  7. Dominik Kriegner,
  8. Christoph Müller,
  9. Werner Wegscheider,
  10. and Yiwen Chu
The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding
the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over one million at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and demonstrate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
arxiv pdf

Transparent Gatable Superconducting Shadow Junctions

  1. Sabbir A. Khan,
  2. Charalampos Lampadaris,
  3. Ajuan Cui,
  4. Lukas Stampfer,
  5. Yu Liu,
  6. S. J. Pauka,
  7. Martin E. Cachaza,
  8. Elisabetta M. Fiordaliso,
  9. Jung-Hyun Kang,
  10. Svetlana Korneychuk,
  11. Timo Mutas,
  12. Joachim E. Sestoft,
  13. Filip Krizek,
  14. Rawa Tanta,
  15. M.C. Cassidy,
  16. Thomas S. Jespersen,
  17. and Peter Krogstrup
Gate tunable junctions are key elements in quantum devices based on hybrid semiconductor-superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes
to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single crystalline InAs, InSb and InAs1−xSbx nanowires with epitaxial superconductors and in-situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in-situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high ICRN, close to the KO−2 limit. This study demonstrates a promising engineering path towards reliable gate-tunable superconducting qubits.
arxiv pdf