Evidence for interacting two-level systems from the 1/f noise of a superconducting resonator.

The performance of a great variety of electronic devices--ranging from semiconductor transistors to superconducting qubits--is hampered by low-frequency noise with spectra proportional to 1/f. The ubiquity and negative impact of 1/f noise has motivated intensive research into its cause, and it is now believed to originate from a bath of fluctuating two-level defect states (TLSs) embedded in the material. This phenomenon is commonly described by the long-established standard tunnelling model (STM) of independent TLS.

Note: cryogenic coaxial microwave filters.

The careful filtering of microwave electromagnetic radiation is critical for controlling the electromagnetic environment for experiments in solid-state quantum information processing and quantum metrology at millikelvin temperatures. We describe the design and fabrication of a coaxial filter assembly and demonstrate that its performance is in excellent agreement with theoretical modelling. We further perform an indicative test of the operation of the filters by making current-voltage measurements of small, underdamped Josephson junctions at 15 mK.

Determination of the Fermi surface of MgB2 by the de Haas-van Alphen effect.

We report measurements of the de Haas-van Alphen (dHvA) effect for single crystals of MgB2, in magnetic fields up to 32 T. In contrast to our earlier work, dHvA orbits from all four sheets of the Fermi surface were detected. Our results are in good overall agreement with calculations of the electronic structure and the electron-phonon mass enhancements of the various orbits, but there are some small quantitative discrepancies.

Heavy quasiparticles in the ferromagnetic superconductor ZrZn2.

We report a study of the de Haas-van Alphen effect in the normal state of the ferromagnetic superconductor ZrZn2. Our results are generally consistent with a linear muffin-tin orbital band structure which predicts four exchange-split Fermi surface sheets. Quasiparticle effective masses are enhanced by a factor of 4.

de Haas-van Alphen effect in single crystal MgB2.

We report observations of quantum oscillations in single crystals of the high temperature superconductor MgB2. Three de Haas-van Alphen frequencies are clearly resolved. Comparison with band structure calculations strongly suggests that two of these come from a single warped Fermi surface tube along the c direction, and that the third arises from cylindrical sections of an in-plane honeycomb network.