7 research outputs found
Grain size in low loss superconducting Ta thin films on c-axis sapphire
In recent years, the implementation of thin-film Ta has led to improved
coherence times in superconducting circuits. Efforts to further optimize this
materials set have become a focus of the subfield of materials for
superconducting quantum computing. It has been previously hypothesized that
grain size could be correlated with device performance. In this work, we
perform a comparative grain size experiment with -Ta on -axis
sapphire. Our evaluation methods include both room-temperature chemical and
structural characterization and cryogenic microwave measurements, and we report
no statistical difference in device performance between small- and
larger-grain-size devices with grain sizes of 924 nm and 1700 nm,
respectively. These findings suggest that grain size is not correlated with
loss in the parameter regime of interest for Ta grown on c-axis sapphire,
narrowing the parameter space for optimization of this materials set
Microscopic Relaxation Channels in Materials for Superconducting Qubits
Despite mounting evidence that materials imperfections are a major obstacle
to practical applications of superconducting qubits, connections between
microscopic material properties and qubit coherence are poorly understood.
Here, we perform measurements of transmon qubit relaxation times in
parallel with spectroscopy and microscopy of the thin polycrystalline niobium
films used in qubit fabrication. By comparing results for films deposited using
three techniques, we reveal correlations between and grain size, enhanced
oxygen diffusion along grain boundaries, and the concentration of suboxides
near the surface. Physical mechanisms connect these microscopic properties to
residual surface resistance and through losses arising from the grain
boundaries and from defects in the suboxides. Further, experiments show that
the residual resistance ratio can be used as a figure of merit for qubit
lifetime. This comprehensive approach to understanding qubit decoherence charts
a pathway for materials-driven improvements of superconducting qubit
performance
Supporting people's placemaking theory and practice The case of support housing in Sri Lanka
SIGLEAvailable from British Library Document Supply Centre- DSC:DX186102 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
A nanoscale gigahertz source realized with Josephson scanning tunneling microscopy
Using the AC Josephson effect in the superconductor-vacuum-superconductor tunnel junction of a scanning tunneling microscope (STM), we demonstrate the generation of GHz radiation. With the macroscopic STM tip acting as a lambda/4-monopole antenna, we first show that the atomic scale Josephson junction in the STM is sensitive to its frequency-dependent environmental impedance in the GHz regime. Further, enhancing Cooper pair tunneling via excitations of the tip eigenmodes, we are able to generate high-frequency radiation. We find that for vanadium junctions, the enhanced photon emission can be tuned from about 25 GHz to 200 GHz and that large photon flux in excess of 10(20) cm(-2) s(-1) is reached in the tunnel junction. These findings demonstrate that the atomic scale Josephson junction in an STM can be employed as a full spectroscopic tool for GHz frequencies on the atomic scale. (C) 2015 AIP Publishing LLC
The stretched harmonic oscillator, a test of semiclassical approximations
SIGLECNRS RP 171 (322) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Superconducting scanning tunneling microscopy tips in a magnetic field: Geometry-controlled order of the phase transition
The properties of geometrically confined superconductors significantly differ from their bulk counterparts. Here, we demonstrate the geometrical impact for superconducting scanning tunneling microscopy (STM) tips, where the confinement ranges from the atomic to the mesoscopic scale. To this end, we compare the experimentally determined magnetic field dependence for several vanadium tips to microscopic calculations based on the Usadel equation. For our theoretical model of a superconducting cone, we find a direct correlation between the geometry and the order of the superconducting phase transition. Increasing the opening angle of the cone changes the phase transition from first to second order. Comparing our experimental findings to the theory reveals first and second order quantum phase transitions in the vanadium STM tips. In addition, the theory also explains experimentally observed broadening effects by the specific tip geometry. (C) 2015 AIP Publishing LLC