Abstract
In this research, we tailor the phonon density of states (DOS) in thin superconducting films to suppress quasiparticle losses. We examine a model system of a proximity-enhanced three-layered Al/Nb/Al heterostructure and show that the local quantized phonon spectrum of the ultrathin Al cladding layers in the heterostructure has a pronounced effect on the superconducting resonator’s quality factors. Instead of a monotonic increase of quality factors with decreasing temperatures, we observe the quality factor reaches a maximum at 1.2 K in 5/50/5 nm Al/Nb/Al microstrip resonators, because of a quantized phonon ladder. The phonon DOS may be engineered to enhance the performance of quantum devices.
Highlights
With tremendous efforts over the last few decades, superconducting devices emerge as one of the most promising candidates for realizing quantum computation[1,2,3,4]
We examine a model system of proximity-enhanced Al/Nb/Al heterostructures, and we consider the size effect of the ultrathin Al cladding layers
When a phonon density of states (DOS) step coincides with the superconducting gap edge, the size effect manifests as an anomalous peak of Q before it levels off with decreasing temperature
Summary
Yong-Chao Tang[1,2], Sangil Kwon[2,3], Hamid R. We examine a model system of a proximity-enhanced three-layered Al/Nb/Al heterostructure and show that the local quantized phonon spectrum of the ultrathin Al cladding layers in the heterostructure has a pronounced effect on the superconducting resonator’s quality factors. In a trilayer heterostructure consisting of one thick core layer and two thin cladding layers, the phonon quantization shows up in the cladding layers due to the local phonon DOS Such superconductor heterostructure can be readily utilized for improving quantum devices. The proximity effect will enhance the performance of resonators under magnetic fields, for example, for pulsed electron spin resonance (pulsed-ESR) based quantum computing[8]. In our Al/Nb/ Al trilayer structures, the Nb core layer can enhance the critical field and critical temperature of Al films through the proximity effect as Nb11 is significantly higher on both parameters. The physical growth of Al/Nb/Al heterostructure and their structural characterization are discussed at last
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