arXiv subjects

Z. Sung

Publications and source records attributed to Z. Sung.

6 recordsLinked to original sources

arXiv2025

Optimizing Superconducting Nb Film Cavities by Mitigating Medium-Field Q-Slope Through Annealing

Niobium films are of interest in applications in various superconducting devices, such as superconducting radiofrequency cavities for particle accelerators and superconducting qubits for quantum computing. In this study, we addressed the persistent medium-field Q-slope issue in Nb film cavities, which, despite their high-quality factor at low RF fields, exhibit a significant Q-slope at medium RF fields compared to bulk Nb cavities. Traditional heat treatments, effective in reducing surface resistance and mitigating the Q-slope in bulk Nb cavities, are challenging for niobium-coated copper cavities. To overcome this challenge, we employed DC biased high-power impulse magnetron sputtering to deposit niobium film onto a 1.3 GHz single-cell elliptical bulk niobium cavity, followed by annealing treatments aimed at modifying the properties of the niobium film. In-situ annealing at 340 {\deg}C increased the quench field from 10.0 to 12.5 MV/m. Vacuum furnace annealing at 600 {\deg}C and 800 {\deg}C for 3 hours resulted in a quench field increase of 13.5 and 15.3 MV/m, respectively. Further annealing at 800 {\deg}C for 6 hours boosted the quench field to 17.5 MV/m. Additionally, the annealing treatments significantly reduced the field dependence of the surface resistance. However, increasing the annealing temperature to 900 {\deg}C induced a Q-switch phenomenon in the cavity. The analysis of RF performance and material characterization before and after annealing has provided critical insights into how the microstructure and impurity levels in Nb films influence the evolution of the Q-slope in Nb film cavities. Our findings highlight the significant roles of hydrides, high local misorientation, and lattice and surface defects in driving field-dependent losses.

physics.acc-ph↗

arXiv2025

Experimental Observation of Short-Range Magnetic Correlations in Amorphous Nb$_2$O$_5$ and Ta$_2$O$_5$ Thin Films

We use muon spin rotation/relaxation/resonance ($\mu$SR) to investigate the magnetic properties of niobium pentoxide (Nb$_2$O$_5$) and tantalum pentoxide (Ta$_2$O$_5$) thin films. In both oxides, we observe a magnetic response at the lowest available temperature of 2.8 K. This response appears to be structurally dependent: thermally oxidized Ta$_2$O$_5$ with low crystallinity demonstrates suppressed magnetism, while fully amorphous Ta$_2$O$_5$ demonstrates local static magnetism. In contrast, amorphous Nb$_2$O$_5$ is dominated by magnetic fluctuations and is strongly magnetically disordered compared to Ta$_2$O$_5$. Our results suggest that these fundamental differences in the magnetism of Ta and Nb oxides could explain the performance limitations in superconducting qubits and resonators.

cond-mat.other↗

arXiv2023

Magnetic Fluctuations in Niobium Pentoxide

Using a spin-polarized muon beam we were able to capture magnetic dynamics in an amorphous niobium pentoxide thin film. Muons are used to probe internal magnetic fields produced by defects. Magnetic fluctuations could be described by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic field. We state that observed fluctuations result from the correlated motion of electron spins. We expect that oxygen vacancies play a significant role in these films and lead to a complex magnetic field distribution which is non-stationary. The characteristic average rate of magnetic field change is on the order of 100~MHz. The observed dynamics may provide insight into potential noise sources in Nb-based superconducting devices, while also highlighting the limitations imposed by amorphous oxides.

cond-mat.mtrl-sci↗

arXiv2022

Key directions for research and development of superconducting radio frequency cavities

Radio frequency superconductivity is a cornerstone technology for many future HEP particle accelerators and experiments from colliders to proton drivers for neutrino facilities to searches for dark matter. While the performance of superconducting RF (SRF) cavities has improved significantly over the last decades, and the SRF technology has enabled new applications, the proposed HEP facilities and experiments pose new challenges. To address these challenges, the field continues to generate new ideas and there seems to be a vast room for improvements. In this paper we discuss the key research directions that are aligned with and address the future HEP needs.

physics.acc-ph↗

arXiv2022

High quality superconducting Nb co-planar resonators on sapphire substrate

We present measurements and simulations of superconducting Nb co-planar waveguide resonators on sapphire substrate down to millikelvin temperature range with different readout powers. In the high temperature regime, we demonstrate that the Nb film residual surface resistance is comparable to that observed in the ultra-high quality, bulk Nb 3D superconducting radio frequency cavities while the resonator quality is dominated by the BCS thermally excited quasiparticles. At low temperature both the resonator quality factor and frequency can be well explained using the two-level system models. Through the energy participation ratio simulations, we find that the two-level system loss tangent is $\sim 10^{-2}$, which agrees quite well with similar studies performed on the Nb 3D cavities.

physics.app-ph↗

arXiv2018

Accelerating fields up to 49 MV/m in TESLA-shape superconducting RF niobium cavities via 75C vacuum bake

In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art.

physics.acc-ph↗