7 Search Results

Here, we study the effects of flux creep on the linear AC response of the vortex lattice in single crystals Ca₃⁢Ir₄⁢Sn₁₃ by measuring the Campbell penetration depth, 𝜆_C⁡(𝑇,𝐻,𝑡). Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter, which is proportional to the pinning well curvature. This effect cannot be detected in conventional magnetic relaxation measurements but is revealed by our observation of a nonmonotonic time evolution of 𝜆_C⁡(𝑇,𝐻,𝑡), which directly probes the average curvature of the occupied pinning centers. The timemore » evolution of 𝜆_C⁡(𝑇,𝐻,𝑡) was measured at different temperatures in samples with different densities of pinning centers produced by electron irradiation. The curves can be collapsed together when plotted on a logarithmic time scale 𝑡 → 𝑇 ⁢ln ⁡(𝑡/𝑡₀) confirming that the time evolution is driven by flux creep. The 𝜆_C⁡(𝑇,𝐻,𝑡) is hysteretic with a noticeable nonmonotonic relaxation in the presence of a vortex density gradient (after zero-field cooling), but is monotonic after field cooling, where the vortex density is uniform. This result quantitatively corroborates the novel picture of vortex creep based on the strong pinning theory.« less

Abstract The London penetration depth, $\lambda \left(T\right)$ , was measured in various forms of niobium, including foils, thin films, single crystals, and samples from superconducting radio-frequency (SRF) cavities. We observed a significant difference in $\lambda \left(T\right)$ at low temperatures, $T<T_{\mathrm{c}}/3$ , due to low-energy quasiparticles. In particular, an unusual downturn of $\lambda \left(T\right)$ on cooling in the SRF cavity samples required to take into account deepmore » in-gap bound states. Theoretical modeling using the generalized Dynes density of states shows that such in-gap states lead to a downturn or a peak in $\lambda \left(T\right)$ upon cooling. Combined, experimental and theoretical findings provide a method for detecting two-level systems or states related to magnetic impurities in the bulk of niobium. This result is particularly relevant for the quantum informatics sciences technologies used in qubits and circuit quantum electrodynamics architecture based on SRF cavities.« less

Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit's structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. Here, in this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electronmore » microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at grain boundary, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.« less

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy (STM) methods. Pieces of the same crystal boule were studied before and after the annealing at 800 °C, 1400 °C, and near the melting point of niobium (2477 °C). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 °C . However, the overall superconducting properties of the annealed samples did not improve and,more » in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 °C, measured, annealed at 1700 °C, and measured again. The STM revealed a 'dirty' superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 °C. The clean gap was recovered after annealing at 1700 °C. This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 °C, while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from Γ = 0.2 to about Γ = 0.4 after annealing at 1400 °C. Finally, these results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.« less

The intrinsically superconducting Dirac semimetal $2M\text{−}{\mathrm{WS}}_2$ is a promising candidate for realizing proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential to understand the nature of surface superconductivity in the system. Here, we report a detailed experimental study of the temperature-dependent London penetration depth, $\lambda \left(T\right)$ , the upper critical field, $H_{c2}\left(T\right)$ , and the effects of nonmagnetic disorder onmore » these quantities, as well as on the superconducting transition temperature $T_c$ in single crystals of $2M\text{−}{\mathrm{WS}}_2$ . We observe a power-law variation of $\lambda \left(T\right)\propto T^3$ at temperatures below $0.35T_c$ . Nonmagnetic pointlike disorder induced by 2.5 MeV electron irradiation at various doses results in a significant suppression of $T_c$ . These observations are markedly different from expectations for a fully gapped isotropic $s\text{−}\mathrm{wave}$ superconductor. Together with the substantial increase of slope, $d{H}_{c2}{/dT|}_{T=T_c}$ , with increasing disorder, our results suggest a strongly anisotropic $s^{++}$ multiband superconducting state. These results have direct consequences for the expected proximity-induced superconductivity of the topological surface states. Published by the American Physical Society 2024« less

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, the London penetration depth - based quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. Here we study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films have a superconducting transition temperature of T_c = 9.35 K and a fairly clean superconducting gap. The estimated superfluid density is enhanced at intermediate temperatures. These observations are consistent with the recent theory ofmore » anisotropic strong-coupling superconductivity in Nb and indicate outstanding quality. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat dissipation (to a substrate), leading to thermomagnetic instabilities. This may present a challenge for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.« less