Diamond Nitrogen-Induced TLS Losses Estimated with Superconducting Micro-Resonators for High-Power Applications

Diamond’s remarkable properties continue to drive innovation in diverse fields, from sensing to high-power technologies, and researchers are now pushing the boundaries of material quality even further. Francesco Mazzocchi, Martin Neidig, and Hideaki Yamada, alongside colleagues at the Karlsruhe Institute of Technology and the National Institute of Advanced Industrial Science and Technology, have developed a new method for precisely estimating dielectric losses in diamond crystals. Their work addresses a critical challenge in creating ultra-low-loss materials, essential for both sensitive quantum sensors and the demanding environments within nuclear fusion reactors. By employing superconducting micro-resonators and analysing samples grown through different processes, the team demonstrates significantly improved sensitivity in measuring these losses at extremely low temperatures, and importantly, links increased nitrogen content within the diamond structure to higher levels of dielectric loss. This advancement offers a deeper understanding of how defects influence material performance and paves the way for fabricating diamond components with enhanced coherence and power handling capabilities.

Research on diamond has intensified due to its exceptional thermal, optical, and mechanical properties, making it a key material for advanced technologies. This work focuses on estimating losses induced by nitrogen-vacancy (NV) defects in both polycrystalline and single-crystalline diamond, a critical factor limiting the performance of diamond-based quantum devices. The team employs superconducting lumped element micro-resonators to probe these losses with high sensitivity, allowing for precise characterisation of the NV defect density and its impact on coherence times.

Low-Temperature Measurement of Diamond Dielectric Loss

Scientists developed a highly sensitive method to measure dielectric losses in single-crystal diamond substrates, crucial for advancing technologies requiring high coherence and power handling. The study employed superconducting thin-film micro-strip resonators, achieving exceptional quality factors, to surpass the limitations of traditional measurement techniques. Four diamond samples underwent analysis at extreme low temperatures, specifically within the sub-Kelvin range, to examine their dielectric losses within the framework of Two-Level Systems (TLS). These resonators function as circuits, enabling precise measurement of the material’s properties.

Researchers carefully designed the devices to minimize unwanted radiative losses, allowing them to focus on discerning the dominant contributions from conduction and dielectric losses. To differentiate between these loss mechanisms, scientists monitored the resonance frequency’s response to temperature variations. At temperatures between 3. 5 and 9 Kelvin, a blue-shift in resonance frequency indicated dominance of conductor losses, stemming from the superconductor’s kinetic inductance. However, measurements at ultra-low temperatures (sub-Kelvin) revealed an opposite trend, with frequency blue-shifts associated with higher temperatures, a signature of TLS dynamics. Defects within the diamond’s crystal lattice create localized energy states, contributing to TLS activity and energy dissipation. By analyzing the temperature-dependent permittivity, researchers established a relationship between the fractional resonance shift and the loss tangent at zero temperature, enabling precise quantification of dielectric losses.

Ultra-Low Dielectric Loss in Single-Crystal Diamond

Researchers have achieved significant breakthroughs in characterizing dielectric losses within single-crystal diamond samples, crucial for applications in high-power technologies and nuclear fusion. The study focused on quantifying ultra-low losses at sub-Kelvin temperatures using superconducting thin-film micro-strip resonators, a method capable of detecting losses with unprecedented sensitivity. Measurements of the tanδ₀ TLS, a key indicator of two-level system related losses, revealed that the J-Clone sample exhibited the lowest values, reaching approximately 7. 31x 10⁻⁶. Further analysis demonstrated a direct correlation between nitrogen inclusion levels and dielectric losses within the single-crystal samples.

The IrSCD sample, exhibiting higher nitrogen content, showed a tanδ₀ TLS of 1. 9525x 10⁻⁵, while the Clone sample registered 1. 508x 10⁻⁵. Complementary Raman spectroscopy confirmed these findings, revealing a strong peak at 1332cm⁻¹ in all samples, influenced by the presence of nitrogen-vacancy (NV) centers. Notably, the IrSCD sample displayed a large peak at 2250cm⁻¹, indicative of stable carbon-nitrogen bonds within the lattice.

While the J-Clone sample demonstrated the lowest TLS losses, measurements of the total loss tangent revealed higher values, due to boundary-induced scattering at the interface between joined tiles. This scattering, confined to a small region, dominated the total loss despite minimized TLS contributions. The Clone sample, grown as a single crystal, avoided this issue, exhibiting a significantly lower total loss tangent. These results demonstrate the potential for producing high-quality, low-loss diamond substrates through advanced growth and joining techniques.

Nitrogen Defects Limit Diamond Dielectric Loss

This research presents a detailed characterization of dielectric losses in synthetic diamond samples grown using different techniques. By employing superconducting micro-resonators, the team achieved highly sensitive measurements of losses at extremely low temperatures, allowing for precise determination of loss tangents within the framework of Two-Level System theory. All single-crystal samples exhibited lower losses compared to commercially available polycrystalline diamond, demonstrating the potential for improved materials in high-power applications. Complementary Raman spectroscopy revealed a correlation between nitrogen content and increased losses, with the presence of specific spectral features indicating the type and stability of nitrogen-related defects within the crystal lattice.

These findings contribute to a better understanding of defect-related losses and provide valuable insights for optimizing diamond growth processes. The authors acknowledge that the J-Clone sample exhibited a higher density of defects, contributing to increased losses, and that further investigation into the nature of these defects could lead to further improvements. Future work may focus on refining growth techniques to minimize defect formation and tailoring diamond properties for specific applications, such as high-power components in fusion reactors.