Nitral Superconducting Density of States Advances Cosmic Radiation Device Quality

Researchers are increasingly focused on nitride-based superconductors to enhance device performance, particularly for sensitive applications like cosmic radiation detection. Jose Antonio Moreno, Pablo García Talavera, and Alba Torras-Coloma, from the Instituto Nicolás Cabrera and IFAE respectively, alongside et al., have investigated the superconducting properties of nitridized Aluminum (NitrAl) , a material recently shown to exhibit improved critical temperatures and magnetic field resilience compared to standard Aluminium. This study is significant because, despite its promise, the fundamental microscopic behaviour of NitrAl remains poorly understood. Using Scanning Tunneling Microscopy, the team measured the superconducting density of states in NitrAl thin films, revealing a near-zero in-gap density and a superconducting gap distribution consistent with theoretical predictions, alongside surprising nanometre-scale variations in gap values , suggesting a more homogeneous superconducting state than typically observed in thin films.

This breakthrough centres on a detailed investigation of the superconducting density of states within NitrAl, a material recently identified as a promising candidate for next-generation superconducting quantum circuits0.4 K with a resistivity of 48.5 μΩcm at 4 K. The study unveils that the in-gap density of states remains zero up to approximately ħω = 250 μeV, and the superconducting gap distribution centres around ∆0 = 360 μeV, aligning closely with the theoretical expectations of the Bardeen-Cooper-Schrieffer (BCS) theory.

The team achieved a precise mapping of the superconducting gap at the nanometer scale, discovering variations of approximately 10% across different regions of the sample. These findings establish a superconducting gap larger than that of pure aluminum, and importantly, demonstrate a significantly more homogeneous distribution of superconducting properties within the NitrAl film compared to typical thin films. Experiments show that the topography of the NitrAl thin film is remarkably smooth, contrasting with the granular structure often observed in other enhanced superconducting materials like Granular Aluminum. This smoothness, coupled with the observed metallic behaviour and fully developed superconducting gap of ∼0.37mV, positions NitrAl as a compelling alternative for quantum circuit fabrication, potentially mitigating decoherence issues associated with oxide-based materials.
This research establishes STM as a powerful screening tool for advanced superconducting materials, enabling detailed assessment of spatial variations in the superconducting density of states. The study reveals that NitrAl remains superconducting even under magnetic fields exceeding 500 mT applied perpendicular to the film surface, further enhancing its suitability for demanding quantum applications. By meticulously analysing the shape of the energy gap and the local fluctuations in the superconducting density of states, scientists are paving the way for the development of more robust and reliable superconducting quantum circuits. The work opens new avenues for engineering materials with tailored superconducting properties, addressing critical challenges in quantum technology and potentially extending the capabilities of devices used in cosmic radiation sensing and beyond.

NitrAl Film Growth and STM Characterisation reveal interesting

Scientists investigated the superconducting properties of nitridized Aluminum (NitrAl) thin films, a promising material for advanced superconducting devices. Recent research indicates NitrAl exhibits enhanced critical temperatures and magnetic field resilience compared to pure Aluminum, prompting a detailed examination of its microscopic characteristics0.99% Al target alongside a nitrogen/argon gas mixture exceeding 5.5 N N2 and Ar. The team meticulously controlled the nitrogen-to-argon flow ratio during deposition, setting it at 8.33% to produce sample “G” as detailed in a prior publication.
This film, exhibiting a room temperature resistivity between pure Al and insulating aluminum nitride, was then affixed to a STM sample holder using conductive silver epoxy, ensuring electrical connection via a silver epoxy path. A platinum-iridium tip, prepared in situ following established protocols, was used to probe the sample’s surface. The STM itself was thermally anchored to a dilution refrigerator, achieving base temperatures of 100 mK, and incorporated home-made ultra-low noise electronics capable of an 8 μeV energy resolution. Experiments involved applying magnetic fields of up to ∼500 mT perpendicular to the film surface using a superconducting coil from Oxford Instruments.

Topographical maps were acquired, revealing a smooth surface, and then converted into maps of tunneling conductance by halting the feedback loop and sweeping the bias voltage to generate current-voltage curves. The derivative of these curves yielded the tunneling conductance as a function of bias, with sufficient data points collected to maintain the desired energy resolution. Researchers normalised the conductance curves at voltages significantly exceeding the superconducting gap to facilitate accurate comparison. Furthermore, the study pioneered a method for mapping the superconducting gap by identifying the position of quasiparticle peaks in the tunneling conductance, averaging peak positions at both positive and negative biases. Maps of the onset voltage for finite conductance were also constructed, determining the voltage at which tunneling current became non-zero within experimental uncertainty. This innovative approach enabled the team to observe a superconducting gap of approximately 0.37mV, aligning with the BCS value of ∆= 1.76kBTc for Tc ∼2.4 K, and to detect spatial variations in the gap of around 10% across the sample, demonstrating a spatially more homogeneous superconducting gap than typically found in thin films.

NitrAl exhibits clean gap and BCS alignment, indicating

Scientists have demonstrated that nitridized Aluminum (NitrAl) exhibits enhanced superconducting properties, positioning it as a promising material for advanced quantum circuit applications. Experiments revealed a remarkably clean superconducting gap in the NitrAl thin film, with in-gap density of states remaining zero up to an energy of ħω = 250 μeV. Data shows a distribution of superconducting gap values centered around ∆₀ = 360 μeV, closely aligning with the BCS expectation of ∆= 1.76kBTc.

The team measured a room temperature resistivity between pure Al and fully insulating aluminum nitride, indicating a successful balance between conductivity and stability. Tests prove that the superconducting gap values vary at the nanometer scale by approximately 10% when probing different regions of the 100nm-thick NitrAl film. Measurements confirm that this spatially homogeneous gap is larger than that found in pure Aluminum, suggesting improved coherence and resilience. The NitrAl sample, grown by sputtering deposition, exhibited a critical temperature of Tc = 2.4 K and a 4K electrical resistivity of ρ₄K = 48.5 μΩcm.

Researchers recorded a fully developed superconducting gap of approximately 0.37mV, again consistent with the BCS value for the observed Tc of around 2.4 K. The breakthrough delivers superconductivity even under magnetic fields reaching up to 500 mT applied perpendicular to the film’s surface. Analysis of tunneling conductance maps, obtained with an energy resolution of 8 μeV, allowed for precise determination of the superconducting gap and its spatial variations. The study demonstrates that STM is a powerful tool for screening materials for quantum devices by mapping the spatial dependence of the superconducting density of states.

Furthermore, the topography of the NitrAl thin film was found to be remarkably smooth, unlike the granular structure often observed in other enhanced superconducting materials like GrAl. The work highlights the potential of NitrAl to overcome limitations associated with oxide-based decoherence sources present in GrAl. Results. This work establishes STM as a valuable tool for screening materials intended for device applications by measuring the spatial dependence of the superconducting density of states. The observed zero density of states over a significant energy range suggests NitrAl is a promising candidate for fabricating highly-coherent superconducting qubit devices. The authors acknowledge that the origin of the spatial inhomogeneities remains unclear, potentially stemming from morphological, structural, or compositional differences inherent in polycrystalline thin films. Future research should focus on optimising thin film-substrate interface processing and deposition conditions, alongside systematic investigation of a wider range of NitrAl thin films, to further refine and tune these spatial inhomogeneities, potentially enhancing device performance.