Bulk Niobium Research Articles

Direct measurement of microwave loss in Nb films for superconducting qubits

Niobium films are a key component in modern two-dimensional superconducting qubits, yet their contribution to the total qubit decay rate is not fully understood. The presence of different layers of materials and interfaces makes it difficult to identify the dominant loss channels in present two-dimensional qubit designs. In this paper, we present the study that directly correlates measurements of RF losses in such films to material parameters by investigating a high-power impulse magnetron sputtered (HiPIMS) film atop a three-dimensional niobium superconducting radio frequency (SRF) resonator. By using a 3D SRF structure, we are able to isolate the niobium film loss from other contributions. Our findings indicate that microwave dissipation in the HiPIMS-prepared niobium films, within the quantum regime, resembles that of record-high intrinsic quality factor of bulk niobium SRF cavities, with lifetimes extending into seconds. Microstructure and impurity level of the niobium film do not significantly affect the losses. These results set the scale of microwave losses in niobium films and show that niobium losses do not dominate the observed coherence times in present two-dimensional superconducting qubit designs, instead highlighting the dominant role of the dielectric oxide in limiting the performance. We can also set a bound for when niobium film losses will become a limitation for qubit lifetimes.

A high throughput facility for the RF characterisation Of planar superconducting thin films

Abstract Accelerator laboratories worldwide are researching copper radio frequency (RF) cavities coated with superconducting thin films to exceed the limits of bulk niobium. The development and RF testing of thin films on small planar samples is vital before cavity depositions. A team at Daresbury Laboratory have developed a cost-effective facility using a novel 7.8 GHz Choke Cavity for the RF characterisation of planar samples. RF chokes ensure that no electrical contact is required between the sample and the cavity. The main advantages are: a simple sample design (90 − 130 mm diameter disk with no sample-cavity welding) and easy operation using a LHe-free cryostat. This enables high sample throughput, with up to 3 sample tests per week, making the facility suitable for quick, systematic scanning of deposition parameters. With the sample thermally and physically isolated from the test cavity, it is possible to measure the average surface resistance, R s, directly using an RF-DC compensation method. Facility commissioning has been performed with bulk and thin film niobium samples. These tests have demonstrated the ability to measure R s at temperatures in the range 4 − 20 K and sample peak magnetic fields up to 3 mT. The minimum resolvable R s is 0.5 μΩ with typical uncertainties of 9 − 15%. The design, operation and commissioning of this facility is reported in this paper.

First Principle Study of Structural, Electronic, Mechanical and Optical Properties of Bulk Niobium Dichalcogenide NbX2 (X=S, SE) within a Visible Phonon Energy Range

An intensive study on structural, electronic mechanical and optical properties of bulk Niobium Dichalcogenides NbX2 (X=S, Se) was carried out using the first principle. The structural parameters such as Equilibrium Lattice Parameters, Volume, Bulk Modulus, and FirstDerivative Modulus were calculated to determine if the materials are energetically stable. Elastic constants were further obtained from which mechanical properties i.e. bulk, Young's and shear moduli and consequently Poisson's ratio were obtained. Based on the well-known Born stability conditions Bulk-NbS2 is most likely mechanically anisotropic ductile material. While Bulk-NbSe2 for the predicted B/G ratio in all three methods is less than a critical value of 1.75, hence this shows that NbSe2 is a brittle material exploring its electronic and optical properties whose motivation was to find out the most stable phase and ascertain if these materials could be used in various fields that suit their mechanical and optical properties. Furthermore, from the calculated optical spectra, plasma frequencies were analyzed which indicated the possibility of applying the material in plasmonic-related fields.

The nontrivial effects of annealing on superconducting properties of Nb single crystals

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, 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 . 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.

Thickness effect on superconducting properties of niobium films for radio-frequency cavity applications

Niobium-coated copper radio-frequency cavities are cost-effective alternatives to bulk niobium cavities, given the lower material costs of copper substrates and their operation in liquid helium at around 4.2 K. However, these cavities historically exhibited a gradual degradation in performance with the accelerating field. This phenomenon, not yet fully understood, limits the application of niobium thin film cavities in accelerators where the real-estate gradient needs to be maximized. Recent studies on niobium films deposited on copper using high power impulse magnetron sputtering (HiPIMS) technique show promising results in mitigating the performance degradation of niobium thin film radio-frequency cavities.This paper examines the effect of film thickness on the superconducting properties of niobium films deposited on copper using HiPIMS. The study provides insights into how the critical temperature, transition width, lower and upper critical fields, and critical current density vary with the film thickness. Increasing the thickness of niobium films deposited through HiPIMS is found to enhance superconducting properties and reduce densities of defects and structural irregularities in the crystalline lattice. This shows potential for enhancing overall performance and potentially mitigating the observed performance degradation in niobium thin film radio-frequency cavities. Additionally, the Ivry’s scaling relation among critical temperature, thickness, and sheet resistance at the normal state appears applicable to niobium films up to approximately 4 µm. This extends the previously confirmed validity for niobium films, which was limited to around 300 nm thickness.

Investigating the Superconducting Properties and Surface Morphology of Sputtered Nb Films on Cu Due to Laser Treatment

Bulk niobium is currently the material of choice for superconducting radio frequency (SRF) cavities and is a well matured process. However, it is possible that SRF cavities could be further improved beyond bulk Nb by sputtering thin Nb films onto Cu cavities. Copper has a greater thermal conductivity than Nb and is also easier to machine, whilst sputtering films on the surface reduces the amount of Nb used to fabricate the whole cavity. However, sputtering Nb on Cu produces other issues, for example, the surface quality of the Cu affects the quality of the Nb deposited on the surface and therefore the superconducting parameters. As the Nb on the surface is not perfect, the magnetic field produced by the RF can enter the cavity earlier than expected, producing RF losses, which can in turn lead to a quench. One approach is to treat the Nb post deposition by irradiating the surface using a laser to polish the surface of the Nb and increase the surface magnetic field that the cavity can maintain whilst remaining in the Meissner state. A magnetic field penetration experiment designed and built at Daresbury Laboratory has been used to measure the field of full flux penetration to characterise the effect of the laser treatment on the superconducting properties of the Nb. Surface characterisation and the response of the Nb in a DC magnetic field have also been performed to try and provide an explanation for the change in the superconducting properties. The results demonstrate that the laser treatment can lead to an increase in the magnetic field at which the flux penetrates from one side of the sample to the other, thus it could potentially improve the performance of Nb coated RF cavities.

Abrasion resistance of a Nb3Sn magnetron-sputtered coating on copper substrates for radio frequency superconducting cavities

The paper presents the first study of the abrasion resistance and degradation processes of magnetron-sputtered Nb3Sn coatings on electropolished (EP) or chemical polished (CP) copper substrates. The coating-substrate system is of interest for application in radio frequency superconducting cavities as an improvement on bulk niobium cavities. The microstructural and chemical properties of the substrates and coating were examined by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), electron backscatter diffraction, X-ray diffraction, and focused ion beam cross-sectioning. The abrasion resistance was investigated by scratch testing under progressive loads up to 200 N, after which the scratched surfaces were characterised by SEM/EDS to determine the failure mechanisms of the coatings. Several stages of coating damage were identified, including nodule compression, cracking, pitting, fragmentation, comminution and detachment. Nevertheless, substantial amounts of fragmented and embedded coating remained attached to the copper up to the highest load. The findings indicated similar abrasion resistance behaviour for the EP and CP conditions.

In Situ Studies of the Formation of Tungsten and Niobium Oxide Nanoparticles: Towards Automated Analysis of Reaction Pathways from PDF Analysis using the Pearson Correlation Coefficient

AbstractUsing Pair Distribution Function (PDF) analysis of in situ total scattering data, we investigate the formation of tungsten and niobium oxides in a simple solvothermal synthesis. We use Pearson Correlation Coefficient (PCC) analysis of the time resolved PDFs to both map the structural changes taking place throughout the synthesis and identify structural models for precursor and product through PCC‐based structure mining. Our analysis first shows that ultra‐small tungsten and niobium oxide nanoparticles form instantaneously upon heating, with sizes between 1.5 and 2 nm. We show that the main structural motifs in the nanoparticles can be described with structures containing pentagonal columns, which is characteristic for many bulk tungsten and niobium oxides. We furthermore elucidate the structure of the precursor complex as clusters of octahedra with O‐ and Cl‐ligands. The PCC based methodology automates the structure characterization and proves useful for analysis of large datasets of for example, time resolved X‐ray scattering studies. The PCC is implemented in ‘PDF in the cloud’, a web platform for PDF analysis.

Enhanced radio-frequency performance of niobium films on copper substrates deposited by high power impulse magnetron sputtering

We present a study of radio frequency properties of niobium films deposited on copper by two different approaches based on High Power Impulse Magnetron Sputtering, namely with a DC voltage biased substrate and with bipolar target voltage. Such approaches enable the synthesis of dense superconducting (SC) layers. The SC radio frequency losses of these films are characterized as a function of the applied RF magnetic field using a dedicated calorimetric method. We report on a significant reduction of the Q-slope phenomenon and the residual surface resistance in the characterized films, achieving similar values as those obtained on bulk niobium surfaces qualified with the same technique. Our results pave the way towards the realization of Nb/Cu coated accelerating cavities featuring a surface resistance 2–3 times lower than the state-of-the-art values at working frequencies of 400 and 800 MHz, making this technology even more appealing for future particle accelerators and colliders.

Low-Temperature Baking Effect of the Radio-Frequency Nb3Sn Thin Film Superconducting CavitySupported by the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2020410), the Major Research Plan of National Natural Science Foundation of China (Grant No. 91426303), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000), and the National Postdoctoral Program for Innovative Talents

The Nb3Sn thin film cavity, having the potential to be operated at a higher temperature and higher gradient compared to the cavity made from bulk niobium, is one of the most promising key technologies for the next-generation radio-frequency superconducting accelerators. In our work, several 1.3 GHz single-cell TESLA-shaped Nb3Sn thin film cavities, coated by the vapor diffusion method, were tested at Peking University and Institute of Modern Physics, Chinese Academy of Sciences. It was observed that the performance of the Nb3Sn thin film cavities in the tests without the slow cooling down procedure and the effective magnetic field shielding was significantly improved by using a low temperature baking at 100 °C for 48 hours. Although the peak electric field of the cavity remained unchanged, the rapid drop of the unloaded Q value (Q 0) with the increasing accelerating field (Q-slope) was effectively eliminated, resulting in an improvement of the Q 0 in the intermediate field region by ∼ 8 times. Furthermore, under better test conditions with the shielded magnetic field less than 5 mG and the slow cooling down procedure in the temperature range of 25–15 K, the Q 0 was still improved by about 20%. Our study shows that the low temperature baking can be an effective supplement to the effective post-treatment for the Nb3Sn thin film cavity.

Electrodeposition of copper applied to the manufacture of seamless superconducting rf cavities

Niobium thin film coated copper superconducting radio frequency elliptical cavities have demonstrated for many years their strong potential as an alternative to bulk niobium cavities. The thin film lower performance at high rf field is often attributed to the defects observed in the elaborated Nb layer, sometimes originated from defects inherited from the substrate itself. The currently used methods of manufacturing the copper elliptical substrates include several steps of electron-beam welding in order to join the half cells and the cutoffs which can contribute to defects and porosities. Seamless methods are nowadays developed in order to avoid welding steps and to decrease the global manufacturing cost of the cavities. We propose in this study an innovative alternative route in which the cavity is formed by electrodeposition of copper on a sacrificial aluminum mandrel. The strength of the process relies on the total absence of welding joints. Two different electroforming techniques using either direct current or pulsed plating have been investigated. The electroformed copper exhibited similar mechanical robustness, cryogenic properties and purity as the oxygen-free copper. In addition, the fabrication process was validated on test mandrels which mimic the geometry of 1.3 GHz cavities.

Characterizing materials for superconducting radiofrequency applications-A comprehensive overview of the quadrupole resonator design and measurement capabilities.

Test cavities to characterize superconductor samples are of great interest for the development of materials suitable for superconducting radio frequency (SRF) accelerator systems. They can be used to investigate fundamental SRF loss mechanisms and to study the material limitations for accelerator applications. Worldwide, this research is based on only few systems that differ in operating frequency, sample size and shape, and the accessible parameter space of frequency, temperature, and RF field strength. For useful performance predictions in future accelerators, it is important that the operating parameter range is close to that employed in accelerating systems. Since 2014, the Helmholtz-Zentrum Berlin has operated such a system built around a redesigned Quadrupole Resonator (QPR). It is based on a system originally developed at CERN. Important new design modifications were developed, along with new measurement techniques and insight into their limitations. In the meantime, an increasing number of laboratories are adopting the QPR for their measurement campaigns. This paper provides a comprehensive overview of the state-of-the-art, the wide spectrum of measurement capabilities, and a detailed analysis of measurement uncertainties, as well as the limitations one should be aware of to maximize the effectiveness of the system. In the process, we provide examples of measurements performed with Nb3Sn and bulk niobium.

Recent Results of High Temperature Vacuum Heat Treatment Program of SRF Resonators at IJCLab

In the framework of ESS, a vacuum furnace dedicated to High Temperature Heat Treatment under Vacuum (VH2T2) of SRF bulk niobium (Nb) cavities was developed and commissioned in May 2016. This furnace is mainly used for the reduction of interstitial hydrogen in niobium. VH2T2 is an effective process for curing SRF cavities from the so-called Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -deases phenomenon, which is due to Nb hydrid formation on the RF surface during the cool down of the resonator to cryogenic temperatures. IJCLab VH2T2 process is done at 650 °C during 10h00. The main differences between IJCLab recipe and the standard VH2T2 (SVH2T2) are 2 folds: 1) SVH2T2 is performed at 800 °C during 3h00, 2) IJCLab recipe is applied to spoke resonators equipped with Ti LHe tank. This process, will be applied to the whole series (26 cavities) of ESS 352 MHz Double-Spoke resonators. More than 28 cavities of various type (ESS Double-Spoke resonators, MYRRHA single spoke, ESS elliptical medium and high β cavities) were successfully heat treated according to our recipe and the last results are reported. Moreover, since 2 years we have launched the R&D program HELOISE with the financial support of IN2P3/CNRS. HELOISE is dedicated to Heat Treatment of SRF niobium cavities: it includes Low Temperature Baking, VH2T2, Nitrogen Infusion and Nitrogen Doping processes. HELOISE main objectives are: 1) understand the complex mechanisms of the physical phenomena involved in heat treatment processes, 2) investigate the effect of the parameters impacting the process, 3) develop and master, at large scale and with good reliability, heat treatment processes in order to produce low RF losses and high gradient SRF bulk niobium cavities. Finally, we will focus on the first experimental data on Nitrogen Infusion we obtained: the corresponding results are reported and discussed.

Vortex Dynamics and Dissipation under High-Amplitude Microwave Drive

In this paper, we describe the vortex dynamics under high-amplitude microwave drive and its effect on the surface resistance of superconductors. The vortex surface resistance is calculated with a Montecarlo approach, where the vortex motion equation is solved for a collection of vortex flux lines each oscillating within a random pinning landscape. This approach is capable of providing a detailed description of the microscopic vortex dynamics and in turn important insights into the microwave field amplitude dependence of the vortex surface resistance. The numerical simulations are compared against experimental data of vortex surface resistance at high microwave amplitude measured by means of bulk niobium superconducting-radio frequency cavities operating at 1.3 GHz. The good qualitative agreement of simulations and experiments suggests that the non-linear dependence of the trapped flux surface resistance with the microwave field amplitude is generated by progressive microwave depinning and vortex jumps.

Kinetically induced low-temperature synthesis of Nb3Sn thin films

Nb3Sn thin films are promising candidates for future application in superconducting radio frequency cavities due to their low surface resistivity, high critical temperature, and critical field, as compared to bulk niobium, which is the current state of the art. In this paper, we report the deposition of Nb3Sn thin films by magnetron co-sputtering at the extremely low temperature of 435°C. These thin films show a critical temperature of 16.3 K, a high critical current density of 1.60×105A/cm2, and a strong shielding effect. The key to achieving low-temperature growth is the independent kinetic control of Nb and Sn species in the sputtering process. From a technological viewpoint, the low-temperature approach paves the way for the use of Nb3Sn as a coating in cryogenic efficient copper based cavities, thereby avoiding the detrimental interdiffusion of Cu.

High-field Q-slope mitigation due to impurity profile in superconducting radio-frequency cavities

In this study, we present the recent insights into the origin of the high-field Q-slope in superconducting radio frequency cavities. Consequent hydrofluoric acid rinses are used to probe the radio frequency performance as a function of the material removal of two superconducting bulk niobium cavities prepared with low temperature nitrogen infusion. The study reveals that nitrogen infusion affects only the first few tens of nanometers below the native oxide layer. The typical high-field Q-slope behavior of electropolished cavities is indeed completely recovered after a dozen hydrofluoric acid rinses. The reappearance of the high-field Q-slope as a function of material removal was modeled by means of London's local description of screening currents in the superconductor, returning good fitting of the experimental data and suggesting that a layer of interstitial impurities with diffusion length on the order of tens of nanometers can mitigate high-field Q-slope.

Continuous Wave Operation of Superconducting Accelerating Cavities With High Loaded Quality Factor

Close to 780 superconducting 1.3-GHz accelerating cavities made of bulk niobium have been installed in the European X-ray free electron laser (E-XFEL) linear accelerator. The linac can operate at a mean gradient (Eacc) of ca. 23 MV/m in the nominal short pulse (SP) operation mode with 1.4-ms pulses and a repetition rate of 10 Hz. An R&D program at Deutsches Elektronen-Synchrotron (DESY) is ongoing since 2011 on the feasibility of a continuous wave (CW) upgrade of the E-XFEL accelerator. In this publication, a modification of the low-level radio frequency (LLRF) control system to the CW mode is presented. Currently, the control system successfully stabilizes accelerating field for the SP mode. The demanding E-XFEL specifications of 0.01% for the field amplitude and 0.01° for the phase stability are required to keep the photon beam quality for all experimental stations. The proposed modification has been implemented and tested to verify the system versatility for SP and CW operation modes. The tests were conducted for a prototype E-XFEL cryomodule XM-3. As for the series cryomodules, this prototype contains eight superconducting 1.3-GHz cavities. The results presented here confirm that the same LLRF system can be used with minor software modifications for either operation mode (CW or SP). However, the CW mode requires a more complex RF-power management and more precise control, especially when cavities operate with very high loaded quality factor (Ql) of the order of 6E7. The achieved stability for such highQl is also presented and discussed.

Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities

We use scanning tunneling microscopy (STM) and spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS) to investigate the effect of nitrogen doping on the surface electronic and chemical structures of cutouts from superconducting $\mathrm{Nb}$ radio-frequency cavities. The goal of this work is to get insights into the fundamental physics and materials mechanisms behind the striking decrease of the surface resistance with the radio-frequency magnetic field, which has been observed on $N$-doped $\mathrm{Nb}$ cavities. Our XPS measurements reveal significantly more oxidized $\mathrm{Nb}$ $3d$ states and a thinner metallic suboxide layer on the $N$-doped $\mathrm{Nb}$ surfaces, which is also confirmed by tunneling spectroscopy measurements. In turn, tunneling measurements performed on native surfaces as well as on $\mathrm{Ar}$-ion sputtered surfaces allow us to separate the effect of $N$ doping on the surface-oxide layer from that on the density of states in the bulk. Analysis of our tunneling spectra in the framework of a model of a proximity-coupled normal layer at the surface [A. Gurevich and T. Kubo, Phys. Rev. B 96, 184515 (2017)] is consistent with the hypothesis that $N$-doping ameliorates lateral inhomogeneities of superconducting properties on the surface and shrinks the metallic suboxide layer. For the $\mathrm{Ar}$ sputtered surfaces, we also find evidence that $N$ doping changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value corresponding to a minimum surface resistance. The totality of our experimental data suggests that the $N$ doping provides an effective tuning of the density of states in such a way that it can result in a decrease of the surface resistance with the radio-frequency field, as predicted by calculations of the nonlinear low-frequency electromagnetic response of dirty superconductors. Furthermore, STM imaging of vortex cores shows a slightly reduced average superconducting gap and a shorter coherence length in the $N$-doped $\mathrm{Nb}$ samples as compared to typically prepared $\mathrm{Nb}$ samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

Parametric Amplification at Ka Band via Nonlinear Dynamics in Superconducting 3D Cavities

Modern parametric amplifiers are based on lithographically produced superconducting thin-film planar transmission line structures. These paramps rely on resonant structures with embedded nonlinear elements to stimulate intermodulation with a stronger pump tone that gives rise to signal gain when certain conditions are satisfied. Such paramps have not yet been realised in superconducting 3D waveguide resonators. Possible applications of these devices include detector systems that are based on 3D waveguide such as dark matter detectors and quantum computers. Reported here are the results of an investigation of a 30.64 GHz series circular waveguide resonance machined from bulk niobium showing parametric gain of up to 2 dB in the presence of a stronger pump tone 10 kHz above in frequency. The gain is largest on abrupt jumps of the transmission spectra of the resonance, which may be a result of weak-link formation on the superconducting surfaces.

Nb Sputtered 325 MHz QWR Cavities for CIADS

The possibility for adopting niobium thin film coated copper (Nb/Cu) quarter wave resonators (QWRs) in the low energy section of CiADS project[1] is being evaluated. Comparing with bulk niobium cavities, the Nb/Cu cavities feature a much better thermal and mechanical stability at 4.5 K. Two 325 MHz Nb/Cu QWR cavities have been fabricated at IMP, to demonstrate whether the niobium coated copper cavity technique can meet the requirements of CiADS. The cavity is coated with biased DC diode sputtering technique. This paper covers resulting film characters, vertical tests with the evolution of the sputtering process, and improvements to mitigate issues we met.