Nb Layers Research Articles

Superconducting routing platform for large-scale integration of quantum technologies

To reach large-scale quantum computing, three-dimensional integration of scalable qubit arrays and their control electronics in multi-chip assemblies is promising. Within these assemblies, the use of superconducting interconnections, as routing layers, offers interesting perspectives in terms of (1) thermal management to protect the qubits from control electronics self-heating, (2) passive device performance with significant increase of quality factors and (3) density rise of low and high frequency signals thanks to minimal dispersion. We report on the fabrication, using 200 mm silicon wafer technologies, of a multi-layer routing platform designed for the hybridization of spin qubit and control electronics chips. A routing level couples the qubits and the control circuits through one layer of Al0.995Cu0.005 and superconducting layers of TiN, Nb or NbN, connected between them by W-based vias. Wafer-level parametric tests at 300 K validate the yield of these technologies while low temperature electrical measurements in cryostat are used to extract the superconducting properties of the routing layers. Preliminary low temperature radio-frequency characterizations of superconducting passive elements, embedded in these routing levels, are presented.

Interfacial strengthening mechanism of C/C/Nb0.74CoCrFeNi2/Nb brazed joints: Stress-relieving effects of the FCC phase

Joining ceramics and metals has long been a key issue for manufacturing structures that are resistant to high temperature. This study joined a carbon-fiber-reinforced carbon-matrix (C/C) composite and Nb using a Nb0.74CoCrFeNi2 eutectic high-entropy alloy filler, and the evolution mechanism of the microstructure and mechanical properties of the joints were systematically studied. The eutectic structure composed of face-centered cubic (FCC) and Laves phases gradually disappeared with the increasing temperature, while the Nb(s,s) layer thickness increased gradually. The lattice mismatch was ∼97% and ∼2.8% at the Cr23C6/C/C and FCC/C/C interfaces, respectively. Considering differences in the coefficient of thermal expansion and lattice mismatch, an increase in FCC phase content at the C/C interface is conducive to reinforcing the joint strength. At 1260 °C, the combined effects of the highly deformable FCC phase and high-strength Cr23C6 increased the strength of the mixing zone, causing fracture to occur in the Nb(s,s) layer. These results are expected to contribute to further improving the joining of dissimilar materials.

Study on Corrosion Resistance and Biological Properties of the Double Glow Plasma Nb-Zr Biological Implantation Alloying Layers

In order to improve the corrosion resistance of implant materials and understand the corrosion mechanisms, we prepared a biomedical Nb-Zr alloying layer on 316L stainless steel using double-layer glow plasma surface-alloying technology and investigated the effects of gas pressures on its surface structure, mechanical properties, and corrosion behavior. In particular, the surface states of the substrate and alloying layers were investigated using 3D confocal micrographs, the water contact angle, and UV reflectance, which aims to study the effect of the surface quality on corrosion resistance and discuss the corrosion mechanisms. The results show that the working pressure has an effect on the current density, the sputtering amount of the alloying elements, and the diffusion process of the alloying elements during glow discharge. The Nb-Zr alloying layer prepared under a pressure of 40 Pa had a uniform and dense surface structure, and the distribution was island-like. A Nb-Zr alloying layer with a thickness of 15 μm was successfully obtained, including the diffusion layer and the deposition layer. Simultaneously, the elements Nb and Zr were gradually distributed along the depth, and a high Nb concentration formed in the Nb-Zr alloying layer. The solid solution formed by Zr in the Nb layer significantly improved the microhardness and corrosion resistance of the substrate. The Nb-Zr alloying layer prepared under a pressure of 40 Pa had the lowest corrosion current density and excellent corrosion resistance, which originated from the passive film formed by the Nb-Zr alloying layer that could inhibit the invasion of corrosive ions and improve the corrosion resistance. In addition, the Nb-Zr alloying layer could promote cell proliferation during long-term use and had good biocompatibility. Our study provides an efficient, high-quality processing method for the surface modification of biomedical metallic materials to form thicker Nb-Zr alloying layers as a cost-effective alternative to bulk Nb-based alloys.

Nb/NiTi laminate composite with high pseudoelastic energy dissipation capacity

NiTi shape memory alloys (SMAs) with marked hysteresis between the forward and reverse martensitic transformations are attractive for the applications in passive damping. However, the stress hysteresis of NiTi SMAs is usually 150–300 MPa, limiting their energy dissipation to ∼10 MJ/m3. Here, we designed a bulk Nb/NiTi laminate composite with a reversible energy dissipation capacity of ∼30 MJ/m3 and a plateau strength of ∼750 MPa, which are outstanding among the common bulk energy dissipation materials. This high performance is attributed to the strong interaction between the pseudoelastic NiTi and the non-pseudoelastic Nb lamellae within the composite. The intrinsically plastic Nb layers deform in a pseudo-super elastic manner, involving elastic lattice deformation, reversible motion of dislocations, and reversed plastic deformation. These are enabled by the unique phenomenon of lattice strain matching the martensitic transforming. This study offers a new pathway for the design of high energy dissipated bulk materials.

Photo-Excitation Band-Structure Engineering of 2H-NbSe2 Probed by Time- and Angle-Resolved Photoemission Spectroscopy

We investigated the nonequilibrium electronic structure of 2H-NbSe$_2$ by time- and angle-resolved photoemission spectroscopy. We find that the band structure is distinctively modulated by strong photo-excitation, as indicated by the unusual increase in the photoelectron intensities around E$_F$. In order to gain insight into the observed photo-induced electronic state, we performed DFT calculations with modulated lattice structures, and found that the variation of the Se height from the Nb layer results in a significant change in the effective mass and band gap energy. We further study the momentum-dependent carrier dynamics. The results suggest that the relaxation is faster at the K-centered Fermi surface than at the $\Gamma$-centered Fermi surface, which can be attributed to the stronger electron-lattice coupling at the K-centered Fermi surface. Our demonstration of band structure engineering suggests a new role for light as a tool for controlling the functionalities of solid-state materials.

Distribution of Hydrogen and Defects in the Zr/Nb Nanoscale Multilayer Coatings after Proton Irradiation.

Radiation damage is one of the significant factors limiting the operating time of many structural materials working under extreme conditions. One of the promising directions in the development of materials that are resistant to radiation damage and have improved physical and mechanical properties is the creation of nanoscale multilayer coatings (NMCs). The paper is devoted to the experimental comprehension of changes in the defect structure and mechanical properties of nanoscale multilayer coatings (NMCs) with alternating layers of Zr and Nb under irradiation. Series of Zr/Nb NMCs with different thicknesses of individual layers were fabricated by magnetron sputtering and subjected to H+ irradiation. The evolution of structure and phase states, as well as the defect state under proton irradiation, was studied using the methods of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction analysis (XRD), glow discharge optical emission spectroscopy (GDOES), and positron annihilation spectroscopy (PAS). The layer-by-layer analysis of structural defects was carried out by Doppler broadening spectroscopy (DBS) using a variable-energy positron beam. To estimate the binding energy and the energy paths for the hydrogen diffusion in Zr/Nb NMCs, calculations from the first principles were used. When the thickness of individual layers is less than 25 nm, irradiation causes destruction of the interfaces, but there is no significant increase in the defect level, the S parameter (open volume defects amount) before and after irradiation is practically unchanged. After irradiation of NMC Zr/Nb with a thickness of layers 50 and 100 nm, the initial microstructure is retained, and the S parameter is significantly reduced. The GDOES data reveal the irregular H accumulation at the interface caused by significant differences in H diffusion barriers in the bulk of Zr and Nb multilayers as well as near the interface’s region.

Anomalous Critical Field Dependence of Cr-Nb-Cr Trilayers on Niobium Thickness

Cr-Nb-Cr trilayer thin films were deposited using DC/RF sputtering with Cr layer thickness maintained at 7.5 nm and Nb thickness varying from 30 nm to 229 nm, in order to study the interplay of antiferromagnetism and superconductivity. Various viewpoints have been expressed in the literature with respect to analyzing the anomalous properties that have been identified in superconductor/magnetic thin films. The study of Cr-Nb-Cr thin films were taken up to identify the existence of similar anomalous superconducting properties on account of antiferromagnetic-superconductor interfaces. X-ray reflectivity measurements showed high quality of thin film structure with minimum surface and interface roughness. Transport measurements down to 2K were carried out in the presence of magnetic fields up to 2 T applied perpendicular to the trilayers to deduce Tc. The suppression of Tc due to Cr layer is larger than proximity effect of similar metal films. It could be explained by proximity effect using antiferromagnetic Cr layer. Upper critical field measurements show a distinct nonmonotonic dependence of upper critical magnetic field and the slope on Nb layer thickness. The analysis due to WHH theory to deduce upper critical fields for the trilayer thin films studied did not match with experimental values. Although studies performed on Cr-Nb-Cr trilayers did not show any anomaly in Tc, it clearly showed a depression of Tc much largerthan proximity effect, non monotonic behavior in Bc2(0) and dBc2/dTc behavior with Nb layer thickness.

Deformation and fracture of Nb/α-Al2O3 composites in compression and bending studied by optical methods

AbstractThe fracture behavior of Nb/α-Al2O3composites was investigated both in compression tests and bending tests using an optical full-field method for the observation of strain fields on the metal surfaces. Joints of polycrystalline materials were prepared by high-vacuum diffusion bonding. The building up of the strain near the interface during the tests, finally leading to the fracture, was studied by examining the strain and displacement fields of the whole metal layer. In compression tests, two types of fracture, fracture of alumina and fracture of interfaces, were detected by cross-examination of the stress –strain curve and the strain maps. The fracture of alumina in compression tests was influenced by the Nb layer thickness and can be rationalized by an extension of Cottrell’s model. For four-point bending tests, the results of the strain maps, determined experimentally by means of the optical method, confirmed the strain maps resulting from finite element simulations.

The influence of orbital moments in anomalous Hall effect in Co/Ni multilayers with perpendicular magnetic anisotropy

We focus on the anomalous Hall effects (AHEs) of Co/Ni multilayers with perpendicular magnetic anisotropy (PMA) by using Ta, Nb, and Cu as the buffer and top layer. An un-conventional AHE behavior was found in which the AH resistance exhibits two anti-symmetric peaks in the presence of a magnetic field. Moreover, Co/Ni multilayers with a Ta neighboring layer show reverse AH resistance compared to the Nb and Cu neighboring layers, except Ta bottom and Cu capping layers. The former can be explained by considering the influence of the external magnetic field on the interfacial spin orbit interaction due to spontaneous symmetry breaking at the ferromagnetic (FM)/FM layer interface. Furthermore, the reverse Co/Ni AHE with a Ta adjacent layer can be interpreted as the leakage spin current of proximity effects from Ta due to its larger spin–orbit coupling, and finally, taking the shunting action of the Cu layer into account, the Co/Ni AHE with Ta and Cu adjacent layers can also be explained. Our results provide a clear physics picture of the AHE in a two-dimensional nano-scaling FM/FM interface with PMA; in particular, this work shows that the non-magnetic adjacent layer with large spin–orbit coupling will play an important role in the understanding of AHE in two-dimensional FM multilayers.

Microstructure, Mechanical and Tribological Properties of Advanced Layered WN/MeN (Me = Zr, Cr, Mo, Nb) Nanocomposite Coatings.

Due to the increased demands for drilling and cutting tools working at extreme machining conditions, protective coatings are extensively utilized to prolong the tool life and eliminate the need for lubricants. The present work reports on the effect of a second MeN (Me = Zr, Cr, Mo, Nb) layer in WN-based nanocomposite multilayers on microstructure, phase composition, and mechanical and tribological properties. The WN/MoN multilayers have not been studied yet, and cathodic-arc physical vapor deposition (CA-PVD) has been used to fabricate studied coating systems for the first time. Moreover, first-principles calculations were performed to gain more insight into the properties of deposited multilayers. Two types of coating microstructure with different kinds of lattices were observed: (i) face-centered cubic (fcc) on fcc-W2N (WN/CrN and WN/ZrN) and (ii) a combination of hexagonal and fcc on fcc-W2N (WN/MoN and WN/NbN). Among the four studied systems, the WN/NbN had superior properties: the lowest specific wear rate (1.7 × 10−6 mm3/Nm) and high hardness (36 GPa) and plasticity index H/E (0.93). Low surface roughness, high elastic strain to failure, Nb2O5 and WO3 tribofilms forming during sliding, ductile behavior of NbN, and nanocomposite structure contributed to high tribological performance. The results indicated the suitability of WN/NbN as a protective coating operating in challenging conditions.

Thickness modulations enable multi-functional spin valves based on Van der Waals hetero-structure

Spintronic devices serve as important components of the next-generation large scale integrated circuits, but until today, spintronic devices of different kinds can only be realized by using different materials and structures, methods of controlling device properties and realizing different functions of devices are still wanted. Here, we report on the first realization of spin valves with three distinct magnetoresistance behaviors in a single hetero-structure of Fe3GeTe2 (FGT)/Nb-doped MoS2 (MoS2: Nb)/FGT by varying the thickness of the sandwiched MoS2: Nb layer. We propose a possible scenario for the spacer layer’s functions and corresponding mechanisms of three different types of spin valves, where the MoS2: Nb layer works as a tunneling barrier, a spin-filter, and a spin potential keeper, respectively. This work demonstrates a novel and promising method for controlling the spin valve properties and functions in a single material and structure system, which significantly benefits the integration of spin devices on a single micro-chip. More importantly, we propose a novel method to tune the properties of different types of spintronic devices.

Superconducting critical temperature and softening of the phonon spectrum in ultrathin Nb and NbN/graphene hybrids

Superconductivity is studied in hybrids consisting of ultrathin superconducting film/few layer graphene. Two different superconductors are used for this purpose, Nb and NbN. An increase in the superconducting critical temperature T c is observed when graphene is put into contact with Nb. The largest increase is obtained for the thinnest Nb layer, which has a T c 8% larger with respect to the single Nb film. In the case of NbN the effect is not as pronounced. Experimental data are discussed by considering the possible modification of the phonon spectrum in the superconductor due to the presence of the graphene. Within an elementary one-dimensional model based on elastic coupling between nearest-neighbor atoms, we demonstrate that the phonon spectrum in the superconductor is modified at low energies with the subsequent enhancement of the effective electron–phonon coupling constant. While the strong oscillating nature of the electron–phonon interaction, α 2(ω), in NbN could lead to the insensitivity of T c on the low-energy phonons generated by the graphene, the almost constant behavior of α 2(ω) in Nb favors the increase of the superconducting critical temperature.

Influence of the position of the Josephson junction in the base Nb layer on modulation characteristics of the Josephson current

We measured the two-dimensional magnetic field dependence of a Josephson current through an Nb/Al-AlOx/Nb Josephson tunnel junction by applying external magnetic fields (Hx, Hy) parallel to the junction plane from two directions. We obtained much information about current distribution within the Josephson junction from a two-dimensional scan of the external magnetic fields (Hx, Hy). We also measured the perpendicular magnetic field Hz dependence of a Josephson current. When the perpendicular magnetic field Hz was applied to the Josephson junction, the Josephson current was modulated by the parallel component of the applied magnetic field Hz produced by the Meissner effect of the base Nb layer. In this study, some Josephson junctions were fabricated on the base Nb layer in our sample, and we investigated how the position of the Josephson junction on the base Nb layer influenced the modulation characteristics of the Josephson current when the perpendicular magnetic field Hz was applied to the junction. The junction at the center of the base Nb layer was not easily affected by the perpendicular magnetic field Hz because of the Meissner effect of the base Nb layer, whereas the junction on the edge of the base Nb layer was susceptible to the perpendicular magnetic field Hz.

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.

Growth and mechanical properties of Nb[sbnd]Cr carbide coatings on graphite by TRD technique

Niobium‑chromium (NbCr) carbide coatings were deposited on graphite using the thermo-reactive deposition and diffusion (TRD) technique. The microstructure, hardness, and coefficient of thermal expansion were characterized to search for the optimal treatment parameters for hard and adherent coatings on graphite. The growth rate increased with treatment temperature for fixed treatment duration and remained constant at a constant temperature for about 6 h. The coatings were found to consist of alternating Nb and Cr layers, and this interesting multilayer structure might be explained by the thermodynamics of oxide reduction and carbide formation reactions. Compared to the single-carbides layer, the hardness of the complex coatings increased from about 2500 HV to above 3000 HV, and displayed a smaller gradient of the depth-dependent hardness from the coating to the substrate. The variation of coefficient of thermal expansion of the samples with temperature was similar to that of the graphite substrate at temperatures from 40 to 450 °C. These results showed the effectiveness of the TRD method for synthesizing hard and adherent carbide coatings on graphite, and for other coating/substrate systems with sharply dissimilar mechanical properties.

Microstructural and Superconducting Radiofrequency Properties of Multilayer Sequentially Sputtered Nb3Sn films

Nb3Sn is considered as a potential candidate for superconducting radiofrequency cavities for particle acceleration due to its higher transition temperature of 18.3 K and higher superheating field of 400 mT. Nb3Sn films can be grown inside the surface of a Nb cavity by sequentially sputtering multiple layers of Nb and Sn thin films followed by annealing at 950 °C for 3 h. We report on the properties of Nb3Sn films grown on Nb substrates by magnetron sputtering. The films' crystal structure, surface morphology, and composition were characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The films had a polycrystalline Nb3Sn structure with a fine-grain surface and an atomic Sn composition of ~23%. The RF surface resistance of the films was measured for different temperatures at 7.4 GHz to understand the feasibility of this method for the SRF application. The RF surface resistance of the films was 5 mΩ at 12 K, which is about 2 orders of magnitude higher than 60 μΩ previously measured in Nb3Sn films grown by Sn vapor diffusion. The sputtered film had a superconducting transition at 17.2 K, which is also lower than 17.9 K observed in Nb3Sn film prepared by vapor diffusion.

Defect depth profiling of sputter-deposited Cu/Nb bilayers using a positron accelerator

Ultrathin bilayers of Cu/Nb viz. Nb/Cu/Si and Cu/Nb/Si have been deposited on Si substrate using DC and RF magnetron sputtering. The structural characterization of bilayers has been carried out using X-ray reflectivity (XRR) to determine the thickness and density of individual layers. The thickness of Nb and Cu layer in the bilayers ranges from ∼ 44–71 nm and 6–11 nm, respectively. Defect depth profiling of bilayers has been carried out using a positron accelerator. The measured depth profiles, i.e., S-parameter profiles have been fitted using VEPFIT, a program that factors the positron implantation profile and its annihilation to estimate S-parameter and positron diffusion length in individual layers. We present a detailed and systematic analysis of the measured S-profiles using VEPFIT to evaluate S-parameter and positron diffusion length in individual layers of the bilayer structures. These parameters are representative of defect characteristics of the layers. In this work we also highlight the nuances of VEPFIT analysis useful to extract defect characteristics in bilayers. The results indicate that defect microstructure of the layer depends on the thickness, deposition method and irradiation. Defect density is seen to increase with the reduction of thickness of the layer. RF sputtered layer has more defects as compared to DC sputtered layers. Positron depth profiling does not reveal distinct interface between the two metallic layers as well as layer and the substrate. A systematic characterization of defect in ultrathin structures is crucial to establish a correlation between defect microstructure and properties of nanolayer composites for advanced applications.

Electrodeposition of Niobium from the CsBr-KBr-NbBr3 Melt

The CsBr-KBr-NbBr3 melt was proposed as a promising electrolyte for obtaining high-quality niobium coatings. This melt is less aggressive and provides a higher deposition rate of dense Nb layers compared to the fluoride and chloride electrolytes commonly used for electrodeposition of niobium. The effect of temperature and cathode current density during electrodeposition from the CsBr-KBr-NbBr3 melt on the structure and morphology of the Nb coatings was investigated. The conditions for the deposition of thick, dense, well-adherent Nb coatings were found. Cyclic voltammetry was used to study the regularities of the cathode process in the CsBr-KBr (57 mol% CsBr − 43 mol% KBr) - NbBr3 melts containing 0.4, 1.6, and 4.45 wt% Nb in a temperature range of 893 to 1013 K. A two-stage mechanism with the participation of Nb(III) and Nb(II) ions was proposed and the effective diffusion coefficient of niobium cations in the bromide melt was estimated based on a comparison of data obtained by cyclic voltammetry and numerical simulation.

Nb Texture Evolution and Interdiffusion in Nb/Si-Layered Systems.

In this paper, we present a detailed study on the microstructure evolution and interdiffusion in Nb/Si-layered systems. Interlayer formation during the early stages of growth in sputter-deposited Nb-on-Si and Si-on-Nb bilayer systems is studied in vacuo using a high-sensitivity low-energy ion-scattering technique. An asymmetric intermixing behavior is observed, where the Si-on-Nb interface is ∼2× thinner than the Nb-on-Si interface, and it is explained by the surface-energy difference between Nb and Si. During Nb-on-Si growth, the crystallization of the Nb layer occurs around 2.1 nm as-deposited Nb thickness with a strong Nb(110)-preferred orientation, which is maintained up to 3.3 nm as-deposited Nb thickness. A further increase in the Nb layer thickness above 3.3 nm results in a polycrystalline microstructure with a reduced degree of texture. High-resolution cross-sectional transmission electron microscopy imaging is performed on Nb/Si multilayers to study the effect of the Nb layer texture on interdiffusion during low-temperature annealing. Nb/Si multilayers with amorphous 2 nm Nb layers and strongly textured 3 nm thick Nb layers, with limited grain-boundary pathways for diffusion, show no observable interdiffusion during annealing at 200 °C for 8 h, whereas in a Nb/Si multilayer with polycrystalline 4 nm thick Nb layers, a ∼1 nm amorphous Nb/Si interlayer is formed at the Si-on-Nb interface during annealing.

Magnonic control of the superconducting spin valve by magnetization reorientation in a helimagnet

We propose a method to control a bilayer superconducting spin valve (SSV) which does not perturb its superconducting state and is suitable for energy saving cryogenic electronics. This SSV consists of a superconducting layer and a helimagnetic layer of B20 family compounds, namely, Nb and spiral antiferromagnet MnSi. Thanks to unique properties of MnSi—broken inversion symmetry and cubic crystal lattice—there are a few ground state magnetic configurations with different directions of the magnetic spiral, divided by a potential barrier. Superconductivity in such a bilayer is controlled by the reorientation of the spiral vector in the MnSi layer, which leads to a change in the critical temperature of the Nb layer due to the proximity effect. The switching is proposed to be carried out by a several hundred ps in duration magnetic field pulse of several kOe in magnitude. Such a pulse does not destroy the superconducting state of the Nb layer by itself but leads to the excitation of magnons in the MnSi layer, which triggers the reorientation process of the magnetic spiral. After the completion of this process, the Nb layer switches into a normal state. Inverse switching returns the spiral to the initial state, opening the valve and turning on the superconducting state. The system can be switched there and back by a magnetic field of opposite signs along one direction in the layers plane, which allows an easy control. The switching time is estimated as several nanoseconds, which coincides with the scales of the STT-MRAM recording time.