Superconducting Niobium Thin Films Research Articles

Formation and Microwave Losses of Hydrides in Superconducting Niobium Thin Films Resulting from Fluoride Chemical Processing

AbstractSuperconducting niobium (Nb) thin films have recently attracted significant attention due to their utility for quantum information technologies. In the processing of Nb thin films, fluoride‐based chemical etchants are commonly used to remove surface oxides that are known to affect superconducting quantum devices adversely. However, these same etchants can also introduce hydrogen to form Nb hydrides, potentially negatively impacting microwave loss performance. Here, comprehensive materials characterization of Nb hydrides formed in Nb thin films as a function of fluoride chemical treatments is presented. In particular, secondary‐ion mass spectrometry, X‐ray scattering, and transmission electron microscopy reveal the spatial distribution and phase transformation of Nb hydrides. The rate of hydride formation is determined by the fluoride solution acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb superconducting properties and lead to increased power‐independent microwave loss in coplanar waveguide resonators. However, Nb hydrides do not correlate with two‐level system loss or device aging mechanisms. Overall, this work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices.

Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3

Oxygen vacancy is known to play an important role for the physical properties in SrTiO3(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO2. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.

Dc magnetometry of niobium thin film superconductors deposited using high power impulse magnetron sputtering

We performed a systematic investigation of the dc magnetic properties of superconducting niobium thin films deposited by high power impulse magnetron sputtering (HiPIMS) as a function of the main deposition parameters: the temperature, T, of the heated substrate and the applied dc bias voltage, V, during the sputtering process. The measured dc magnetization curves between 0 and 1000 mT were used to calculate the relative volume of each sample into which the applied magnetic field had penetrated, ðΔV=VÞM. The sample deposited at 700°C with −80 V biased substrate exhibited the least penetration by the magnetic field. ðΔV=VÞM appeared to be highly dependent on the bias voltage at both room temperature and 500°C; however, a broad range of bias voltages showed comparatively similar results at increased temperatures of 700°C. Samples deposited at 700°C exhibit smaller upper critical fields, HC2, than samples deposited at room temperature and 500°C, with the lower temperatures exhibiting a greater dependency on the applied bias. The films deposited at 700°C also display a more stable magnetization curve suggesting that an enhanced flux pinning was achieved when compared to lower temperatures. Consequently, films with stable pinning were found to have the most repeatable dc magnetic behavior. Our results are particularly relevant to the superconducting radio-frequency accelerator scientific community where thin films have been suggested as a technology which may ultimately surpass the performance of bulk niobium. They are also relevant to the fundamental area of superconducting thin films and any applied area where thin films produced by HiPIMS are used, such as superconducting electronics.

Fabrication and characterization of SmCo5/Nb ferromagnetic/superconducting hybrid thin films grown by RF magnetron sputtering technique

Ferromagnet/Superconductor (F/S) bilayer hybrids show exclusive states due to the mutual interaction between the superconductor and the underlying ferromagnetic substructures in micron scale. In this work, we aimed to observe the effects of the interaction between superconductivity and magnetism, especially the phenomenon involving the orientation and the size of magnetic stripes has been investigated in a coupled ferromagnetic/superconducting thin-film structure. In the proposed F/S hybrid system by this work, superconducting niobium thin-films were combined with underlying segments of ferromagnetic SmCo5 substructures. 300 nm thick magnetic films fabricated by RF magnetron sputtering techniques were topographically grown in patterns with stripes oriented either transverse to or along the direction of current flow. The elemental and microstructural analyses were conducted by EDX, SEM and GIXRD characterization tools. Low-temperature DC transport measurements were conducted by means of four point probe method in a 9T closed-cycle cryogenic refrigeration system. Transport superconducting properties, transition temperature TC(H) and second critical field HC2(T) were measured in a range of applied magnetic field between H = 0–9 kOe for the hybrid system. The results revealed that the artificial periodic modulation of applied field through preferentially-oriented magnetic stripes could introduce normal and superconducting channels or barriers for the current flow.

Unusual commensurability effects in quasiperiodic pinning arrays induced by local inhomogeneities of the pinning site density

We experimentally investigate the magnetic field dependence of the critical current Ic(B) of superconducting niobium thin films patterned with periodic and quasiperiodic antidot arrays on the submicron scale. For this purpose we monitor current–voltage characteristics at different values of B and temperature T. We investigate samples with antidots positioned at the vertices of two different tilings with quasiperiodic symmetry, namely the Shield Tiling and the Tuebingen Triangle Tiling. For reference we investigate a sample with a triangular antidot lattice. We find modulations of the critical current for both quasiperiodic tilings, which have partly been predicted by numerical simulations but not observed in experiments yet. The particularity of these commensurability effects is that they correspond to magnetic field values slightly above an integer multiple of the matching field. The observed matching effects can be explained by the caging of interstitial vortices in quasiperiodically distributed cages and the formation of symmetry-induced giant vortices.

Anisotropy of the critical temperature of a superconducting niobium thin film with an array of nanoscale holes in an external magnetic field

We report on magnetotransport experiments investigating the effect of a regular array of nanoscale holes on the anisotropic response in the transition temperature of a superconducting niobium thin film. We find that the angle dependence of the critical temperature exhibits two strong anisotropic effects: Little-Parks oscillations whose period varies with field direction and a smooth background arising from one-dimensional confinement by the finite lateral space between neighboring holes. The two components of the anisotropy are intrinsically linked and appear in concert with one superimposed on top of the other.

The phase boundary of superconducting niobium thin films with antidot arrays fabricated with microsphere photolithography

The experimental investigation of the Ic(B)–Tc(B) phase boundary of superconducting niobium films with large area quasihexagonal hole arrays is reported. The hole arrays were patterned with microsphere photolithography. We investigate the perforated niobium films by means of electrical directed current transport measurements close to the transition temperature Tc in perpendicularly applied magnetic fields. We find pronounced modulations of the critical current with applied magnetic field, which we interpret as a consequence of commensurable states between the Abrikosov vortex lattice and the quasihexagonal pinning array. Furthermore, we observe Little–Parks oscillations in the critical temperature versus magnetic field.

Study of Stress and Morphology of Superconducting Niobium Thin Films

Intrinsic stress, surface morphology and crystallographic structure of superconducting thin films play an important role in determining the quality of their superconducting properties and the Josephson junctions made of them. Nb thin films deposited by DC magnetron sputtering with various deposition conditions were studied using curvature method, atomic force microscopy and X-ray diffraction. The role of sputtering pressure, gun operating parameters, substrate temperature and substrate materials on film morphology, crystallographic structure and intrinsic stress are investigated. Superconducting transition temperatures of the films were measured and relationship between the transition temperatures and deposition conditions were studied.

Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from $8\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth $\ensuremath{\lambda}(0)$ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the absolute values of $\ensuremath{\lambda}(0)$ were determined by a microwave transmission method. We found that the reduction of the film thickness below $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ leads to a significant increase of the magnetic field penetration depth from about $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for $300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film up to $230\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for a $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Huge compact flux avalanches in superconducting Nb thin films

Using a magneto-optical technique we perform a quantitative analysis of magnetic flux penetration in superconducting niobium thin films on A-plane ( 1 1 2 ¯ 0 ) sapphire substrates as a function of temperature (1.5 K ⩽ T ⩽ T c = 9.2 K). In these samples we observe huge compact avalanches (HCAs for brevity), very much like some snow-avalanches and unlike the rough dendritic flux penetration observed on R-plane ( 1 1 ¯ 0 2 ) sapphire. The behavior observed is consistent with the occurrence of thermo-magnetic avalanches as proposed by Aranson et al. [Phys. Rev. Lett. 87 (2001) 067003]. For increasing temperature, we find: (1) an increased branching of the HCA. (2) An increased applied field necessary for the first HCA to occur. (3) A decrease in the number of HCAs, accompanied by more ‘regular’ flux penetration (above 6.2 K avalanches are completely absent). (4) An increase of the total amount of flux as well as the area of the HCA.

Metallurgical analysis and RF losses in superconducting niobium thin film cavities

Copper cavities with a thin niobium film - as used in the large electron positron collider LEP - would be also attractive for future linear colliders, provided the decrease of the Q-value with the accelerating gradient could be reduced. We aim at extracting the important parameters that govern this decrease. The dependence on the RF frequency is studied by exciting 500 MHz and 1500 MHz cavities in different modes. In addition we combined RF measurements for two 1500 MHz cavities of different RF performance with microscopic tests (AFM,TEM) on samples cut out of the same cavities. Their microstructural characterization in plan-view allows the extraction of the grain size and the defect densities.

Observation of magnetic-field penetration via dendritic growth in superconducting niobium films.

Using a high-resolution magneto-optical technique, we have studied the penetration and expulsion of magnetic flux in superconducting niobium thin films. We have found that over a wide region of the phase diagram, the picture of uniform flux fronts as described by a critical-state model breaks down and that the penetration of the field takes place through the growth of magnetic dendrites. Magnetic decoration studies with single vortex resolution show that the dendrites have a complex structure and consist of walls of both high and low magnetic field and are filled with a state of matter which structurally appears to be a vortex fluid. Some of the dendrites are seen to grow by nucleating regions with hundreds of antivortices in the film.

Superconducting niobium thin film sputtering onto copper quarter wave resonators for heavy ion accelerators

Niobium sputtered copper quarter wave resonators (QWRs) represent an innovative and promising alternative to lead electroplated copper cavities, or to those made of niobium, both bulk and explosively bonded on copper. The authors describe the R&D efforts of the first niobium sputtered copper prototypes. The results obtained and the progressive improvements achieved test by test make Nb sputtered QWRs an intriguing possibility for the acceleration of heavy ions whenever the need for changing from Pb to Nb is encountered. A prototype with a Q-value of 7*10/sup 8/ and maximum accelerating field of 4 MV/m has been produced. >

Low- and high-temperature superconducting microwave filters

Stripline and microstrip filters at X-band were designed and fabricated using low- and high-temperature superconductors in quarter-wave, parallel-coupled section configurations. Low-temperature superconducting niobium thin films, deposited on single-crystal sapphire, were used to build to six-pole stripline filters with adjacent passbands and approximately 3 dB crossovers and 1.2% bandwidth. Four- and six-pole microstrip filters were made with in situ epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films on LaAlO/sub 3/ substrates. All the YBCO filters showed 77 K passbands with clean skirts and high out-of-band rejection. The six-pole filters had adjacent passbands with -28 dB crossovers and 1.5% bandwidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Requirements and an approach for coating the gravity probe B gyroscope rotor

The Gravity Probe B gyroscope test of general relativity places a number of difficult requirements on the coating of the rotating element. The gyroscopes each consist of two parts: a rotor (3.8 cm diameter) and a housing, both made of fused quartz. The requirements for uniformity, film adhesion, and superconducting properties are discussed. We have achieved a uniformity of 1.5% (38 nm) peak to valley with good adhesion and transition temperatures of 9.8 K. Approaches used to meet the requirements and to verify the properties of the superconducting niobium thin film rotor coating are described. Problems concerning abrasive damage and electrical arc damage during earthbound testing have been observed.

Epitaxial growth of superconducting niobium thin films by ultrahigh vacuum evaporation

A series of thin films of niobium have been grown on various orientations of single crystal sapphire by e-beam evaporation in an ultrahigh vacuum molecular beam epitaxy system. The films were grown at temperatures between 600 and 950 °C and at growth rates from 0.1 to 10 Å/s. The films were single-crystal as determined by both in situ reflection high energy electron diffraction patterns and by Read photographs. The best films had rocking curve widths of the order of 1/10 of a degree. The superconducting transition temperature was measured as a function of film thickness and crystal orientation and ranged up to 9.35 K for a thick sample grown on [11̄02] sapphire.

Flux pinning in superconducting niobium thin films

The effect of flux pinning on the resistive behavior of superconducting niobium thin films was investigated. No resistive voltage, longitudinal or transverse, was observed below the upper critical field, H c2 , determined by a two coil technique.