Utilizing difference in temperature dependencies we decoupled BCS and residual components of the microwave surface resistance of superconducting niobium at all rf fields up to $B_\mathrm{rf}\sim110$ mT. We reveal that the residual resistance decreases with field at $B_\mathrm{rf} \lesssim$ 40 mT and strongly increases in chemically treated niobium at $B_\mathrm{rf} > 80$ mT. We find that BCS surface resistance is weakly dependent on field in the clean limit, whereas a strong and peculiar field dependence emerges after $120^\circ$C vacuum baking.
We propose a model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium thin film superconducting RF cavities. The physical mechanism that we explore is due to the constriction of surface electrical current flow at grain interface boundaries. This constriction creates an additional electrical contact resistance which induces localized punctual heat dissipation. The temperature at these spots is derived; and the electrical contact resistance is shown to depend on the magnetic field, on the gain contact size over which dissipation occurs, and on other …
Identifying the loss mechanisms of niobium cavities enables an accurate determination of applications for future accelerator projects and points to research topics required to mitigate current limitations. For several cavities an increasing surface resistance above a threshold field, saturating at higher field has been observed. Measurements on samples give evidence that this effect is caused by the surface electric field. The measured temperature and frequency dependence is consistent with a model that accounts for these losses by interface tunnel exchange between localized states in oxides f…
In niobium superconducting radio frequency (SRF) accelerating cavities a decrease of the quality factor at lower fields - a so called \emph{low field Q slope or LFQS} - has been a long-standing unexplained effect. By extending the high $Q$ measurement techniques to ultralow fields we discover two previously unknown features of the effect: i) saturation at rf fields lower than $E_\mathrm{acc} \sim 0.1$~MV/m; ii) strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner …
Microwave spectroscopy is a powerful experimental tool to reveal information on the intrinsic properties of superconductors. Superconducting stripline resonators, where the material under study constitutes one of the ground planes, offer a high sensitivity to investigate superconducting bulk samples. In order to improve this measurement technique, we have studied stripline resonators made of niobium, and we compare the results to lead stripline resonators. With this technique we are able to determine the temperature dependence of the complex conductivity of niobium and the energy gap $Δ(0)=2.1…
We report a strong effect of the cooling dynamics through $T_\mathrm{c}$ on the amount of trapped external magnetic flux in superconducting niobium cavities. The effect is similar for fine grain and single crystal niobium and all surface treatments including electropolishing with and without 120$^\circ$C baking and nitrogen doping. Direct magnetic field measurements on the cavity walls show that the effect stems from changes in the flux trapping efficiency: slow cooling leads to almost complete flux trapping and higher residual resistance while fast cooling leads to the much more efficient flu…
Superconducting niobium film cavities deposited on copper substrates (Nb/Cu) have suffered from strong field-dependent surface resistance, often referred to as the Q-slope problem, since their invention. We argue that the Q-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions. In this study, extrinsic effects were carefully minimized in a series of experiments on a seamless cavity. The origin of the Q-slope in low frequency cavities is traced back to two contributions with different temperature …
Recently, Nb superconducting radio frequency cavities vacuum heat treated between 300-400 C for a few hours have exhibited very high quality factors (~5x10^10 at 2.0 K). New secondary ion mass spectrometry measurements of O, N and C show this enhancement in RF surface conductivity is primarily associated with interstitial O alloying via dissolution and diffusion of the native oxide. We use a theory of oxide decomposition and O diffusion to quantify previously unknown parameters crucial in modeling this process. RF measurements of a vacuum heat treated Nb superconducting radio frequency cavity …
Long standing data from niobium thin film accelerating cavities will be revisited and analysed by a modified London model of RF superconductivity. Firstly, the applicability of this model is explored using data of the BCS surface resistance and its dependence on the RF magnetic field, temperature and mean free path. Secondly, the RF losses from trapped magnetic flux are analysed with regard to their dependence on these same parameters.
The thermal conductivity, DC magnetization and penetration depth of large-grain niobium hollow cylindrical rods fabricated from ingots, manufactured by CBMM subjected to chemical and heat treatment were measured. The results confirm the influence of chemical and heat-treatment processes on the superconducting properties, with no significant dependence on the impurity concentrations in the original ingots. Furthermore, RF properties, such as the surface resistance and quench field of the niobium rods were measured using a TE011 cavity. The hollow niobium rod is the center conductor of this cavi…
The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. This model is found to yield consistent results for the coherence lengths in high-kappa superconducting samples, and can be used with HTSC even at temperatures much below T_c. It is shown that for samples with high values of depinning frequency, one should measure both components of the compl…
We present the results of from DC magnetization and penetration depth measurements of cylindrical bulk large-grain (LG) and fine-grain (FG) niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities. The surface treatment consisted of electropolishing and low temperature baking as they are typically applied to SRF cavities. The magnetization data were fitted using a modified critical state model. The critical current density Jc and pinning force Fp are calculated from the magnetization data and their temperature dependence and field dependence are presented. The…
We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the …
Niobium thin film wires were fabricated using electron beam lithography with a four layer liftoff mask system, and subsequently thinned by anodisation. The resistance along the wire was monitored in situ and trimmed by controlling the anodisation voltage. Depending on the room temperature sheet resistance, samples showed either superconducting or insulating behaviour at low temperatures. A Coulomb blockade was observed for samples exceeding 6 kOhm per square. Samples were also made in a single electron transistor-like geometry with two weak links made by combined angular evaporation and anodis…
In this work, we find that Al cladding on Nb microstrip resonators is an efficient way to suppress nonlinear responses induced by local Joule heating, resulting in improved microwave power handling capability. This improvement is likely due to the proximity effect between the Al and the Nb layers. The proximity effect is found to be controllable by tuning the thickness of the Al layer. We show that improving the film quality is also helpful as it enhances the microwave critical current density, but it cannot eliminate the local heating.
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 agai…
Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization sh…
We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3 $μ$m to 50 $μ$m, taken at temperatures in the range 100-800 mK, far below the 9.2 K transition temperature of niobium. These data agree well with the two-level system (TLS) theory. Fits to this theory provide information on the number of TLS that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLS, and the data are consistent with a TLS surface layer thickness of order a…
The field distribution inside the superconducting radiofrequency (SRF) film with different mean free path is studied using niobium (Nb) as an example. The surface resistance of clean Nb film with different substrate and different film thickness is calculated. We also show the study of a special structured multilayer superconducting film called Superconductor-Insulator-Superconductor (SIS) structure.
We investigate the performance of superconducting nanowire photon detectors fabricated from ultra-thin Nb. A direct comparison is made between these detectors and similar nanowire detectors fabricated from NbN. We find that Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. We show that the devices can be stabilized by reducing the input resistance of the readout. Nb detectors optimized in this way are shown to have approximately 2/3 the reset time of similar large-active-area NbN detectors of the same geometry, with approximately 6% detect…
Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size grains of MgB_2 separated powder are presented and discussed. The microwave surface resistance data is compared to experimental results of Nb, Bi_2Sr_2CaCu_2O_{8+δ} (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS).
Oval-shaped resonance distortion as a signature of quasiparticle heating effect in a niobium superconducting resonator
School of Integrated Circuits, Tsinghua University（清华大学集成电路学院）；Cavendish Laboratory, University of Cambridge（剑桥大学卡文迪许实验室）；Department of Astronomy, Tsinghua University（清华大学天文系）
Microwave-field distribution, dissipation, and surface impedance are theoretically investigated for superconductors with laminar grain boundaries (GBs). In the present theory we adopt the two-fluid model for intragrain transport current in the grains, and the Josephson-junction model for intergrain tunneling current across GBs. Results show that the surface resistance $R_s$ nonmonotonically depends on the critical current density $J_{cj}$ at GB junctions, and $R_s$ for superconductors with GBs can be smaller than the surface resistance $R_{s0}$ for ideal homogeneous superconductors without GBs…
Here we report the observation of extraordinary superconductivity in a pressurized commercial niobium-titanium alloy. We find that its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record high pressure up to which a known superconducting state can continuously survives. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (HC2) at 1.8 K has be…
The effect of the patterning process on the nonlinearity of the microwave surface resistance $R_S$ of YBCO thin films is investigated. With the use of a sapphire dielectric resonator and a stripline resonator, the microwave $R_S$ of YBCO thin films was measured before and after the patterning process, as a function of temperature and the rf peak magnetic field in the film. The microwave loss was also modeled, assuming a $J_{rf}^2$ dependence of $Z_S(J_{rf})$ on current density $J_{rf}$. Experimental and modeled results show that the patterning has no observable effect on the microwave residual…
We characterize low-loss electron-beam evaporated niobium thin films deposited under ultra-high vacuum conditions. Slow deposition yields films with a high superconducting transition temperature ($9.20 \pm 0.06 \rm ~K$) as well as a residual resistivity ratio of $4.8$. We fabricate the films into coplanar waveguide resonators to extract the intrinsic loss due to the presence of two-level-system fluctuators using microwave measurements. For a coplanar waveguide resonator gap of $2~μ\rm m$, the films exhibit filling-factor-adjusted two-level-system loss tangents as low as $1.5 \times 10^{-7}$ wi…
The field-induced variations of the microwave surface resistance are investigated in two different samples of powdered MgB2. The experimental results can be justified in the framework of the Coffey and Clem model with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account. Assuming the angular dependence of the upper critical field expected from the anisotropic Ginzburg-Landau theory, we determine the anisotropy factor by fitting the experimental data of the field dependence of the microwave surface resistance. We show that th…
We report experimental results of the field-induced variations of the microwave surface resistance in samples of MgB2, produced by different methods. By sweeping the DC magnetic field at increasing and decreasing values, we have detected a magnetic hysteresis that can be ascribed to the different magnetic induction, due to the critical state of the fluxon lattice. The hysteresis observed in the bulk samples has an unusual shape, which cannot be justified in the framework of the critical-state models.
Field-induced variations of the microwave surface resistance, Rs(H), have been investigated in high-density ceramic MgB2. At low temperatures, several peculiarities of the Rs(H) curves cannot be justified in the framework of models reported in the literature. We suggest that they are ascribable to the unconventional vortex structure in MgB2, related to the presence of two gaps. On the contrary, the results near Tc can be accounted for by the Coffey and Clem model, with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account.
The microwave power, dc magnetic field, frequency and temperature dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by a number of characteristics, the most clear being the breakdown in the omega squared law for poor quality samples. Analysis of the experimental data suggests the most attractive procedure for high quality film growth for technical applications.
Two distinct models account for the microwave residual surface resistance of superconducting cavities with equally good agreement with the measured temperature and frequency dependence. In presenting his phonon-generation model, Passow claimed that Rabinowitz' fluxoid power-loss model of residual resistance does not fit the experimental data, whereas his does. In fact, the two models have essentially the same temperature and frequency dependence. Furthermore, Passow's phonon-generation model cannot explain the observed sensitivity to details of sample preparation and history, while the fluxoid…
The anomalous decrease in microwave surface resistance, R_{s}, of superconducting YBa_{2}Cu_{3}O_{7-d} (YBCO) thin films in the presence of a low dc magnetic field is studied using a microstrip resonator technique. We have done a dc field hysteresis measurement of R_{s} to study the effects of vortex penetration on the anomalous effect. It is shown that the anomaly happens at a field level far below the low critical field, H_{c1,strip}, of the superconducting microstrip and vortex (Abrikosov) penetration would eliminate the anomalous effect observed at low field. This implies that the anomalou…
We discuss the microwave energy losses in superconductors in the critical state. The field-induced variations of the surface resistance are determined, in the framework of the Coffey and Clem model, by taking into account the distribution of the vortex magnetic field inside the sample. It is shown that the effects of the critical state cannot generally be disregarded to account for the experimental data. Results obtained in bulk niobium at low temperatures are quantitatively justified.
The dependence of microwave impedance of a dc-biased Josephson Fluxonic Diode (JFD) under application of both dc magnetic field and rf excitation is calculated with a variety of conditions. For finite length of a JFD excited by a very low microwave excitation below its plasma frequency, applied dc magnetic field increases the rate of Vortex and Anti-Vortex (VAV) pair generation which fine-tunes the microwave resistance up to several factors more than its zero field microwave resistance (R0). Under this circumstance, adding a dc bias for moving VAVs causes oscillation-like features in microwave…
We discuss on the electromagnetic response of a polycrystalline sample of LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and high-frequency responses have been investigated by measuring the AC susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At low as well as high DC magnetic fields, the susceptibility strongly depends on the amplitude of the AC driving field, highlighting enhanced nonlinear effects. The field dependence of the AC susceptibility exhibits a magnetic hysteresis that can be justified considering the intragrain-field-penetration e…
Here we report a novel nitridation technique for transforming niobium into hexagonal Nb2N which appears to be superconducting below 1K. The nitridation is achieved by high temperature annealing of Nb films grown on Si3N4/Si (100) substrate under high vacuum. The structural characterization directs the formation of a majority Nb2N phase while the morphology shows granular nature of the films. The temperature dependent resistance measurements reveal a wide metal-to-superconductor transition featuring two distinct transition regions. The region close to the normal state varies strongly with the f…
The microwave surface resistance Rs of MgB2 films with the zero-resistance temperature of - 39 K was measured at 8.0 - 8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a vaporized magnesium environment. The Rs appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed Rs of ~ 0.8 mohm at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced Rs of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer e…
We report on the microwave surface resistance of melt-textured Y_1.6Ba_2.3Cu_3.3O_7-x samples, doped with different amount of B_2O_3 and, subsequently, irradiated by thermal neutrons at the fluence of 1.476 \times 10^17 cm^-2. The microwave surface resistance has been measured as a function of temperature and DC magnetic field. The experimental results are quantitatively discussed in the framework of the Coffey and Clem theory, properly adapted to take into account the d-wave nature of cuprate superconductors. By fitting the experimental data at zero DC field, we have highlighted the effects o…
We report the growth of NdBa2Cu3O7-d films on (100) MgO substrate by Molecular Beam Epitaxy (MBE). Large area NdBa2Cu3O7-d films with homogeneous superconducting properties were grown by precise control of stoichiometry and the optimisation of growth parameters. The stoichiometric ratio of Nd:Ba:Cu close to 1:2:3 yields films with TC of 94 K and JC values above 3.5 MA/cm2 at 77 K on bare MgO substrate. The NdBa2Cu3O7-d films grown under optimised conditions had excellent in-plane texture and good metallicity. The most significant characteristic of our MBE grown NdBa2Cu3O7-d films is the very l…
Superconducting spintronics in hybrid superconductor/ferromagnet (S-F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, $T_c$, of the S-layer can be controlled by the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using field history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both $T_c$ and the shape of the resistive tran…
We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($ν=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films grown on SrTiO$_3$ substrates, as a consequence of the introduction of sub-micrometric BaZrO$_3$ particles. The field increase of the surface resistance is smaller by a factor of $\sim$3 in the film with BaZrO$_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO$_3$ inclusions determine very deep and steep pinning wells and …
We demonstrate the operation of superconducting coplanar microwave resonators in a very large frequency range up to 50 GHz. The resonators are fabricated from niobium thin films on sapphire substrates and optimized for these high frequencies by small chip sizes. We study numerous harmonics of the resonators at temperatures between 1.5 K and 6 K, and we determine quality factors of up to 25000 at 1.5 K. As an example for spectroscopy applications of such resonators we detect the superconducting transition of a bulk tin sample at multiple probing frequencies.
We have measured the temperature dependence of the microwave surface impedance Zs = Rs + iΩμ_0 \Lamda of two c-axis oriented MgB2 films at a frequency Ω/(2π) of 17.9 GHz employing a dielectric (sapphire) resonator technique. The temperature dependence of the magnetic field penetration depth \Lamda can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 110 nm and 115 nm were determined from the fit. Our results clearly…
