Superconducting Nb Films Research Articles

Low Temperature Vortex Dynamics in Superconducting Nb Films Containing Square and Rectangular Arrays of Ni Nanodots

We examine the vortex lattice (VL) dynamics in superconducting Nb films containing square and rectangular arrays of Ni nanodots, using ac susceptibility techniques. A remarkable robust feature is the increase in pinning that occurs for magnetic fields that create integer n or half integer flux lines per pinning cell. This phenomenology has been reported in transport experiments performed very close to the sample critical temperature TC. In our contactless experiments we determined vortex mobility and pinning properties in an extended temperature range 0.65 <; T/TC <; 0.99. This was possible after growing larger samples, in the square millimeters range, with top down techniques. For the square pinning array, matching up to n = 8 was observed and pinning for n = 3, 4 and 5 show different characteristics, in agreement with published simulations. For the rectangular array, some of the matching orders are missing and we do not find clear evidence of a reconfiguration transition, where vortex commensuration changes from a rectangular cell to a square cell, as reported near TC in earlier transport experiments.

Determination of relative pinning strength of hexagonally arranged composite pinning centers

A method to determine the relative pinning strength of different pinning sites in composite hexagonal pinning arrays is proposed and experimentally realized. Six different samples of superconducting Nb films with composite hexagonal arrays were fabricated and measured. Two types of pinning sites have different sizes. The density of one type of pinning site is twice that of the other. The critical current versus the applied magnetic fields were also measured. The vortex configurations at the matching fields can be deduced from these curves and verified by molecular dynamic simulations. The relative pinning strength of different pinning sites can be determined from these configurations. The method could be extended to decide the optimal pinning size or to compare the pinning strength of pinning sites made of different materials.

Stochastic Resonance in Superconducting Nb Film with Periodic Array of Holes

Stochastic fluxon dynamics and spatial ordering are studied in a superconducting Nb film patterned with a periodic (3×3 micron) square array of holes. First, imaging of the film is performed using Scanning Hall Probe Microscope following application of the random noise magnetic drive. We observe formation of the fluxon stripe patterns oriented preferentially along the principal axes of the array. Next, ac magnetic field is applied as superposition of harmonic and random noise components and the local magnetic response is probed with the micron-sized Hall sensor at a single array site. We find that hopping of the fluxons becomes synchronized with the harmonic driving at certain amplitude of the random noise; the latter is found to be dependent upon the harmonic drive frequency. We attribute our observations to the phenomenon of stochastic resonance that is ubiquitous in non-linear multi-level systems with dynamic threshold and discuss a possibility of flux manipulation using stochastically-resonant periodic driving.

Reconfiguration and hysteresis in superconducting Nb film with honeycomb arrays

Superconducting Nb films with honeycomb array of holes are studied using transport measurements. The oscillating magneto-resistance curves are observed up to large flux density. Two types of resistance minima with different field intervals are observed, indicating the reconfiguration of the overall flux lattice from honeycomb to triangular arrangement. Moreover, hysteretic effects are found in a very large field span from H = 2H1 to H = 8.5H1. It is revealed that the hysteresis is related to the presence of interstitial vortices.

Superconducting vortex dynamics on arrays with bicrystal-like structures: matching and rectifier effects

Hybrid nanostructures of magnetic Ni dots embedded in superconducting Nb films have been fabricated. The dot arrays show bicrystal-like structures with an interface which separates two different dot arrangements in the same array. Vortex lattice dynamics is studied on these particular bi-arrays using magnetotransport measurements. Commensurability and rectifier (ratchet) effects have been observed. Matching between the vortex lattice and the pinning bi-array is only governed by the interplay between the densities of vortices and pinning centers. In bi-arrays with dissimilar pinning densities, for vortex flow parallel to the boundary, the highest pinning center density governs the matching effects. Hybrid samples with only half of the array with asymmetric potentials show a ratchet effect; that is input ac currents yield a net flow of vortices.

Vortex Confinement in Planar Superconductor/Ferromagnet Hybrid Structures

We use low temperature magnetic force microscopy and global magnetometry measurements to study the influence of magnetic domains on the Abrikosov vortex pinning in planar superconducting/ferromagnet bilayers. The superconducting/ferromagnet bilayers consist of a 200 nm superconducting Nb film covering a Permalloy film, with an insulating layer in between to avoid proximity effect. The periodic stripe domain in the Permalloy film produces a potential for directing vortex motion in the adjacent superconducting film. We observed an enhancement of vortex pinning by a factor of 3 that occurs in bilayers with a magnetic stripe domains <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</i> ≈ 500 nm, close to the superconducting critical temperature (T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0.9) . At lower temperatures, when T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0.6 the channeled vortex motion and the intrinsic pinning favor vortex avalanches.

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated. Due to small edge-to-edge separation of the holes, the patterned films are similar to multi-connected superconducting islands. Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima. Moreover, in between these two regions, variation of the minima spacing was observed. Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.

Abnormal magnetoresistance behavior in Nb thin films with rectangular arrays of antidots

Magnetoresistance in superconducting Nb films perforated with rectangular arrays of antidots (holes) is investigated at various temperatures and currents. Normally, the magnetoresistance increases with the increasing magnetic field. In this paper, we report a reverse behavior in a certain range of high fields after vortex reconfiguration transition, where the resistances at non-matching fields are smaller than those in the low field regime. This phenomenon is due to a strong caging effect, in which the interstitial vortices are trapped among the pinned multiquanta vortices. This effect is temperature and current dependent.

Wire network behavior in superconducting Nb films with diluted triangular arrays of holes

We present results of transport measurements on superconducting Nb films with diluted triangular arrays (honeycomb and kagomé) of holes. The patterned films have large disk-shaped interstitial regions even when the edge-to-edge separations between nearest neighboring holes are comparable to the coherence length. Changes in the field interval of two consecutive minima in the field dependent resistance R(H) curves are observed. In the low field region, fine structures in the R(H) and Tc(H) curves are identified in both arrays. Comparison of experimental data with calculation results reveals that these structures observed in honeycomb and kagomé hole arrays resemble those in wire networks with triangular and T3 symmetries, respectively. The findings suggest that even in these specified periodic hole arrays with very large interstitial regions, the low field fine structures are determined by the connectivity of the nanostructures.

Domain structure and magnetic pinning in ferromagnetic/superconducting hybrids

Magnetic patterns in three iron-garnet films with different magnetic properties covered with 100-nm superconducting Nb film are studied using a magneto-optical imaging technique. In all samples the strong coupling between ferromagnetic domains and vortices noticeably modifies the magnetization process. However, depending on the type and width of the magnetic domains, we observe different flux dynamics in the Nb film. Wide domains give rise to a combined domain structure and type I-like superconducting response resulting in enhanced attenuation of the flux motion due to cooperative pinning of vortices and magnetic domain walls. These combined domains strongly shrink in the ac fields due to a dynamic instability triggered by oscillating domain walls. Combined domains formed on narrow magnetic domains do not shrink but they guide the motion of vortices and, in turn, align in the vortex motion direction. This introduces superconducting current anisotropy due to strong pinning on the magnetic domain walls. Irregular magnetic domain structures with immobilized domain walls stochastically modify the vortex entry patterns. The presence of magnetic domains essentially arrests thermomagnetic avalanches in the superconducting layer. The studied magnetic pinning has a good potential for slowing down vortices in high-Tc superconductors.

Bitter decoration of vortex patterns in superconducting Nb films with random, triangular, and Penrose arrays of antidots

We imaged Abrikosov vortex patterns in thin Nb films with random, periodic (triangular), and quasiperiodic (Penrose) arrays of antidots. Vortex positions were visualized by Bitter decoration for a range of applied fields $B$, antidot radii $r$, and densities ${n}_{p}$ after field-cooling through the transition temperature ${T}_{c}$ to a base temperature $T\ensuremath{\approx}2\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The observed vortex patterns correspond to snapshots of vortex positions at the time of decoration. The effectiveness of antidots as artificial pinning sites for vortices is found to be sensitive to several factors: array geometry, antidot size and density, and applied field. Overall, the triangular lattice provides the most effective pinning landscape, with antidots trapping the highest proportion of vortices, but for a wide range of parameters the Penrose lattice is equally effective. For a quantitative analysis, we determined the occupation number $n$ (average number of vortices trapped per antidot) from each image. This revealed a significantly more complicated dependence of antidot occupation on applied field and/or antidot density than that predicted by simple models considering pinning by an isolated antidot. In particular, upon increasing the antidot density ${n}_{p}$, we find a marked increase in $n$ for triangular arrays, which we attribute to the additional repulsion from interstitial vortices, pushing more vortices into antidots with decreasing antidot separation. This effect is also present but less pronounced for Penrose arrays, which can be explained by the variation of antidot spacing inherent to the Penrose geometry and accordingly more options for accommodating interstitial vortices.

Magnetic pinning of flux lattice in superconducting-nanomagnet hybrids

Strong superconducting pinning effects are observed from magnetic landscapes produced by arrays of circular rings with varying magnetic remanent states. The collective and the background pinning of superconducting Nb films is strongly enhanced by the stray magnetic field produced by an array of circular Ni rings magnetized to form “onion” (bidomain) states. On the other hand, when the same rings are magnetized into vortex (flux-closed) states, or are randomly magnetized, the superconducting pinning is much smaller. The greatest pinning is produced when the superconducting vortex lattice motion is along a direction in which there is a strong magnetic field variation.

Probing the magnetization inside a superconducting Nb film by nuclear resonant scattering

We present an approach to probe the magnetization inside superconducting films using ultrathin 57Fe probe layers excited by synchrotron radiation. We investigate the evolution of the 57Fe hyperfine field orientation as a function of magnetic field above and below the superconducting transition temperature Tc for a Nb(50 nm)/57Fe(0.6 nm)/Nb(50 nm) trilayer. It is found that significant screening of the external field in the superconductor occurs only at low field, leading to a change in the hyperfine field angle below Tc. The presented approach allows to study the influence of magnetic fields and vortex induced electron correlations in complex layered structures incorporating superconductors.

Quasiparticle scattering time in niobium superconducting films

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured current-voltage curves we derive the vortex critical velocity $v^{*}$ for several temperatures. From the $v^{*}(T)$ values, the quasiparticle energy relaxation time $\tau_{\epsilon}$ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of $\tau_{\epsilon}(T)$ for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process as the dirty limit is approached from the clean regime.

Classical Analogue of Electromagnetically Induced Transparency with a Metal-Superconductor Hybrid Metamaterial

Metamaterials are engineered materials composed of small electrical circuits producing novel interactions with electromagnetic waves. Recently, a new class of metamaterials has been created to mimic the behavior of media displaying electromagnetically induced transparency (EIT). Here we introduce a planar EIT metamaterial that creates a very large loss contrast between the dark and radiative resonators by employing a superconducting Nb film in the dark element and a normal-metal Au film in the radiative element. Below the critical temperature of Nb, the resistance contrast opens up a transparency window along with a large enhancement in group delay, enabling a significant slowdown of waves. We further demonstrate precise control of the EIT response through changes in the superfluid density. Such tunable metamaterials may be useful for telecommunication because of their large delay-bandwidth products.

Vortex ratchet reversal: Role of interstitial vortices

Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.

Low loss and magnetic field-tunable superconducting terahertz metamaterial

Superconducting terahertz (THz) metamaterial (MM) made from niobium (Nb) film has been investigated using a continuous-wave THz spectroscopy. The quality factors of the resonance modes at 0.132 THz and 0.418 THz can be remarkably increased when the working temperature is below the superconducting transition temperature of Nb, indicating that the use of superconducting Nb is a possible way to achieve low loss performance of a THz MM. In addition, the tuning of superconducting THz MM by a magnetic field is also demonstrated, which offers an alternative tuning method apart from the existing electric, optical and thermal tuning methods.

Vortices trapped in the damaged surroundings of antidots in Nb films – Depinning transition

The depinning transition of Vortex Matter in the presence of antidots in superconducting Nb films has been investigated. The antidots were fabricated using two different techniques, resulting in samples with arrays of diverse pinning efficiency. At low temperatures and fields, the spatial arrangement of Vortex Matter is governed by the presence of the antidots. Keeping the temperature fixed, an increase of the field induces a depinning transition. As the temperature approaches T c , the depinning frontier exhibits a characteristic kink at the temperature T k , above which the phase boundary exhibits a different regime. The lower-temperature regime is adequately described by a power-law expression, whose exponent n was observed to be inversely proportional to the pinning capability of the antidot, a feature that qualifies this parameter as a figure of merit to quantify the pinning strength of the defect.

Measurement of the nonlinear Meissner effect in superconducting Nb films using a resonant microwave cavity: A probe of unconventional pairing symmetries

We report observation of the nonlinear Meissner effect (NLME) in Nb films by measuring the resonance frequency of a planar superconducting cavity as a function of the magnitude and the orientation of a parallel magnetic field. Use of low power rf probing in films thinner than the London penetration depth, significantly increases the field for the vortex penetration onset and enables NLME detection under true equilibrium conditions. The data agree very well with calculations based on the Usadel equations. We propose to use NLME angular spectroscopy to probe unconventional pairing symmetries in superconductors.

Direct measurements of the penetration depth in a superconducting film using magnetic force microscopy

We report the local measurements of the magnetic penetration depth λ in a superconducting Nb film using magnetic force microscopy (MFM). We developed a method for quantitative extraction of the penetration depth from single-parameter simultaneous fits to the lateral and height profiles of the MFM signal, and demonstrate that the obtained value is in excellent agreement with that obtained from the bulk magnetization measurements.