Session Z23: Superconductivity: Magnetic Field & Vortex Related (Experimental)

Flux avalanches triggered by microwave depinning of superconducting vortices

We observe abrupt changes in broadband microwave permeability of thin Pb superconducting films as functions of the microwave frequency and intensity, as well as of external magnetic field. These changes are attributed to vortex avalanches generated by microwave induced depinning of vortices close to the sample edges. We map the experimental results on the widely used theoretical model assuming reversible response of the vortex motion to a. c. drive. It is shown that our measurements provide an efficient method of extracting the main parameter of the model- depinning frequencies for different pinning centers. The observed dependences of the extracted depinning frequencies on the microwave power, magnetic field and temperature support the idea that the flux avalanches are generated by microwave induced thermomagnetic instabilities.   

Free flux flow: a probe into the field dependence of vortex core size in clean single crystals

The free-flux-flow (FFF) phase has been attained successfully in a number of clean, weak-pinning, low-anisotropy, low-$T_{c}$, single-crystal samples as a unique probe into type II superconductivity that is independent of composition. The ``clean'' quality of the samples have been confirmed by reversible magnetization, high residual resistivity ratio, and low critical current densities $J_{c}$ with a re-entrant ``peak'' effect in $J_{c}(H)$ just below the critical field $H_{c2}$. The necessity of high current densities presented technical challenges that had been successfully addressed, and FFF is confirmed by a field-dependent ohmic state that is also well below the normal state. In these studies, the FFF resistivity \textit{$\rho $}$_{f}(H)$ has been measured in order to observe the field-dependent core size of the quantized magnetic flux vortices as modeled recently by Kogan and Zelezhina (KZ) who predicted a specific deviation from Bardeen-Stephen flux flow, dependent on normalized temperature and scattering parameter $\lambda $. The compounds studied are: V$_{3}$Si, LuNi$_{2}$B$_{2}$C, and NbSe$_{2}$, and results have shown consistency with the KZ model. Other applications of this method could also be used to probe normal-state properties, especially for the new iron arsenides, as will be discussed.   

Vortex exclusion transition

Ordinarily, Abrikosov vortices penetrate a type-II superconductor in a magnetic field larger than the lower critical field. However if the sample dimensions/s transverse to the applied magnetic field become smaller than some limit (found by Likharev to be $d<4.4 \xi$ for a film of thickness $d$ where $\xi$ is the coherence length), the formation of a parallel vortex in a film is excluded. We measured the transport response of a Molybdenum-Germanium superconducting film in parallel magnetic field and observed a sharp change in the V-I characteristics at a temperature which corresponds to the condition that $d \approx 4.4 \xi(T)$. We present the evolution of the observed transport behavior as a function of temperature and magnetic field.   

Vortex motion in superconducting single-crystal microstructures of NbSe$_2$

Superconducting microstructures prepared by advanced nanofabrication methods can be used to address long-standing, fundamental questions concerning vortex motion, including vortex tunneling and the Aharonov-Casher effect of vortices. The observation of these phenomena requires devices with minimal disorder and the fewest dissipative normal electrons. We have developed a process to fabricate superconducting microstructures from single-crystal ultrathin flakes of the layered Type II superconductor NbSe$_2$. Our process utilizes a multi-step electron beam lithography technique, whereby a NbSe$_2$ flake is cut into a desired microstructure with appropriate electrical leads. Despite the small device dimensions, which feature line widths less than 40 nanometers, our devices are superconducting. We are currently working on superconducting microstructures of NbSe$_2$ that involve integration of aluminum leads, aiming at the control and measurement of vortices in these novel structures.   

Evidence of Vortex Jamming in Abrikosov Vortex Flux Flow Regime

We report on dynamics of non-local Abrikosov vortex flow in mesoscopic superconducting Nb channels. Magnetic field dependence of the non-local voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly moderates vortex dynamics in mesoscopic geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with engineered pinning centers.   

Vortex Pinning Enhancement in Patterned MoGe Thin Films Coated with Permalloy

Resistivity measurements on permalloy (Py) coated MoGe thin films containing periodic hole arrays were carried out to study the effects of magnetic material filling the hole array. Thin films of MoGe were patterned via focused-ion-beam (FIB) milling to create pinning sites for the vortex lattice. A Py layer was deposited onto the hole array using magnetron sputtering. We investigate periodic hole arrays coated with magnetic material to determine the change in transport properties with varying magnetic fields. Samples with and without a Py coating were tested to determine the effect of magnetic material filling the hole array on the vortex lattice. Significant pinning enhancement was found in the Py coated samples.   

Vortex Confinement in Planar S/F Hybrid Structures

We have investigated the effect of periodic stray fields of the ferromagnet on the vortex dynamics in superconductor/ferromagnet (S/F) systems. Magnetization measurements were performed using SQUID magnetometry for Permalloy/Niobium (Py/Nb) samples of varying Py domain widths and Nb thicknesses. The hysteresis loops show an increase of the critical current for some values of magnetic domain width and superconductor thickness in some portion of the H-T phase diagram. However, below a threshold temperature sudden jumps in magnetization are observed during a slow sweep of the external magnetic field, which indicate the occurrence of vortex avalanches. These avalanches have been confirmed by scanning probe microscopy and they can cause a collapse of the critical state below a threshold temperature. Static and dynamics of these systems will be discussed.   

Spatial resolution of MFM measurements of penetration depth

The penetration depth and its temperature dependence are key ways to characterize superconductors. Measurements of the local Meissner response of a superconductor can determine the local penetration depth. To quantify the spatial resolution of such measurements, we seek to characterize the point spread function of magnetic force microscope (MFM) measurements of the penetration depth both numerically and experimentally. Modeling various geometries of MFM tips (pyramid, dipole, and long thin cylinder) in the presence of various geometries of spatial variation in the penetration depth (point variation, columnar defects, and planar defects or twin boundaries) shows the importance of the MFM tip geometry to achieving both excellent spatial resolution and quantitatively interpretable results. We compare these models to experimental data on pnictides and cuprates to set upper limits on the sub-micron-scale variation of the penetration depth. These results demonstrate both the feasibility and the technical challenges of submicron penetration depth mapping.   

Direct observation of jamming of superconducting vortices in a funnel structure

Arrest of the vortex dynamics at elevated temperatures is an imperative for using benefits of high Tc in cuprate superconductors. Here we explore experimentally a possibility of using funnel geometries for attenuation of the vortex motion. A single funnel structure is sculptured into a twin free single crystal of YBCO using laser lithography and ion milling. The magnetic field penetration patterns are studied using magneto-optical imaging technique at different temperatures below Tc. In ramping up fields we observe an increase of the vortex density in the throat of the funnel structure showing the jamming of vortices at the entry into narrow vortex channel. The effect is discussed using results of recent numerical simulations. This work was supported by UChicago Argonne, LLC, under contract No. DE-AC02-06CH11357.   

Interplay between Superconducting Vortices in the Dynamic Regime and Magnetism in Borocarbides

In superconductors with coexisting magnetic order an interaction is expected between vortices moving above a certain speed and the intrinsic magnetic moments. It has been predicted that in this dynamic regime vortices emit energy through the radiation of spin waves, thus slowing down and reducing the dissipation associated with their movement.$^{1}$ While of potential interest for applications, this effect has not yet been proven experimentally. In order to elucidate the phenomenon, we have carried out electrical transport measurements on ErNi$_2$B$_2$C single crystals, covering a broad range of temperatures, applied magnetic fields and field orientations. We observe a distinct change in the shape of current-voltage curves measured above and below the N\'{e}el temperature ($T_N$), which implies that the features seen below $T_N$ are related to the material's antiferromagnetism. We complement these results with measurements of the irreversible magnetization, which also show significant anomalies around $T_N$. Angular critical current measurements have also been performed to investigate the influence of the material's intrinsic magnetic moments on its current-carrying capabilities. \\[4pt] $^{1}$A.~Shekhter, L.~N.~Bulaevskii, and C.~D.~Batista, Phys. Rev. Lett. 106, 037001 (2011).   

Vortex pinning effects in the Corbino geometry

We probed a dynamic system of superconducting vortices with an artificial pinning landscape in the Corbino geometry. Current was applied from the center of the disc and propagated radially outward to produce a circular force with strength proportional to 1/r on the vortices. For small injection currents the vortex lattice is rigid, however large currents can cause shearing of the lattice. In order to investigate the temperature, current, and pinning lattice dependencies in different samples, we have defined Nb discs with a diameter of 60 $\mu $m on a circularly symmetric lattice of magnetic dots. In particular we present data that show steps instead of minima in the magnetoresistance curves at the position of the matching field indicating an unexpected influence of the pinning array on the motion of the vortex lattice.   

Zero average and net flows of vortices in hybrid nanostructures with asymmetric pinning potentials

We have fabricated hybrid nanostructures with superconducting film on top of an array of magnetic nanotriangles. In these structures, non-zero DC and AC voltages (V$_{DC}$, V$_{AC})$ are generated by alternating currents injected in the hybrid device. The V$_{DC }$and V$_{AC}$ behaviors give us an overall picture of the vortex dynamics and the rectification effects in these superconducting devices.   

Vortex configurations and geometrical shapes of superconducting MoGe networks revealed by scanning SQUID microscope

We prepared square networks of MoGe films by a standard photolithographic technique to observe vortex penetration into the network by means of a scanning SQUID microscope under different conditions of applied magnetic fields and temperatures. We found that vortex distribution in network evolves with applied magnetic field. At half matching field, vortices showed a checkerboard pattern, being in good agreement with theoretical predictions. We also investigated how vortices occupy network holes at different temperatures. Vortices tend to align in a ``diagonal'' direction at high temperatures while vortices repel each other to become isolated vortices at lower temperatures. Our results are consistent with theoretical calculation for nanoscopic superconducting network using the nonlinear Ginzburg-Landau equation. We also demonstrate that our data processing method appreciably improved a spatial resolution of the SQUID microscope.   

Threshold Critical Current Density to Trigger Flux Avalanches in Superconducting Thin Films

Under certain conditions of temperature and magnetic field, sudden flux bursts (avalanches) develop into superconducting films, as a consequence of thermomagnetic instabilities, which occur when heat dispersion is slower than magnetic diffusion. Based on a systematic study of the magnetic response (including magneto-optical imaging) of two Nb films - one plain and the other decorated with a square array of square antidots - we have found the existence of a threshold critical current density above which vortex avalanches are triggered. The experimental results reveal that this threshold value is nearly constant within the whole range of temperatures and magnetic fields investigated. The fact that an avalanche is triggered once the critical current reaches the threshold is in close correspondence with the behavior of granular material in sandpiles, which slides down whenever the slope exceeds the threshold repose angle. Our results are in perfect agreement with the predictions of a model for thermomagnetic instabilities in superconducting films, published previously by Yurchenko and coworkers [PRB 76, 092504 (2007)].   

Vortex-state electrodynamics in superconducting thin films studied by infrared spectroscopy

In a type-II superconductor, a magnetic field above the lower critical field creates vortices and dramatically changes the superconductor's electrodynamic response. Such changes have been observed in thin film samples by our infrared magneto-spectroscopy experiments with field normal to the film surface. The complex optical conductivity was extracted, and was compared to existing models for the effective electrodynamic response of the vortex state. We found a good agreement between our optical data and a calculation using Maxwell Garnett theory, which treats the mixed state as having normal-metal disks (representing the vortex cores) surrounded by superconductor. Our data also show the effect of magnetic-field-induced pair breaking on the superconducting fraction outside of the vortices.