No abstract

We have established a link between the global ac response and the local flux distribution of superconducting films by combining magnetic ac susceptibility, dc magnetization, and magneto-optical measurements. The investigated samples are three Nb films: a plain specimen, used as a reference sample, and other two films patterned with square arrays of antidots. At low temperatures and small ac amplitudes of the excitation field, the Meissner screening prevents penetration of flux into the sample. Above a certain ac drive threshold, flux avalanches are triggered during the first cycle of the ac excitation. The subsequent periodic removal, inversion, and rise of flux occurs essentially through the already-created dendrites, giving rise to an ac susceptibility signal weakly dependent on the applied field. The intradendrite flux oscillation is followed, at higher values of the excitation field, by a more drastic process consisting of creation of new dendrites and antidendrites. In this more invasive regime, the ac susceptibility shows a clear field dependence. At higher temperatures a smooth penetration occurs, and the flux profile is characteristic of a critical state. We have also shown that the regime dominated by vortex avalanches can be reliably identified by ac susceptibility measurements.

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first penetration field, we directly observe a terraced critical state as proposed theoretically by Cooley and Grishin [Phys. Rev. Lett. 74, 2788 (1995)]. However, at higher fields, the flux front tends to disorder and the classical Bean profile is restored. This study allows us to establish a clear link between the widely used integrated response measurements in the superconducting state and the nanoscale landscape defined by individual 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 ac 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.

A combination of scanning Hall microscopy and scanning ac-susceptibility measurements in superconducting stripes ͑ribbons͒ of width w Ͻ 10 m was used to observe the dimensional phase transitions of the vortex lattice and its stability under alternating fields. At low dc magnetic fields applied perpendicularly to the plane of the stripes, vortices form a one-dimensional chain at the center of the stripes. Above a certain field H ‫ء‬ ͑w͒, the vortex chain splits in two parallel rows displaced laterally in such a way that a zigzag vortex pattern is observed. By shaking the vortices with an external magnetic ac field and detecting their in-phase motion locally, we can identify the degree of mobility of each individual vortex. This technique allows us ͑i͒ to directly visualize the transition from intravalley ͑Campbell regime͒ to intervalley vortex motion as the amplitude of the ac modulation is increased and ͑ii͒ to accurately determine the temperature at which the vortex lattice freezes in a field-cooling experiment. The hallmark of type II superconductivity is the presence of quantized magnetic flux encircled by a rotating condensate of paired electrons when a sufficiently strong magnetic field is applied. The free motion of these fluxons leads to dissipation thus destroying the perfect conductivity of the system. The ever growing electricity demand has motivated for several decades a fierce strive for understanding, improving, and optimizing the mechanisms to prevent the fluxon's motion by introducing a rich diversity of pinning centers. 1Among the most powerful experimental methods used to determine the efficiency of pinning sites is the ac-susceptibility technique which consist of shaking the flux-line lattice with a small alternating magnetic field while recording its in-phase magnetic response.2 Pioneering theoretical works by Campbell and co-workers substantially contributed to comprehend and interpret the ac response of a superconducting system. 3,4 In the most simple version, two different regimes with distinctive ac responses can be identified. For a gentle ac shacking, flux lines oscillate inside the pinning potential following a reversible and low dissipative motion ͑Campbell regime͒. By increasing the strength of the ac excitation, the intravalley vortex motion first senses the anharmonicities of the potential well and eventually switches to an intervalley motion where vortices can hop from one pinning site to another. 5So far, all the experimental and theoretical reports are focused on global ac-susceptibility investigations where the recorded signal represents an average over millions of flux lines each of which trapped in a different pinning potential and subjected to a different environment. [6][7][8] In this paper, we introduce a local vortex imaging technique with singlevortex resolution which allows us to directly visualize the oscillatory motion of individual vortices submitted to harmonic ac excitations. The system to investigate consists of superconducting ribbons which, due to the proximity o...