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[en] We study the effect of magnetic fields on the resistance R of a superconducting La1.84Sr0.16CuO4 film patterned into a 'double' network comprising nanosized square loops having their vertexes linked by relatively long wires. The results are compared with those obtained in a regular network of square loops of the same size. Both networks exhibit periodic dependence of R on the ratio Φ/Φ0 between the flux penetrating a loop and the superconducting flux quantum. However, while the regular network exhibit features characteristics of collective behavior of the loops, the double network exhibits a single-loop behavior. This observation indicates uncorrelated arrangements of fluxoids in the double network, in agreement with a recent theoretical prediction.
[en] A superconducting La1.84Sr0.16CuO4 film patterned into a network of 100 x 100 nm2 noninteracting square loops exhibits large magnetoresistance oscillations superimposed on a background which increases monotonically with the applied magnetic field. Neither the oscillations amplitude nor its temperature dependence can be explained by the Little-Parks effect. Conversely, a good quantitative agreement is obtained with a recently proposed model ascribing the oscillations to the interaction between thermally excited moving vortices and the oscillating persistent currents induced in the loops. Extension of this model, allowing for direct interaction of the vortices and antivortices magnetic moment with the applied field, accounts quantitatively for the monotonic background as well. Analysis of the background indicates that in the patterned film both vortices and antivortices are present at comparable densities. This finding is consistent with the occurrence of Berezinskii-Kosterlitz-Thouless transition in La1.84Sr0.16CuO4 films.
[en] Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometeres. Here, we report the results of measurements on loops of La1.84Sr0.16CuO4, a high-temperature superconductor that loses its resistance to electric currents when cooled below ∼38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little-Parks effect. Moreover, unlike Little-Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry or with a period of h/4e, as predicted for superconductors that exhibit stripes.
[en] Time-resolved magneto optical measurements in Bi2Sr2CaCu2O8+δ show oscillatory relaxation of the local magnetization. The oscillatory behaviour is observed in a limited range of temperatures and fields below the vortex order-disorder phase transition line. We ascribe this phenomenon to coupled effects of thermally activated flux creep and annealing of transient disordered vortex states injected into the sample during field increase
[en] Local nonlinear AC magnetic measurements were employed in the study of the vortex phase diagram in Bi2Sr2CaCu2O8+x. The melting and the Bragg-Glass to Vortex-Glass (solid-solid) transitions are manifested by a sharp peak and a sharp drop in the third harmonic response, respectively. The peak at the solid-liquid transition signifies the hysteretic nature of the first-order melting transition. The disappearance of the nonlinear response in the vicinity of the solid-solid phase transition line suggests the existence of an intermediate soft lattice vortex state in-between the ordered and disordered vortex phases.
[en] Energy oscillations and vortex occupation as a function of the external magnetic field are calculated for the recently realized superconducting double network consisting of two interlaced sub-networks of small and large loops. The calculations, based on fluxoid quantization and energy minimization, show that at the low or high field range in each energy period only the population of the large loops increases, whereas all small loops remain equally populated with fluxoids. The small loops gain an extra fluxoid over a small field range around the center of each period, resembling the behavior of a single loop. This range increases as the ratio between the areas of the large and small loops decreases.
[en] We describe a study of fluxoid quantization effects in a novel superconducting network consisting of two interlaced sub-networks of small and large loops. Computer simulations show different behavior for the sub-networks in this double network. In particular, while the occupation of the large loops by fluxoids grows linearly with the external magnetic field, the small loops occupation grows in steps, similar to the occupation of a single loop. Magnetoresistance measurements in a double network made of MBE grown La1.84Sr0.16CuO4 reveal periodic oscillations resembling that of a single loop with field periodicity as found in the Little-Parks effect. However, the amplitude of the oscillations is found to be larger by almost two orders of magnitude than that expected from this effect. We propose a new model attributing these oscillations to the interaction between moving vortices and the periodic persistent current induced in the loops by the external field. This model explains the large magnetoresistance amplitude as well as its temperature dependence.