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We demonstrate an improved YBa2Cu3O7−δ -based microwave kinetic inductance detector with a quality factor Q i ∼ 2.5 × 10 4 and noise equivalent power, ∼ 10 − 12 W Hz − 1 at 10 K. Zero field cooled (ZFC) and field cooled (FC) measurements of the magnetic field dependence of the resonance characteristics, show substantially different behavior, indicating that both the screening currents and vortices play a role. The ZFC measurements exhibit a sharp decrease of the resonance frequency, f r , and Q i at low fields, up to the full penetration field, revealing the dominant role of the screening currents. In contrast, the FC measurements exhibit a moderate decrease of f r and Q i with field, revealing the role of vortices and reflecting the field dependence of the penetration depth in a d-wave superconductor.

By implementing a unique magneto-optical system with ultrafast magnetic-field ramping-rate capability (up to 3 kT/s), we have been able to routinely generate and image dendritic flux instabilities in YBa2Cu3O7-x films. In the present work we study the effect of the film thickness on the dendritic instability. Dendritic avalanches in 50 - 600 nm thick films were magneto-optically imaged at 7 K, after ramping the magnetic field from zero to 60 mT at different rates. The data reveal a remarkable change in flux morphologies between the thin and the thicker films. While the former (50-250 nm) display well-developed dendritic patterns, the latter (350-600 nm) exhibit few avalanches with favored branch directions parallel to the film's edges. Several possible explanations for this behavior are discussed.

Nb nano-rings connected serially by Nb wires exhibit, at low bias currents, the typical parabolic Little-Parks magnetoresistance oscillations. As the bias current increases, these oscillations become sinusoidal. This result is ascribed to the generation of Josephson junctions caused by the combined effect of current-induced phase slips and the non-uniformity of the order parameter along each ring due to the Nb wires attached to it. This interpretation is validated by further increasing the bias current, which results in magnetoresistance oscillations typical of a SQUID.

Magnetic flux instability limits potential applications of superconductors such as MgB2 in practical devices. Previous studies in MgB2 films exposed to magnetic fields revealed the occurrence of dendritic flux avalanches at temperatures below T ~ 10 K. In the present work it is shown that films of MgB2 exposed to a fast-ramped magnetic field display a dendritic flux instability at elevated temperatures, up to 23 K. Such instability can therefore cause malfunctioning of practical devices based on MgB2 films even when operating at liquid hydrogen temperature.

Magnetic decoration of BaFe2(As1-xPx)2 revealed an hexagonal vortex lattice in overdoped single crystals (x>0.4, Tc=14-18 K). A disordered vortex lattice was observed in a crystals with optimal content of phosphorus (x ≈ 0.4, Tc = 25-30 K). Magneto-optical studies and global magnetization measurement of the optimally-doped crystal have led to an estimated critical current density of ~ 10 5 A/cm 2 at 5-12 K in fields up to 0.7 T. This high critical current density suggests strong pinning, probably due to defects, a result consistent with the observed disordered vortex lattice. The critical current density of the overdoped crystal is much smaller, i.e. ~ 10 4 A/cm 2 at 5 K in fields up to 0.1 T, decreasing to virtually zero at 0.3 T.

The authors have investigated the effects of 2.5 MeV electron irradiation on the irreversibility line of a sintered YBa2Cu3O7 by applying DC and AC measuring techniques. They find that after electron irradiation, both the superconducting transition temperature Tc and the irreversibility temperature Tirr, determined by either technique, decrease; the decrease in Tirr scales with the decrease in Tc.

Magneto-transport measurements in YBa2Cu3O7−δ nanowires, comprising a single layer of a small number of grains, reveal periodic magnetoresistance oscillations in a wide temperature range below the transition. The rich structure of these oscillations manifests a negative magnetoresistance in the initial part of each cycle beginning at zero magnetic field. The salient features of these unique oscillations indicate the presence of π Josephson junctions in the granular wires, arising from the d-wave pairing symmetry in YBa2Cu3O7−δ.

Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions., 10 pages, 4 figures

Magneto-optical imaging was employed to study magnetic flux propagation through an interface separating between heavy-ion irradiated and non-irradiated parts of the same Bi2Sr2CaCu2O8+δ crystal. As the flux front hits the interface the local induction increases abruptly at a rate considerably larger than that of the external field. This induction jump is accompanied by the appearance of a finger-like flux pattern at the interface. At low temperatures or high field ramp-rates, the induction at the interface increases gradually, finger patterns do not form and the flux front crosses the interface smoothly. We explain the flux behavior at the interface on the basis of the magnetic diffusion equation, and propose that the induction jump triggers the formation of finger patterns.

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.

Mossbauer spectra of amorphous Fe83B12Si5 ribbons at temperatures 90 to 800 K and their isothermal time evolution have been recorded. The analysis of the experimental spectra reveals that crystallisation begins at about 600 K and crystalline Fe2B, Fe metal and FeSi are formed either isothermally in time or by raising the temperature above 700 K.

Commercial Manganin gauges were used in a series of planar impact experiments in order to determine the Hugoniot elastic limits (HEL) of differently heat-treated 2024 aluminium samples. The gauges were used in an indirect configuration by which they were embedded in a soft backing material (polymethylmethacrylate (PMMA) in this case) at the back surface of the specimen. The resulting HEL values were compared with the corresponding yield stress values from uniaxial tension tests. It is found that the simple elastic relation between yield in uniaxial strain and uniaxial stress is obeyed. An additional measurement of the HEL of a pressurised sample also conforms with this simple relation.

The electric field gradients in ZrO2 and HfO2 have been measured by applying the differential perturbed angular correlation method to the 133-482 keV cascade in 181Ta. The results for HfO2 have been compared with previous results. In general, a fairly good agreement has been found except for the electric field asymmetry parameter eta , in which the older result of Gerdau (1969) et al, rather than the more recent one of Gardner and Prestwich (1970) has been confirmed. The results for ZrO2 show the electric field gradient to be (13.3+or-0.7)*1017 V cm-2 which is slightly higher than the value of (12.8+or-0.6)*1017 V cm-2 obtained for HfO2. The corresponding values for the field asymmetry are 0.335+or-0.015 for ZrO2 and 0.348+or-0.016 for HfO2. Despite the small difference of only 4% between the corresponding means, the 't' distribution shows, with confidence of better than 99%, that the results for ZrO2 and HfO2 are different.