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A phenomenological theory is developed to explain the recently observed negative magnetoresistance slope in ultra-thin granular YBa 2 Cu 2 O 7− δ films. Viewing this system as a two-dimensional array of extended Josephson junctions, we numerically solve the sine-Gordon equations including a viscosity term that increases linearly with the external field. The solution yields a negative magnetoresistance slope setting in at a field that is determined by the geometry and thus independent of temperature, in agreement with the experimental results.

We report on magnetoresistance measurements in 10 nm thick and submicron-wide granular YBa 2 Cu 3 O 7− δ bridges. The results show a strong dependence of the resistance on the magnetic field at low fields crossing over to a relatively weak field dependence at high fields. The field derivative of the resistance at high fields decreases as the temperature is lowered and eventually changes sign, exhibiting a negative slope in a wide field range in the Tesla regime. This negative slope is sensitive to the bias current, turning to be positive as the bias current increases. This complex magnetoresistance behavior is attributed to both phase slips in a distribution of strongly and weakly linked superconducting grains, and tunneling of quasiparticles between grains. The latter dominates at low temperatures and high fields, giving rise to the negative magnetoresistance slope.

Time-resolved magneto optical measurements in Bi 2 Sr 2 CaCu 2 O 8+ 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.

We highlight interesting thermomagnetic history effects across the transition line between the quasi-ordered and disordered vortex states in single crystal YBa 2 Cu 3 O 7− δ (YBCO), and argue that these features are indicative of the first-order nature of the transition line. We suggest that the destruction of the ordered vortex state in YBCO leading to vortex liquid (at high temperatures and low fields) and amorphous vortex solid (at low temperatures and high fields), takes place along a unified first-order transition line. The non-monotonic behavior of this first-order transition line gives rise to the possibility of more than one Kauzmann point where the entropies of the ordered and disordered vortex states are equal. In the high temperature region, one may order the vortex lattice by warming it, giving rise to an inverse melting effect.

We report on measurements of the critical current in a single and bifilar multifilamentary Bi-2223/Ag tapes, at 77 K, under various DC magnetic fields applied perpendicular to the tape plane. At low applied fields the bifilar tape exhibits higher critical current, as expected. However, above ∼50 G the situation is reversed, i.e., the critical current of the single tape is higher. These results are attributed to the different spatial distribution of the self-field in these two configurations. Our data and analysis imply that, in terms of critical current, the bifilar configuration has over advantage over the single tape configuration only in a limited range of external magnetic fields.

The time evolution of the vortex structure in a Bi2Sr2CaCu2O8+δ crystal is magneto-optically recorded after a sudden change of the external magnetic field applied to the sample. The magneto-optical images reveal dynamic coexistence of a quasi-ordered vortex phase and a transient disordered vortex phase. The front of the quasi-ordered state moves with time toward the sample edges (center) when the field is suddenly increased (decreased), until a quasi-ordered state is established throughout the entire sample, as dictated by thermodynamics. Effects of magnetic relaxation on the growth rate of the quasi-ordered state are discussed.

We exploit the magnetic flux rotation effect under tilted fields, and its associated magnetization peak, to study the temperature induced crossover from a 2D to 3D behavior in La 1.85 Sr 0.15 CuO 4 . Measurements of the peak field versus temperature show a pronounced anomaly around the theoretical 2D–3D crossover temperature (∼12 K). Analysis of the data on both sides of this anomaly shows two distinct regimes corresponding to a 2D and 3D behavior.

Magneto-optical measurements in Bi2Sr2CaCu2O8+d demonstrate enhanced effects of transient disordered vortex states (TDVS) in a prism-shaped sample. The existence of such transient states is indicated by the appearance of a sharp change (‘break’) in the slope of the induction profiles. As the external magnetic field is swept up or down, the break, which signifies the front of the TDVS, moves towards the sample center at a constant velocity determined by the rate of change of the external field. We discuss the results in terms of a competition between injection and annealing processes, the latter being ineffective in the prism due to direct injection of disorder into the bulk through surface barriers distributed along the tilted facets of the prism. � 2007 Elsevier B.V. All rights reserved.

The interplay between AC and DC currents in a High- T c Superconducting (HTS) coil, made of multifilamentary silver-sheathed Bi-2223 tape, was investigated. We observed that the application of a small sinusoidal current in the frequency range of 50–500 Hz into the coil, while it is already carrying a DC current in the range of 16–22.5 A, caused an increase in the coil DC voltage. The DC voltage increment due to the AC signal is found to increase linearly with frequency and quadratically with amplitude. The DC voltage increment increases as the coil current grows towards its critical value of 22.2 A. This result may be important in some power applications such as fault current limiters (FCL) and superconducting magnet energy storage (SMES) based on HTS coils.

The onset temperature, Tirr, for magnetic irreversibility in high-temperature superconductors has been investigated as a function of frequency (0.2 Hz⩽f⩽65 000 Hz) and DC magnetic field (0

We have investigated the irreversibility line (IRL) of a ceramic YBa 2 Cu 3 O 7 sample by probing the onset of irreversibility in four DC and AC techniques. All techniques yield a line which obeys a power law H=A( 1−T T c ) n but A and n depend on the technique. Also, the location of the peak in the AC signal obeys this power-law but it does not scale with the onset of irreversibility. We have also investigated the effect of two doses of 2.5 MeV electron irradiation on the IRL and find that after irradiation both the transition temperature T c and the irreversibility temperature T irr decrease. The decrease in T irr scales with the decrease in T c . We propose that this scaling indicates that electron-irradiation does not induce pinning centers stronger than those present in the unirradiated sample.

Magnetization curves of La 1.85 Sr 0.15 CuO 4 in fields slightly tilted from the ab plane exhibit an additional magnetization peak (AMP) in between the well-known first and second magnetization peaks. Measurements of the induction vector versus field reveals flux rotation from the ab plane towards the direction of the external field, beginning at the onset of the AMP. Characterization of the AMP and flux rotation as a function of temperature, time and tilting angle, shows a strong correlation between the two phenomena. This correlation is explained by considering variations in the magnetization caused by a tilt of the current flow plane associated with the flux rotation. Simulations of the magnetization curves based on the proposed model agree well with the experimental results.

We explain theoretically an oscillatory behaviour in space and time of the magnetic induction in Bi2Sr2CaCu2O8+δ crystals during magnetic relaxation. This new “flux waves” phenomenon appears near the order–disorder vortex phase transition, under specific conditions of temperature and induction gradient. Our theory is based on two coupled equations: The Landau–Ginzburg equation for the order parameter of the vortex phase transition and the diffusion equation for the magnetic induction. Linear stability analysis of these equations shows an appearance of oscillatory instabilities characterized by a period and wavelength that are in accordance with the experimental results.

Time evolution of the induction profile across a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded in two experiments: during ramping the external magnetic field or after a sudden application of the field. The results of these experiments reveal a front-like propagation or retreat of a transient disordered vortex state, respectively. Analysis of the front velocity shows that while the retreat is continuously decelerated, the propagation is accelerated, approaching a constant velocity when the front induction reaches the vortex order–disorder phase transition field Bod. The latter observation suggests a method for determining the value of Bod, eliminating transient state effects.

Time relaxation of the local persistent current in a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order–disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively.

Local magnetization curves at different times, extracted from high-temporal resolution magneto-optical measurements in Bi 2 Sr 2 CaCu 2 O 8+ δ , demonstrate the absence of the second magnetization peak (SMP) at short times, its appearance at longer times, and the movement of its onset toward higher induction fields approaching the limit of the thermodynamic order–disorder transition field. We utilize theoretical analysis, based on the Landau–Khalatnikov dynamic equation adopted for the vortex matter order parameter, to simulate the measured time evolution of the SMP. We show that the observed time evolution is a manifestation of a dynamic coexistence of transient-disordered and ordered vortex phases.

A high temporal resolution magneto-optical system was employed to image the induction distribution on the surface of a Bi 2 Sr 2 CaCu 2 O 8+ δ crystal while the external magnetic field was ramped up at a constant rate. These data reveal coexistence of quasi-ordered and disordered vortex phases, near the order–disorder phase transition line. The coexistence region in the B – T phase diagram narrows down with increasing temperature or decreasing sweep rate. These observations clarify previous interpretations of phenomena associated with the fishtail, e.g. the shift of the fishtail onset to higher inductions for slower sweep rates and the absence of a fishtail at low temperatures.

The vortex order–disorder phase transition line in (La 0.937 Sr 0.063 ) 2 CuO 4 exhibits a steep concave decrease throughout the whole temperature range. This unusual behavior is explained postulating that in (La 0.937 Sr 0.063 ) 2 CuO 4 , both thermal and disorder-induced fluctuations take part in destabilizing the vortex lattice. Irradiation of the samples with electrons causes a significant decrease in both the magnitude of the transition field and the curvature of the transition line. These results are interpreted as caused by the enhanced role of disorder-induced fluctuations as compared with thermal fluctuations.

Magnetic induction profiles across the width of a Bi2Sr2CaCu2O8þd crystal were measured while the external magnetic field was ramped down from above the vortex solid–solid phase transition field down to zero. These profiles indicate dynamic coexistence of two vortex phases: a disordered phase in the sample interior and a quasi-ordered phase near the sample edges. The border between these two phases,marked by an abrupt change in the slope of the profiles, progresses towards the sample center and appears at the same induction, Bf ,independent of the location. By increasing the sweep rate of the external field, Bf can be monotonically decreased to values well below the transition field. These results emphasize the role of the electric field in the vortex solid–solid phase transition,and question the interpretation of such ‘‘supercooling’’ experiments as signifying the first order nature of this transition. 2003 Elsevier Science B.V. All rights reserved.

Transient vortex states, created after a sudden exposure of a Bi 2 Sr 2 CaCu 2 O 8+δ crystal to a steady magnetic field, are observed using a high-temporal resolution magneto-optical imaging system. The images reveal dynamic coexistence of a disordered phase near the sample edges and a quasi-ordered phase in the sample interior, prior to the establishment of a quasi-ordered state throughout the entire sample as dictated by the thermodynamic conditions. Different points on the borderline between these two phases move toward the sample edges at different rates, depending on the local induction.

The reversible magnetization M is measured in an YBa2Cu3O6.5 crystal (Tc=45.2 K) as a function of temperature T for various fields H between 0.2 and 3.5 T. All isochamps for H > 1 T intersect at T 2 D ∗ ≈42.8 K and collapse into a single curve when m = M(H·T)(D−1)/D is plotted as a function of t=(T−Tc(H))/(H·T)(D−1)/D where the dimension D=2 (“2D scalling”). Surprinsingly, the low field curves also intersect, but at a different temperature T 3 D ∗ ≈43.4 K , and they obey a 3D scaling.

We study the frequency dependence of the irreversibility temperature T irr in single crystal, thin film and ceramic Y-Ba-Cu-O samples. T irr , measured by the onset of third harmonic signal, depends weakly on frequency for crystals, moderately for thin films and strongly for ceramic samples. The data is analyzed in the framework of the flux creep and the vortex-glass models. Although we cannot rule out the flux creep scenario, the data is better described by the vortex-glass phase transition model.

We have studied the effect of 5.3 GeV Pb irradiation on irreversible magnetic properties of YBa 2 Cu 3 O 7 crystal, using an ultra-sensitive local Hall-probe technique. In this article we briefly summarize our previous results and present new data at 90 K (=0.97 T c ) which demonstrate that this type of irradiation enhances flux pinning in the vicinity of T c .