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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.

A study of the DC I–V curves of bifilar BSCCO/Ag tapes at 77 K is presented. Tapes under study were carrying DC transport current and exposed to external AC (0–20 mT, 50–400 Hz) or DC (0–20 mT) magnetic fields perpendicular to the plane of the tape. The measurements were carried out with bifilar tape to avoid the influence of the self-field. The measured I–V curves are well described by power law in the range of electric field 0.1–10 μV/cm. The AC field affects the I–V curves in a much stronger way than the DC field. The application of the AC field results in a linear decrease of the critical current (criterion 1 μV/cm) independent of the frequency of the magnetic field. The exponent n also exhibits a strong dependence on the AC magnetic field decreasing with increasing AC field amplitude and increasing with increasing frequency. The results are interpreted by assuming a confinement of the DC transport current in the central AC field free region of the tape. A low current density is assumed to flow in the region to which the AC magnetic field has penetrated.

We report on measurements of I–V curves in microbridges of thin Y–Ba–Cu–O films of different thickness, in the presence of external magnetic fields up to 6 T. A discontinuity is observed at a critical voltage, V*, in the flux flow regime, reflecting an electronic instability, as predicted by Larkin and Ovchinnikov (LO), and in agreement with results reported by Doettinger et al. [Phys. Rev. Let. 73 (1994) 1691]. The critical voltage, V*, and the flux flow resistance, R0, in the limit V→0, are calculated by fitting the data to the LO model. We find that the vortex critical velocity, v*, at the instability, derived from V*, decreases with magnetic field and film thickness. These results, not predicted by the LO theory, reflect the dependence of the (spatially averaged) quasiparticle energy relaxation rate on magnetic field and film thickness.

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.

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.

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.

Global transport measurements and local Hall-probe array measurements of a section of silver clad BSCCO tape, are reported. Transport measurements are in good agreement with the `brick wall' model. The induction profiles obtained from magnetic measurements agree with the assumption of a uniform bulk current at low (40–60 K) temperatures. At higher temperatures and relatively high fields (700 G and up) local measurements seem to reveal the existence of surface currents. These currents are the result of small current loops, flowing within the grains, which result in a net current only at the edges of the sample. Thus, the local magnetic measurements do not agree with global transport ones, and we contend that the `brick wall' model aptly describes the flow of transport currents, but fails to describe the flow of currents induced by an external magnetic field.

The relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated. An efficient way to determine the value of x and residual BaCuO2 content during Nd123SS powder synthesis is presented. The method is based on calculation of the orthorhombic splitting (OS) factor from the unit cell parameters obtained from XRD data. The final phase in the Nd123SS system is formed through a diffusion controlled reaction between BaCuO2 and Nd123SS and a kinetic model is developed to describe the formation of the Nd123 (x=0) superconducting powder. Finally, high values of Tc (measured by DC magnetization) were found in the powders. The highest Tc of 98.7 K was measured for the x=0 case. This value is the highest ever reported for the NdBaCuO system. The Tc was found to be insensitive to the value of x, ranging between 98.7 and 94 K for x between 0 and 0.25, respectively. This data is contrast to published results on samples synthesized at higher temperatures, where Tc fell to 40 K at x = 0.25.

We study the angular dependence of the magnetic properties of Y-ba-Cu-O thin films irradiated with 0.86 GeV Pb and 5 GeV Xe ions. For the Pb irradiated samples the critical current has a maximum when the direction of the external magnetic field coincides with the direction of the columnar defects induced by the irradiation. For the Xe irradiated samples such anisotropy in the critical current was not found. The pinning force density obeys a scaling law of the form b p (1 − b ) q , where b = H ext / H irr ( T ), and H ext and H irr are the external and the irreversibility fields, respectively. The exponents p and q depend on the type and direction of the irradiation.

We report on a study of the magnetic properties of the intermetallic superconductor YNi 2 B 2 C in a wide range of magnetic fields (up to 8 T) and temperatures (4.4−16 K). The results yield the characteristic length scales and fields for this compound: λ (0) ≈ (3.5 ± 0.5) × 10 −5 cm, ξ(0) ≈ (10 ±2) × 10 −7 cm, H c1 (0) ≈ 83 G and H c2 (0)≈ 47600 G. We compare our results with those obtained by other groups and add new information on thermodynamic properties, in particular on the lower critical field, and on irreversible magnetic features such as the critical current, pinning force and magnetic relaxation. We conclude that YNi 2 B 2 C is a type-II superconductor with relatively weak pinning centers.

Pinning by columnar defects in irradiated YBa 2 Cu 3 O 7 (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals is most efficient when the field is parallel to the defect. However, in the low field limit the remanent magnetization is independent of the direction of the field, indicating that the fluxons flop towards the columnar defect. The flux-flop is almost temperature independent in YBCO and strongly temperature dependent in BSCCO due to the different nature of the vortices

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.

DC magnetic measurements on the high- T c superconductors YBa 2 Cu 4 O 8 (124, T c =80 K) and Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 ( T c =87K) are reported. The irreversible lines of 124 and ceramic YBa 2 Cu 3 O 7 (123) are quite similar. The results show that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase. Conversions of 124 into 123+CuO does not change the grain size of the material but enhances its critical current density.

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.

Magnetization curves of Tl 2 Ba 2 CaCu 2 O 8 crystal were measured in applied fields up to 8 Tesla. At temperatures above 25 K we observe reversible magnetization at high magnetic fields. Equilibrium magnetization value was derived from these data. The penetration length λ and the bulk critical field H cl were determined by fitting the theoretical curves to the reversible magnetization data. The magnetization corresponding to the first maximum is much higher than the calculated value based on the measured equilibrium magnetization at high fields. We conclude that geometrical barrier effects change the magnetization curves drastically.

Magnetization curves have been measured for several Tl2Ba2CaCu2O8 crystals. For crystals with relatively low critical currents, an anomalous maximum is recorded over a wide range of temperatures. This maximum completely disappears after heavy ion irradiation. We interpreted the results in terms of a phase transition from different vortex phases.

We study the influence of irradiation with Pb and Xe ions on the magnetic properties of thin YBa 2 Cu 3 O 7−x films. Using analysis of scaled pinning force we found unidirectional magnetic anisotrophy in the case of Pb irradiation. For Xr irradiated samples, such anistrophy was not found. The irreversibility lines and their frequency dependencies vary with the type of defects and the direction of the external magnetic field.

We present new measurements of the field distribution in untwinned single crystal of YBa2Cu3O7, using an ultra-sensitive local Hall-probe technique. In the temperature range 75 K to Tc (92.5 K), there is no field gradient along the sample when the external field is reduced. This indicates that Bean-Livingston surface barriers are the main source of magnetic irreversibility in this temperature range.

We have studied the angular dependence of the zero-field-cooled (M ZFC ), field-cooled (M FC ) and remanent magnetization (M rem ) of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 single crystals, by rotating the crystal relative to the applied field direction. Two compatible mechanisms, namely shape anisotropy and intrinsic anisotropy are used to describe the angular dependence of these quantities. At sufficiently low temperatures, the pinning forces are strong and the relation M FC =M ZFC +M rem holds for all orientations. The angular dependence of M ZFC is due only to variation of the demagnetization factor by rotation; whereas the angular dependence of the remanent magnetization can be interpreted by using the two mechanisms. For larger fields or higher temperature the pinning forces are weaker and |M FC | ZFC +M rem|.

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 .