M. Oussena

Comparative AC susceptibility and resistivity studies of the vortex lattice melting transition in detwinned YBa2Cu3O7-δ single crystals

Abstract We have studied the transition from the vortex liquid to the vortex solid state in clean, detwinned YBa2Cu3O7−δ single crystals in fields up to 12 T using AC susceptibility and resistivity measurements. For μ0H 1 , 2 ) was found to be accompanied by an AC field independent abrupt onset of both χ′ and χ″ components. The AC susceptibility data suggest that just below Tm there is a transition region where the liquid and solid phases coexist. The width of the transition region was found to be sample dependent and can be related to structural inhomogeneity. Above the multicritical field μ0Hmc=7 T simultaneous broadening of both the ρ(T) drop and the χ″(T) onset show that the first order melting transition is suppressed.

Scaling features in M-H loops of YBa2Cu3O7-x single crystal

Abstract Magnetic hysteries curves in YBa 2 Cu 3 O 7-x single crystal are scaled by dividing the magnetic field H and the magnetisation M by the full penetration field H p and the associated magnetisation M p . The scaling parameters, H p and M p are determined experimentally as the merging point between the virgin magnetic curve and the M-H loop envelope. For magnetic fields higher than H p , the M-H loops, once scaled, lie on a unique envelope, independent of temperature up to the irreversibility line. This result suggests that below the irreversibility line there is no evidence of any transition between different types of pinning for H > H p .

Effect of oxygen stoichiometry on the out-of-plane anisotropy of YBa2Cu3O7- single crystals near optimal doping

We have performed magnetic measurements on pure detwinned YBa2Cu3O7- single crystals in order to investigate the influence of variations in the oxygen stoichiometry on the out-of-plane anisotropy ck in the region close to optimal doping (6.90 < 7- < 6.98). The anisotropy is estimated using the anisotropic London model from the period of the commensurability oscillations of the hysteretic magnetization as observed for fields applied parallel to the a-b-planes. For a variation in the oxygen deficiency by as much as 300%, we find that ck decreases but only slightly (less than 12%) with decreasing oxygen deficiency. The possibility of a temperature-dependent ck is also discussed.

Organisation of vortex motion in detwinned YBa2Cu3O7−δ single crystals

Abstract The dynamic behaviour of a driven vortex lattice in the vicinity of the melting line has been investigated. We have shown that a square wave driving current with asymmetric period or amplitude can induce a slow periodic variation in the voltage response. We also show that this effect can be observed for current modulations with sinusoidal and triangular wave forms thereby demonstrating that these oscillations are not due to the sharp fronts of the probing current which are a feature of the square wave modulation.

The critical state in YBa2Cu3O7-x high temperature superconductor

Abstract A practical method was devised for measurements of the diamagnetic response of high temperature superconductor specimens. The method is useful for the study of the temperature and field dependencies of the local magnetic induction and its two-dimensional visualisation B ( T , H a , x , y ) with a resolution of 0.1 gauss and spatial resolution of 0.1 mm in the plane of the specimen surface. The proposed method facilitates quite simple and quick measurements at liquid nitrogen temperature and in applied H a magnetic field in excess of 1 T. Cross-shaped miniature Hall probes (HPs) of thin film n -InSb on semi-insulating substrate i -GaAs with a small magnetically sensitive zone of 0.1 × 0.1 mm 2 were used as measuring transducers. The high degree of doping of n -InSb layers ( n ~ 10 18 cm −3 ) resulted in a sensitivity of 5–7 μV mT −1 (with nominal current up to 10 mA through HPs) and thermosensitivity of less than 0.03% K −1 , and made it possible to operate without the need for thermal stabilization of transducers. This method enabled us to study anisotropy of the critical current densities ( J c ) of elt-grown YBa 2 Cu 3 O 7 − x pellets. Also, we have been able to determine local cracks and grain connectivity in the YBa 2 Cu 3 O 7 − x pellet samples.

Fish-Tail Effect and Its Evolution Under Neutron Irradiation in Li-Doped YBa2Cu3O7−x

The evolution of the critical current density of Li-doped YBa2Cu3O7−x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 ≤ B ≤ 6 T) temperature (5 ≤ T ≤ 85 K) and neutron fluence (0 ≤ Φ ≤ 9.98 × 1017 cm−2). At fluences lower than 1017 cm−2, a second peak in js vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 1017 cm−2, the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field Binfl. A dependence of Binfl on fluence is proposed.

Field induced suppression of the vortex lattice melting transition in twinned YBa2Cu3O7−δ

Abstract We present magneto-resistance data for a high quality, twinned YBa 2 Cu 3 O 7−δ crystal, taken with the current applied along the ab plane. The crystal was examined at a number of angles to an applied magnetic field, of up to 20T, in order to observe the influence of correlated and point-like disorder on the vortex dynamics. When the applied field was orientated at 15° to the crystalline c-axis (θ=15°), for fields below H * =12T, we observed the kink in ϱ(T) associated with vortex lattice melting. We found that when the field exceeded this value, there was a complete suppression of this kink and the ϱ(T) curves resembled those of crystals with extensive point disorder. This suppression in melting occurs abruptly between 11T and 12T. The melting transition is recovered when the angle between the c-axis and the field is increased. An analysis of features around 12T in ϱ(T) and ϱ(θ) has been performed for a number of fields and angles.

Novel vortex dynamics in untwinned YBa2Cu3O7−δ single crystals

Magnetotransport measurements on a clean, untwinned YBa2Cu3O7−δ single crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition are highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the “shaking” of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small asymmetry (∼10%) in the durations of the positive and negative parts of the square-wave period induces periodical oscillations of the voltage response amplitude. The period of oscillations (∼100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage oscillations to a periodically ordered vortex motion.

The peak effect as a phase boundary in detwinned YBa2Cu3O7−δ

Abstract We present magneto-resistance data from a high quality detwinned YBa 2 Cu 3 O 7−δ single crystal, taken using the conventional four-point method with the current applied along the ab plane and the field along the c-axis. Measurements were taken in fields of up to 12T. At temperatures just below the melting line, a sharp feature (a ‘peak effect’) appeared in the resistance when sufficiently high current densities were applied. We saw this feature as both a function of temperature, ϱ(T), and field, ϱ(H). At higher fields, above the multi-critical point H * =6T, the peak effect in ϱ(H) splits into two and spreads apart in field. This forms two branches in the H-T phase diagram, and their implications for the dynamics of the vortices are discussed.

a.c. susceptibility studies of the vortex lattice melting transition in clean, detwinned YBa2Cu3O6.92 single crystals

Abstract We present data from a.c. susceptibility and resistivity measurements in an applied d.c. magnetic field on clean detwinned YBa 2 Cu 3 O 6.92 single crystals. Our investigations have revealed an extremely sharp amplitude and frequency independent onset of both the in phase and out of phase a.c. susceptibility components. For fields below H * =6T the position of this onset in H-T plane agrees with the melting transition obtained from resistivity measurements on the same crystal. At H≥H * this transition becomes criteria dependent. We found that H * accurately correlates with the multi-critical point obtained from resistivity measurements.