MAGNETORESISTANCE OF YBA2CU3-ZALZO7-Δ SINGLE CRYSTALS WITH A SYSTEM OF UNIDIRECT PLANAR DEFECTS

Abstract

The work carried out electroresistive studies under the influence of a constant magnetic field up to 12.7 kOe at the orientation of the magnetic field vector Н ^ с and Н || с on different conductivity regimes of single crystals                 YBa₂Cu_3-zAl_zO_7-δ (z ≤ 0.5) with a unidirectional system of twin boundaries (TB) at the geometry of the flow of the transport current I || TB, when the influence of twins on the processes of scattering of current carriers is minimized. It was established that twin boundaries in single crystals YBa₂Cu_3-zAl_zO_7-δ (z ≤ 0.5) are effective centers of fluctuation carriers. The deviation from linearity of the dependences of the specific electrical resistance in the base ab-plane ρ_ab(Т) at a temperature T in the interval T_с < T < 1.35 T_с can be satisfactorily explained within the framework of the theory of fluctuation superconductivity. At the same time, in the immediate vicinity of the critical temperature T_c, the fluctuation conductivity (FC) is well described by the three-dimensional Aslamazov – Larkin model. The application of a magnetic field leads to a significant narrowing of the temperature interval of the existence of three-dimensional superconducting fluctuations. The non-monotonic dependence of the coherence length along the c axis at T → 0 x_с(0) on the magnetic field can probably be associated with the suppression of the excess fluctuation conductivity in the region of weak magnetic fields. The absence of the “fan-shaped” expansion of resistive transitions in the magnetic field, characteristic of impurity-free YBa₂Cu_3-zAl_zO_7-δ samples, is due to the suppression of the phase transition to the state of an unpinned vortex liquid due to the enhancement of the pinning of the vortex lattice at the twin boundaries.