Pinning in type II superconductors

Abstract

Large and randomly arranged pinning centers cause a strong deformation of a flux line lattice, so that each pinning center acts on the lattice with a maximum force. The average force for such single-particle pinning can be inferred from a simple summing procedure and has a domelike dependence on magnetic field. Pinning centers of average ]orce, such as clusters of dislocations, strongly deform the flux line lattice only in weak fields and in fields close to the critical field, where there is a peak in the dependence of the critical current on magnetic field. In the range of intermediate fields there is a weak collective pinning. A large concentration of weak centers leads to collective pinning in all fields. In this case, near the critical field a critical current peak should be observed. To explain this peak and to define the boundaries between the regions of collective and single-particle pinning the possible break -off of the flux line lattice from the lines of magnetic force should be taken into consideration, which leads to extra softening of the lattice. 409

Current-driven vortex dynamics in untwinned superconducting single crystals

“…The coherently moving bundles consist of a nearly regular vortex lattice because the large applied current effectively 'heals' the defect-induced inhomogeneous vortex structure, as illustrated in figure 4(a). As the temperature increases to T pc < T < T pl , the moving vortex bundles become increasingly softened due to the rapidly decreasing c 66 [15,20], and therefore may be partially pinned by local defects [15,20], resulting in the onset of plasticity with smaller and defected vortex bundles which undergo predominantly coherent motion [28], as illustrated in figure 4(b). The occurrence of partially pinned vortex bundles gives rise to an increasing J c (the peak effect) and therefore decreasing vortex dissipation.…”

“…In the weak-pinning limit, measurements of thermodynamic quantities such as magnetization and heat capacity have confirmed the existence of a first-order melting transition in both Bi 2 Sr 2 CaCu 2 O x [9] and untwinned YBa 2 Cu 3 O 7 single crystals [10], while the resistive hysteresis observed in untwinned YBa 2 Cu 3 O 7 single crystals below the first-order vortexsolid melting transition temperature T M [1][2][3] has been attributed to the current-driven non-equilibrium effects below the thermodynamic transition [3]. Another current-induced phenomenon in weak-pinning systems is the 'peak effect', which refers to a peak feature in the critical current density (J c ) as a function of the temperature or magnetic field [17][18][19][20][21]. The peak effect has been observed just below the upper critical field H c2 (T ) in conventional superconductors such as niobium [17] and Nb 3 Ge films [18], and has been attributed to the softening of the elastic moduli in the vortex lattice first by Pippard [19] and then in a generalized way by Larkin and Ovchinnikov [20].…”

“…Another current-induced phenomenon in weak-pinning systems is the 'peak effect', which refers to a peak feature in the critical current density (J c ) as a function of the temperature or magnetic field [17][18][19][20][21]. The peak effect has been observed just below the upper critical field H c2 (T ) in conventional superconductors such as niobium [17] and Nb 3 Ge films [18], and has been attributed to the softening of the elastic moduli in the vortex lattice first by Pippard [19] and then in a generalized way by Larkin and Ovchinnikov [20]. Recently similar peak effects have been reported for conventional superconductors such as 2H-NbSe 2 single crystals [4], and for high-temperature superconductors (HTSs) such as YBa 2 Cu 3 O 7 single crystals either with very few twin boundaries [5,6] or completely untwinned [7].…”

“…Recently similar peak effects have been reported for conventional superconductors such as 2H-NbSe 2 single crystals [4], and for high-temperature superconductors (HTSs) such as YBa 2 Cu 3 O 7 single crystals either with very few twin boundaries [5,6] or completely untwinned [7]. In the case of hightemperature superconductors, it has been theoretically argued that the peak effect occurs just below the vortex-solid melting line H M (T ) rather than near H c2 (T ) [15], because the shear modulus c 66 in HTSs vanishes at H M [3,15] due to large thermal fluctuations, and the rapid decrease of c 66 associated with the softening vortex solid below H M (T ) is known to result in enhancement of the pinning and therefore the peak effect in the critical current density J c [15,[19][20][21].…”

Microwave absorption study of pinning regimes in Ba(Fe1−xCox)2As2 single crystals

“…The second effect pointed out in Ref. 26 is the formation of vortex bundles. Both effects can result in the increase of the vortex shift √ < u 2 > under the action of thermal fluctuations, as compared with the rigid (elastic) vortex lattice.…”