Anisotropic flux dynamics in single-crystalline and melt-texturedYBa2Cu3

Current-driven vortex dynamics in untwinned superconducting single crystals

“…Various types of current-driven vortex dynamics at T < T M are proposed, and the possibility of an accompanying dimensional crossover in the vortex correlation is also discussed in the context of the different temperature variations in c 66 and c 44 near T M . The latter viewpoint of a dimensional crossover near the occurrence of the peak effect is analogous, but not identical, to that described in reference [21]. Second, the general agreement of the angular and magnetic field dependence of ρ 0 with equation (2) provides further quantitative support for the existence of current-driven plastic motion below T M .…”

“…Comparing with previous reports of resistive peaks in similar systems [5,6,21], we note the following new aspects of this work. First, our quantitative analysis of the E-J characteristics in the as-grown untwinned YBa 2 Cu 3 O 7 single crystal provides evidence for the motion of vortex bundles with finite elastic moduli, and the physical origin of the peak effect is attributed to the motion of softened vortex bundles below the thermodynamic melting transition T M .…”

“…It is also interesting to compare our empirical relation with the current-voltage characteristics of charge-density-wave (CDW) behaviour in the mean-field approximation for a strongpinning system, where E ∝ [J − J c ] 3/2 is satisfied [26]. Such behaviour has been reported for YBa 2 Cu 3 O 7 melt-textured samples and twinned crystals [21] where significant pinning defects are present. Although the mean-field CDW behaviour is similar to our currentvoltage characteristics, we note that the power α for our untwinned single crystal is always smaller than 3/2 for all of the temperatures of our investigation.…”

“…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].…”

“…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].…”