Observation of a nonlocal conductivity in the mixed state ofYBa2Cu3

Abstract

%e report on transport measurements in the mixed state of YBa2Cu307 single crystals using the dc flux transformer geometry. In the vortex fluid regime we have observed the onset of a nonlocal conductivity. At lower temperatures, close to but above the phase transition temperature the vortices become eA'ectively three dimensional in character. This is in sharp contrast to recent flux transformer experiments on Bi2Sr2CaCupO which have failed to see a nonlocal conductivity, confirming the twodimensional character at high temperatures of the vortices in that system. These observations suggest that in a finite temperature interval, the vortex state in YBa2Cu307 is a line liquid.The magnetic phase diagram of the oxide superconductors is a rich system with many novel physical phenomena playing a significant role. Effects such as thermal Auctuations, disorder, anisotropy, and dimensionality are all important in explaining the experimentally observed behavior [1]. It is now clear from experiments that there is a phase transition in the mixed state which separates a low temperature ordered phase from a high temperature vortex fluid. In the limit of strong pinning disorder, this low temperature phase appears to be a vortex glass [2] and the phase transition is known to be second order [3].In the weak pinning limit, the low temperature phase is a vortex lattice with long-ranged positional order and the transition is theoretically [41 and experimentally [5] found to be first order. However, there remain many open and important questions regarding the role of the dimensionality of the vortices.In a layered superconductor with the magnetic field perpendicular to the layers, one may ask whether the vortices in the different layers will tend to align forming vortex lines. This should occur when the interactions between vortices in adjacent layers are not overwhelmed by either thermal Auctuations or random pinning in the layers [2,6]. When the vortices do form lines this reAects strong correlations between layers and so the material is behaving three dimensionally.However, as the temperature is increased, the thermal Auctuations may destroy the correlations between layers [7], breaking the vortex lines up into point or "pancake" vortices in each layer [81 and causing a crossover to two-dimensional behavior. In YBa2Cu307 (YBCO) this crossover occurs in the vortex liquid regime and can be studied in a new way using multiterminal transport measurements as reported and analyzed in this Letter.In this paper, we present transport measurements on YBCO in the vortex liquid regime using the modified dc Aux transformer geometry [9]. We show that the data are highly inconsistent with the material being simply an anisotropic, local conductor. Thus we report the first observation of nonlocal conductivity in the mixed state of an 1272 oxide superconductor. This observation allows us to "measure" the vortex dimensionality as a function of temperature and directly show that at a temperature close to but above the vortex phase transition temperatu...

Hydrodynamics and Nonlocal Conductivities in Vortex States of Type II Superconductors

“…(8) between χ and s T is removed, and in principle the phase fluctuation behaves like in zero field case at such short lengths, while, if q ⊥ l < 1 and a gauge disturbance δA ⊥ (q ⊥ = 0) is not substituted, v s ≃ 0 is established and thus, (7) reduces to (5). Variational equations resulting from the action (7) agree with those following from a dual representation used in ref.12, where a role of nonzero v s at nonzero frequency was stressed in a context of superfluid 4 He. The action (7) does not include the term…”

“…Interestingly, the expressions (12) smoothly lead to the results (11) for the vortex lattice at zero q z by taking η to be infinity. The terms 1 + η(q ⊥ r B ) 4 /2γρ 0 a in the denominators of expressions (12) again originates, through a nonzero v s (i.e., a higher spatial gradient), from the spectrum of the shear mode ω ∼ −iC 66 (q ⊥ r B ) 4 , which cannot be detected in eq. (1), while the nonlocality in the…”

“…Next, let us here comment on eigen modes of the action (3) in 2D and nondisssipative (γ = 0) case, formally corresponding to the rotating superfluid 4 He at zero temperature described according to the Gross-Pitaevskii equation. Consistently with refs.12 and 13, the order parameter in eq.…”

“…This equation is essentially an extension 3) of the single vortex dynamics to interacting vortex states. Subsequently, the presence of other nonlocal terms was proposed 2) neglecting any sample disorder, although experiments have been performed in heavily twinned (disordered) samples 4) , and, the presence deep in the liquid regime of a large conductivity nonlocal in the field direction was argued. It is important to theoretically clarify to what degree these proposals can be justified beyond the phenomenology 1,2,5) and within a basic dynamical model for type II superconductors such as the time-dependent GinzburgLandau equation (TDGL) which correctly describes the uniform linear dissipation in terms of Kubo formula.…”

Shear-induced vortex decoupling inBi2Sr2CaCu2Khaykovich1, Fuchs2, Teitelbaum3 et al. 2000Phys. Rev. B311490View full textAdd to dashboardCite

“…Computer simulations 18 of Josephson-coupled planes also show sharp current-induced decoupling. Vortex cutting was also observed in YBCO crystals at elevated currents in flux transformer measurements 19,20 . The much lower anisotropy of YBCO, however, causes only a small change in the transport properties.…”