Measurements of the paraconductivity in the a-direction of untwinned Y1Ba2Cu3O7−δ single crystals

Pomar, A.; Díaz, A.; Ramallo, M. V.; Torrón, C.; Veira, J.A.; Vidal, F.

doi:10.1016/0921-4534(93)90291-w

Cited by 58 publications

(79 citation statements)

References 53 publications

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“…Similar results were reported for the zero-field DC conductivity [10,11]. Frequency dependent microwave conductivity experiments yield z ∼ 2.3 − 3.0 [12].…”

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confidence: 85%

Critical Dynamics of a Vortex-Loop Model for the Superconducting Transition

Aji

Goldenfeld

2001

Phys. Rev. Lett.

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We calculate analytically the dynamic critical exponent zMC measured in Monte Carlo simulations for a vortex loop model of the superconducting transition, and account for the simulation results. In the weak screening limit, where magnetic fluctuations are neglected, the dynamic exponent is found to be zMC = 3/2. In the perfect screening limit, zMC = 5/2. We relate zMC to the actual value of z observable in experiments and find that z ∼ 2, consistent with some experimental results.PACS Numbers: 05.70.Jk, 74.40.+k, 75.40.Gb, 75.40.Mg The discovery of the short coherence length cuprate superconductors has allowed heretofore inaccessible fluctuation effects in superconductors to be probed. Beginning with the penetration depth measurements of Kamal et al. [1], and including measurements of magnetic susceptibility [2,3], resistivity [3,4] and specific heat [5], static and dynamic fluctuation effects have been convincingly observed and accurately quantified. These measurements are consistent with the theory of a strongly type-II superconductor, with a weak coupling of the order parameter to the electromagnetic field, described by the 3D XY model coupled to a gauge field [6].The dynamic critical exponent, z, characterizes the relaxation to equilibrium of fluctuations in the critical regime of systems exhibiting a second order phase transition [7,8]. In particular it relates the time scale of relaxation, τ , to a relevant length scale, x: τ ∼ x z . For infinite systems x is the correlation length, ξ. Near the critical point, the correlation length diverges and the relaxation time tends to infinity, a phenomenon known as critical slowing down. In finite size scaling studies, x is identified as the system size L.The dynamic critical exponent, obtained from the measurement of longitudinal dc-resistivity for YBCO is z = 1.5 ± 0.1 in finite but small magnetic fields [9]. Similar results were reported for the zero-field DC conductivity [10,11]. Frequency dependent microwave conductivity experiments yield z ∼ 2.3 − 3.0 [12]. On reanalysis it was found that the data were consistent with z ∼ 2 provided one neglected the region close to T c [13]. Moloni et al. obtained z = 1.25 ± 0.05 at low magnetic fields [14], but a later, more complicated analysis by these authors gave z = 2.3 ± 0.2. More recently, DC conductivity measurements on single crystal BSCCO samples were interpreted to give evidence for z ∼ 2 [15]. In summary, experiments do not yet yield a consistent picture of the critical dynamics.If the dynamic exponent were indeed z ∼ 1.5 , then this would be surprising. Precisely this value is obtained for the superfluid transition in He 4 where the combination of second sound (a propagating mode, therefore z = 1) and order parameter dynamics (diffusive, therefore z ∼ 2) lead to z = 3/2 (model E dynamics) [7]. In YBCO, however, the combination of a momentum sink arising from the lattice, and the Coulomb interaction destroying the longitudinal current fluctuations should lead to pure order parameter dynamics and a prediction t...

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“…Similar results were reported for the zero-field DC conductivity [10,11]. Frequency dependent microwave conductivity experiments yield z ∼ 2.3 − 3.0 [12].…”

supporting

confidence: 85%

Critical Dynamics of a Vortex-Loop Model for the Superconducting Transition

Aji

Goldenfeld

2001

Phys. Rev. Lett.

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“…This work serves as a basis for examining more complicated models, such as model F of Hohenberg and Halperin [21] involving reversible couplings to a conserved energy-mass density field, as in superfluid 4 He. In three dimensions z = 3/2 for model F [30,36] which, although not observed in the a.c. conductivity data [18], is seen in some d.c. conductivity experiments [4,12,14] and simulations [37]. Another extension of the present theory is to consider a non-zero magnetic field, with the aim of examining the crossover from the zero-field critical scaling of the 3D XY-model to the lowest-Landau-level scaling which obtains in high fields [13,38].…”

Section: Discussionmentioning

confidence: 99%

“…(2.13) is a four-point orderparameter average since J s (2.14) is quadratic in ψ. Quite generally, this four-point average can be written as the sum of a "disconnected" product, σ (2) , of two two-point averages, and a "connected" four point-average σ (4) :…”

Section: Formalismmentioning

confidence: 99%

“…It is natural then to ask: If scaling and universality exist in the critical region, to which universality class does the transition belong? From observations of the effects of critical superconducting fluctuations on thermodynamic properties, such as the penetration depth [1,2], magnetic susceptibility [3][4][5], specific heat [3,6] and thermal expansivity [7] a consensus is emerging that the zero-field normalsuperconducting transition is in the static universality class of the three-dimensional, complex order-parameter (3D XY) model. In contrast, the effect of critical fluctuations on transport properties, such as the conductivity, depends on the nature of the dynamics near T c and is much less explored.…”

Section: Introductionmentioning

confidence: 99%

“…In these theories the dynamic exponent z associated with the growth of the characteristic orderparameter time-scale near T c appears in the conductivity and takes the value z = 2. By examining the deviation of z from 2 inside the critical region through linear d.c. [3,4,[11][12][13][14], non-linear d.c. [15][16][17] and linear a.c. [18] conductivity measurements, the dynamic universality class can, in principle, be determined. Currently, however, there is much variation in the measured values for z and the dynamic universality class of the zerofield normal-superconducting transition remains uncertain.…”

Section: Introductionmentioning

confidence: 99%

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