Universal critical behavior in single crystals and films ofYBa2Cu3O7−δ

Abstract

We have studied the normal-to-superconducting phase transition in optimally doped YBa 2 Cu 3 O 7−␦ in zero external magnetic field using a variety of different samples and techniques. Using dc transport measurements, we find that the dynamical critical exponent z = 1.54Ϯ 0.14, and the static critical exponent = 0.66Ϯ 0.10 for both films ͑when finite-thickness effects are included in the data analysis͒ and single crystals ͑where finitethickness effects are unimportant͒. We also measured thin films at different microwave frequencies and at different powers, which allowed us to systematically probe different length scales to avoid finite-thickness effects. dc transport measurements were also performed on the films used in the microwave experiments to provide a further consistency check. These microwave and dc measurements yielded a value of z consistent with the other results, z = 1.55Ϯ 0.15. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for and z previously reported in the literature.

Temperature range of superconducting fluctuations aboveTcin YBa2CuGrbić1, Požek2, Paar3 et al. 2011Phys. Rev. B9216922View full textAdd to dashboardCite

“…9,10 Recently, it has been shown that the original experimental results in thin films can be analyzed properly only by accounting for the finite size effects and the value of z then extracted. 86 In microwave measurements, the phase of the complex fluctuation conductivity at T c is often taken as a sensitive probe for the value of z. However, the extracted value of z may still be incorrect if the effect of the short wavelength 144508-8 cutoff at T c is not accounted for properly.…”

Scaling analysis of the static and dynamic critical exponents in underdoped, overdoped, and optimally dopedPr2−xCexCuO4

“…Most notably, we have shown that the finite thickness of the films, even of "thick" films ͑d Ն 3000 Å͒, obscures the phase transition. 14 We have found that when we account for these effects, the phase transition in hole-doped YBa 2 Cu 3 O 7−␦ ͑YBCO͒ films and crystals yields consistent exponents for both dc and microwave conductivity measurements, = 0.68Ϯ 0.07, as predicted by three-dimensional ͑3D͒ XY theory; and z = 1.5Ϯ 0.2, 15 indicating Model E dynamics. 16 In this paper we report on the zero-field normalsuperconducting phase transition of the electron-doped su-perconductors and present data on the critical regime of Pr 2−x Ce x CuO 4−y ͑PCCO͒ thin films ͑ranging in thickness from d Ϸ 2000 Å to d Ϸ 3000 Å͒ for a variety of different cerium dopings ͑x͒.…”

“…Our data confirm this hypothesis: at the smallest transition widths, we find z Ϸ 1.5 in optimally doped and x = 0.16 films, similar to results in YBCO. 15 However, as the transition width increases the dynamic critical exponent tends toward z = 1. Our results imply that only in more homogenous films with transition widths smaller than current transition widths will we be able to unambiguously see critical dynamics for all dopings ͑though whether all dopings will have the same dynamic critical exponent as optimally doped PCCO and YBCO is unknown͒.…”

“…29 Our recent results in YBCO find z = 1.5Ϯ 0.2 for both dc and ac conductivity measurements in crystals and films, when we account for finite-size effects. 15 To properly account for finite-size effects in dc measurements, we must limit ourselves to the high current regime ͑J Ͼ 10 6 A / m 2 ͒ to determine the dynamic critical exponent z. We recognize that the high currents obey the opposite concavity criterion, and we use the horizontal portion of the critical isotherm in Fig.…”

“…2 Recent measurements on thin-film YBCO using this method found = 0.68Ϯ 0.05. 15 A typical measurement is shown in the inset of Fig. 4 which shows the log-log plot of T c ͑0͒ − T g ͑H͒ vs H for a film of doping x = 0.14.…”