Dynamic scaling and two-dimensional high-Tcsuperconductors

Abstract

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12Å thick YBa2Cu3O 7−δ film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.

Zero-magnetic-field dynamic scaling inBi2Sr2CaCu2O8Sefrioui1, Arias2, León3 et al. 2004Phys. Rev. B10020View full textAdd to dashboardCite

“…Critical exponents were = 1.4 and z = 4.85 similar to those found for YBa 2 Cu 3 O 7−␦ in presence of magnetic field, 20 T g = 77 K and D =3 ( see Fig. 6).…”

“…20 It is important to notice here that the applicability of this scaling to the BKT transition has been recently questioned on the basis of size effects (discussed later) 25 and on an inconsistency with power law dependent isotherms below the transition temperature in the very small current limit. 20 Attempts to reanalyze the data of Fig. 1 using 3D We now discuss the occurrence of a BKT-like transition in connection with sample details.…”

“…It is worth noting that the resistance data drops abruptly below 87 K suggesting a true superconducting state in our pure 2D system below 86 K. It is interesting to remark that the same set of parameters z , b, and T BKT consistently accounted for the resistance measurements and for the scaling, which is not necessarily the case if the opposite concavity criterion by Strachan et al is not observed. 19,20 Due to the large scaling flexibility equally satisfactory scaling plots can be constructed for different sets of b , z, and T BKT . In fact, if we tentatively fix the value of z = 2 for conserving the concept of universality of the BKT, it is actually possible to scale the data of Fig.…”

“…In the following we use scaling arguments to show that this is actually a BKTlike transition and to obtain the critical exponent z and the transition temperature. The transition temperature is obtained from the derivative plot as suggested by Strachan et al 19,20 The proposed criterion for I-V data is that log V vs log I isotherms above and below T BKT must have opposite concavities at the same applied currents. Figure 1(b) shows the current dependence of the derivative of the log V vs log I isotherms of Fig.…”

“…A zero field BKT transition has been reported for the highly anisotropic BSCCO and TBCCO superconductors 11-14 assuming but not testing the value z =2. In fact quite high values of z ϳ 6 have been reported 5 19,20 is used to arrive at the correct exponent and critical temperature from scaling arguments. Fresh Bi 2 Sr 2 CaCu 2 O 8 films show a BKT-like transition with z = 0.8, smaller than expected for the BKT transition but similar to that observed in frustrated Josephson junction arrays (JJA).…”