Fluctuation effects in underdoped YBCO from susceptibility and nuclear relaxation

Fluctuating diamagnetism in underdoped high-temperature superconductors

“…These observations allow for the following conclusions: (i) The experiments in Ref. 6 can be much better accounted for by phase excitations than by Gaussian fluctuations of the pairing field. This is particularly manifest in the activated T -dependence of the susceptibility, but also in the existence of two field regimes with quite different behaviours of the magnetization.…”

“…In this context we have, however, to admit that the very sharp crossover between these two regimes observed in the experiment may also be due to sample inhomogeneities as it was suggested in the experimental papers. 6 (ii) Except for the data taken close to the zero-field critical temperature, the diamagnetic response of the underdoped compound presented in Ref.6 seems to be "precritical", in the sense that the relevant lengths in the lattice planes as well as in the perpendicular direction do not show any true critical (i.e., singular) behaviour (which would be supposed to belong to the 3D XY universality class 3 ). In section II we develop the theoretical formalism that allows to express χ(T ) and M (T, B) in terms of the vortex line structure factor, and in section III we compare our theoretical results with the data presented in Ref.…”

“…Moreover, the molar quantities like those reported in Ref. 6 are obtained by multiplying expression (23) by the molar volume Ω M = N A a 2 d = 115 cm 3 and by a reduction factor λ which accounts for the fact that in the materials we are interested in only a fraction of the unit cell volume actually carries the current densities and thus contributes to the diamagnetic response.…”

“…6, for a very low applied field of 0.02T, cover the range from T = 63K, which is just above the zero-field transition temperature, to T = 110K. Above T = 80K the diamagnetic susceptibility χ is essentially equal to zero, a background consisting of spin susceptibility and free-electron orbital diamagnetism having been subtracted.…”

“…4 More refined calculations based on the Lawrence-Doniach model have taken into account the lattice structure. 5 Recent measurements 6 have shown that the Gaussian approximation can indeed well describe the diamagnetic fluctuations in optimally doped YBCO, whereas underdoped specimens of the same compound, such as YBa 2 Cu 3 O 6.67 , show dramatic deviations from this behaviour. The fluctuation region where the zero-field orbital susceptibility shows appreciable values extends over a much larger temperature range than in the optimally doped system.…”