Measurements of fluctuation-induced diamagnetism above the superconducting transition in YBa2Cu3O7−δ single crystals in low magnetic fields: Comparison with paraconductivity

Abstract

We report detailed experimental results on the magnetic susceptibility of high-quality YBa2Cu3O7−δ single crystals for the applied magnetic field H parallel, χab, and perpendicular, χc, to the c direction. The data were obtained from 5 to 300 K, and with μ0H≤0.6 T, for which χab and χc above the superconducting transition correspond to the so-called weak magnetic-field limit, even for reduced temperatures, ɛ≡(T-TCO)/TCO, of the order of ɛ=2×10−3, where TCO is the mean-field-like normal-superconducting transition temperature. For T>TCO, χc is found to be temperature independent and its average value for the samples measured here is (2.7±0.5)×10−5. In contrast, χab(T) is found to be temperature independent, and equal to (5.2±1)×10−5, only above T-TCO≊25 K and up to the highest temperature studied here (300 K). Between TCO and T-TCO≊25 K, χab(T) is appreciably rounded. The excess diamagnetism above but near TCO, for H parallel to the c axis, Δχab, is analyzed in terms of the existing approaches for independent Gaussian fluctuations of the superconducting-order-parameter amplitude in layered materials and also compared with the paraconductivity measured in the same samples. The so-called mean-field-like region will correspond to, approximately, 10−2≲ɛ≲0.15. Beyond the lower ɛ limit, closer to TCO, both the paraconductivity and Δχab(ɛ) separate from the mean-field region behavior. Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these data for ɛ≲10−2 may be explained on the grounds of the three-dimensional XY model, with a critical exponent of x=-1/3 for the paraconductivity and x=-2/3 for the fluctuation-induced diamagnetism. A scenario compatible with the experimental results is characterized by the values of the correlation length amplitude in the ab plane and in the c direction of, respectively, ξab(0)=(1.1±0.2) nm and ξc(0)=(0.12±0.02) nm, two Josephson coupled superconducting planes in the unit-cell length, and a complex order parameter with two real components, i.e., conventional 1s0-wave pairing or one-complex-component unconventional pairing.

Source-reported materials — not catalogue approval

Similar papers