Istituto dei materiali per l'elettronica ed il magnetismo     
Lamura G., Le Cochec J., Gauzzi A., Licci F., Di Castro D., Bianconi A., Bok J. Doping dependence of the low-energy excitations in superconducting Bi2Sr2CaCu2O8+delta: Evidence for thermal phase fluctuations. In: Physical Review B, vol. 67 (14) article n. 144518. American Physical Society, 2003.
By using a single-coil technique, we study the low-temperature dependence of the in-plane magnetic penetration depth lambda(ab) in several Bi2Sr2CaCu2O8+delta (BSCCO) single crystals prepared with various doping levels delta ranging from the under- to the overdoped regime. Four samples exhibit a linear dependence of thermally activated low-energy excitations Deltalambda(ab)equivalent tolambda(ab)(T)-lambda(ab)(0)similar toT. The thermal activation rate proportional topartial derivativelambda(ab)/partial derivativeT is minimum, 8 Angstrom/K, at optimum doping, in good quantitative agreement with the d-wave model of superconducting order parameter. The much larger rates, greater than or similar to40 Angstrom/K, observed in all under- and overdoped samples indicate that, in these samples, another type of low-energy excitations is relevant. These large rates are quantitatively consistent with a model of thermal phase fluctuations suitable for granular superconductors with short coherence length like cuprates. One underdoped sample exhibits a quadratic dependence suggesting an incipient crossover from thermal to quantum fluctuations.
DOI: 10.1103/PhysRevB.67.144518
Subject Magnetic Penetration Depth; T-C superconductors; D-wave superconductors; Single-crystals; Single-crystals; Thin-films; Oxygen stoichiometry; Temperature; State; Transition; Density

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