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Abstract

Microwave absorption measurements in magnetic fields from 0 up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature T(c) in YBa(2)Cu(3)O(7)-(delta). Measurements were performed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above T(c) is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the ab plane, and c-axis conductivity, by identifying the onset temperature T'. Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above T(c). Its width increases from 7 K in the overdoped sample (T(c) = 89 K) to, at most, 23 K in the deeply underdoped sample (T(c) = 57 K), so that T' falls well below the pseudogap temperature T(*). Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.