Temperature range of superconducting fluctuations above Tc in YBa2Cu3O7-δ single crystals

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 YBa2Cu3O7-(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.