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−δ . 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.

The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. This model is found to yield consistent results for the coherence lengths in high-κ superconducting samples, and can be used with HTSC even at temperatures much below Tc. It is shown that for samples with high values of depinning frequency, one should measure both components of the complex frequency shift in order to determine the flow resistivity.The thick Nb film (160 nm) has low resistivity at 10 K, comparable to the best single crystals, and low κ value. In contrast, the thinnest (10 nm) film has κ ≈ 9.5 and exhibits a high depinning frequency (≈ 20 GHz). The upper critical field determined from microwave measurements is related to the radius of nonoverlaping vortices, and appears to be larger than the one determined by the transition to the normal state.

An approach to microwave measurements is used in order to determine both, the in-plane and out-of-plane conductivity of the high-T c superconductor HgBa 2 CuO 4+␦ near optimal doping. Unlike the ab-plane conductivity, the c-axis conductivity is highly sensitive to superconducting fluctuations. From a single c-axis data set, we can clearly discern the opening of the pseudogap at T ‫ء‬ = 185͑15͒ K, the appearance of the superconducting fluctuations at a much lower temperature TЈ = 105͑2͒ K, and the full transition to the superconducting state at the critical temperature T c = 94.3 K. Thus, with the present high sensitivity, we establish that the extent of the superconducting fluctuations is only about 10 K above T c .

No abstract