Superconducting fluctuation regime in HgBa$_{2}$CuO4$_{+\delta }$ revealed by microwave measurements

There have been many attempts to measure the fluctuations preceding the superconducting long-range order in the cuprates with various experimental techniques, yet the onset temperature of the fluctuation regime has proven difficult to determine. We used a novel approach to microwave conductivity measurements in order to elucidate the phase diagram of the cuprates. Initial measurements were performed on the single-layer material HgBa$_{2}$CuO$_{4+\delta }$. From c-axis data for a sample close to optimal doping, we clearly discern the opening of the pseudogap at T*=185 K, the appearance of the superconducting fluctuations at the much lower temperature T'=105 K, and the transition to the superconducting state at T$_{c}$=94.3 K. Our result implies that the superconducting fluctuations extend only to about 10 K above T$_{c}$. Using the same approach, a narrow fluctuation regime is also found in other cuprates.