Distinctive behavior of superconducting fluctuations and pseudogap in nearly optimally doped single crystal of HgBa₂CuO_4+δ

We have applied an unconventional microwave measurement approach to a nearly optimally doped HgBa₂CuO_4+δ single crystal. The sample geometry assured the total lateral penetration of microwaves due to weak c-axis screening currents. With this configuration, one can achieve excellent sensitivity to small changes in conductivity. The data show that the pseudogap opens at T=185(15)K, which is almost twice the superconducting critical temperature Tc=94.3K. In contrast, the superconducting fluctuation regime is clearly confined to a narrow temperature range Tc<T<T′≈ 105(2)K, far below T. This is confirmed by the magnetic field dependence of the microwave absorption. Hence, our results support the distinction between the physical processes of pseudogap and the superconducting ordering.