Percolative nature of the direct-current paraconductivity in cuprate superconductors

Despite extraordinary scientific efforts over the past three decades, the cuprate high-temperature superconductors continue to pose formidable challenges. A pivotal problem, essential for understanding both the normal and superconducting states, is to clarify the nature of the superconducting pre-pairing above the bulk transition temperature Tc. Different experimental probes have given conflicting results, in part due to difficulties in discerning the superconducting response from the complex normal-state behavior. Moreover, it has proven challenging to separate common properties of the cuprates from compound-specific idiosyncrasies. Here we investigate the paraconductivity—the superconducting contribution to the direct-current (dc) conductivity—of the simple-tetragonal model cuprate material HgBa2CuO4+δ. We are able to separate the superconducting and normal-state responses by taking advantage of the Fermi-liquid nature of the normal state in underdoped HgBa2CuO4+δ; the robust and simple quadratic temperature-dependence of the normal-state resistivity enables us to extract the paraconductivity above the macroscopic Tc with great accuracy. We find that the paraconductivity exhibits unusual exponential temperature dependence, and that it can be quantitatively explained by a simple superconducting percolation model. Consequently, the emergence of superconductivity in this model system is dominated by the underlying intrinsic gap inhomogeneity. Motivated by these insights, we reanalyze published results for two other cuprates and find exponential behavior as well, with nearly the same characteristic temperature scale. The universal intrinsic gap inhomogeneity is not only essential for understanding the supercoducting precursor, but will also have to be taken into account in the analysis of other bulk measurements of the cuprates. Paraconductivity, the superconducting contribution to conductivity, is investigated in the normal state of the cuprates. An international team led by Miroslav Požek, Neven Barisic and Martin Greven from Croatia, Austria and USA investigate the paraconductivity of the single-tetragonal model cuprate material HgBa2CuO4+δ and separate the superconducting and normal-state contributions by taking advantage of the Fermi-liquid nature of the normal state. They extract the paraconductivity above the superconducting transition temperature by eliminating the normal-state conductivity, which follows a robust quadratic temperature dependence. The paraconductivity shows exponential temperature dependence and can be explained by a superconducting percolation model which underlines the role of intrinsic gap inhomogeneity. They analyze published results for YBa2Cu3O6+δ and La2-xSrxCuO4 and find similar exponential temperature dependence. These results provide useful information for the understanding of the pre-pairing regime of cuprate superconductors.