Systematics of two-component superconductivity in $YBa_{2}Cu_{3}O_{6.95}$ from microwave measurements of high quality single crystals

摘要

Systematic microwave surface impedance measurements of YBCO single crystalsgrown in $BaZrO_3$ crucibles reveal new properties that are not directly seenin similar measurements of other YBCO samples. Two key observations obtainedfrom complex conductivity are: a new normal conductivity peak at around 80K andadditional pairing below 65K. High pressure oxygenation of one of the crystalsstill yields the same results ruling out any effect of macroscopic segregationof O-deficient regions. A single complex order parameter cannot describe thesedata, and the results suggest at least two superconducting components.Comparisons with model calculations done for various decoupled two-componentscenarios (i.e. s+d, d+d) are presented. Systematics of three single crystalsshow that the 80K quasiparticle peak is correlated with the normal stateinelastic scattering rate. Close to Tc, the data follow a mean-field behavior.Overall, our results strongly suggest the presence of multiple pairingtemperature and energy scales in $YBa_{2}Cu_{3}O_{6.95}$.