Strong- to weak-coupling superconductivity in high-$T_c$ bismuthates: revisiting the phase diagram via $\hat{I}¼$SR

Abstract

Several decades after the discovery of superconductivity in bismuthates, the strength of their electron-phonon coupling and its evolution with doping remain puzzling. To clarify these issues, polycrystalline hole-doped Ba${}_{1-x}$K${}_x$BiO${}_3$ ($0.1\le x\le 0.6$) samples were systematically synthesized and their bulk- and microscopic superconducting properties were investigated by means of magnetic susceptibility and muon-spin rotation/relaxation ($\hat{I}¼$SR), respectively. The phase diagram of Ba${}_{1-x}$K${}_x$BiO${}_3$ was reliably extended up to $x=0.6$, which is still found to be a bulk superconductor. The lattice parameter $a$ increases linearly with K-content, implying a homogeneous chemical doping. The low-temperature superfluid density, measured via transverse-field (TF)-$\hat{I}¼$SR, indicates an isotropic fully-gapped superconducting state with zero-temperature gaps $\hat{I}{”}_0/k_{\mathrm{B}}{T}_c$ = 2.15, 2.10, and 1.75, and magnetic penetration depths $\hat{I}{»}_0$ = 219, 184, and 279 nm for $x$ = 0.3, 0.4, and 0.6, respectively. A change in the superconducting gap, from a nearly ideal BCS value (1.76 $k_{\mathrm{B}}$$T_c$ in the weak coupling case) in the overdoped $x$ = 0.6 region, to much higher values in the optimally-doped case, implies a gradual decrease in electron-phonon coupling with doping.

accepted by Phys. Rev. B, 6 pages, 5 figuures