Strong Coupling S-wave Superconductivity in Bi$_{4}$O$_{4}$S$_{3}$

We investigate the superconducting properties and pairing symmetry in recently discovered Bi$_{4}$O$_{4}$S$_{3}$ superconductor. A series of Bi$_{6}$O$_{4}$S$_{4}$(SO$_{4}$)$_{1-x}$ samples were synthesized by solid-state reaction. The optimally doped sample Bi$_{4}$O$_{4}$S$_{3}$ which is 50\% SO$_{4}$ deficient shows maximum T$_{c}$ of 5.3K as confirmed by resistivity and magnetization measurement. The upper critical field at zero temperature is found to be $\sim$ 2.75 T and Ginzburg Landau coherence length is estimated to be $\sim$ 110{\AA}. Hall measurement confirmed the dominant role played by the electrons with charge carrier density of 4.405x10$^{19}$ cm$^{-3}$ at 10 K. The Sommerfeld constant $\gamma$ is calculated to be 1.113 mJ/K$^{2}$mol. Superconducting pairing symmetry and superconducting gap was studied from penetration depth measurement using tunnel diode oscillator technique. It is shown that Bi$_{4}$O$_{4}$S$_{3}$ is a strong coupling s-wave type superconductor with fully developed gap. Below T$_{c}$, superfluid density is best fitted with single gap s wave model with zero-temperature value of the superconducting energy gap $\Delta$$_{0}$= 1.54 meV, corresponding to the ratio 2$\Delta$$_{0}$/k$_{B}$T$_{c}$=7.2 which is much higher than the BCS value of 3.53.