Penetration depth study of LaOs${}_4$Sb${}_{12}$: Multiband s-wave superconductivity

We measured the magnetic penetration depth $\lambda (T)$ in single crystals of LaOs${}_4$Sb${}_{12}$ ($T_c$=0.74 K) down to 85 mK using a tunnel diode oscillator technique. The observed low-temperature exponential dependence indicates a s-wave gap. Fitting the low temperature data to BCS s-wave expression gives the zero temperature gap value $\Delta (0)= (1.34 \pm 0.07) k_B T_c$whichissignificantlysmallerthantheBCSvalueof1.76$k_B T_c$. In addition, the normalized superfluid density $\rho (T)$ shows an unusually long suppression near $T_c$, and are best fit by a two-band s-wave model.