Muon-spin-rotation measurements of the penetration depth in Li_2Pd_3B

Measurements of the magnetic field penetration depth $\lambda$ in the ternary boride superconductor Li${}_2$Pd${}_3$B ($T_c\simeq 7.3$ K) have been carried out by means of muon-spin rotation ($\mu$SR). The absolute values of $\lambda$, the Ginzburg-Landau parameter $\kappa$, and the first $H_{c1}$ and the second $H_{c2}$ critical fields at T=0 obtained from $\mu$SR were found to be $\lambda (0)=252(2)$ nm, $\kappa (0)=27(1)$, ${\mu }_0{H}_{c1}(0)=9.5(1)$ mT, and ${\mu }_0{H}_{c2}(0)=3.66(8)$ T, respectively. The zero-temperature value of the superconducting gap ${\mathrm{\Delta }}_0=$1.31(3) meV was found, corresponding to the ratio $2{\mathrm{\Delta }}_0\mathrm{/}{k}_B{T}_c=4.0(1)$. At low temperatures $\lambda (T)$ saturates and becomes constant below $T\simeq 0.2T_c$, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Li${}_2$Pd${}_3$B is a s-wave BCS superconductor with the only one isotropic energy gap.