Effect of tunable spin-orbit coupling on the superconducting properties of LaRu3Si2 containing kagome-honeycomb layers

Abstract

We report a detailed investigation of the superconducting properties of the kagome-honeycomb lattice compound $\mathrm{La}{\mathrm{Ru}}_{3}{\mathrm{Si}}_{2}$ by systematically tuning the spin-orbit coupling (SOC) via doping of heavier elements Rh and Ir at the Ru site. All doped samples (for a doping level of 10 at. %) preserve the pristine hexagonal crystal structure in the space group $P{6}_{3}/mmc$, though a marginal lattice compression was noted for Rh doping. Based on the results of dc magnetization, resistivity, and heat capacity measurements, we derived the normal and superconducting state electronic and thermodynamic properties of the pristine and doped samples. Substitution of Ir/Rh at the Ru site of $\mathrm{La}{\mathrm{Ru}}_{3}{\mathrm{Si}}_{2}$ resulted in a rather slow but linear suppression of superconducting transition temperature (${T}_{\mathrm{c}}$), which may be related to the decrease in the density of states. As manifested by the estimated electron-phonon (el-ph) coupling constant (${\ensuremath{\lambda}}_{\mathrm{el}\text{\ensuremath{-}}\mathrm{ph}}\ensuremath{\sim}0.58\text{--}0.66$) and the normalized specific heat jump at ${T}_{\mathrm{c}}$ ($\mathrm{\ensuremath{\Delta}}C/\ensuremath{\gamma}{T}_{c}\ensuremath{\sim}1.5$), the observed superconductivity in $\mathrm{La}{\mathrm{Ru}}_{3}{\mathrm{Si}}_{2}$ and the doped variants is moderately coupled. We observed a nonmonotonous variation of the upper critical field $[{\ensuremath{\mu}}_{0}{H}_{c2}(0)]$ with respect to the doping concentration, as it is influenced by the effective SOC and the coherence length. Most strikingly, we found an enhancement of the superconducting gap parameter $({\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{\mathrm{c}})$ with doping concentration even though ${\ensuremath{\lambda}}_{\mathrm{el}\text{\ensuremath{-}}\mathrm{ph}}$ remains essentially unchanged. Moreover, we also notice a nonzero residual electronic specific heat coefficient (${\ensuremath{\gamma}}_{r}$) in the limit $T$ \ensuremath{\rightarrow}0 for all compositions. Interestingly, the evolution of the ${\ensuremath{\gamma}}_{r}$ with the magnetic field can be well described by a $\sqrt{H}$ dependence, which was attributed to multiband superconductivity.