Nodeless superconductivity in the charge density wave superconductor ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$

We have studied the superconducting gap structure of ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ by measuring the temperature dependence of the London penetration depth shift $\mathrm{\Delta }\lambda \left(T\right)$ and point contact spectroscopy of single crystals. $\mathrm{\Delta }\lambda \left(T\right)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density ${\rho }_s\left(T\right)$ can be well described by a single-gap $s$-wave model. The point-contact conductance spectra can also be well fitted by an $s$-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$, with moderately strong electron-phonon coupling.

Figure 1

Temperature dependence of the resistivity $\rho \left(T\right)$ of ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$, measured upon both cooling and warming, as indicated by the arrows. The inset shows the low-temperature $\rho \left(T\right)$ near $T_c$ = 1.8 K.

Figure 2

$\mathrm{\Delta }\lambda \left(T\right)$ of ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$, where samples were measured with fields perpendicular and parallel to the $c$ axis. The dashed and dash-dotted lines represent fitting using an $s$-wave model and a power-law dependence, respectively. The inset shows $\mathrm{\Delta }f\left(T\right)$ from 2.5 K down to 0.35 K, where there is a sharp superconducting transition around 1.8 K.

Figure 3

Normalized superfluid density ${\rho }_s$ with $H\perp c$ as a function of the reduced temperature $T/T_c$. The solid, dashed, and dash-dotted lines here represent the fits for $s$-, $p$-, and $d$-wave models, respectively.

Figure 4

(a) A set of representative point-contact conductance curves at 0.3 K in comparison with optimal BTK fitting curves (black solid lines). (b) A statistical plot of the conductance peak enhancement intensity (half-filled blue stars) and the extracted superconducting gap (olive green spheres) as a function of the smearing factor $\mathrm{\Gamma }$ divided by the gap $\mathrm{\Delta }, \mathrm{\Gamma }/\mathrm{\Delta }$, for different contacts.

Figure 5

(a) and (b) The temperature and magnetic field evolution of the soft PCS on ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ in comparison with optimal BTK fitting curves. The curves are shifted for clarity, and the magnetic field is applied perpendicular to the sample surface. (c) and (d) The extracted superconducting gap value $\mathrm{\Delta }$ from BTK fitting as a function of temperature and magnetic field, respectively. Solid lines in (c) and (d) show the BCS gap function and fitted curve.