Specific-Heat Evidence for Two-Gap Superconductivity in the Ternary-Iron Silicide ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$

We report low-temperature specific-heat studies on the single-crystalline ternary-iron silicide superconductor ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ with $T_c=6.1\mathrm{K}$ down to $\sim T_c/20$. We confirm a reduced normalized jump in specific heat at $T_c$, and find that the specific heat divided by temperature $C/T$ shows a sudden drop at $\sim T_c/5$ and goes to zero with further decreasing temperature. These results indicate the presence of two distinct superconducting gaps in ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$, similar to the typical two-gap superconductor ${\mathrm{MgB}}_2$. We also report Hall coefficients, band structure calculations, and the anisotropy of upper critical fields for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$, which support the anisotropic multiband nature and reinforce the existence of two superconducting gaps in ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$.

Figure 1

The specific heat divided by temperature $C/T$ as a function of $T^2$ for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$. The dashed line represents the fit to the data in the normal state $C={\gamma }_{n}T+{\beta }_n{T}^3+{\alpha }_n{T}^5$. The solid line is the fit to the data in the superconducting state $C={\gamma }_{s}T+{\beta }_s{T}^3$. The inset shows the temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ at 5 Oe.

Figure 2

Temperature dependence of the normalized electronic specific heat for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$. Dashed and solid lines are the BCS normalized specific heat and a two-gap fit calculated by Eqs. (1, 2), respectively. The inset shows a semilogarithmic plot of $C_e/{\gamma }_n{T}_c$ vs $T_c/T$ for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$. The solid line is a linear fit below $\sim T_c/5$.

Figure 3

Temperature dependence of the Hall coefficients for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ with (a) $I\parallel a$, $H\parallel c$ and (b) $I\parallel c$, $H\parallel a$.

Figure 4

Fermi surfaces of ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ calculated by a FLAPW method: holelike Fermi surfaces from the (a) 155th and (b) 156th bands, and (c) electronlike Fermi surface from the 157th band.

Figure 5

Temperature dependence of the upper critical field ${\mu }_0{H}_{c2}$ along the $a$ axis (●) and $c$ axis (○) obtained by resistivity measurements. Dashed lines are linear fits to the data. The inset shows the temperature dependence of the zero-field inter- ($I\parallel c$) and in-plane ($I\parallel a$) resistivity.