Low-Temperature Magnetothermal Transport Investigation of a Ni-Based Superconductor ${\mathrm{BaNi}}_2{\mathrm{As}}_2$: Evidence for Fully Gapped Superconductivity

We have performed low-temperature specific heat and thermal conductivity measurements of the Ni-based superconductor ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ ($T_{\mathrm{c}}=0.7\mathrm{K}$) in a magnetic field. In a zero field, thermal conductivity shows $T$-linear behavior in the normal state and exhibits a BCS-like exponential decrease below $T_c$. The field dependence of the residual thermal conductivity extrapolated to zero temperature is indicative of a fully gapped superconductor. This conclusion is supported by the analysis of the specific heat data, which are well fit by the BCS temperature dependence from $T_c$ down to the lowest temperature of 0.1 K.

Figure 1

(a) Zero-field low-temperature specific heat, plotted as $C/T$ vs $T$, of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ [21] (with $T_c$ and $\gamma T_c$ scaled to that of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$) for comparison. The solid curve is a fit to the theoretical expectation for BCS superconductivity including a nuclear quadrupolar Schottky contribution. (b) $C/T$ vs $T$ in fields of 0, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.1 T (right to left as indicated by the arrow) for $H\parallel ab$. (c) Upper critical field $H_{c2}$ for $H\parallel ab$ as determined by the midpoint of the jump in $C/T$. The solid line represents a least-squares fit to the data.

Figure 2

Temperature dependence of the thermal conductivity $\kappa (T)$ in ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ for heat current $q\parallel [100]$, in zero field and in the normal state (0.5 T) for $H\parallel ab$. Arrows indicate $T_c$ determined from the specific heat measurement. Solid square and cross symbols represent the electronic thermal conductivity ${\kappa }_e=L_{0}T/\rho$, with $L_0=2.44\times {10}^{-8}\mathrm{W}\Omega /{\mathrm{K}}^2$ for 0 and 0.5 T, respectively, derived from resistivity data using the Wiedemann-Franz law. The dotted line in the main figure and the solid line in the inset show a fit to $\kappa (T)$ with a BCS curve defined as $\kappa =C\exp (-a{T}_c/T)$.

Figure 3

Low-temperature dependence of $\kappa /T$ vs $T^2$ for $q\parallel [100]$ in several fields for $H\perp q$. The solid lines are fits to the data in the temperature range by $\kappa /T={\kappa }_0/T+b{T}^2$.

Figure 4

(a) Residual thermal conductivity ${\kappa }_0/T$ of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$, normalized by the normal state value ${\kappa }_n/T$ above $H_{c2}$, as a function of $H/H_{c2}$ for $H\perp q$ (#1). A small contribution from the Pb flux has been subtracted. For comparison, data for several superconductors with different superconducting gap characteristics are shown: Nb (clean, fully gapped $s$-wave) [27], InBi (dirty, fully gapped $s$-wave) [17], ${\mathrm{MgB}}_2$ (multiband gap) [18], and Tl-2201 ($d$-wave with line nodes) [16]. The dotted lines are guides to the eyes. (b) $({\kappa }_0/T)/({\kappa }_n/T)$ vs $H/H_{c2}$ of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ for $H\perp q$ (#1 and #2) and $H\parallel q$ (#1).