Physical Properties of the Noncentrosymmetric Superconductor ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$

Specific heat, electrical resistivity, and magnetic susceptibility measurements on a high quality sample of ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ provide a self-consistent determination of its superconducting properties. They indicate that ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ is a type-II superconductor [$T_C=4.45\mathrm{K}$, $\kappa (0)\approx 20$], with an electron-phonon coupling constant ${\lambda }_{\mathrm{ep}}=0.66$. An analysis of the $T$-dependent specific heat shows that superconducting properties are dominated by an $s$-wave gap ($\Delta =0.7\mathrm{meV}$). Point contact tunneling data provide evidence for multiple superconducting gaps, as expected from strong asymmetric spin-orbit coupling.

Figure 1

(a) Specific heat divided by temperature ($C/T$) as a function of $T^2$ under magnetic field from 0 to 1 T increasing by steps of $\Delta {\mu }_{0}H=0.1\mathrm{T}$. The inset shows the superconducting transition measured in zero-field-cooled and field-cooled mode. (b) Temperature dependent electronic contribution to the specific heat of ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$. Lines are fits described in the text. The inset shows lattice specific heat ($C_{\mathrm{ph}}$) as a function of temperature from 2 to 300 K. The blue dotted line represents a Debye model expectation (${\Theta }_D=280\mathrm{K}$) and black solid line is a fit to a Debye model (${\Theta }_D=740\mathrm{K}$) plus an Einstein mode fit (${\Theta }_D=147\mathrm{K}$). See text for details.

Figure 2

Magnetic field dependence of $\gamma (H)$ as a function of magnetic field ${\mu }_{0}H$. The solid (black) line corresponds to a linear ($\sim H$) relation and dotted (blue) line represents the nonlinear ($\sim H^{1/2}$) relation. The inset shows $C/T$ vs $T^2$ under magnetic field (0–0.6 T) in the lowest temperature region.

Figure 3

The upper critical field (${\mu }_0{H}_{C2}$) from specific heat (red squares) and from resistivity (black circles) as a function of temperature. The vertical bars show 5% and 95% of the resistively determined superconducting transition. The dashed curve is predicted by the WHH expression. The upper right inset is the temperature dependence of the lower critical field (${\mu }_0{H}_{C1}$) for ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$. The lower left inset shows the resistivity near $T_C$ for representative applied fields. From the initial slope $-d{H}_{C2}/d{T}_C=2700\mathrm{Oe}/\mathrm{K}$ (using specific heat data), $\gamma =52.6\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$, and the relation $\frac{d{H}_{C2}}{dT}{|}_{T_C}=4.48\times {10}^4\gamma \rho (\mathrm{Oe}/\mathrm{K})$ (with $\gamma$ in $\mathrm{erg}/{\mathrm{cm}}^3{\mathrm{K}}^2$ and $\rho$ in $\Omega \mathrm{cm}$), we estimate the intrinsic intragranular resistivity $\rho (T\ge T_C)\approx 80\mu \Omega \mathrm{cm}$. This intragranular resistivity is only about 14% of the value seen in the inset, consistent with strong intergranular scattering and an associated larger $H_{C2}$ measured resistively.

Figure 4

Normalized conductance spectra at 2.405 K (circles) and $4.825\mathrm{K}>T_C$ (thin black line). Arrows indicate the presence of two energy scales in the superconducting state. The dashed (solid) line is a fit assuming pure Andreev reflection with one (two) gap(s). Relaxing this assumption within the Blonder-Tinkham-Klapwijk formalism did not improve the fits. The inset shows the temperature dependence of the two gap scales compared with theoretical expectations.