Unconventional superconducting phase in the weakly correlated noncentrosymmetric ${\text{Mo}}_3{\text{Al}}_2\text{C}$ compound

Electrical resistivity, specific-heat, and NMR measurements classify noncentrosymmetric ${\text{Mo}}_3{\text{Al}}_2\text{C}$ ($\beta \text{-Mn}$ type, space group $P{4}_{1}32$) as a strong-coupled superconductor with $T_c=9\text{K}$ deviating notably from BCS-type behavior. The absence of a Hebbel-Slichter peak, a power-law behavior of the spin-lattice relaxation rate (from ${}^{27}\text{A}\text{l}$ NMR), an electronic specific heat strongly deviating from BCS model and a pressure enhanced $T_c$ suggest unconventional superconductivity with possibly a nodal structure of the superconducting gap. Relativistic density-functional theory calculations reveal a splitting of degenerate electronic bands due to the asymmetric spin-orbit coupling, favoring a mix of spin-singlet and spin-triplet components in the superconducting condensate, in absence of strong correlations among electrons.

Figure 1

(Color online) Rietveld refinement (Guinier-Huber image plate system, $\text{Cu}{K}_{\alpha 1}$) and crystallographic data of ${\text{Mo}}_3{\text{Al}}_2\text{C}$. The inset shows a three-dimensional view of the crystal structure. Traces of ${\text{Mo}}_2\text{C}$ are indicated by vertical bars.

Figure 2

(Color online) Temperature-dependent electrical resistivity $\rho$ of ${\text{Mo}}_3{\text{Al}}_2\text{C}$. The dashed line is a least-squares fit according to the parallel resistance model. The inset shows the pressure dependence of $T_c$.

Figure 3

(Color online) (a) Electronic specific-heat contribution $C_{eS}\left(T\right)$ of ${\text{Mo}}_3{\text{Al}}_2\text{C}$ plotted as $C_{eS}/T$ vs $T^2$. The dashed line is a guide for the eye and indicates an idealized superconducting phase transition together with a $T^3$ dependence of $C_p\left(T\right)$ for $T<T_c$. The solid line represents $C_p\left(T\right)$ of a spin-singlet fully gapped BCS superconductor according to Mühlschlegel (Ref. 19). The dashed-dotted line indicates an exponential temperature dependence suggested in Ref. 30. (b) Temperature-dependent $1/T_1$ ${}^{27}\text{A}\text{l}$ NMR relaxation rate deduced at ${\mu }_{0}H=1.24$ and 6.95 T. The solid lines are guides for the eye. The dashed line represents an exponential temperature dependence.

Figure 4

(Color online) (a) Temperature- and field-dependent specific heat $C_p$ of ${\text{Mo}}_3{\text{Al}}_2\text{C}$. (b) Temperature-dependent upper critical field ${\mu }_0{H}_{c2}$ and thermodynamic critical field ${\mu }_0{H}_c$ as obtained from specific-heat measurements. The solid and the long-dashed lines are fits according to the WHH model for different values of the Maki parameter. The horizontal bar indicates the upper critical field ${\mu }_0{H}^{\ast }\left(0\right)$ in absence of Pauli limiting. The dashed-dotted line is an extrapolation of the thermodynamic critical field toward zero.

Figure 5

(Color online) (Upper panel) Section of relativistic total and atom-projected DOS in states per electron volt for ${\text{Mo}}_3{\text{Al}}_2\text{C}$ summed over all three Mo atoms for the energy range $\pm 1.5$ eV around the Fermi energy $E_F$. (Lower panel) Relativistic electronic band structure along high symmetry directions for ${\text{Mo}}_3{\text{Al}}_2\text{C}$ in the energy range $\pm 1\text{eV}$ around the Fermi energy $E_F$.