Pressure-Induced Superconductivity up to 9 K in the Quasi-One-Dimensional ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$

The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ is an antiferromagnetic metal with $T_N\approx 75\mathrm{K}$. By measuring resistance and ac magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic anvil cell apparatus, we find that its antiferromagnetic transition can be suppressed completely at a critical pressure of $P_c\approx 13\mathrm{GPa}$, around which bulk superconductivity emerges and displays a superconducting dome with the maximal $T_c^{\text{onset}}=9.3\mathrm{K}$ achieved at about 14 GPa. The close proximity of superconductivity to a magnetic instability in the temperature-pressure phase diagram of ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ and an unusually large ${\mu }_0{H}_{c2}(0)$ exceeding the Pauli paramagnetic limit suggests an unconventional magnetism-mediated paring mechanism. In contrast to the binary MnP, the flexibility of the crystal structure and chemical compositions in the ternary $A{\mathrm{Mn}}_6{\mathrm{Bi}}_5$ ($A=\text{alkali}$ metal) can open a new avenue for finding more Mn-based superconductors.

Figure 1

(a) Crystal structure of single-crystal ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$. (b) Temperature dependence of resistivity $\rho (T)$ and its derivative $d\rho /dT$ along the $b$ axis at ambient pressure (left axis). The antiferromagnetic transition at $T_N$ is marked by vertical broken line. Temperature dependence of magnetic susceptibility $\chi (T)$ (right axis) recorded at a magnetic field of 0.2 T. The Curie-Weiss fitting is shown by the dotted line.

Figure 2

(a) Electrical resistance of ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ (#1) under various hydrostatic pressures up to 14.2 GPa measured in CAC. (b) The corresponding $dR/dT$ curves. (c) The $R(T)$ and corresponding $dR/dT$ curves at 4 and 6.2 GPa highlighting the distinct features around $T_N$. Inset of (a) shows the sample configuration in CAC. The electric current ($I$) was applied along the $b$ axis, while the magnetic field ($H$) has an angle about 45° with respect to the $b$ axis. Noted that the angle between the $H$ and $b$ axis may change slightly due to the rotation or movement of the sample upon compression.

Figure 3

(a) The enlarged view of low-$T$ resistance data ($0\le T\le 11\mathrm{K}$), in which the $T_c^{\text{onset}}$ and $T_c^{\text{offset}}$ are marked by arrows. (b) The ac magnetic susceptibility $4\pi {\chi }_{\mathrm{ac}}(T)$ of ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ and Pb with similar volume. The arrows show the superconducting transition temperature $T_c^{\chi }$ of ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ crystal. (c) The low-temperature normalized $R/R_{13\mathrm{K}}(T)$ under different fields at 14.2 GPa. (d) The temperature dependences of ${\mu }_0{H}_{c2}(T)$ fitted by the Ginzburg-Landau (GL) equation; the Pauli limit of ${\mu }_0{H}_p=1.84T_c$ at 14.2 GPa is indicated.

Figure 4

(a) Temperature-pressure phase diagrams for ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$. The characteristic temperatures for the antiferromagnetic transition $T_N$ and the superconducting transition $T_c^{\text{onset}}$, $T_c^{\text{offset}}$ and $T_c^{\chi }$ are determined from the resistance and ac magnetic susceptibility measurement on samples #1 to #4. AFM and SC denote the antiferromagnetic and superconducting phase, respectively. Pressure dependences of (b) ${\mu }_0{H}_{c2}(0)$ and $R_0$, and (c) the coefficient $A$ and the exponent $n$.