Superconductivity at the Polar-Nonpolar Phase Boundary of SnP with an Unusual Valence State

Structural, magnetic, and electrical characterizations reveal that SnP with an unusual valence state (nominally ${\mathrm{Sn}}^{3+}$) undergoes a ferroelectriclike structural transition from a simple NaCl-type structure to a polar tetragonal structure at approximately 250 K at ambient pressure. First-principles calculations indicate that the experimentally observed tetragonal distortion enhances the charge transfer from Sn to P, thereby making the polar tetragonal phase energetically more stable than the nonpolar cubic phase. Hydrostatic pressure is found to promptly suppress the structural phase transition in SnP, leading to the emergence of bulk superconductivity in a phase-competitive manner. These findings suggest that control of ferroelectriclike instability in a metal can be a promising way for creating novel superconductors.

Figure 1

Crystal structure of SnP for (a) cubic and (b) tetragonal phases. For the cubic phase, we show half of the conventional cell for clear comparison with the tetragonal phase. (c),(d) Band structure, (e),(f) density of states, and (g),(h) Fermi surfaces for cubic and tetragonal phases are shown on the left and right, respectively.

Figure 2

(a) Powder x-ray diffraction profiles for SnP (sample $B$; see the experimental details in Supplemental Material [21]) from 300 to 5 K. (b) Temperature dependence of the molar fraction of cubic (red circles) and polar tetragonal (blue circles) phases for sample $B$. (c) Temperature dependence of the magnetic susceptibility for sample $A$ under a magnetic field of 1 T. (d) Temperature dependence of the resistivity for sample $A$. Black arrows indicate the directions of the temperature scans.

Figure 3

(a) Temperature dependence of the resistivity under hydrostatic pressure normalized by the value at 300 K (sample $A$). The data are vertically offset by $-0.2$ at every pressure value for clarity. (b) Temperature dependence of the resistivity under hydrostatic pressure at a low-temperature region for sample $A$. (c) Temperature dependence of the magnetic susceptibility under hydrostatic pressure in the field-cooling process with a magnetic field of 1 mT for sample $B$.

Figure 4

(a) Pressure dependence of the magnitude of the magnetic susceptibility (Meissner signal) at 1.8 K. (b) Electronic and lattice phase diagram of SnP in the plane of pressure and temperature. Red and blue circles indicate the cubic (nonpolar)–tetragonal (polar) transition temperature $T_s$ observed in the warming and cooling processes, respectively. Only $T_s$ at ambient pressure is determined from the magnetic susceptibility measurement. The open circles represent the superconducting transition temperature with a filamentary (not bulk) component. The superconductivity of the bulk nature, where $T_c$ is plotted with solid circles, was identified from the superconducting fraction of the Meissner signal combined with the resistivity data. $T_c$ was determined from the value of zero resistivity. We plotted $T_c$ multiplied by 20 for clarity.