Lattice melting and superconductivity in a group IV-VI compound

Inspired by the rich physical properties of IV-VI compounds, we choose polycrystalline ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ to investigate its structural, vibrational, and electrical transport properties under pressure up to 50 GPa. The structural transitions from the $B1$ to $Pnma$ phase and then to the $B2$ phase in this sample are verified by the x-ray diffraction and Raman scattering measurements. The formation of the intermediate phase is suggested to be mediated by Peierls distortion, and the broad hump in the temperature-dependent resistivity in the intermediate phase gives further evidence of this phenomenon. When the material evolves into the $B2$ phase, superconductivity is observed to emerge, accompanied by suppressing the broad hump of resistivity at intermediate temperatures. Meanwhile, Hall coefficient measurements indicate that the carrier type changes during the structural transitions. These results suggest that the superconductivity in the $B2$ phase for this material is originated by “melting” the Peierls lattice distortion. By extending the present findings to other similar IV-VI semiconductors, we propose that all group IV-VI compounds could exhibit superconductivity in their $B2$ phase due to the lattice melting at high pressures.

Figure 1

(a) Synchrotron x-ray diffraction patterns of ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ at room temperature and various pressures up to 48.5 GPa. (b) Typical data points and refinement results at pressure of 1.2, 14.5, and 46.3 GPa using the Rietveld method. The experimental data points are presented as small open circles. The calculated values based on the different structures are shown by the thin curves. The Bragg peak positions are marked by the sticks in each panel. The differences between the experiments and calculations are shown by the thin curve at the bottom of each panel. (c) The structures of the $B1$, $Pnma$, and $B2$ phases, with olive circles for Pb(Cr) atoms and pink circles for Se atoms.

Figure 2

(a) Pressure dependence of lattice parameters and (b) the unit-cell volumes as a function of pressure of the $B1$, $Pnma$, and $B2$ phases. The curves are the fitting results of the obtained volumes based on the Murnaghan equation of state. The dashed line represents a rough boundary between the phases.

Figure 3

(a) The selected Raman spectra of ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ at room temperature and various pressures up to 41.5 GPa. (b) The pressure dependence of the obtained phonon frequencies. The dashed line represents a rough boundary between the phases.

Figure 4

(a) Temperature dependence of the resistivity of ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ at various pressures from 0.5 to 15 GPa. (b) Temperature dependence of the resistivity measured at different pressures between 19 and 50 GPa. The drop in resistivity and zero-resistivity behavior can be seen clearly. (c) Temperature dependence of the resistivity at a pressure of 40.2 GPa with the applied magnetic fields. (d) The upper critical field $H_{c2}$ as a function of temperature at 40.2 GPa. The color area represents the calculated $H_{c2}$ from the Werthamer-Helfand-Hohenberg (WHH) equation.

Figure 5

(a) Pressure dependence of the resistivity at selected temperatures of 10, 100, and 300 K. (b) Pressure dependence of the Hall coefficient $R_H$ measured at a temperature of 15 K. $R_H$ has different signs in the different phases.

Figure 6

(a) $T_c$ of ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ as a function of pressure. (b) The pressure dependence of the carrier concentration $n_H$ measured at a temperature of 15 K.