Physical properties and pressure-induced superconductivity in the single-crystalline band insulator SnO

Abstract

We report the growth and physical properties of high-quality single-crystal SnO via electrical transport, specific heat, Hall coefficients, and the high-pressure effect. Apart from polycrystalline SnO showing an insulating behavior in the whole temperature range, the in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}$ of single-crystal SnO exhibits a metal-insulator transition around the characteristic temperature ${T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$. The anisotropic resistivity ratio ${\ensuremath{\rho}}_{\mathrm{c}}/{\ensuremath{\rho}}_{\mathrm{ab}}$ is \ensuremath{\sim}1 for $T\ensuremath{\ge}{T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$ and increases quickly up to \ensuremath{\sim}400 for $T<{T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$, which implies the enhanced anisotropic electronic structures and electronic correlations. Its multiband electronic character with dominant hole-type carriers is revealed via the Hall coefficient and the appearance of low-lying phonon models, evidenced by specific heat showing an evident peak at \ensuremath{\sim}10 K in $({C}_{\mathrm{p}}\ensuremath{-}{\ensuremath{\gamma}}_{\mathrm{n}}T)/{T}^{3}$ vs $T$. The appearance of a metal-insulator phase transition in single-crystal SnO was attributed to the slight difference in the lattice parameters ratio $c/a$, the atomic coordinate of $Z(\mathrm{Sn})$, and the chemical pressure effect. Under hydrostatic pressures generated in a cubic-anvil pressure cell, the insulating state melts at the critical pressure ${P}_{\mathrm{c}}\ensuremath{\sim}3\ensuremath{-}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, and the temperature exponent of resistivity $\ensuremath{\rho}\ensuremath{\propto}{T}^{n}$ in the metallic state increases gradually from $n=2$ to 3 with increasing the pressure. A domelike superconductivity is achieved in the diamond pressure cell with the pressure up to 13.5 GPa and the temperature to 80 mK, with the superconducting transition temperatures and the upper critical fields close to those of polycrystals. Several possible physical mechanisms are proposed.