Concurrent Pressure‐Induced Superconductivity and Photoconductivity Transitions in PbSe_0.5Te_0.5

Concurrent superconductivity and negative photoconductivity (NPC) are rarely observed. Here, the discovery in PbSe_0.5Te_0.5 of superconductivity and photoconductivity transitions between positive photoconductivity (PPC) and NPC during compression is reported to ≈40 GPa and subsequent decompression, which are also accompanied by reversible structure transitions (3D Fm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:annotation>${{\bar{3}}}$</mml:annotation></mml:semantics> </mml:math> m ⇌ 2D Pnma ⇌ 3D Pm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:annotation>${{\bar{3}}}$</mml:annotation></mml:semantics> </mml:math> m). Superconductivity with a maximum T_c of ≈6.7 K coincides with NPC and structure transition of Pnma to Pm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:annotation>${{\bar{3}}}$</mml:annotation></mml:semantics> </mml:math> m at ≈18 GPa and the latter phase is preserved down to ≈5 GPa with enhanced T_c of ≈6.9 K during decompression. The observations imply the simultaneous superconducting and photoconductive transitions are closely related to the metallic Pm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:annotation>${{\bar{3}}}$</mml:annotation></mml:semantics> </mml:math> m phase. First-principles calculations suggest the enhanced p-p hybridization and charge transfer between Pb-5p and ligand-p orbitals near the Fermi surface play key roles in electron-phonon interaction of mediating the Cooper pairs in PbSe_0.5Te_0.5. Hall coefficient measurements reveal that photothermal effect enhances electron-phonon interplay, which decreases carrier concentration and mobility and results in the reversal of PPC-NPC. Structure-dependent superconductivity and NPC are jointly mediated by electron-phonon interplay, which is tunable through illumination or cooling at high-pressure. The findings shed light on the origin of superconductive and photoconductive transitions in versatile materials of lead chalcogenides.