A research team led by Prof. WANG Xianlong and Dr. WANG Pei from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has discovered a concurrent negative photoconductivity (NPC) and superconductivity in PbSe_0.5Te_0.5 by pressure-induced structure transition.

This research has been published in Advanced Materials.

NPC is a unique phenomenon where the conductivity of a material decreases due to the trapping of charge carriers in localized states, leading to a reduction in the number of free carriers, which is contrary to the more common behavior of positive photoconductivity (PPC). Though NPC holds great promise in next-generation semiconductor optoelectronics and its application potential has recently reached far beyond photodetection, the phenomenon has rarely been reported and remains unclear, especially concurrent NPC and superconductivity are rarely observed at high-pressure owing to the lack of in situ experimental measuring facility.

In this study, the team systematically investigated the response of PbSe_0.5Te_0.5 to pressure modulation under visible-light and low-temperature stimuli using a self-developed facility. Through detailed experimental and theoretical approaches, the researchers examined how changes in the material's crystal and electronic structures influenced both its photoconductive and superconducting properties.

They found that the pressure-driven PPC-NPC transition arises from a strong nonequilibrium distribution of excited carriers. This is due to an enhanced electron-phonon interaction resulting from the photothermal effect, which reduces carrier concentration and mobility. 

Density functional theory (DFT) theory calculations proved that the dramatically enhanced p-p and s-p hybridizations lead to enhanced electron-phonon interplay at the Fermi level, facilitating the semiconductor-to-superconductor transition. Structure-dependent superconductivity and NPC are switchable by pressure-mediated electron-phonon interplay under illumination or cooling.

This study sheds light on the origin of superconductive and photoconductive transitions in versatile materials of lead chalcogenides.

Phase diagrams and carrier concentration n_e as a function of pressure during compression and decompression for superconducting and photoconducting PbSe_0.5Te_0.5. (Image by WANG Pei)