Pressure-induced 18K superconductivity and two superconducting phases in CuIr2S4 - Dr.Yang Ding

New research led by Dr. Yang Ding at the Center for High Pressure Science and Technology Advanced Research (HPSTAR), in collaboration with Dr. Jiangping Hu at the Institute of Physics, Chinese Academy of Sciences, reports two superconducting phases and a superconducting critical temperature, T_c, as high as 18.2 K in the spinel compound CuIr₂S₄ under high pressure. The work sets a new record for the critical temperature of bulk spinel superconductors, and was published in Physical Review Letters under the title“Pressure-Induced 18 K Superconductivity and Two Superconducting Phases in CuIr₂S₄,” and was selected as an Editors’ Suggestion.

Spinel compounds host metal–insulator transitions, magnetic order, and superconductivity due to strong coupling among spin, charge, and orbital degrees of freedom. Yet for decades, the highest Tc for bulk spinel superconductors has remained around 13.7 K. CuIr₂S₄ is a special member of the family: at ambient pressure it has a cubic structure with a geometrically frustrated Ir pyrochlore lattice and undergoes a charge-order-driven metal–insulator transition near 230 K, leaving it non-superconducting under conventional conditions. Theory suggested pressure could suppress the insulating state and induce superconductivity, but direct evidence had been lacking.

Using diamond anvil cells, the team measured the crystal and electronic structures of CuIr₂S₄ up to 224 GPa. Combining electrical transport, synchrotron X-ray diffraction, and first-principles calculations, they constructed an electronic–crystal-structure phase diagram and identified two distinct superconducting phases intertwined with multiple structural transitions.

The first superconducting phase (SC-I) appears at approximately 18 GPa, forming a dome-shaped superconducting region: in this region, the resistivity drops significantly and is sensitive to magnetic field and current, but strict zero resistance is not achieved above the lowest experimental temperature. Above 111.8 GPa, a second superconducting phase (SC-II) with a lower transition temperature appears and coexists with SC-I over a wide pressure range; above 122.2 GPa, SC-II finally develops a true zero-resistance state and diamagnetism.

Further analysis shows that the emergence of the two superconducting phases in CuIr₂S₄ is closely related to a series of structural phase transitions. Under pressure, the structural phase transitions in CuIr₂S₄ systematically modify the original geometrically frustrated pyrochlore lattice composed of iridium atoms, weaken the dimer order, and alleviate electron localization caused by frustration, thereby promoting electrons to become more itinerant and creating conditions for superconducting pairing.

“The two-step superconducting transition highlights the complex interactions and rich diversity of pressure-driven superconductivity in CuIr₂S₄,” said Principal Investigator Yang Ding.

“Notably, CuIr₂S₄ exhibits a rare direct transition from an insulating state to a superconducting state, skipping the intermediate metallic state common in other systems. Conventional superconducting materials usually need to be tuned into a metallic state before becoming superconducting upon cooling. This unique phenomenon provides a new route to realize high-temperature superconducting pairing directly from correlated insulating states via lattice engineering.” Dr. Bijuan Chen, the first author of the work added. “Compared with chemical doping, high pressure is a relatively “impurity-free” tuning method, which avoids the disturbance from structural disorder and more clearly reveals the intrinsic correlation between lattice distortion and superconductivity. This approach provides an exemplary paradigm for exploring superconductivity in other correlated electron materials.”

Caption: Pressure-induced distortion of the CuIr₂S₄ spinel lattice and the emergence of two superconducting phases.

北京高压科学研究中心的丁阳团队联合中国科学院物理研究所的胡江平团队揭示了尖晶石化合物 CuIr₂S₄ 在高压下奇特的双超导现象，以及高达 18 K 的超导临界温度（Tc），该温度刷新了尖晶石超导体的临界温度纪录。该工作由论文第一作者陈碧娟博士牵头完成。相关成果以 “Pressure-Induced 18 K Superconductivity and Two Superconducting Phases in CuIr₂S₄” 为题发表于《物理评论快报》（PRL），并被编辑选为科研亮点进行专题报道。