[论文解读] Structure responsible for the superconducting state in La3Ni2O7 at high pressure and low temperature conditions

Luhong Wang, Yan Li+16 more|arXiv (Cornell University)|Nov 15, 2023

Magnetic and transport properties of perovskites and related materials被引用 12

一句话总结

本文在高压低温下发现 La3Ni2O7 的四方晶系 I4/mmm 相（大约在 19 GPa 与 40 K）很可能与超导相关，费米能级附近的电子态主要由 Ni eg 轨道主导，重点关注 Ni–O–Ni 键角几何。

ABSTRACT

Very recently, a new superconductor with Tc = 80 K was reported in nickelate (La3Ni2O7) at around 15 - 40 GPa conditions (Nature, 621, 493, 2023) [1], which is the second type of unconventional superconductor, beside the cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement at low temperature and high pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results was carried out at room temperature. This encouraged us to carry out in situ high pressure and low temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from orthorhombic Amam structure to orthorhombic Fmmm structure, a tetragonal phase with space group of I4/mmm was discovered when the sample was compressed to 19 GPa at 40 K where the superconductivity takes palce in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states approached to the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, are analyzed. This work sheds new lights to identify what is the most likely phase responsible for superconductivity in the double layered nickelate.

研究动机与目标

• 在高压低温条件下确定与 La3Ni2O7 超导性相关的晶体结构。
• 识别与室温 XRD 假设之外的与超导态相关的相变。
• 将结构演变与费米能量附近的电子态及超导转变温度 Tc 联系起来。
• 评估 Ni–O 八面体的规则性以及面内 Ni–O–Ni 键角与 Tc 的相关性。

提出的方法

• 在原位高压低温的同步辐射 X 射线衍射（XRD）下绘制超导条件下的结构相图。
• 表征从 Amam 到 Fmmm 的相变，并在约19 GPa 和约40 K 时识别出四方 I4/mmm 相。
• 进行电子结构分析，显示在该四方相中 Ni eg（3dz2, 3dx2−y2）态在费米能量附近的主导地位。

实验结果

研究问题

• RQ1 在高压低温下，La3Ni2O7 与超导性相关的晶体结构是什么？
• RQ2 压力诱导的结构变化（Amam→Fmmm→I4/mmm）如何与超导性的出现相关？
• RQ3 Ni–O 八面体几何和 Ni–O–Ni 键角在确定 Tc 中的作用是何？
• RQ4 费米能量附近参与超导态的电子态是哪些？

主要发现

• 在约19 GPa和40 K 的条件下出现了四方晶系 I4/mmm 相，且此处发生超导。
• 在该相中费米能量附近的电子态受 Ni eg 轨道（3dz2 和 3dx2−y2）主导，处于氧八面体场中。
• 该研究将 Tc 与结构演化联系起来，特别是 Ni–O 八面体的规则性及面内 Ni–O–Ni 键角。
• 通过原位测量，该研究将超导相与其他压力驱动的斜方相（Amam 到 Fmmm）区分开来。
• 研究结果提供了线索，指出在双层镍酸盐中最有可能导致超导的相是哪个。