标题： 压缩应变薄膜La$_2$PrNi$_2$O$_7$的电子结构 显示英文标题

标题： Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$

摘要： 高压下块状镍酸盐中发现超导性是物理学的重大进展。 最近，在常压下通过角分辨光电子能谱（ARPES）直接表征超导相成为可能，这是因为在压缩应变的双层镍酸盐薄膜中观察到了超导性。 在这里，我们展示了通过氧化物分子束外延生长的压缩应变La$_2$PrNi$_2$O$_7$薄膜及其臭氧处理对应物的原位ARPES研究结果，起始转变温度T$_c$为40 K，并补充了脉冲激光沉积薄膜的结果，其具有相似的T$_c$。 我们解析出相对于块状晶体的系统性应变驱动的电子能带偏移，这在定性上与密度泛函理论（DFT）计算一致。 然而，强重整化的平坦3$d_{z2}$能带的偏移量仅为DFT预期的五分之一到十分之一，而且它大约位于费米能级以下70 meV处，这与超导性源于该能带在费米能级附近高态密度的预期相矛盾。 我们还观察到一个非平凡的 k$_z$色散的类似铜氧化物的 3$d_{x2-y2}$带。 结合 X 射线衍射和密度泛函理论 (DFT) 的结果，我们认为拉伸薄膜下承受着约 5 GPa 的有效压力，这比从 DFT 放松结构得到的天真预期要大得多。 最后，约 70 meV 的能量位置令人感兴趣地接近集体模耦合，在薄膜中更为突出，且处于与氧相关声子和自旋激发谱最大值的能量范围内。 显示英文摘要

摘要： The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the na\"ive expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum.