Ruddlesden-Popper镍酸盐薄膜中的热力学相变、配对对称性与费米面拓扑
Thermodynamic phase transition, pairing symmetry and Fermi surface topology in Ruddlesden-Popper nickelate films
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中文总结 AI 辅助
本研究通过角分辨光电子能谱探究新型RP镍酸盐超导异质结的电子结构，证实其超导热力学相变、无节点序参量及多轨道费米面，填补了镍酸盐超导相变的热力学证据空白。
中文摘要 AI 辅助
Ruddlesden-Popper（RP）镍酸盐为探索高转变温度（高$T_C$）超导性及超导机制提供了一片未知领域。本文中，我们利用角分辨光电子能谱研究了新型高$T_C$超导RP镍酸盐异质结$\boldsymbol{\rm La_2PrNi_2O_7/NdAlO_3}$的电子结构。我们观测到不存在赝能隙态的超导态，从而能够直接测量超导序参量并从微观层面提取电子比热。观测到的超导能隙在$T_C$处打开，伴随显著的相干峰，表明进入超导态时出现非零序参量。电子比热在$T_C$处出现跃变，进一步证明了热力学相变的存在。我们通过观测到的超导相干峰量化了超导序参量的大小，并明确证实了无赝能隙情况下的无节点行为。底层费米面由$\boldsymbol{\rm α}$、$\boldsymbol{\rm β}$和$\boldsymbol{\rm γ}$口袋组成，呈现出多轨道特性。应变依赖测量进一步显示，在具有不同外延应变的所有超导和非超导薄膜中均存在$\boldsymbol{\rm γ}$口袋。我们的结果填补了镍酸盐中超导相变的热力学证据空白，同时为超导序参量的对称性提供了直接证据，并阐明了RP镍酸盐薄膜中费米面拓扑与超导出现之间的关系。
英文摘要
Ruddlesden-Popper (RP) nickelates provide an uncharted territory to explore high-transition-temperature (high-$T_C$) superconductivity and superconducting mechanism. Here, we investigate the electronic structure of a new type of high-$T_C$ superconducting RP nickelate heterostructure $\mathrm{La_2PrNi_2O_7/NdAlO_3}$ by angle-resolved photoemission spectroscopy. A superconducting state is observed without a pseudogap state, enabling a direct measurement of the superconducting order parameter and a microscopic extraction of the electronic specific heat. The observed superconducting gap opens at $T_C$ with prominent coherence peaks, illustrating the emergence of nonzero order parameter upon entering the superconducting state. An electronic specific heat jump appears at $T_C$, further demonstrating a thermodynamic phase transition. The magnitude of the superconducting order parameter is quantified by the observed superconducting coherence peaks, and a nodeless behavior is unambiguously established in the absence of pseudogap. The underlying Fermi surface consists of $α$, $β$ and $γ$ pockets, exhibiting a multi-orbital nature. Strain dependent measurements further reveal the $γ$ pocket in all superconducting and non-superconducting films with different epitaxial strain. Our results establish the missing thermodynamic evidence for superconducting phase transition in nickelates. They also provide direct evidence for the symmetry of the superconducting order parameter and illustrate the relationship between Fermi surface topology and the emergence of superconductivity in RP nickelate films.
