Angle-resolved photoemission spectroscopy, visualizing the superconducting gap in k-space, plays a pivotal role in research on cuprates and other high-T_c superconducting materials. However, there has always been an imminent doubt whether this technique truly represents the intrinsic bulk spectral function, whose response can be distorted by energy- and k-dependence of the photoexcitation matrix element, and by a small photoelectron escape depth of few surface atomic layers. Here, we address this fundamental question with soft-X-ray photoemission measurements of the superconducting gap in the paradigm high-T_c cuprate Bi₂Sr₂CaCu₂O₈. We vary the matrix element by spanning a dense k-space grid, formed by the lattice superstructure, and the probing depth by changing the emission angle. The measured gap appears independent of the matrix element effects, probing depth or photoexcitation energy. This fact proves the relevance of photoemission studies for the bulk superconductivity in Bi₂Sr₂CaCu₂O₈, and calls for similar verification experiments on other high-T_c compounds, in particular more three-dimensional ones. Bi₂Sr₂CaCu₂O₈ shows an anomalously fast decay of the coherent spectral weight with photon energy, tracing back to strong electron–phonon interaction or relaxation of the lattice coherence.

角分辨光发射光谱学可以看到k空间中的超导间隙，在研究铜酸盐和其他高T _c超导材料方面起着关键作用。然而，一直存在一个迫在眉睫的疑问，该技术是否真正代表了本征体光谱函数，其响应会因光激发矩阵元素的能量和k依赖性以及少数几个表面原子层的较小光电子逸出深度而失真。。在这里，我们用软X射线光发射测量范式高T _c铜酸盐Bi ₂ Sr ₂ CaCu ₂ O中的超导间隙来解决这个基本问题。₈。我们通过跨越由晶格上层结构形成的致密k空间网格来改变矩阵元素，并通过改变发射角来改变探测深度。所测量的间隙与基质元素效应，探测深度或光激发能无关。这一事实证明了光发射研究与Bi ₂ Sr ₂ CaCu ₂ O _8中的本体超导性相关，并要求对其他高T _c化合物，尤其是三维化合物进行类似的验证实验。Bi ₂ Sr ₂ CaCu ₂ O ₈ 显示了相干光谱权重随光子能量的异常快速衰减，追溯到强的电子-声子相互作用或晶格相干的弛豫。