Abstract
Angle-resolved photoemission spectroscopy, visualizing the superconducting gap in k-space, plays a pivotal role in research on cuprates and other high-Tc 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-Tc 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-Tc 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.
Highlights
Since the discovery of high-temperature superconductors (HTSCs) and angle-resolved photoemission spectroscopy (ARPES) has been playing a pivotal role in the research towards the exact mechanisms of this phenomenon[1]
The corresponding cation displacements, projected onto the ab-plane, form replicas of the fundamental reciprocal vectors along one-dimensional lines in k-space (Fig. 1d). This lattice superstructure (LSS) manifests itself in the ARPES spectra as multiple replicas of the fundamental band structure and Fermi surface (FS) displaced by QLSS ~ (1/5,1/5)[9,10]
We have used SX-ARPES to investigate the SC gap magnitude in Bi2212 as apparent in the photoemission spectra under variation of the matrix element (ME) effects and of the probing depth. The former was varied by measurements of the SC gap over a dense grid of equivalent kpoints formed by the LSS-replicas of the FS, taking into advantage strengthening of the LSS-induced atomic displacements into the bulk of Bi2212
Summary
Since the discovery of high-temperature superconductors (HTSCs) and angle-resolved photoemission spectroscopy (ARPES) has been playing a pivotal role in the research towards the exact mechanisms of this phenomenon[1]. These studies are based on the assumption that the ARPES spectra directly represent the oneelectron spectral function A(ω, k), essentially neglecting matrix element (ME) effects. Throughout the immensity of ARPES studies, in particular on cuprate HTSCs, most works interpreted the data in terms of A(ω, k) without paying much attention to the ME effects Another question is related to the extreme surface sensitivity of ARPES, whose probing depth in the conventional VUV-ARPES range of photon energy hv below ~100 eV hardly exceeds 3–5 Å. No SXARPES studies have so far addressed the SC gap
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