Correlation-driven insulating behaviour in double perovskite ruthenate thin films A2DyRuO6 (A = Ba, Sr)

Abstract

The structural, optical and electronic properties of double perovskite ruthenate thin films, specifically A₂DyRuO₆ (where A = Ba, Sr), have been thoroughly examined using various analytical methods. These methods include X-ray diffraction (XRD), UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and X-ray absorption spectroscopy (XAS). XRD analysis of the films' structure shows the formation of highly crystalline, single-phase thin films with a preferred c-axis orientation. Furthermore, a contraction in the lattice and an increase in octahedral distortion are observed, which is attributed to the substitution of the larger Ba²⁺ ion with the smaller Sr²⁺ ion. Using UV-visible diffuse reflectance spectroscopy and the Kubelka–Munk method to analyze the results, we found that Ba₂DyRuO₆ and Sr₂DyRuO₆ have wide band gaps and behave like insulators, with optical band gaps of about 3.66 eV and 4.16 eV, respectively. XPS studies show that most of the ruthenium is in the Ru⁵⁺ oxidation state, with a small amount in the Ru⁴⁺ state. This suggests that there are mixed valence states that are probably caused by oxygen vacancies or local distortions. XAS measurements at the O K-edge show the unoccupied electronic states, and strong hybridization between Ru 4d and O 2p orbitals is shown by a clear pre-edge feature. Complementary UPS measurements investigate the occupied states, and the integrated UPS–XAS analysis demonstrates a distinct reduction of spectral weight at the Fermi level, thereby validating the insulating ground state in both compounds. The spectral features, such as the creation of lower and upper Hubbard bands, show that the insulating behavior is caused by electron–electron correlations in the Ru 4d manifold. Even though the structure changes when the A-site is replaced, both systems still show strong Mott–Hubbard insulating properties that are driven by correlation. This puts them close to the metal–insulator transition regime.