Computational Condensed Matter Physics Group

Abstract:

<jats:title>Abstract</jats:title><jats:p>Double perovskite oxides, with generalized formula A<jats:sub>2</jats:sub>BB<jats:inline-formula><jats:alternatives><jats:tex-math>$$^{\prime}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math></jats:alternatives></jats:inline-formula>O<jats:sub>6</jats:sub>, attract wide interest due to their multiferroic and charge transfer properties. They offer a wide range of potential applications such as spintronics and electrically tunable devices. However, great practical limitations are encountered, since a spontaneous order of the B-site cations is notoriously hard to achieve. In this joint experimental-theoretical work, we focused on the characterization of double perovskites La<jats:sub>2</jats:sub>TiFeO<jats:sub>6</jats:sub> and La<jats:sub>2</jats:sub>VCuO<jats:sub>6</jats:sub> films grown by pulsed laser deposition and interpretation of the observed B-site disorder and partial charge transfer between the B-site ions. A random structure sampling method was used to show that several phases compete due to their corresponding configurational entropy. In order to capture a representative picture of the most relevant competing microstates in realistic experimental conditions, this search included the potential formation of non-stoichiometric phases as well, which could also be directly related to the observed partial charge transfer. We optimized the information encapsulated in the potential energy landscape, captured via structure sampling, by evaluating both enthalpic and entropic terms. These terms were employed as a metric for the competition of different phases. This approach, applied herein specifically to La<jats:sub>2</jats:sub>TiFeO<jats:sub>6</jats:sub>, highlights the presence of highly entropic phases above the ground state which can explain the disorder observed frequently in the broader class of double perovskite oxides.</jats:p>

Abstract:

Halide double perovskites with alternating silver and pnictogen cations are an emerging family of photoabsorber materials with robust stability and band gaps in the visible range. However, the nature of optical excitations in these systems is not yet well understood, limiting their utility. Here, we use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate the electronic structure and optical excitations of the double perovskite series Cs2AgBX6, with B=Bi3+, Sb3+, X = Br−, Cl−. We find that these materials exhibit strongly localized resonant excitons with energies from 170 to 434 meV below the direct band gap. In contrast to lead-based perovskites, the Cs2AgBX6 excitons are computed to be non-hydrogenic, with anisotropic effective masses and sensitive to local field effects, a consequence of their chemical heterogeneity. Our calculations demonstrate the limitations of the Wannier-Mott and Elliott models for this class of double perovskites and contribute to a detailed atomistic understanding of their light-matter interactions.