Materials Science (cond-mat.mtrl-sci)

Abstract: Bismuth vanadate - bismuth molybdate solid-solution was prepared to elaborate ceramics with different amounts of cation vacancies. Dense ceramics with similar microstructures were obtained and the evolution of their melting point, specific heat, thermal diffusivity, and conductivity as a function of the amount of vacancy was evaluated. At room temperature, the thermal conductivity decreases from 1.74 W m$^{-1}$ K$^{-1}$ for BiVO$_{4}$ (x=0) to 1.12 W m$^{-1}$ K$^{-1}$ for Bi$_{0.867}$$\square$$_{0.133}$Mo$_{0.4}$V$_{0.6}$O$_{4}$ (x=0.4). Moreover, we show that a very small amount of vacancy (1.7%, x=0.05) is enough to provide a large decrease in thermal conductivity by more than 15%, in agreement with a mass fluctuation scattering model. However, the temperature of the melting point also decreases with increasing amount of vacancy. Our results suggest adding only a very small amount of vacancy as the best strategy to obtain superior materials for thermal barriers and thermoelectric devices, with ultra-low thermal conductivity and high-temperature stability.

Abstract: We explore the nonlinear response of ultrafast strong-field driven excitons in a one-dimensional solid with ab initio simulations. We demonstrate from our simulations and analytical model that a finite population of excitons imprints unique signatures to the high-harmonic spectra of materials. We show the exciton population can be retrieved from the spectra. We further demonstrate signatures of exciton recombination and that a shift of the exciton level is imprinted into the harmonic signal. The results open the door to high-harmonic spectroscopy of excitons in condensed-matter systems.

Abstract: Why are materials with specific characteristics more abundant than others? This is a fundamental question in materials science and one that is traditionally difficult to tackle, given the vastness of compositional and configurational space. We highlight here the anomalous abundance of inorganic compounds whose primitive unit cell contains a number of atoms that is a multiple of four. This occurrence - named here the 'rule of four' - has to our knowledge not previously been reported or studied. Here, we first highlight the rule's existence, especially notable when restricting oneself to experimentally known compounds, and explore its possible relationship with established descriptors of crystal structures, from symmetries to energies. We then investigate this relative abundance by looking at structural descriptors, both of global (packing configurations) and local (the smooth overlap of atomic positions) nature. Contrary to intuition, the overabundance does not correlate with low-energy or high-symmetry structures; in fact, structures which obey the 'rule of four' are characterized by low symmetries and loosely packed arrangements maximizing the free volume. We are able to correlate this abundance with local structural symmetries, and visualize the results using a hybrid supervised-unsupervised machine learning method.

Abstract: In this work we use a phenomenological theory of ferroelectric switching in BiFeO$_3$ thin films to uncover the mechanism of the two-step process that leads to the reversal of the weak magnetization of these materials. First, we introduce a realistic model of a BiFeO$_3$ film, including the Landau energy of isolated domains as well as the constraints that account for the presence of the substrate and the multidomain configuration found experimentally. We use this model to obtain statistical information about the switching behavior - by running dynamical simulations based on the Landau-Khalatnikov time-evolution equation, including thermal fluctuations - and we thus identify the factors that drive the two-step polarization reversal observed in the experiments. Additionally, we apply our model to test potential strategies for optimizing the switching characteristics.

Abstract: Identifying a suitable water-soluble sacrificial layer is crucial to fabricating large-scale freestanding oxide membranes, which stimulates intriguing functionalities and enables novel integrations with semiconductor technologies. In this work, we introduce a new water-soluble sacrificial layer, "super-tetragonal" Sr4Al2O7 (SAOT). Its unique atomic structure ensures a coherent growth of perovskite ABO3/SAOT heterostructures, effectively inhibiting crack formation in the water-released membranes. For various non-ferroelectric oxide membranes with lattice constants ranging from 3.85 to 4.04 A, the crack-free areas can span up to millimeter-scale. The high water-solubility of SAOT shortens the exfoliation duration to a few minutes only. Our findings highlight the SAOT as an effective and versatile sacrificial layer for freestanding oxide membranes with superior integrity, crystallinity, and functionalities, further promoting their potential for innovative device applications.

Abstract: We predict energetically and dynamically stable ternary Carbon-Phosphorous-Arsenic (CPAs2) monolayers in buckled geometric structure by employing density functional theory based calculations. We consider three different symmetric configurations, namely, inversion (i), mirror (m) and rotational (r). The low-energy dispersions in electronic band structure and density of states (DOS) around the Fermi level contain two contrasting features: (a) parabolic dispersion around highly symmetric Gamma point with a step function in DOS due to nearly-free-particle-like Schroedinger-Fermions and (b) linear dispersion around highly symmetric K point with linear DOS due to massless Dirac-Fermions for i-CPAs2 monolayer. The step function in DOS is a consequence of two-dimensionality of the system in which the motion of nearly-free-particles is confined. However, a closer look at (b) reveals that the ternary monolayers possess distinct characters, namely (i) massless-gapless, (ii) slightly massive-gapped and (iii) unpinned massless-gapless Dirac-Fermions for i, m and r-CPAs2 configurations respectively. Thus, the nature of states around the Fermi level depends crucially on the symmetry of systems. In addition, we probe the influence of mechanical strain on the properties of CPAs2 monolayer. The results indicate that the characteristic dispersions of (a) and (b) move in opposite directions in energy which leads to a metal-to-semimetal transition in i and r-CPAs2 configurations, for a few percentages of tensile strain. On the other hand, a strain induced metal-to-semiconductor transition is observed in m-CPAs2 configuration with a tunable energy band gap. Interestingly, unlike graphene, the Dirac cones can be unpinned from highly symmetric K (and K') point, but they are restricted to move along the edges (K-M'-K') of first Brillouin zone due to C2 symmetry in i and r-CPAs2 configurations.