CaCu3Ti4O12 single crystals: insights on growth and nanoscopic investigation

The combination of scanning impedance microscopy and conductive atomic force microscopy was applied to single crystals of the perovskite-type oxide CaCu3Ti4O12 (CCTO) in order to provide a local dielectric characterization on ingot sections. Both techniques clearly showed dielectric heterogeneities due to the presence of inclusions within crystals grown in different laboratories. Despite macroscopic characterizations, such as Laue diffraction, gave no indications for the presence of inclusions within the crystals, the discovery of dielectric heterogeneities prompted a careful structural analysis, which revealed the presence of crystalline CaTiO3 (CTO) precipitates. Thus, the scanning probe investigation provided the evidence for the electrical homogeneity within the CCTO crystal and the presence of internal barriers due to the CCTO/CTO interfaces

Evidence for a Ru4+^{4+} Kondo Lattice in LaCu3_3Ru4_4O12_{12}

Rare $d$-electron derived heavy-fermion properties of the solid-solution series LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ were studied for $1 \leq x \leq 4$ by resistivity, susceptibility, specific-heat measurements, and magnetic-resonance techniques. The pure ruthenate ($x = 4$) is a heavy-fermion metal characterized by a resistivity proportional to $T^2$ at low temperatures $T$. The coherent Kondo lattice formed by the localized Ru 4$d$ electrons is screened by the conduction electrons leading to strongly enhanced effective electron masses. By increasing titanium substitution the Kondo lattice becomes diluted resulting in single-ion Kondo properties like in the paradigm $4f$-based heavy-fermion compound Ce$_x$La$_{1-x}$Cu$_{2.05}$Si$_2$ [M. Ocko {\em et al.}, Phys. Rev. B \textbf{64}, 195106 (2001)]. In LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ the heavy-fermion behavior finally breaks down on crossing the metal-to-insulator transition close to $x = 2$.Comment: 9 pages, 8 figure

Tunable Optical Properties and the Role of Defects on the Carrier Lifetimes of Cs₃Sb₂I₉ Synthesized in Various Solvents

Pb‐free halide perovskites have recently attracted immense attention due to the number of advantages in their optical and electronic properties. However, tuning the optical bandgap with minimized amounts of point defects is a particularly challenging task in photovoltaics. It is pivotal to clearly understand the detailed relationship between the bandgap change with defect generation and charge carrier lifetime. In this study, Cs₃Sb₂I₉ crystals are synthesized by varied choice of solvents, namely, γ‐butyrolactone, a mixture of dimethylformamide and dimethyl sulfoxide, and hydroiodic acid. Although the same principles of decreasing solubility and crystallization are applied, Cs₃Sb₂I₉ crystals with different size and shape in microscopic and macroscopic scale are obtained during heating and cooling of the solution. The synthesized crystals are investigated using a combination of different spectroscopies including Raman, UV–visible, and time‐resolved photoluminescence. In the results, it is suggested that there is a strong relationship between Urbach energy and the lifetime of charge carriers. In this research, readily applicable practical principles and examples of how to control the defects for the advancement in Pb‐free perovskite photovoltaics are provided

Site‐selective substitution and resulting magnetism in arc‐melted perovskite ATiO₃₋δ (A = Ca, Sr, Ba)

Magnetic properties in perovskite titanates ATiO₃₋δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X‐ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X‐ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X‐ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc‐melted perovskite titanates ATiO₃₋δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc‐melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc‐melted perovskite ATiO₃₋δ (A = Ca, Sr, Ba)

CaCu3Ru4O12: a high-kondo-temperature transition-metal oxide

遷移金属酸化物の近藤効果を初めて実証 --電子相関物性の設計・探索の新たなプラットホームを開拓--. 京都大学プレスリリース. 2022-01-31.Open access publication funded by the Max Planck Society.We present a comprehensive study of CaCu₃Ru₄O₁₂ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu 2p core-level spectra, we deduce the presence of magnetic Cu²⁺ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and, thus, to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide CaCu₃Ru₄O₁₂ must be classified as a Kondo system and that the Kondo temperature is in the range of 500–1000 K