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Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high‐temperature superconductor YBa2Cu3O7‐δ (Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration‐corrected scanning transmission microscopy has been used for exploring, on a single unit‐cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect‐driven ferromagnetism, and X‐ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects. A dilute ferromagnetic system in the superconducting state of YBa2Cu3O7−δ is reported. The interplay between the local structure, composition, and physical properties of previously unforeseen point defects buried within the YBa2Cu3O7−δ is explored. These defects, composed of Cu and O vacancies, lead to short‐range ferromagnetic coupling of surrounding Cu spins, which ultimately form ferromagnetic clusters embedded within the superconductor.

The Ag–C composite anodes facilitate stable LixAg deposition in solid‐state batteries. However, the role of carbon and the kinetics of lithium migration and deposition in the composite structure remain unclear. Few studies have focused on this critical research area owing to a shortage of effective, non‐destructive characterization methods that can directly observe the Li alloy deposition process in solid‐state batteries in real time. In this study, Li alloy formation on the Ag–C anode is investigated through operando X‐ray diffraction (XRD) analysis and scanning electron microscopy combined with in situ probing. This enables the observation of the composition and morphology of the LixAg alloy as it evolves within the Ag–C composite anode during discharge. Further insights from scanning transmission electron microscopy–electron energy loss spectroscopy and first‐principles simulations on lithiophilicity and the energy barrier for lithium migration reveal a complex lithium migration and deposition mechanism within the Ag–C anode. Operando XRD, in situ SEM, and first‐principles simulations uncover the complex mechanism underlying the continuous and stable deposition in the Ag–C anode and complex interplay of the selective diffusion and deposition enabled by a nuanced balance of diffusivity and Li adsorption energy among constituent nanoparticles.