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Machine learning is changing how we design and interpret experiments in materials science. In this work we show how unsupervised learning, combined with ab initio random structure searching, improves our understanding of structural metastability in multicomponent alloys. We focus on the case of Al-O-N alloys where the formation of aluminum vacancies in wurtzite AlN upon the incorporation of substitutional oxygen can be seen as a general mechanism of solids where crystal symmetry is reduced to stabilize defects. The ideal AlN wurtzite crystal structure occupation cannot be matched due to the presence of an aliovalent hetero-element into the structure. The traditional interpretation of the c-lattice shrinkage in sputter-deposited Al-O-N films from X-ray diffraction (XRD) experiments suggests the existence of a solubility limit at 8 at.% oxygen content. Here we show that such naive interpretation is misleading. We support XRD data with accurate ab initio modeling and dimensionality reduction on advanced structural descriptors to map structure-property relationships. No signs of a possible solubility limit are found. Instead, the presence of a wide range of non-equilibrium oxygen-rich defective structures emerging at increasing oxygen contents suggests that the formation of grain boundaries is the most plausible mechanism responsible for the lattice shrinkage measured in Al-O-N sputtered films. We further confirm our hypothesis using positron annihilation lifetime spectroscopy.

Generating and detecting radiation in the technologically relevant range of the so-called terahertz gap (0.1–10 THz) is challenging because of a lack of efficient sources and detectors. Quantum dots in carbon nanotubes have shown great potential to build sensitive terahertz detectors, usually based on photon-assisted tunneling. A recently reported mechanism combining resonant quantum dot transitions and tunneling barrier asymmetries results in a narrow linewidth photocurrent response with a large signal-to-noise ratio under weak THz radiation. That device was sensitive to one frequency, corresponding to transitions between equidistant quantized states. In this work we show, using numerical simulations together with scanning tunneling spectroscopy studies of a defect-induced metallic zigzag single-walled carbon nanotube quantum dot, that breaking simultaneously various symmetries in metallic nanotube quantum dots of arbitrary chirality strongly relaxes the selection rules in the electric dipole approximation and removes energy degeneracies. This leads to a richer set of allowed optical transitions spanning frequencies from 1 THz to several tens of THz, for a ∼10 nm quantum dot. Based on these findings, we propose a terahertz detector device based on a metallic single-walled carbon nanotube quantum dot defined by artificial defects. Depending on its length and contacts transparency, the operating regimes range from a high-resolution gate-tunable terahertz sensor to a broadband terahertz detector. Our calculations indicate that the device is largely unaffected by temperatures up to 100 K, making carbon nanotube quantum dots with broken symmetries a promising platform to design tunable terahertz detectors that could operate at liquid nitrogen temperatures.

The mechanisms for the dehydrogenation reaction of cyclohexaphenylene at a copper surface are investigated with the help of density functional theory and metadynamics. Our results represent a showcase for an approach that describes the surface using many-body classical potentials and molecule-surface interactions with a van der Waals model. Starting from the experimental observation that dispersion-assisted mechanisms are at least as important as catalytic processes for the description of the reaction, we fully describe the former, we identify intermediate states and estimate the free energy barriers that characterize the reaction

The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental data

The mechanisms for the dehydrogenation reaction of cyclohexaphenylene at a copper surface are investigated with the help of density functional theory and metadynamics. Our results represent a showcase for an approach that describes the surface using many-body classical potentials and molecule-surface interactions with a van der Waals model. Starting from the experimental observation that dispersion-assisted mechanisms are at least as important as catalytic processes for the description of the reaction, we fully describe the former, we identify intermediate states and estimate the free energy barriers that characterize the reaction

The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental data