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We perform ab initio calculations, based on density functional theory, of substitutional and vacancy defects in the monoclinic hafnium oxide (m-HfO2) and alpha-quartz (SiO2). The neutral oxygen vacancies and substitutional Si and Hf defects in HfO2 and SiO2, respectively, are investigated. Our calculations show that, for a large range of Hf chemical potential, Si substitutional defects are most likely to form in HfO2, leading to the formation of a silicate layer at the HfO2/Si interface. We also find that it is energetically more favorable to form oxygen vacancies in SiO2 than in HfO2, which implies that oxygen deficient HfO2 grown on top of SiO2 will consume oxygen from the SiO2., Comment: Revtex4, 3 pages and 1 figure, submitted to Appl. Phys. Lett

Extensive effort has been devoted recently to develop nanoscale films of three-dimensional (3D) topological insulators, but preserving the topological state character. Usually, the surface topological states of ultra thin 3D systems are drastically affected or even suppressed due to quantum tunnelling, interactions with the substrate, as well the enlargement of the band gap due to confinement effects. Here we show that an ultra thin system composed by 2 and 3 quintuple layers of Bi2Se3 adsorbed on AlN surface present pairs of topological states with opposite spin-momentum locking. The preservation of the topological spin texture is a result of a unique combination presented in the system: a strong bond between Bi2Se3 and AlN substrate, and the enhancement of spin-orbit couplings due to a large interface potential gradient.

Ab initio calculations are used to investigate the structural and electronic properties of vacancies in SixGe1−x. The (++), (+), (0), and (−) charge states are studied and the substitutional disorder of the alloy is considered explicitly. We found a linear relationship between the effective-U for the system formed by the (++), (+), and (0) charge states and the number of Si atoms in the first neighborhood of a vacancy (NSi). The effective-U is positive when NSi is zero, and it is negative when NSi is 2 and 4. In all cases, the absolute value of the effective-U in the alloy is significantly smaller than its value for pure Si and pure Ge.

Using ab initio total-energy calculations, we investigate the effects of stacking faults on the properties of dopants in pure and n-type doped GaAs. We find that the Si impurity segregates towards a GaAs stacking fault. A Si atom at a Ga site in the stacking fault, in either a neutral or a negative charge state, is energetically favorable as compared to a Si atom at a Ga site in a crystalline environment by as much as 0.2 eV. We also find that a Si impurity in the stacking fault cannot occupy metastable positions, as occurs in the formation of DX centers. Thus, stacking faults can prevent the formation of DX-like centers in GaAs.

We study the carbon substitutional impurity in GaAs at both cation and anion sites. We use first‐principle nonlocal pseudopotentials, and we find for the acceptor CAs impurity the expected shallow‐level defect behavior. The CGa impurity, however, behaves as a deep amphoteric impurity, not as a shallow donor, and in the neutral and negative charge states should cause virtually no lattice shrinking. We expect that this unusual behavior is the reason why the center was not directly detected up to now, although compensating for the shallow acceptor is known to occur with high carbon concentrations.

Whereas the conventional practice of referring binding energies of deep donors and acceptors to the band edges of the host semiconductor does not produce transparent chemical trends when the same impurity is compared in different crystals, referring them to the vacuum level through the use of the photothreshold reveals a remarkable material invariance of the levels in III-V and II-VI semiconductors. It is shown that this is a consequence of the antibonding nature of the deep gap level with respect to the impurity atom-host orbital combinations.

International audience; Abstract In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moiré materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. We stress that this article is not meant to be a fully comprehensive review but rather an up-to-date snapshot of different areas of research on quantum materials with a minimal number of references focusing on the latest developments.