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Register a new userPoint defects, ferromagnetism and transport in calcium hexaboride
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The formation energy and local magnetic moment of a series of point defects in CaB$_6 $ are computed using a supercell approach within the generalized gradient approximation to density functional theory. Based on these results, speculations are made as to the influence of these defects on electrical transport. It is found that the substitution of Ca by La does not lead to the formation of a local moment, while a neutral B$_6 $ vacancy carries a moment of 2.4 Bohr magnetons, mostly distributed over the six nearest-neighbour B atoms. A plausible mechanism for the ferromagnetic ordering of these moments is suggested. Since the same broken B-B bonds appear on the preferred (100) cleavage planes of the CaB$_6$ structure, it is argued that internal surfaces in polycrystals as well as external surfaces in general will make a large contribution to the observed magnetization.
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Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the conductivity distribution $P(sigma)$ is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.
Defects are inevitably present in two-dimensional (2D) materials and usually govern their various properties. Here a comprehensive density functional theory-based investigation of 7 kinds of point defects in a recently produced {gamma} allotrope of 2D phosphorus carbide ({gamma}-PC) is conducted. The defects, such as antisites, single C or P, and double C and P and C and C vacancies, are found to be stable in {gamma}-PC, while the Stone-Wales defect is not presented in {gamma}-PC due to its transition metal dichalcogenides-like structure. The formation energies, stability, and surface density of the considered defect species as well as their influence on the electronic structure of {gamma}-PC is systematically identified. The formation of point defects in {gamma}-PC is found to be less energetically favourable then in graphene, phosphorene, and MoS2. Meanwhile, defects can significantly modulate the electronic structure of {gamma}-PC by inducing hole/electron doping. The predicted scanning tunneling microscopy images suggest that most of the point defects are easy to distinguish from each other and that they can be easily recognized in experiments.
Electrides are an emerging class of materials with highly-localized electrons in the interstices of a crystal that behave as anions. The presence of these unusual interstitial quasi-atom (ISQ) electrons leads to interesting physical and chemical properties, and wide potential applications for this new class of materials. Crystal defects often have a crucial influence on the properties of materials. Introducing impurities has been proved to be an effective approach to improve the properties of a material and to expand its applications. However, the interactions between the anionic ISQs and the crystal defects in electrides are as yet unknown. Here, dense FCC-Li was employed as an archetype to explore the interplay between anionic ISQs and interstitial impurity atoms in this electride. This work reveals a strong coupling among the interstitial impurity atoms, the ISQs, and the matrix Li atoms near to the defects. This complex interplay and interaction mainly manifest as the unexpected tetrahedral interstitial occupation of impurity atoms and the enhancement of electron localization in the interstices. Moreover, the Be impurity occupying the octahedral interstice shows the highest negative charge state (Be8-) discovered thus far. These results demonstrate the rich chemistry and physics of this emerging material, and provide a new basis for enriching their variants for a wide range of applications.
To understand the magnetic properties of Fe$_3$GeTe$_2$, we performed the detailed first-principles study. Contrary to the conventional wisdom, it is unambiguously shown that Fe$_3$GeTe$_2$ is not ferromagnetic but antiferromagnetic carrying zero net moment in its stoichiometric phase. Fe defect and hole doping are the keys to make this material ferromagnetic, which are shown by the magnetic force response as well as the total energy calculation with the explicit Fe defects and the varied system charges. Further, we found that the electron doping also induces the antiferro- to ferromagnetic transition. It is a crucial factor to understand the notable recent experiment of gate-controlled ferromagnetism. Our results not only unveil the origin of ferromagnetism of this material but also show how it can be manipulated with defect and doping.
Close-packed chalcogenide spinels, such as MgSc$_2$Se$_4$, MgIn$_2$S$_4$ and MgSc$_2$S$_4$, show potential as solid electrolytes in Mg batteries, but are affected by non-negligible electronic conductivity, which contributes to self-discharge when used in an electrochemical storage device. Using first-principles calculations, we evaluate the energy of point defects as function of synthesis conditions and Fermi level to identify the origins of the undesired electronic conductivity. Our results suggest that Mg-vacancies and Mg-metal anti-sites (where Mg is exchanged with Sc or In) are the dominant point defects that can occur in the systems under consideration. While we find anion-excess conditions and slow cooling to likely create conditions for low electronic conductivity, the spinels are likely to exhibit significant $n$-type conductivity under anion-poor environments, which are often present during high temperature synthesis. Finally, we explore extrinsic aliovalent doping to potentially mitigate the electronic conductivity in these chalcogenide spinels. The computational strategy is general and can be easily extended to other solid electrolytes (and electrodes) to aid in the optimization of the electronic properties of the corresponding frameworks.
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