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Register a new userFinite-temperature materials modeling from the quantum nuclei to the hot electrons regime
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Atomistic simulations provide insights into structure-property relations on an atomic size and length scale, that are complementary to the macroscopic observables that can be obtained from experiments. Quantitative predictions, however, are usually hindered by the need to strike a balance between the accuracy of the calculation of the interatomic potential and the modelling of realistic thermodynamic conditions. Machine-learning techniques make it possible to efficiently approximate the outcome of accurate electronic-structure calculations, that can therefore be combined with extensive thermodynamic sampling. We take elemental nickel as a prototypical material, whose alloys have applications from cryogenic temperatures up to close to their melting point, and use it to demonstrate how a combination of machine-learning models of electronic properties and statistical sampling methods makes it possible to compute accurate finite-temperature properties at an affordable cost. We demonstrate the calculation of a broad array of bulk, interfacial and defect properties over a temperature range from 100 to 2500 K, modeling also, when needed, the impact of nuclear quantum fluctuations and electronic entropy. The framework we demonstrate here can be easily generalized to more complex alloys and different classes of materials.
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Topology, a mathematical concept, has recently become a popular and truly transdisciplinary topic encompassing condensed matter physics, solid state chemistry, and materials science. Since there is a direct connection between real space, namely atoms, valence electrons, bonds and orbitals, and reciprocal space, namely bands and Fermi surfaces, via symmetry and topology, classifying topological materials within a single-particle picture is possible. Currently, most materials are classified as trivial insulators, semimetals and metals, or as topological insulators, Dirac and Weyl nodal-line semimetals, and topological metals. The key ingredients for topology are: certain symmetries, the inert pair effect of the outer electrons leading to inversion of the conduction and valence bands, and spin-orbit coupling. This review presents the topological concepts related to solids from the viewpoint of a solid-state chemist, summarizes techniques for growing single crystals, and describes basic physical property measurement techniques to characterize topological materials beyond their structure and provide examples of such materials. Finally, a brief outlook on the impact of topology in other areas of chemistry is provided at the end of the article.
Hot electrons role in shock generation and energy deposition to hot dense core is crucial for the shock ignition scheme implying the need for their characterization at laser intensities of interest for shock ignition. In this paper we analyze the experimental results obtained at the PALS laboratory and provide an estimation of hot electrons temperature and conversion efficiency using a semi analytical approach, including Harrach-Kidders model.
Properties of electrons in superlattices (SLs) of a finite length are described using standing waves resulting from the fixed boundary conditions (FBCs) at both ends. These electron properties are compared with those predicted by the standard treatments using running waves (Bloch states) resulting from the cyclic boundary conditions (CBCs). It is shown that, while the total number of eigenenergies in a miniband is the same according to both treatments, the number of different energies is twice higher according to the FBCs. It is also shown that the wave vector values corresponding to the eigenenergies are spaced twice as densely for the FBCs as for the CBCs. The reason is that a running wave is characterized by a single value of wave vector k, while a standing wave in a finite SL is characterized by a pair of wavevectors +/- q. Using numerical solutions of the Schroedinger equation for an electron in an increasing number N of periodic quantum wells (beginning with N = 2) we investigate the birth of an energy miniband and of a Brillouin zone according to the two approaches. Using the Fourier transforms of the computed wave functions for a few quantum wells we follow the birth of electrons momentum. It turns out that the latter can be discerned already for a system of two wells. We show that the number of higher values of the wave vector q involved in an eigenenergy state is twice higher for a standing wave with FBCs than for a corresponding Bloch state. Experiments using photons and phonons are proposed to observe the described properties of electrons in finite superlattices.
The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.
Ultrafast dynamics of graphite is investigated by time-resolved photoemission spectroscopy. We observe spectral features of direct photoexcitations, non-thermal electron distributions, and recovery dynamics occurring with two time scales having distinct pump-power dependences. Additionally, we find an anomalous increase of the spectral intensity around the Fermi level, and we attribute this to spectral broadenings due to coupled optical phonons in the transient. The fingerprints of the coupled optical phonons occur from the temporal region where the electronic temperature is still not definable. This implies that there is a mechanism of ultrafast-and-efficient phonon generations beyond a two-temperature model.
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