Materials Science (cond-mat.mtrl-sci)

Abstract: We present an artificial intelligence (AI) method for automatically computing the melting point based on coexistence simulations in the NPT ensemble. Given the interatomic interaction model, the method makes decisions regarding the number of atoms and temperature at which to conduct simulations, and based on the collected data predicts the melting point along with the uncertainty, which can be systematically improved with more data. We demonstrate how incorporating physical models of the solid-liquid coexistence evolution enhances the AI method's accuracy and enables optimal decision-making to effectively reduce predictive uncertainty. To validate our approach, we compare our results with approximately 20 melting point calculations from the literature. Remarkably, we observe significant deviations in about one-third of the cases, underscoring the need for accurate and reliable AI-based algorithms for materials property calculations.

Abstract: The all-temperature magnon (ATM) theory [J. Phys. Condens. Matter 21, 336003/1-14, 2009] has been used to analyze the temperature dependence of magnetization as well as the internal energy components of a mono-domain ferromagnetic solid. The critical exponents have been in better agreement with experiment than their mean-field theory and critical phenomenon theory counterparts, and unlike in the latter theories, vary from one ferromagnet to another. Expressions have been derived for the thermally averaged spin-center force constants and their break-up in terms of the base-line related (solid) and exchange-cum-field mediated (magnetic) components. These components give rise to expressions for phonon frequencies and their modifications by magnon-phonon coupling. The derived expressions are suitable for a correct quantum chemical evaluation of the involved properties. A detailed numerical calculation using spin configurations at varying crystal geometries is hardly possible even today and beyond the scope of the present work. The focus here is on the correctness and explaining the trends of properties. It has been shown that the frequency modification by magnon-phonon interaction can be negative for certain phonon branches near the ferromagnetic transition temperature. Also, the ratio of frequency modification and phonon frequency is approximately proportional to the ratio of curvatures of the involved energy surfaces.

Abstract: The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising two-dimensional material for electronic, optoelectronic, nanophotonic, and quantum devices. Here we report on the changes in hBN's properties induced by isotopic purification in both boron and nitrogen. Previous studies on isotopically pure hBN have focused on purifying the boron isotope concentration in hBN from its natural concentration (approximately 20 at$\%$ $^{10}$B, 80 at$\%$ $^{11}$B) while using naturally abundant nitrogen (99.6 at$\%$ $^{14}$N, 0.4 at$\%$ $^{15}$N), i.e. almost pure $^{14}$N. In this study, we extend the class of isotopically-purified hBN crystals to $^{15}$N. Crystals in the four configurations, namely h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N, and h$^{11}$B$^{15}$N, were grown by the metal flux method using boron and nitrogen single isotope ($>99\%$) enriched sources, with nickel plus chromium as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate the high quality of the monoisotopic hBN crystals with vibrational and optical properties of the $^{15}$N-purified crystals at the state of the art of currently available $^{14}$N-purified hBN. The growth of high-quality h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N, and h$^{11}$B$^{15}$N opens exciting perspectives for thermal conductivity control in heat management, as well as for advanced functionalities in quantum technologies.

Abstract: In the field of plane elasticity, complexities arising from geometric properties and applied forces often present substantial challenges to existing methods. Some specific situations where these limitations emerge are when considering problems with a non-vanishing traction, non-zero forces of specific forms, and ring-shaped domains. This paper addresses these issues by developing an approach that leverages established knowledge of the Neumann problem for the inhomogeneous Cauchy-Riemann problem. We propose an integral representation method for solutions applicable to domains that can be conformally mapped from a unit disk or an annulus. We demonstrate the practicality and applicability of this method through specific examples, including a notch problem in a cardioid domain, a ring domain with shifted hole, and gear-like domain. We hope the techniques presented in this work will find themselves useful for people working on isotropic planar elastostatics problems.

Abstract: X-ray diffraction from dislocation half-loops consisting of a misfit segment and two threading arms extending from it to the surface is calculated by the Monte Carlo method. The diffraction profiles and reciprocal space maps are controlled by the ratio of the total lengths of the misfit and the threading segments of the half-loops. A continuous transformation from the diffraction characteristic of misfit dislocations to that of threading dislocations with increasing thickness of an epitaxial film is studied. Diffraction from dislocations with edge and screw threading arms is considered and the contributions of both types of dislocations are compared.

Abstract: Modulation of magnetic properties through voltage-driven ion motion and redox processes, i.e., magneto-ionics, is a unique approach to control magnetism with electric field for low-power memory and spintronic applications. So far, magneto-ionics has been achieved through direct electrical connections to the actuated material. Here we evidence that an alternative way to reach such control exists in a wireless manner. Induced polarization in the conducting material immersed in the electrolyte, without direct wire contact, promotes wireless bipolar electrochemistry, an alternative pathway to achieve voltage-driven control of magnetism based on the same electrochemical processes involved in direct-contact magneto-ionics. A significant tunability of magnetization is accomplished for cobalt nitride thin films, including transitions between paramagnetic and ferromagnetic states. Such effects can be either volatile or non-volatile depending on the electrochemical cell configuration. These results represent a fundamental breakthrough that may inspire future device designs for applications in bioelectronics, catalysis, neuromorphic computing, or wireless communications.

Abstract: Oxidation of graphite and subsequent exfoliation leads to single layer graphene oxide (GO). GO has found many applications across diverse fields including medicinal chemistry, catalysis as well as a precursor for graphene. One of the key structural features of GO is the presence of different kinds of defects. Molecular dynamics simulations with ReaxFF force fields have been widely used to model realistic representations of GO that include defects of various types. In these simulations, one can vary the extent and distribution of the defects by changing the initial O/C ratio. It is therefore very important to employ a proper measure of the defect density. Traditionally, the total number of non-graphitic carbon atoms have been employed to quantify the amount of defects. Our simulations suggest that this parameter may not be a good measure at low defect densities. Herein, we introduce a hitherto unexplored metric, relative area of the defects, to gauge the defect density. We show that this metric has desirable properties at both low and high defect densities. Additionally, we investigate the changes in the defect distribution and mechanical properties upon varying the size of the simulation cell.

Abstract: In this theoretical study, we examine the influence of dimensionality, size reduction, and heattransport direction on the phonon-drag contribution to the Seebeck coefficient of silicon nanostructures. Phonon-drag contribution arises from the momentum transfer between out-of-equilibrium phonon populations and charge carriers, and significantly enhances the thermoelectric coefficient. Our implementation of the phonon drag term accounts for the anisotropy of nanostructures such as thin films and nanowires through the boundary- and momentum-resolved phonon lifetime. Our approach also takes into acconout the spin-orbit coupling, which turns out to be crucial for hole transport. We reliably quantify the phonon drag contribution at various doping levels, temperatures, and nanostructure geometries for both electrons and holes in silicon nanstructures. Our results support the recent experimental findings, showing that a part of phonon drag contribution survives in 100 nm silicon nanostructures.

Abstract: Indentation tests are largely exploited in experiments to characterize the mechanical and fracture properties of the materials from the resulting crack patterns. This work proposes an efficient theoretical and computational framework, whose implementation is detailed for 2D axisymmetric and 3D geometries, to simulate indentation-induced cracking phenomena caused by non-conforming contacts with indenter profiles of arbitrary shape. The formulation hinges on the coupling of the MPJR (eMbedded Profile for Joint Roughness) interface finite elements which embed the indenter profile to solve the contact problem between non-planar bodies efficiently and the phase-field for brittle fracture to simulate crack evolution and nonlocal damage in the substrate. The novel framework is applied to predict cone-crack formation in the case of indentation tests with smooth spherical indenters, with validation against experimental data. Then, the methodology is employed for the very first time in the literature to assess the effect of surface roughness superimposed on the shape of the smooth spherical indenter. In terms of physical insights, numerical predictions quantify the dependencies of the critical load for crack nucleation and the crack radius on the amplitude of roughness in comparison with the behavior of smooth indenters. Again, the consistency with available experimental trends is noticed.

Abstract: Zinc phosphide (Zn3P2) has had a long history of scientific interest largely because of its potential for earth-abundant photovoltaics. To realize high-efficiency Zn3P2 solar cells, it is critical to understand and control point defects in this material. Using hybrid functional calculations, we assess the energetics and electronic behavior of intrinsic point defects and hydrogen impurities in Zn3P2. All intrinsic defects are found to act as compensating centers in p-type Zn3P2 and have deep levels in the band gap, except for zinc vacancies which are shallow acceptors and can act as a source of doping. Our work highlights that zinc vacancies rather than phosphorus interstitials are likely to be the main source of p-type doping in as-grown Zn3P2. We also show that Zn-poor and P-rich growth conditions, which are usually used for enhancing p-type conductivity of Zn3P2, will facilitate the formation of certain deep-level defects (P_Zn and P_i) which might be detrimental to solar cell efficiency. For hydrogen impurities, which are frequently present in the growth environment of Zn3P2, we study interstitial hydrogen and hydrogen complexes with vacancies. The results suggest small but beneficial effects of hydrogen on the electrical properties of Zn3P2.

Abstract: The Si/SiO$_2$ interface is populated by isolated trap states which modify its electronic properties. These traps are of critical interest for the development of semiconductor-based quantum sensors and computers, as well as nanoelectronic devices. Here, we study the electric susceptibility of the Si/SiO$_2$ interface with nm spatial resolution using frequency-modulated atomic force microscopy to measure a patterned dopant delta-layer buried 2 nm beneath the silicon native oxide interface. We show that surface charge organization timescales, which range from 1-150 ns, increase significantly around interfacial states. We conclude that dielectric loss under time-varying gate biases at MHz and sub-MHz frequencies in metal-insulator-semiconductor capacitor device architectures is highly spatially heterogeneous over nm length scales.