Materials Science (cond-mat.mtrl-sci)

Abstract: Phonon splitting of the longitudinal optical and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional (3D) polar materials, is essential for the formation of the 3D phonon polaritons. Theories predict that the LO-TO splitting will break down in two-dimensional (2D) polar systems, but direct experimental verification is still missing. Here, using monolayer hexagonal boron nitride (h-BN) as a prototypical example, we report the direct observation of the breakdown of LO-TO splitting and the finite slope of the LO phonons at the center of the Brillouin zone in 2D polar materials by inelastic electron scattering spectroscopy. Interestingly, the slope of the LO phonon in our measurements is lower than the theoretically predicted value for a freestanding monolayer due to the screening of the Cu foil substrate. This enables the phonon polaritons (PhPs) in monolayer h-BN/Cu foil to exhibit ultra-slow group velocity (~ 5 x 10^-6 c, c is the speed of light) and ultra-high confinement (~ 4000 times smaller wavelength than that of light). Our work reveals the universal law of the LO phonons in 2D polar materials and lays a physical foundation for future research on 2D PhPs.

Abstract: Defects play an important role in determining the type of carriers as well as on tuning the physical properties of layered materials. In this study, we have demonstrated that by varying the growth kinetics one can control the defects and can achieve electrons or holes dominated Bi2Te3 single crystals using modified Bridgman method. The correlation between structural defects and the type of dominant charge carriers in crystals are discussed using X-Ray diffraction and Hall resistivity. Electrons are found to be originating from Te vacancy type defects, while holes are manifested from predominant structural defects viz. Bi_Te antisite defects or interstitial Te atoms. We observe that the alteration of charger carriers from electrons to holes have enhanced magnetoresistance (MR) from 103% to 224%. The enhancement in MR emerges from 2D multichannel quantum coherent conduction mechanism.

Abstract: The discovery of (4x4) silicene formation on Ag(111) raised the question on whether silicene maintains its Dirac fermion character, similar to graphene, on a supporting substrate. Previous photoemission studies indicated that the {\pi}-band forms Dirac cones near the Fermi energy, while theoretical investigations found it shifted at deeper binding energy. By means of angle-resolved photoemission spectroscopy and density functional theory calculations we show instead that the {\pi}-symmetry states lose their local character and the Dirac cone fades out. The formation of an interface state of free-electron-like Ag origin is found to account for spectral features that were theoretically and experimentally attributed to silicene bands of {\pi}-character.

Abstract: We report a flexible multi-modal mechanics language model, MeLM, applied to solve various nonlinear forward and inverse problems, that can deal with a set of instructions, numbers and microstructure data. The framework is applied to various examples including bio-inspired hierarchical honeycomb design, carbon nanotube mechanics, and protein unfolding. In spite of the flexible nature of the model-which allows us to easily incorporate diverse materials, scales, and mechanical features-it performs well across disparate forward and inverse tasks. Based on an autoregressive attention-model, MeLM effectively represents a large multi-particle system consisting of hundreds of millions of neurons, where the interaction potentials are discovered through graph-forming self-attention mechanisms that are then used to identify relationships from emergent structures, while taking advantage of synergies discovered in the training data. We show that the model can solve complex degenerate mechanics design problems and determine novel material architectures across a range of hierarchical levels, providing an avenue for materials discovery and analysis. Looking beyond the demonstrations reported in this paper, we discuss other opportunities in applied mechanics and general considerations about the use of large language models in modeling, design, and analysis that can span a broad spectrum of material properties from mechanical, thermal, optical, to electronic.

Abstract: The van der Waals (vdWs) forces between monolayers has been a unique distinguishing feature of exfoliable materials since the first isolation of graphene. However, the vdWs interaction of exfoliable materials with their substrates and how this interface force influences their interaction with the environment is yet to be well understood.Here, we experimentally and theoretically unravel the role of vdWs forces between the recently rediscovered wide band gap p-type vdW semiconductor violet phosphorus (VP), with various substrates (including, SiO$_2$, mica, Si, Au) and quantify how VP stability in air and its interaction with its surroundings is influenced by the interface force.Using a combination of infrared nanoimaging and theoretical modeling we find the vdWs force at the interface to be a main factor that influences how VP interacts with its surroundings.In addition, the hydrophobicity of the substrate and the substrate surface roughness modify the vdWs force there by influencing VP stability. Our results could guide in the selection of substrates when vdW materials are prepared and more generally highlight the key role of interface force effects that could significantly alter physical properties of vdWs materials.

Abstract: Quaternary mixed-metal chalcohalides (Sn$_2$BCh$_2$X$_3$) are emerging as promising lead-free perovskite-inspired photovoltaic absorbers. Motivated by recent developments of a first Sn$_2$BCh$_2$X$_3$-based device, we used density functional theory to identify lead-free Sn$_2$BCh$_2$X$_3$ materials that are structurally and energetically stable within Cmcm, Cmc2$_1$ and P2$_1$/c space groups and have a band gap in the range of 0.7 to 2.0 eV to cover out- and indoor photovoltaic applications. A total of 27 Sn$_2$BCh$_2$X$_3$ materials were studied, including Sb, Bi, In for B-site, S, Se, Te for Ch-site and Cl, Br, I for X-site. We identified 12 materials with a direct band gap that meet our requirements, namely: Sn$_2$InS$_2$Br$_3$, Sn$_2$InS$_2$I$_3$, Sn$_2$InSe$_2$Cl$_3$, Sn$_2$InSe$_2$Br$_3$, Sn$_2$InTe$_2$Br$_3$, Sn$_2$InTe$_2$Cl$_3$, Sn$_2$SbS$_2$I$_3$, Sn$_2$SbSe$_2$Cl$_3$, Sn$_2$SbSe$_2$I$_3$, Sn$_2$SbTe$_2$Cl$_3$, Sn$_2$BiS$_2$I$_3$ and Sn$_2$BiTe$_2$Cl$_3$. A database scan reveals that 9 out of 12 are new compositions. For all 27 materials, P2$_1$/c is the thermodynamically preferred structure, followed by Cmc2$_1$. In Cmcm and Cmc2$_1$ mainly direct gaps occur, whereas mostly indirects in P2$_1$/c. To open up the possibility of band gap tuning in the future, we identified 12 promising Sn$_2$B$_{1-{a}}$B$'_{a}$Ch$_{2-{b}}$Ch$'_{b}$X$_{3-{c}}$X$_{c}$ alloys which fulfill our requirements and additional 69 materials by combining direct and indirect band gap compounds.

Abstract: Crack front waves (FWs) are dynamic objects that propagate along moving crack fronts in 3D materials. We study FW dynamics in the framework of a 3D phase-field framework that features a rate-dependent fracture energy $\Gamma(v)$ ($v$ is the crack propagation velocity) and intrinsic lengthscales, and quantitatively reproduces the high-speed oscillatory instability in the quasi-2D limit. We show that in-plane FWs feature a rather weak time dependence, with decay rate that increases with $d\Gamma(v)/dv\!>\!0$, and largely retain their properties upon FW-FW interactions, similarly to a related experimentally-observed solitonic behavior. Driving in-plane FWs into the nonlinear regime, we find that they propagate slower than predicted by a linear perturbation theory. Finally, by introducing small out-of-plane symmetry-breaking perturbations, coupled in- and out-of-plane FWs are excited, but the out-of-plane component decays under pure tensile loading. Yet, including a small anti-plane loading component gives rise to persistent coupled in- and out-of-plane FWs.

Abstract: Relaxor ferroelectrics are a class of materials that are widely perceived as deriving their exotic properties from structural heterogeneities. Understanding the microscopic origin of the superior electromechanical response requires knowledge not only concerning the formation of polar nanodomains (PNDs) built from individual atoms but more importantly the spatial distribution of PNDs over longer distances. The mesoscale PND arrangement is shaped by the interactions between these domains and, in turn, dictates the electric-field driven PND response directly relevant to the macroscopic material properties. Here, we show the emergence of mesoscale lattice order that we name "polar laminates" in the canonical relaxor ferroelectric 0.68PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-0.32PbTiO$_{3}$ (PMN-0.32PT) using X-ray coherent nano-diffraction. These laminates are nematic with a size of ~350 nm and arise from the staggered arrangement of ~13 nm monoclinic PNDs along the <110> of the pseudocubic lattice. The spatial distribution of c-axis strain is directly correlated with the tilting of the PNDs and is most prominent between the laminates. Further operando nano-diffraction studies demonstrate heterogeneous electric-field-driven responses. The most active regions tend to reside inside the laminates while the spatial pinning centers are between the laminates. This observation reveals the hierarchical assembly of lattice order as a novel form of electron and lattice self-organization in heterogenous materials and establishes the role of such mesoscale spatial arrangement in connecting the nanoscale heterogeneity and macroscopic material properties. These findings provide a guiding principle for the design and optimization of future relaxors and may shed light on the existence of similar behavior in a wide range of quantum and functional materials.

Abstract: The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn$_{1-x}$Fe$_x$NiSi$_{0.95}$Al$_{0.05}$, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni$_2$In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.

Abstract: 2D materials present an interesting platform for device designs. However, oxidation can drastically change the system's properties, which need to be accounted for. Through {\it ab initio} calculations, we investigated freestanding and SiC-supported As, Sb, and Bi mono-elemental layers. The oxidation process occurs through an O$_2$ spin-state transition, accounted for within the Landau-Zener transition. Additionally, we have investigated the oxidation barriers and the role of spin-orbit coupling. Our calculations pointed out that the presence of SiC substrate reduces the oxidation time scale compared to a freestanding monolayer. We have extracted the energy barrier transition, compatible with our spin-transition analysis. Besides, spin-orbit coupling is relevant to the oxidation mechanisms and alters time scales. The energy barriers decrease as the pnictogen changes from As to Sb to Bi for the freestanding systems, while for SiC-supported, they increase across the pnictogen family. Our computed energy barriers confirm the enhanced robustness against oxidation for the SiC-supported systems.

Abstract: Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact. Here, we measure the chemical composition in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices, revealing a one-to-one correlation between the distribution of oxygen vacancies and the electric and magnetic properties. Using atom probe tomography, we observe oxygen vacancies arranging in a layered three-dimensional structure with a local density on the order of 10$^{14}$ cm$^{-2}$, congruent with the formula-unit-thick ferrimagnetic LuFe$_2$O$_4$ layers. The vacancy order is promoted by the locally reduced formation energy and plays a key role in stabilizing the ferroelectric domains and ferrimagnetism in the LuFeO$_3$ and LuFe$_2$O$_4$ layers, respectively. The results demonstrate the importance of oxygen vacancies for the room-temperature multiferroicity in this system and establish an approach for quantifying the oxygen defects with atomic-scale precision in 3D, giving new opportunities for deterministic defect-enabled property control in oxide heterostructures.

Abstract: Despite ongoing efforts aimed at increasing energy density in all-solid-state-batteries, the optimal composite cathode morphology, which requires minimal volume change, small void development, and good interfacial contact, remains a significant concern within the community. In this work, we focus on the theoretical investigation of the above-mentioned mechanical defects in the composite cathode during electrochemical cycling. It is demonstrated that these mechanical defects are highly dependent on the SE material properties, the external stack pressure and the cathode active material (CAM) loading. The following conclusions are highlighted in this study: (1) Higher CAM loading (>50 vol. %) causes an increase in mechanical defects, including large cathode volume change (>5%), contact loss (50%) and porosity (>1%). (2) High external stack pressure up to 7MPa reduces mechanical defects while preventing internal fracture in the cathode. (3) Soft SE materials with small Youngs modulus (<10GPa) and low hardness (<2GPa) can significantly minimize these mechanical defects during cycling. (4) A design strategy is proposed for high CAM loading with minimal mechanical defects when different SE materials are utilized in the composite cathode, including oxide-type SE, sulfide-type SE, and halide-type SE. The research provides specific guidelines to optimize the composite cathode in terms of mechanical properties. These guidelines broaden the design approach towards improving the performance of SSB, by highlighting the importance of considering the mechanical properties of battery materials.

Abstract: In this paper a geometric field theory of dislocation dynamics and finite plasticity in single crystals is formulated. Starting from the multiplicative decomposition of the deformation gradient into elastic and plastic parts, we use Cartan's moving frames to describe the distorted lattice structure via differential $1$-forms. In this theory the primary fields are the dislocation fields, defined as a collection of differential $2$-forms. The defect content of the lattice structure is then determined by the superposition of the dislocation fields. All these differential forms constitute the internal variables of the system. The evolution equations for the internal variables are derived starting from the kinematics of the dislocation 2-forms, which is expressed using the notions of flow and of Lie derivative. This is then coupled with the rate of change of the lattice structure through Orowan's equation. The governing equations are derived using a two-potential approach to a variational principle of the Lagrange-d'Alembert type. As in the nonlinear setting the lattice structure evolves in time, the dynamics of dislocations on slip systems is formulated by enforcing some constraints in the variational principle. Using the Lagrange multipliers associated with these constraints, one obtains the forces that the lattice exerts on the dislocation fields in order to keep them gliding on some given crystallographic planes. Moreover, the geometric formulation allows one to investigate the integrability -- and hence the existence -- of glide surfaces, and how the glide motion is affected by it. Lastly, a linear theory for small dislocation densities is derived, allowing one to identify the nonlinear effects that do not appear in the linearized setting.

Abstract: For a given class of materials, universal displacements are those displacements that can be maintained for any member of the class by applying only boundary tractions. In this paper we study universal displacements in compressible anisotropic linear elastic solids reinforced by a family of inextensible fibers. For each symmetry class and for a uniform distribution of straight fibers respecting the corresponding symmetry we characterize the respective universal displacements. A goal of this paper is to investigate how an internal constraint affects the set of universal displacements. We have observed that other than the triclinic and cubic solids in the other five classes (a fiber-reinforced solid with straight fibers cannot be isotropic) the presence of inextensible fibers enlarges the set of universal displacements.