The mechanical strength properties of carbon fibers are generally thought to be correlated with the presence of underlying atomic-level defects. These defects serve to break the underlying translational symmetry of the graphitic or graphene subunits of the fiber construction, resulting in the emergence of new Raman-active spectral features. However, historical attempts to classify the precise origin of defect contributions to the Raman spectra have been challenging because of indistinct and overlapping features in the carbon fiber Raman spectra. Further, while substantial research exists on high-temperature exposure in inert atmospheres for carbon fiber composites, comparatively less addresses microscale behaviors and Raman spectral alterations to monofilament carbon fibers exposed to high-temperature atmospheric environments. Here, we report Raman spectral responses of nine commercially available high-performance, polyacrylonitrile-based carbon fibers exposed to various atmospheric heat treatments. We introduce a model-independent characterization method, the integrated absolute difference, to quantify the spectral responses to heat treatment of different fiber modulus classes. We combine this new method with a newly reported strategy to standardize spectral fitting for carbon fibers. With this approach, we show that atomic-scale defects in the underlying fiber microstructure manifest in measurably distinct manners and have distinct responses to thermal perturbation. These combined approaches may lay the foundation for disentangling contributions from specific defects in the Raman spectra of carbon fibers.

The recent discovery of axion states in materials such as antiferromagnetic topological insulators has boosted investigations of the magnetoelectric response in topological insulators and their promise towards realizing dissipationless topological electronics. In this paper, we develop a tight-binding methodology to explore the emergence of axion states in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in proximity to magnetic insulators on the top and bottom surfaces. The topological protection of the surface states is lifted by a time-reversal-breaking perturbation due to the proximity of a magnetic insulator, and a gap is opened on the surfaces, giving rise to half-quantized Hall conductance and a zero Hall plateau—evidencing an axion insulator state. We developed a real-space tight-binding Hamiltonian for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using first-principles data. Transport properties of the system were obtained within the Landauer-Büttiker formalism, and we discuss the creation of axion states through Hall conductance and a zero Hall plateau at the surfaces, as a function of proximitized magnetization and corresponding potentials at the surfaces, as well as the thickness of the topological insulator.

Materials exhibiting metal-insulator transitions (MITs) are proposed platforms for next-generation low-power electronics. Many of these materials exhibit strong coupling between the electronic and lattice degrees of freedom, which makes them ideal systems to examine the interplay between lattice dynamics, electronic structure, and magnetic order. The rutile structure, featuring edge-connected octahedral chains along its $c$ axis, permits metal-metal interactions that can induce metal-insulator transitions. Although the MITs in rutile-structured compounds have been thoroughly studied, the derivative trirutile phase, which accommodates similar metal-metal interactions, has yet to be examined in the context of MITs. Here we use density functional theory calculations to investigate a carrier-driven MIT in trirutile ${\mathrm{MgTa}}_2{\mathrm{O}}_6$, a $d^0$ insulator with a suitable axial ratio. Our calculations suggest the existence of four distinct phases in ${\mathrm{MgTa}}_2{\mathrm{O}}_6$ with increasing electron concentration: nonmagnetic insulator, ferromagnetic (FM) metal, FM half-metal, and nonmagnetic insulator. We explain how the resulting electronic phases arise from changes in atomic structure with increasing carrier density. Our results indicate that trirutile oxides may be a promising materials class for which to access and functionalize MITs.

We study nonequilibrium roughening during compressive plastic flow of initially flat ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$ metallic glass using large-scale molecular dynamics simulations. Roughness emerges at atomically flat interfaces beyond the yield point of the glass. A self-affine rough topography is imprinted at yield and is reinforced during subsequent deformation. The imprinted topographies have Hurst exponents that decrease with increasing strain rate and temperature. After yield, the root-mean-square roughness amplitude grows as the square root of the applied strain with a prefactor that also drops with increasing strain rate and temperature. Our calculations reveal the emergence of spatial power-law correlations from homogeneous samples during plastic flow with exponents that depend on the rate of deformation and the temperature. The results have implications for interpreting and engineering roughness profiles.

Alloying has long been used to control the properties of crystalline materials. However, the microscopic details of the effects of adding impurities remains poorly understood, as directly imaging these processes is challenging in atomic systems. Here we study how crystallization and grain growth are impacted by impurities in a quenched monolayer of a colloidal polycrystal. The rates of crystallization are unaffected by the impurity concentration and well described by the Johnson-Mehl-Avrami-Kolmogorov model. However, with an increased impurity concentration, more of the system becomes frustrated and does not crystallize. The rate of grain growth is significantly slowed down due to grain boundary pinning. Nevertheless, the evolution of the polycrystalline structure is well described by a single correlation length, and dynamic scaling applies up to relatively large impurity concentrations. Finally, we develop a simple model accounting for both crystallization and grain growth to describe the time evolution of the correlation length, and we find good agreement with our experimental data at all impurity concentrations. Our results shed new light on the interplay between crystallization, grain growth and the presence of impurities in impurity-doped polycrystalline systems.

Wood deformation, in particular when subject to compression, exhibits scale-free avalanche-like behavior as well as structure-dependent localization of deformation. We have taken three-dimensional (3D) x-ray tomographs during compression with constant stress rate loading. Using digital volume correlation, we obtain the local total strain during the experiment and compare it to the global strain and acoustic emission. The wood cells collapse layer by layer throughout the sample starting from the softest parts, i.e., the spring wood. As the damage progresses, more and more of the softwood layers throughout the sample collapse, which indicates damage spreading instead of localization. In 3D, one can see a fat-tailed local strain rate distribution, indicating that inside the softwood layers, the damage occurs in localized spots. The observed log-normal strain distribution is in agreement with this view of the development of independent local collapses or irreversible deformation events. A key feature in the mechanical behavior of wood is then in the complex interaction of localized deformation between or among the annual rings.

We demonstrate the gradual shift from scale-free intermittent microplasticity to a scale-dependent behavior via the introduction of a variety of microstructural features within the Al-Cu binary alloy system. As long as the obstacles to dislocation motion remain shearable, the statistics of intermittent microplasticity has fat-tailed contributions. The introduction of incoherent precipitates leads to a complete transition from scale-free power-law scaling to an exponential and scale-dependent distribution. These results demonstrate how non-Gaussian interactions survive across different microstructures and further suggest that characteristic microstructural length scales and obstacle pinning-strengths are of secondary importance for the intermittency statistics, as long as dislocations can shear their local environment.

The interaction between accumulated spins on the surface of a heavy metal (HM) and the magnetization of an adjacent magnetic material leads to various spin phenomena, such as spin-orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low charge-to-spin conversion efficiency of the HM. Authors of recent studies have suggested that topological insulators (TIs) are promising candidates for device applications due to their potentially higher charge-to-spin conversion efficiency. Here, we report a multifaceted study of a bilayer structure consisting of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Y}}_3{\mathrm{Fe}}_5\mathrm{O}$ (YIG) and demonstrate an approach based on angle-dependent magnetoresistance (ADMR) measurements to determine the effective charge-to-spin conversion efficiency in TIs. Our ferromagnetic resonance measurements demonstrate efficient spin pumping from YIG to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, which is further confirmed by detection of an electromotive force generated in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ via spin-to-charge conversion. Our ADMR measurements show that the interfacial spin diffusion can significantly affect the charge transport in a way like the SHMR effect and provide an estimate of the charge-to-spin conversion efficiency in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ of $\sim 0.1–0.4$. Neglecting to account for the large out-of-plane magnetoresistance of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ results in a fivefold overestimate of the charge-to-spin conversion efficiency.

Topological insulator spin-polarized surface states are attractive for spintronic applications, in particular for spin-charge current interconversion, where extremely high conversion efficiencies are predicted. However, the contribution of topologically trivial bulk states is often disregarded although it may play a crucial role in the experimental results and extracted conversion efficiencies. The presence of bulk states at the Fermi level can be avoided by increasing the gap using the confinement effect appearing as the film thickness is reduced. We address this topic by growing ${\mathrm{Bi}}_{1-\mathrm{x}}{\mathrm{Sb}}_x$ thin films (2.5–15 nm) by molecular beam epitaxy on InSb, ${\mathrm{BaF}}_2$, and Si substrates. The surface electronic band structure is studied by angle-resolved photoemission spectroscopy. Two ${\mathrm{Bi}}_{1-\mathrm{x}}{\mathrm{Sb}}_x$ surface states are observed in the gap for several Sb concentrations and thicknesses, across the topological insulator phase, scanning $x$ between 7% and 30%. Tight-binding calculations of the surface states are in good agreement with the experiments, revealing their polarized nature. Surface states are still present at the Γ point for the thinnest films (2.5 nm), suggesting highly confined polarized states at the surface.

We propose two different two-dimensional topological Dirac materials, planar ${\mathrm{PtN}}_4{\mathrm{C}}_2$ and ${\mathrm{Pt}}_2{\mathrm{N}}_8{\mathrm{C}}_6$, which exhibit graphenelike electronic structures with linearly dispersive Dirac-cone states exactly at the Fermi level. Moreover, the Dirac cone is anisotropic, resulting in anisotropic Fermi velocities and making it possible to realize orientation-dependent quantum devices. Using first-principles electronic structure calculations, we have systemically studied the structural, electronic, and topological properties. We find that spin-orbit coupling opens a sizable topological band gap so that the materials can be classified as quantum spin Hall insulators as well as quantum valley Hall insulators. Helical edge states that reside in the insulating band gap connecting the bulk conduction and valence bands are observed. Our work not only expands the Dirac-cone material family, but also provides another avenue to search for more two-dimensional topological quantum spin and valley Hall insulators.

${\mathrm{Mn}}_{1-x}{\mathrm{Ga}}_x$ (MnGa) with the $L{1}_0$ structure is a ferromagnetic material with strong perpendicular magnetocrystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L{1}_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x\text{−}{k}_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2}\times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

${\mathrm{VO}}_2$, of interest for decades due to both its phenomenology and its potential applications, has a monoclinic distortion of the rutile crystal structure at ambient temperature that is coupled to its metal-insulator transition. In contrast, ${\mathrm{RuO}}_2$ has three electrons more per formula unit, is a metallic conductor, and has an undistorted rutile structure. Here, we report a systematic study of Ru-doped ${\mathrm{VO}}_2$ (${\mathrm{V}}_{1\text{−}x}{\mathrm{Ru}}_x{\mathrm{O}}_2, 0.01\le x\le 0.9)$, generally characterizing its crystal structure, magnetic and electronic properties, and heat capacity. The composition-dependent Wilson ratio is determined. We find that an unusually high Ru doping value (80%, $x=0.8$) is required to achieve a metallic state in ${\mathrm{V}}_{1\text{−}x}{\mathrm{Ru}}_x{\mathrm{O}}_2$. No superconductivity was observed down to 0.1 K in the metallic materials. We propose a possible understanding for how the insulating state can exist in ${\mathrm{V}}_{1\text{−}x}{\mathrm{Ru}}_x{\mathrm{O}}_2$ at high Ru contents.

Nanostructured tungsten has been proposed as a promising option for plasma facing materials in future fusion reactors, because compared to conventional tungsten it shows advantages such as a better radiation resistance and, in particular, a retardation of tungsten-fuzz growth. Besides these aspects, the sputtering yield of nanostructured tungsten under ion bombardment is of interest, since it would affect the atomic density of tungsten emitted into the fusion plasma, which leads to radiative heat losses. In this work, we present a multiscale approach for investigating the sputtering yield of nanocolumnar tungsten surfaces under 1 keV and 2 keV ${\mathrm{Ar}}^{+}$ irradiation. Our results cover experiments and also computational simulations, which operate either on the basis of the binary collision approximation and ray tracing or use a full molecular dynamics implementation. In our studied case, both computational approaches can predict the sputtering yield of nanocolumnar tungsten surfaces very well. In comparison to flat W, we observe a much reduced dependence on the ion incidence angle, similar as reported for conventional rough surfaces in literature. However, an additional global reduction of the sputtering yield was identified, which can be attributed to geometrical redeposition effects between the separated nanocolumns. These results support the applicability of nanocolumnar tungsten as a first wall coating.

We have developed a simulational methodology for calculating the nanoscale frequency-dependent complex dielectric function of a wide range of materials using a combination of Langevin dynamics and Metropolis Monte Carlo methods. The premise of such a scheme is to use the atomistic structure of materials and designate appropriate interatomic interactions as well as internal field couplings to accommodate correlated materials. We validate our model by recreating the dielectric functions of well-studied representative materials including insulator ${\text{SiO}}_2$ thin film that has phonon resonances in the midinfrared, and semiconductor monolayer ${\text{MoS}}_2$ that exhibits strong excitonic resonances in the visible frequency range. To further showcase the capability of the model in calculating nanoscale dielectric modulation of complex materials, we simulate the dielectric response of ${\text{SmNiO}}_3$, a correlated perovskite oxide, with respect to differing levels of hydrogenation, oxygen vacancy formation, and external fields. This is accomplished by inserting and tracking the movement of dopants at the nanoscale using Metropolis Monte Carlo methods that explicitly include interactions with each other as well as external fields. Simulated nanoscale dielectric spectra agree very well with high-resolution near-field experimental measurements based on scattering type scanning near-field microscopy. We find that this modeling scheme carries a broad utility in describing and predicting the nanoscale dielectric behavior of a broad range of materials exposed to changing local environments.

Accurate prediction of vacancy migration energy barriers, $\mathrm{\Delta }{E}_{\text{a}}$, in multicomponent alloys is extremely challenging yet critical for the development of diffusional transformation kinetics needed to model alloy behavior in many technological applications. In this paper, results from $\mathrm{\Delta }{E}_{\text{a}}$ and the energy driving force $\mathrm{\Delta }E$ of many $(>1000)$ vacancy migration events calculated using density functional theory and the nudged elastic band method show large changes $(\sim 1\mathrm{eV})$ of $\mathrm{\Delta }{E}_{\text{a}}$ in different local chemical environments of the model face-centered cubic (fcc) Al-Mg-Zn alloys. Due to local lattice distortion effects induced by solute atoms (such as Mg) with different sizes than the matrix element (Al), the changes of $\mathrm{\Delta }{E}_{\text{a}}$ for one type of migrating atom originate primarily from fluctuations of $\mathrm{\Delta }{e}_{\text{a}}\equiv \mathrm{\Delta }{E}_{\text{a}}-\frac{1}{2}\mathrm{\Delta }E$ (instead of $\frac{1}{2}\mathrm{\Delta }E$ according to the widely used kinetic Ising model). To understand these fluctuations, a quartic function of the reaction coordinate is shown to accurately describe the energy landscape of the minimum energy path (MEP) for each vacancy migration event studied in this paper. Analyses of the quartic function show that $\mathrm{\Delta }{e}_{\text{a}}$ can be approximated with $\mathrm{\Delta }{e}_{\text{a}}\approx \alpha k_f{D}^2$, where $\alpha \sim 0.022$ a constant value for all types of migrating atoms in the Al lattice. Here $D$ is the distance of a migrating atom between two adjacent equilibrium positions and $k_f$ is the average vibration spring constant of this atom at these two equilibrium positions. $k_f$ and $D$ quantitatively describe the lattice distortion effects on the curvatures and locations of the MEP at its initial and final states in different local chemical environments. We also used the local lattice occupations as inputs to train surrogate models to predict the coefficients of the quartic function, which accurately and efficiently output both $\mathrm{\Delta }{E}_{\text{a}}$ and $\mathrm{\Delta }E$ as the necessary inputs for the mesoscale studies of diffusional transformation in Al-Mg-Zn alloys.

A mathematical relationship between magnetic material constants is proposed for which highly reversible microstructures form during phase transformations. The microstructures formed during paramagnetic-to-ferromagnetic phase transformation are systematically studied, and specific combinations of material constants—namely, magnetocrystalline anisotropy ${\kappa }_1,{\kappa }_2$ and magnetostriction ${\lambda }_{100},{\lambda }_{111}$—for which stress-free interfaces with divergence-free magnetization can form are identified. The microstructural analysis shows that magnetic materials with ${\kappa }_1\le 0$, ${\kappa }_2\ge \frac{9|{\kappa }_1|}{4}$, and ${\lambda }_{111}=-\frac{{\lambda }_{100}}{3}$ form exactly compatible and divergence-free phase boundaries. Furthermore, the analysis shows that, consistent with experimental observation, magnetic alloys that undergo cubic-to-monoclinic-II transformation have multiple solutions satisfying the high-reversibility condition and are suitable candidates for alloy development. Broadly, the results could guide the design of magnetic materials with low fatigue and hysteresis for energy applications.

The family of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = K, Rb, Cs) kagome metals exhibits charge density wave (CDW) order, proposed to be chiral, followed by a lower temperature superconducting state. Recent studies have proposed the importance of band structure saddle points proximal to the Fermi energy in governing these two transitions. Here we show the effects of hole doping achieved via chemical substitution of Sn for Sb on the CDW and superconducting states in both ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ and generate a phase diagram. Hole doping lifts the $\mathrm{\Gamma }$ pocket and van Hove singularities toward $E_F$ causing the superconducting $T_C$ in both systems to increase to about 4.5 K, while rapidly suppressing the CDW state.