We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ was epitaxially grown on a van der Waals ferromagnet ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$ (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in six quintuple-layer (6QL) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in its 2QL counterpart. Intriguingly, this gap is much larger than those for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.

A study of the layered triangular lattice materials ${\mathrm{K}}_2\mathrm{Co}{\left({\mathrm{SeO}}_3\right)}_2$ and ${\mathrm{Rb}}_2\mathrm{Co}{\left({\mathrm{SeO}}_3\right)}_2$ is reported. These isostructural compounds crystallize in the trigonal space group $R-3m$ (No. 166). The magnetic $\mathrm{C}{\mathrm{o}}^{2+}$ ions form a spin-1/2 triangular lattice and display strongly frustrated magnetism. We find no evidence in the susceptibility or heat capacity for in-plane ordering of the spins; rather the in-plane inverse susceptibility is typical of that seen for frustrated magnets. At around 10 K, a ferromagnetic component appears in the magnetic susceptibility perpendicular to the triangular planes. The heat capacity at low applied fields does not show sharp features, but in larger applied fields a sharp ferromagneticlike transition appears. Field- and direction-dependent magnetization measurements at 2 K indicate that both materials have a magnetic easy axis perpendicular to the triangular planes. The magnetic properties are sensitive to the field applied perpendicular to the triangular planes, which induces a metamagnetization transition at 0.75 T. Spin polarization develops along with the increasing out-of-plane field, resulting in a saturated ferromagnetic state at high fields. In contrast, the magnetic properties are resistant to the in-plane field. Our results suggest that these materials are embodiments of Ising spins on triangular lattices with easy-axis anisotropy.

The effects of Ni doping in Eu(${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{)}_{2-y}{\mathrm{As}}_2$ single crystals with $x=0$ to 1 grown out of self-flux are investigated via crystallographic, electronic transport, magnetic, and thermal measurements. All compositions adopt the body-centered-tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure with space group $I4/mmm$. We also find 3%–4% of randomly distributed vacancies on the Co/Ni site. Anisotropic magnetic susceptibility ${\chi }_{\alpha }(\alpha =ab,c)$ data versus temperature $T$ show clear signatures of an antiferromagnetic (AFM) $c$-axis helix structure associated with the ${\mathrm{Eu}}^{+2}$ spins $\frac{7}{2}$ for $x=0$ and 1 as previously reported. The ${\chi }_{\alpha }\left(T\right)$ data for $x=0.03$ and 0.10 suggest an anomalous $2q$ magnetic structure containing two helix axes along the $c$ axis and in the $ab$ plane, respectively, whereas for $x=0.75$ and 0.82 a $c$-axis helix is inferred as previously found for $x=$ 0 and 1. At intermediate compositions $x=0.2$, 0.32, 0.42, 0.54, and 0.65, a magnetic structure with a large ferromagnetic (FM) $c$-axis component is found from magnetization versus field isotherms, suggested to be an incommensurate FM $c$-axis cone structure associated with the Eu spins, which consists of both AFM and FM components. In addition, the $\chi \left(T\right)$ and heat capacity $C_{\mathrm{p}}\left(T\right)$ data for $x=0.2$–0.65 indicate the occurrence of itinerant FM order associated with the Co/Ni atoms with Curie temperatures from 60 to 25 K, respectively. Electrical resistivity $\rho \left(T\right)$ measurements indicate metallic character for all compositions with abrupt increases in slope on cooling below the Eu AFM transition temperatures. In addition to this panoply of magnetic transitions, ${}^{151}\mathrm{Eu}$ Mössbauer measurements indicate that ordering of the Eu moments proceeds via an incommensurate sine amplitude-modulated structure with additional transition temperatures associated with this effect.

Information regarding precipitate shapes is critical for estimating material parameters. Hence, we considered estimating a region of material parameter space in which a computational model produces precipitates having shapes similar to those observed in the experimental images. This region, called the lower-error region (LER), reflects intrinsic information of the material contained in the precipitate shapes. However, the computational cost of LER estimation can be high because the accurate computation of the model is required many times to better explore parameters. To overcome this difficulty, we used a Gaussian-process-based multifidelity modeling, in which training data can be sampled from multiple computations with different accuracy levels (fidelity). Lower-fidelity samples may have lower accuracy, but the computational cost is lower than that for higher-fidelity samples. Our proposed sampling procedure iteratively determines the most cost-effective pair of a point and a fidelity level for enhancing the accuracy of LER estimation. We demonstrated the efficiency of our method through estimation of the interface energy and lattice mismatch between $\mathrm{Mg}{\mathrm{Zn}}_2$ and $\alpha -\mathrm{Mg}$ phases in an Mg-based alloy. The results showed that the sampling cost required to obtain accurate LER estimation could be drastically reduced.

The heat capacities of nanocrystalline ${\mathrm{Ni}}_3\mathrm{Fe}$ and control materials with larger crystallites were measured from $0.4–300\mathrm{K}$. The heat capacities were integrated to obtain the enthalpy, entropy, and Gibbs free energy and to quantify how these thermodynamic functions are altered by nanocrystallinity. From the phonon density of states (DOS) measured by inelastic neutron scattering, we find that the Gibbs free energy is dominated by phonons and that the larger heat capacity of the nanomaterial below 100 K is attributable to its enhanced phonon DOS at low energies. Besides electronic and magnetic contributions, the nanocrystalline material has an additional contribution at higher temperatures, consistent with phonon anharmonicity. The nanocrystalline material shows a stronger increase with temperature of both the enthalpy and entropy compared to the bulk sample. Its entropy exceeds that of the bulk material by 0.4 $k_{\mathrm{B}}$/atom at 300 K. This is insufficient to overcome the enthalpy of grain boundaries and defects in the nanocrystalline material, making it thermodynamically unstable with respect to the bulk control material.

Density functional theory (DFT) approaches have been ubiquitously used to predict topological order and nontrivial band crossings in real materials, like Dirac, Weyl semimetals, and so on. However, the use of less accurate exchange-correlation functional often yields a false prediction of nontrivial band order leading to misguide the experimental judgment about such materials. Using relatively more accurate hybrid functional exchange correlation, we explore a set of (already) experimentally synthesized materials (crystallizing in space group $P{6}_3/mmc$). Our calculations based on more accurate functional helps to correct various previous predictions for this material class. Based on point group symmetry analysis and ab initio calculations, we systematically show how lattice symmetry breaking via alloy engineering manifests different fermionic behavior, namely, Dirac, triple point, and Weyl in a single material. Out of various compounds, $X\mathrm{AgBi}$ ($X$=Ba,Sr) turns out to be two ideal candidates, in which the topological nodal point lies very close to the Fermi level, within minimal/no extra Fermi pocket. We further studied the surface states and Fermi arc topology on the surface of Dirac, triple point, and Weyl semimetallic phases of BaAgBi. We firmly believe that, while the crystal symmetry is essential to protect the band crossings, the use of accurate exchange correlation functional in any DFT calculation is an important necessity for the correct prediction of band order which can be trusted and explored in future experiments.

We report the discovery of a trigonal polymorph of the prospective Li-ion battery material ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ and its synthesis in bulk, single-crystal form. Crystal growth of trigonal ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ is strongly dependent upon the quality of a polycrystalline $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$ precursor consumed in the synthesis process. The crystal structure of the trigonal phase is composed of ordered honeycomb layers of $\mathrm{Li}{\mathrm{O}}_6$ and $\mathrm{Mn}{\mathrm{O}}_6$ octahedra segregated by layers of $\mathrm{Li}{\mathrm{O}}_6$ octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of $2.47\pm 0.11$ eV and a series of charge excitations that are well explained by the expected ${\mathrm{Mn}}^{4+}3d^3$ valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.

I study dynamical instabilities in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability.

Spin-polarized scanning tunneling microscopy is used to identify the magnetic state of different thin films on a Re(0001) substrate, which becomes superconducting below 1.7 K. All magnetic films contain an Fe/Ir interface, which is known to facilitate the emergence of noncollinear magnetic order. For Fe monolayers on ultrathin Ir films of different thicknesses we find several different atomic-scale magnetic states. For Pd/Fe bilayers we find nanoscale spin spirals as magnetic ground states for Ir thicknesses of three and four atomic layers. In applied magnetic fields skyrmions emerge, and in remanence nontrivial magnetic textures survive. This demonstrates the possibility to prepare skryrmion-hosting magnetic films on superconducting substrates.

Spin coating yields thin films of non-equilibrated polymers, with severe consequences, observable by the naked eye, in creep experiments on molten filamentlike samples derived from these films. While filaments from annealed films showed the expected monotonic elongation, filaments made from as-cast films initially showed the opposite behavior: They contracted and thereby lifted loads up to a thousand times the weight of the filament over a distance of almost up to $40\%$ its initial length. We discuss the molecular origin of such large macroscopic manifestations of deviations from equilibrium in terms of reduced conformational entropy.

We present the significant spin-driven dielectric anomaly ($\sim 40\%$ drop) and magnetoelectric coupling near the magnetic ordering temperature in single crystal ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. By combining neutron and x-ray single crystal diffraction techniques, we unambiguously determined its magnetic symmetry and studied the structural phase transition at $T_S$ = 70 K. The temperature-dependent static dielectric constant is strongly anisotropic, rendering two dielectric anomalies along the $a$ axis in the hexagonal lattice with the first one coupled to the magnetic ordering around $T_N$ = 97 K and the second one accompanying with a first-order structural transition around $T_S$ = 70 K. Below $T_N$, we found that the anomalous dielectric constant is practically proportional to the square of the magnetic moment from neutron diffraction data, indicating that the exchange striction is likely responsible for the strong spin-lattice coupling. Magnetic-field-induced magnetoelectric coupling was observed and is compatible with the determined magnetic structure that is characteristic of antiferromagnetically arranged ferromagnetic chains in the honeycomb plane. We propose that such magnetic symmetry should be immune to external magnetic fields to some extent favored by the freedom of rotation of moments in the honeycomb plane, laying out a promising system to control the magnetoelectric properties by magnetic fields.

In this work, we investigate the magnetic structures of ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_2\mathrm{Al}{\mathrm{B}}_2$ solid-solution quaternaries in the $x=0$ to 1 range using x-ray and neutron diffraction, magnetization measurements, and mean-field theory calculations. While ${\mathrm{Fe}}_2\mathrm{Al}{\mathrm{B}}_2$ and ${\mathrm{Mn}}_2\mathrm{Al}{\mathrm{B}}_2$ are known to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively, herein we focused on the magnetic structure of their solid solutions, which is not well understood. The FM ground state of ${\mathrm{Fe}}_2\mathrm{Al}{\mathrm{B}}_2$ becomes a canted AFM at $x\approx 0.2$, with a monotonically diminishing FM component until $x\approx 0.5$. The FM transition temperature ($T_{\mathrm{C}}$) decreases linearly with increasing $x$. These changes in magnetic moments and structures are reflected in anomalous expansions of the lattice parameters, indicating a magnetoelastic coupling. Lastly, the magnetocaloric properties of the solid solutions were explored. For $x=0.2$ the isothermal entropy change is smaller by 30% than it is for ${\mathrm{Fe}}_2\mathrm{Al}{\mathrm{B}}_2$, while the relative cooling power is larger by 6%, due to broadening of the temperature range of the transition.

We apply first-principles calculations to metastable B-doped Fe-Ni compounds to compute their magnetic properties. We focus on crystal structures with tetragonal and orthorhombic lattices. Boron atoms, doped at interstitial sites, help in stabilizing noncubic Fe-Ni structures. At the same time, the dopants improve the magnetic properties, such as magnetocrystalline anisotropy, compared to FeNi alloys in the tetragonal $L{1}_0$-ordered structures. The calculated magnetocrystalline anisotropy constants of $1–2 \mathrm{MJ}/{\mathrm{m}}^3$ along with sufficient magnetic polarization saturation $(\ge 1 \mathrm{T})$ show the potential of our metastable Fe-Ni-B compounds for replacing rare-earth-based magnets in permanent-magnet applications.

We utilize coarse-grained molecular-dynamics simulations to probe the influence of grafting on the conformation and aggregation of methylcellulose chains in water, inspired by the recent experiments on solutions of methylcellulose (MC) chains grafted with polyethylene glycol (PEG) that showed inhibition of methylcellulose fibril formation at high PEG-grafting densities [S. Morozova et al., Macromolecules (Washington, DC, U. S.) 51, 9413 (2018)]. These simulations reveal three features of the grafted system that should frustrate fibril assembly. First, multichain simulations indicate that the distance between the centers of mass of the chains increases at high grafting densities, suggesting that the ability to form collapsed structures is disrupted. Second, single-chain simulations using grafted MC show that the formation of the precursor toroidal structure responsible for fibril formation is hampered at high grafting densities. Third, the frequency spectrum of conformational fluctuations indicates that low-frequency modes dominate at higher grafting densities, suggesting a larger decorrelation time in conformational fluctuations. Together, these results provide a macromolecular basis for the suppression in fibril formation in grafted methylcellulose solutions for grafting densities exceeding approximately 10%.

The vapor-liquid-solid growth of semiconductor nanowires proceeds via the sequential addition of individual atomic or biatomic monolayers at the interface between the nanowire stem and a liquid catalyst droplet. Each monolayer growth cycle comprises a period of nucleation and extension of the monolayer followed by a waiting time before the next nucleation. The cyclic evolution of the liquid-solid system and the duration of these periods affect the properties of individual nanowires and the statistics of nanowire ensembles. Based on recent in situ growth experiments, we investigate theoretically the thermodynamics and kinetics of monolayer formation, with emphasis on nanowires of III-V materials. We first study the thermodynamics of the process in a quasistatic approximation. We calculate and minimize the work of formation of a partial monolayer, accounting for the depletion and refill of the liquid phase and all relevant interface energies. We focus on the regime where the liquid does not contain enough nanowire material at nucleation to form a full monolayer and where growth comprises a fast stage where a partial monolayer forms rapidly using the available material and a slower stage during droplet refill. We explore the effect of the nonmonotonous variation of the total monolayer edge energy on these two growth stages and determine the partition between liquid and monolayer of the atoms provided during refill. We then develop a kinetic model that accounts in simple terms for the competition between delivery and attachment of atoms to the monolayer and droplet refill. We perform fully analytical calculations that describe a continuum between the pulsed regime explored in our thermodynamic study and a smooth regime where the monolayer extends at the pace of refill. The various characteristic times of the monolayer cycle are calculated in presence of desorption of the volatile group V species from the liquid droplet. Simplified asymptotic formulas corresponding to fast attachment or low desorption are derived. Desorption is found to affect weakly the monolayer formation time but strongly the waiting time.

The emerging interest in van der Waals heterostructures as new materials for optoelectronics and photonics poses questions about their stability and structure-property relations. In the framework of density-functional and many-body perturbation theory, we investigate the structural, electronic, and optical properties of periodic heterostructures formed by graphene and hexagonal boron nitride (h-BN). To understand how the constituents affect each other depending on the layer stacking, we examine 12 commensurate arrangements. We find that interaction with graphene improves the stability of bulk h-BN also in those configurations that are predicted to be energetically metastable. In return, the interaction with h-BN can open a band gap of a few hundred meV in graphene. Its actual size can be tuned by the arrangement of the layers. In the semiconducting configurations, the character and spatial distribution of optical excitations are affected by the specific stacking that determines the electronic states involved in the transitions. Remarkably, six out of the 12 explored heterostructures remain semimetallic.

We present a comprehensive study of the short-ranged nuclear and magnetic order in the two-dimensional spin glass, ${\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{PS}}_3$. Nuclear neutron scattering data reveal a random distribution of ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ ions within the honeycomb layers, which gives rise to a spin glass state through inducing competition between neighboring exchange interactions, indicated in magnetic susceptibility data by a cusp at the glass transition, $T_g=35$ K. Analysis of magnetic diffuse neutron scattering data collected for both single-crystal and polycrystalline samples gives further insight into the origin of the spin glass phase, with spin correlations revealing a mixture of satisfied and unsatisfied correlations between magnetic moments within the honeycomb planes, which can be explained by considering the magnetic structures of the parent compounds, ${\mathrm{MnPS}}_3$ and ${\mathrm{FePS}}_3$. We found that, on approaching $T_g$ from above, an ensemble-averaged correlation length of $\xi =5.5\left(6\right)\AA$ developed between satisfied correlations, and below $T_g$, the glassy behavior gave rise to a distance-independent correlation between unsatisfied moments. Correlations between the planes were found to be very weak, which mirrored our observations of rodlike structures parallel to the c* axis in our single-crystal diffraction measurements, confirming the two-dimensional nature of ${\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{PS}}_3$.

Combining quasistatic and time-resolved transport measurements with x-ray and neutron diffraction experiments we study the nonequilibrium states that arise in pure and in Ti-substituted ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ under the application of current densities. Time-resolved studies of the current-induced switching find a slow conductance relaxation that can be identified with heating and a fast one that unambiguously proves an intrinsic mechanism. The current-induced phase transition leads to complex diffraction patterns. Separated Bragg reflections that can be associated with the metallic and insulating phases by their lattice parameters, indicate a real structure with phase coexistence that strongly varies with temperature and current strength. A third contribution with a $c$ lattice constant in between those of metallic and insulating phases appears upon cooling. At low current densities, this additional phase appears below $\sim 100$ K and is accompanied by a suppression of the antiferromagnetic order that otherwise can coexist with current carrying states. A possible origin of the intermediate phase is discussed.

The effects of hydrogen incorporation into $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ thin films have been investigated by chemical, electrical, and optical characterization techniques. Hydrogen incorporation was achieved by remote plasma doping without any structural alterations of the film; however, x-ray photoemission reveals major changes in the oxygen chemical environment. Depth-resolved cathodoluminescence (CL) reveals that the near-surface region of the H-doped $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ film exhibits a distinct red luminescence (RL) band at 1.9 eV. The emergence of the H-related RL band is accompanied by an enhancement in the electrical conductivity of the film by an order of magnitude. Temperature-resolved CL points to the formation of abundant H-related donors with a binding energy of $28\pm 4\mathrm{meV}$. The RL emission is attributed to shallow donor-deep acceptor pair recombination, where the acceptor is a $V_{\mathrm{Ga}}$-H complex and the shallow donor is interstitial H. The binding energy of the $V_{\mathrm{Ga}}$-H complex, based on our experimental considerations, is consistent with the computational results by Varley et al., [J. Phys.: Condens. Matter 23, 334212 (2011)].