Magnetic-field-dependent small-angle neutron scattering (SANS) has been utilized to study the magnetic microstructure of bulk metallic glasses (BMGs). In particular, the magnetic scattering from soft magnetic ${\mathrm{Fe}}_{70}{\mathrm{Mo}}_5{\mathrm{Ni}}_5{\mathrm{P}}_{12.5}{\mathrm{B}}_{2.5}{\mathrm{C}}_5$ and hard magnetic ${\left({\mathrm{Nd}}_{60}{\mathrm{Fe}}_{30}{\mathrm{Al}}_{10}\right)}_{92}{\mathrm{Ni}}_8$ alloys in the as-prepared, aged, and mechanically deformed state is compared. While the soft magnetic BMGs exhibit a large field-dependent SANS response with perturbations originating predominantly from spatially varying magnetic anisotropy fields, the SANS cross sections of the hard magnetic BMGs are only weakly dependent on the field, and their angular anisotropy indicates the presence of scattering contributions due to spatially dependent saturation magnetization. Moreover, we observe an unusual increase in the magnetization of the rare-earth-based alloy after deformation. Analysis of the SANS cross sections in terms of the correlation function of the spin misalignment reveals the existence of field-dependent anisotropic long-wavelength magnetization fluctuations on a scale of a few tens of nanometers. We also give a detailed account of how the SANS technique relates to unraveling displacement fields on a mesoscopic length scale in disordered magnetic materials.

We investigate the superconducting ternary lithium borohydride phase diagram at pressures of 0 and 200 GPa using methods for evolutionary crystal structure prediction and linear-response calculations for the electron-phonon coupling. Our calculations show that the ground state phase at ambient pressure, ${\mathrm{LiBH}}_4$, stays in the $Pnma$ space group and remains a wide band-gap insulator at all pressures investigated. Other phases along the $1:1:x$ Li:B:H line are also insulating. However, a full search of the ternary phase diagram at 200 GPa revealed a metallic ${\mathrm{Li}}_2{\mathrm{BH}}_6$ phase, which is thermodynamically stable down to 100 GPa. This superhydride phase, crystallizing in a $Fm\overline{3}m$ space group, is characterized by sixfold hydrogen-coordinated boron atoms occupying the fcc sites of the unit cell. Due to strong hydrogen-boron bonding this phase displays a critical temperature of $\sim 100$ K between 100 and 200 GPa. Our investigations confirm that ternary compounds used in hydrogen-storage applications should exhibit high-$T_c$ conventional superconductivity in diamond anvil cell experiments, and suggest a viable route to optimize the superconducting behavior of high-pressure hydrides, exploiting metallic covalent bonds.

Solid-state sodium electrolytes are expected to improve next-generation batteries on the basis of favorable energy density and reduced cost. ${\mathrm{Na}}_3{\mathrm{SbS}}_4$ represents a new solid-state ion conductor with high ionic conductivities in the mS/cm range. Here, we explore the tetragonal phase of ${\mathrm{Na}}_3{\mathrm{SbS}}_4$ and its interface with metallic sodium anode using a combination of experiments and first-principles calculations. The computed Na-ion vacancy migration energies of 0.1 eV are smaller than the value inferred from experiment, suggesting that grain boundaries or other factors dominate the experimental systems. Analysis of symmetric cells of the electrolyte—${\mathrm{Na}/\mathrm{Na}}_3{\mathrm{SbS}}_4/\mathrm{Na}$—show that a conductive solid electrolyte interphase forms. Computer simulations infer that the interface is likely to be related to ${\mathrm{Na}}_3{\mathrm{SbS}}_3$, involving the conversion of the tetrahedral ${\mathrm{SbS}}_4^{3-}$ ions of the bulk electrolyte into trigonal pyramidal ${\mathrm{SbS}}_3^{3-}$ ions at the interface.

Solid-state electrolytes that are compatible with high-capacity electrodes are expected to enable the next generation of batteries. As a promising example, ${\mathrm{Li}}_2\mathrm{OHCl}$ was reported to have good ionic conductivity and to be compatible with a lithium metal anode even at temperatures above $100{}^{\circ }\mathrm{C}$. In this work, we explore the fundamental properties of ${\mathrm{Li}}_2\mathrm{OHCl}$ by comparing simulations and experiments. Using calculations based on density functional theory, including both static and dynamic contributions through the quasiharmonic approximation, we model a tetragonal ground state, which is not observed experimentally. An ordered orthorhombic low-temperature phase was also simulated, agreeing with experimental structural analysis of the pristine electrolyte at room temperature. In addition, comparison of the ordered structures with simulations of the disordered cubic phase provide insight into the mechanisms associated with the experimentally observed abrupt increase in ionic conductivity as the system changes from its ordered orthorhombic to its disordered cubic phase. A large Haven ratio for the disordered cubic phase is inferred from the computed tracer diffusion coefficient and measured ionic conductivity, suggesting highly correlated motions of the mobile Li ions in the cubic phase of ${\mathrm{Li}}_2\mathrm{OHCl}$. We find that the OH bond orientations participate in gating the Li ion motions which might partially explain the predicted Li-Li correlations.

Plastic deformation in solids induced by external stresses is of both fundamental and practical interest. Using both phase field crystal modeling and molecular dynamics simulations, we study the shear response of monocomponent polycrystalline solids. We subject mesocale polycrystalline samples to constant strain rates in a planar Couette flow geometry for studying its plastic flow, in particular its grain deformation dynamics. As opposed to equilibrium solids where grain dynamics is mainly driven by thermal diffusion, external stress/strain induce a much higher level of grain deformation activity in the form of grain rotation, coalescence, and breakage, mediated by dislocations. Despite this, the grain size distribution of this driven system shows only a weak power-law correction to its equilibrium log-normal behavior. We interpret the grain reorganization dynamics using a stochastic model.

The homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. To account for the complex shape with low symmetry and the peculiar intrachain conformational order of polymer, we introduce a shape descriptor $O_{CB}$ coupling conformational order and interchain rotational symmetry, which is able to differentiate hexagonal and orthorhombic clusters from melt. With the shape descriptor $O_{CB}$, we find that coupling between conformational and interchain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. After the formation of hexagonal clusters, the nucleation of orthorhombic structure occurs inside of them, which proceeds via the coalescence of neighboring hexagonal clusters rather than a standard stepwise growth process. This demonstrates that nucleation of PE crystal is an $O_{CB}$ order assisted two-step process, which is different from previous models for polymer crystallization but similar with that proposed for spherical “atoms” such as colloid and metal.

A relatively easy-to-fabricate nanolayered metal composite with superior mechanical properties is introduced. The matrix is a nanocrystalline nickel in which the grain size is engineered to optimize the strength, and monolayer particles of graphene are embedded into the matrix as reinforcing interlayers. Atomistic-scale deformation mechanisms, and mechanics of hindering the dislocations propagation by graphene nanoplatelets with different configurations in the nanocrystalline metallic matrix, are investigated by molecular dynamics simulations. Molecular dynamics findings are utilized to engineer the nanostructure of a metal matrix composite. Nanocrystalline nickel-graphene nanolayered systems with optimum mechanical properties are identified and fabricated with a cost-efficient method. The nanostructure of the fabricated composites is examined via electron microscopy, and their mechanical performance is inspected via nanoindentation tests. The experimental results show that a nickel-graphene nanolayered system with 14% areal coverage of graphene particles at the interlayers has improved the hardness of the nanocrystalline nickel by almost 40%.

A series of $\mathrm{Pb}{\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x\right)}_2\mathrm{T}{\mathrm{e}}_4$ topological-insulator crystals with various Sb molar ratios $x$ was fabricated, and a systematic study of their transport properties was performed, aiming at realizing enhanced bulk insulation in this system. Hall-effect measurements showed that $n$- to $p$-type transition in bulk conduction occurred at $x\approx 0.80$. Semiconducting behavior with a negative temperature coefficient of resistivity was observed with resistivity values as high as 180 mΩ cm at 2 K around the transition composition. The conduction mechanism in the semiconducting samples is discussed in relation to an impurity band formed within the bandgap.

The valence energy loss spectrum which is characterized by valence-to-conduction band and plasmon excitations is rarely used in spectroscopy of Li-ion battery materials. One reason being the large number of different excitations observed in this region as well as the difficulty in interpreting their nature and origin. We have determined the nature and origin of spectral features observed in Li-Mn-Ni-O spinel oxides with respect to Mn valency changes during the insertion of lithium ion. The lithiation process is accompanied by a Mn valency change from Mn 4+ in ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ to Mn 3+ in lithium rich ${\mathrm{Li}}_2{\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$. The valence energy loss spectrum of ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ is characterized by sharp peaks in the 7–10 eV energy loss range whose intensity decrease with lithiation to ${\mathrm{Li}}_2{\mathrm{Ni}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$. Using electronic structure calculations and molecular orbital considerations we show that the intense peaks in the valence loss spectra of ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ have a large contribution from ligand-metal charge transfer transitions. These transitions arise from the mainly O $2p$ nonbonding $t_{2u}$ and bonding $t_{1u}$ orbitals to the mainly Mn $3d$ antibonding $t_{2g}^{*}$ and $e_g^{*}$ orbitals. We discuss the origins of the observed valence spectra differences between the two phases in relation to peaks shift, variations in occupancy, and variations in covalency as a result of Mn valency changes occurring during lithiation.

We show that the previously unreported ternary $\sigma$-phase material $\mathrm{N}{\mathrm{b}}_{20.4}\mathrm{R}{\mathrm{u}}_{5.7}\mathrm{G}{\mathrm{e}}_{3.9}$ $\left(\mathrm{N}{\mathrm{b}}_{0.68}\mathrm{R}{\mathrm{u}}_{0.19}\mathrm{G}{\mathrm{e}}_{0.13}\right)$ is a superconductor with a critical temperature of 2.2 K. Temperature-dependent magnetic susceptibility, resistance, and specific-heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for $\mathrm{N}{\mathrm{b}}_{20.4}\mathrm{R}{\mathrm{u}}_{5.7}\mathrm{G}{\mathrm{e}}_{3.9}$ is $91\mathrm{mJ}\mathrm{mol}\mathrm{f}.\mathrm{u}{.}^{-1}{\mathrm{K}}^{-2}$ $\left(\sim 3\mathrm{mJ}\mathrm{mol}\mathrm{ato}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}\right)$ and the specific-heat anomaly at the superconducting transition, $\mathrm{\Delta }C/\gamma T_c$, is approximately 1.38. The zero-temperature upper critical field $\left[{\mu }_{0}H{c}_2\left(0\right)\right]$ was estimated to be 2 T by resistance data. Field-dependent magnetization data analysis estimated ${\mu }_{0}H{c}_1\left(0\right)$ to be 5.5 mT. Thus, the characterization shows $\mathrm{N}{\mathrm{b}}_{20.4}\mathrm{R}{\mathrm{u}}_{5.7}\mathrm{G}{\mathrm{e}}_{3.9}$ to be a type-II BCS superconductor. This material appears to be the first reported ternary phase in the Nb-Ru-Ge system, and the fact that there are no previously reported binary Nb-Ru, Nb-Ge, or Ru-Ge $\sigma$ phases shows that all three elements are necessary to stabilize the material. An analogous $\sigma$ phase in the Ta-Ru-Ge system did not display superconductivity above 1.7 K, which suggests that electron count cannot govern the superconductivity observed. Preliminary characterization of a possible superconducting $\sigma$ phase in the Nb-Ru-Ga system is also reported.

Charged defects in two-dimensional (2D) materials have emerging applications in quantum technologies such as quantum emitters and quantum computation. The advancement of these technologies requires a rational design of ideal defect centers, demanding reliable computation methods for the quantitatively accurate prediction of defect properties. We present an accurate, parameter-free, and efficient procedure to evaluate the quasiparticle defect states and thermodynamic charge transition levels of defects in 2D materials. Importantly, we solve critical issues that stem from the strongly anisotropic screening in 2D materials, that have so far precluded the accurate prediction of charge transition levels in these materials. Using this procedure, we investigate various defects in monolayer hexagonal boron nitride ($h-\mathrm{BN}$) for their charge transition levels, stable spin states, and optical excitations. We identify ${\text{C}}_{\text{B}}{\text{V}}_{\text{N}}$ (nitrogen vacancy adjacent to carbon substitution of boron) to be the most promising defect candidate for scalable quantum bit and emitter applications.

Trirutile-type ${\mathrm{LiFe}}_2{\mathrm{F}}_6$ is a charge-ordered material with an ${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$ configuration. Here, its physical properties, including magnetism, electronic structure, phase transition, and charge ordering, are studied theoretically. On one hand, the charge ordering leads to improper ferroelectricity with a large polarization. On the other hand, its magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate compressive strain. Thus, ${\mathrm{LiFe}}_2{\mathrm{F}}_6$ can be a rare multiferroic with both large magnetization and polarization. Most importantly, since the charge ordering is the common ingredient for both ferroelectricity and magnetization, the net magnetization may be fully switched by flipping the polarization, rendering intrinsically strong magnetoelectric effects and desirable functions.

The two-dimensional electron gas occurring between the band insulators ${\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3$ continues to attract considerable interest, due to the possibility of dynamic control over the carrier density and due to ensuing phenomena such as magnetism and superconductivity. The formation of this conducting interface is sensitive to the growth conditions, but despite numerous investigations there are still questions about the details of the physics involved. In particular, not much is known about the electronic structure of the growing ${\mathrm{LaAlO}}_3$ layer at the growth temperature (around $800{}^{\circ }\mathrm{C}$) in oxygen (pressure around $5\times {10}^{-5}$ mbar), since analysis techniques at these conditions are not readily available. We developed a pulsed laser deposition system inside a low-energy electron microscope in order to study this issue. The setup allows for layer-by-layer growth control and in situ measurements of the angle-dependent electron reflection intensity, which can be used as a fingerprint of the electronic structure of the surface layers during growth. By using different substrate terminations and growth conditions we observe two families of reflectivity maps, which we can connect either to samples with an ${\mathrm{AlO}}_2$-rich surface and a conducting interface or to samples with a LaO-rich surface and an insulating interface. Our observations emphasize that substrate termination and stoichiometry determine the electronic structure of the growing layer, and thereby the conductance of the interface.

Doping lithium-ion battery electrode materials $\mathrm{Li}M{\mathrm{O}}_2$ ($M=$ Co, Ni, Mn) with impurities has been shown to be an effective way to optimize their electrochemical properties. Here, we report a detailed first-principles study of layered oxides ${\mathrm{LiCoO}}_2$, ${\mathrm{LiNiO}}_2$, and ${\mathrm{LiMnO}}_2$ lightly doped with transition-metal (Fe, Co, Ni, Mn) and non-transition-metal (Mg, Al) impurities using hybrid-density-functional defect calculations. We find that the lattice site preference is dependent on both the dopant's charge and spin states, which are coupled strongly to the local lattice environment and can be affected by the presence of codopant(s), and the relative abundance of the host compound's constituting elements in the synthesis environment. On the basis of the structure and energetics of the impurities and their complexes with intrinsic point defects, we determine all possible low-energy impurity-related defect complexes, thus providing defect models for further analyses of the materials. From a materials modeling perspective, these lightly doped compounds also serve as model systems for understanding the more complex, mixed-metal, $\mathrm{Li}M{\mathrm{O}}_2$-based battery cathode materials.

We report a detailed first-principles study of doping in ${\mathrm{Li}}_2{\mathrm{MnO}}_3$, in both the dilute doping limit and heavy doping, using hybrid density-functional calculations. We find that Al, Fe, Mo, and Ru impurities are energetically most favorable when incorporated into ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ at the Mn site, whereas Mg is most favorable when doped at the Li sites. Nickel, on the other hand, can be incorporated at the Li site and/or the Mn site, and the distribution of Ni over the lattice sites can be tuned by tuning the material preparation conditions. There is a strong interplay among the lattice site preference and charge and spin states of the dopant, the electronic structure of the doped material, and the delithiation mechanism. The calculated electronic structure and voltage profile indicate that in Ni-, Mo-, or Ru-doped ${\mathrm{Li}}_2{\mathrm{MnO}}_3$, oxidation occurs on the electrochemically active transition-metal ion(s) before it does on oxygen during the delithiation process. The role of the dopants is to provide charge compensation and bulk electronic conduction mechanisms in the initial stages of delithiation, hence enabling the oxidation of the lattice oxygen in the later stages. This work thus illustrates how the oxygen-oxidation mechanism can be used in combination with the conventional mechanism involving transition-metal cations in design of high-capacity battery cathode materials.

Here, we describe the direct imaging—with four-dimensional ultrafast electron microscopy—of the emergence, evolution, dispersion, and decay of photoexcited, hypersonic coherent acoustic phonons in nanoscale germanium wedges. Coherent strain waves generated via ultrafast in situ photoexcitation were imaged propagating with initial phase velocities of up to 35 km/s across discrete micrometer-scale crystal regions. We observe that, while each wave front travels at a constant velocity, the entire wave train evolves with a time-varying phase-velocity dispersion, displaying a single-exponential decay to the longitudinal speed of sound (5 km/s) and with a mean lifetime of 280 ps. We also find that the wave trains propagate along a single in-plane direction oriented parallel to striations introduced during specimen preparation, independent of crystallographic direction. Elastic-plate modeling indicates the dynamics arise from excitation of a single, symmetric (dilatational) guided acoustic mode. Further, by precisely determining the experiment time-zero position with a plasma-lensing method, we find that wave-front emergence occurs approximately 100 ps after femtosecond photoexcitation, which matches well with Auger recombination times in germanium. We conclude by discussing the similarities between the imaged hypersonic strain-wave dynamics and electron/hole plasma-wave dynamics in strongly photoexcited semiconductors.

Ferroelectrics with suitable band gaps have recently attracted attention as candidate solar absorbing materials for photovoltaics. The inversion symmetry breaking may promote the separation of photoexcited carriers and allow voltages higher than the band gap. However, these effects are not fully understood, in part because of a lack of suitable model systems for studying these effects in detail. Here, we report properties of ferroelectric ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{Se}}_6$ using first principles calculations. Results are given for the electronic structure, carrier pocket shapes, optical absorption, and transport. We find indirect band gaps of 2.20 eV and 1.55 eV, respectively, and favorable band structures for carrier transport, including both holes and electrons. Strong absorption is found above the direct gaps of 2.43 eV and 1.76 eV. Thus these compounds may serve as useful model systems for understanding photovoltaic effects in ferroelectric semiconductors.

We demonstrate an interface-sensitive NMR in a semiconducting nanostructure, where an NMR signal from the minute heterojunction region of a model heterojunction structure $\left(\mathrm{I}{\mathrm{n}}_{0.48}\mathrm{G}{\mathrm{a}}_{0.52}\mathrm{P}/\mathrm{GaAs}\right)$ is detected by using nuclear hyperpolarization created by optical pumping. The key to the detection is the use of minute lattice distortions occurring at the heterojunction due to the lattice mismatch, which enables us to create and localize hyperpolarization at the heterojunction and distinguish it from the other parts. In particular, the suppression of nuclear spin diffusion by the spatial variation in the strain and the resultant unexpectedly stable hyperpolarization at the heterojunction are the keys to successful detection.

Large crystals of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (La-Ca-2126) with $x=0.10$ and 0.15 have been grown and converted to bulk superconductors by high-pressure oxygen annealing. The superconducting transition temperature, $T_c$, is as high as 55 K; this can be raised to 60 K by postannealing in air. Here we present structural and magnetic characterizations of these crystals using neutron scattering and muon spin rotation techniques. While the as-grown, nonsuperconducting crystals are single phase, we find that the superconducting crystals contain three phases forming coherent domains stacked along the $c$ axis: the dominant La-Ca-2126 phase, very thin (1.5 unit-cell) intergrowths of ${\mathrm{La}}_2{\mathrm{CuO}}_4$, and an antiferromagnetic ${\mathrm{La}}_8{\mathrm{Cu}}_8{\mathrm{O}}_{20}$ phase. We propose that the formation and segregation of the latter phases increases the Ca concentration of the La-Ca-2126, thus providing the hole doping that supports superconductivity.

Materials design based on first-principles electronic calculations has proven a fruitful strategy to identify new thermoelectric materials with a favorable figure of merit. Recent electronic structure calculations predict that in cobalt-based half-Heusler systems a power factor higher than in CoTiSb can be achieved upon $p$-type doping of CoVSn, CoNbSn, CoTaSn, CoMoIn, and CoWIn. Here, using a first-principles approach and semiclassical Boltzmann transport theory, we investigate the electrical and thermal transport properties of these materials. The calculated thermal conductivity at room temperature of all the systems is lower than that of CoTiSb, with CoMoIn and CoWIn having an almost threefold lower thermal conductivity than CoTiSb. We also provide conservative estimates of the figure of merit for these systems which all turn out to be higher than in CoTiSb and to have a maximum value for CoWIn.