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.

The phases of $A_2{\mathrm{Mn}}_8{\mathrm{O}}_{16}$ hollandite group oxides emerge from the competition between ionic interactions, Jahn-Teller effects, charge ordering, and magnetic interactions. Their balanced treatment with feasible computational approaches can be challenging for commonly used approximations in density functional theory. Three examples ($A$ = Ag, Li, and K) are studied with a sequence of different approximate exchange-correlation functionals. Starting from a generalized gradient approximation (GGA), an extension to include van der Waals interactions and a recently proposed meta-GGA are considered. Then local Coulomb interactions for the Mn $3d$ electrons are more explicitly considered with the DFT + $U$ approach. Finally, selected results from a hybrid functional approach provide a reference. Results for the binding energy of the A species in the parent oxide highlight the role of van der Waals interactions. Relatively accurate results for insertion energies can be achieved with a low-$U$ and a high-$U$ approach. In the low-$U$ case, the materials are described as band metals with a high-symmetry, tetragonal crystal structure. In the high-$U$ case, the electrons donated by A result in formation of local ${\mathrm{Mn}}^{3+}$ centers and corresponding Jahn-Teller distortions characterized by a local order parameter. The resulting degree of monoclinic distortion depends on charge ordering and magnetic interactions in the phase formed. The reference hybrid functional results show charge localization and ordering. Comparison to low-temperature experiments of related compounds suggests that charge localization is the physically correct result for the hollandite group oxides studied here. Finally, while competing effects in the local magnetic coupling are subtle, the fully anisotropic implementation of DFT + $U$ gives the best overall agreement with results from the hybrid functional.

Small-scale mechanics of solute atom segregation and incipient plasticity in nanotwinned Ag containing trace concentrations of Cu were studied by using large-scale hybrid Monte Carlo and molecular-dynamic simulations. It is found that solute Cu atoms are segregated concurrently to grain boundaries and intrinsic twin-boundary kink-step defects during thermal annealing. Low Cu dopant contents below 1 at. % are predicted to substantially increase twin stability in nanotwinned Ag, accompanied with a pronounced rise in yield strength at 300 K. Incipient plasticity is associated with kink-step migration, grain-boundary sliding, and dislocation nucleation from grain boundaries and twin-boundary defects, which are affected by doping. Cu-dependent yield strengthening in doped nanotwinned Ag is shown to correlate with the critical stress required to initiate crystal slip emitted from grain boundaries and twin-boundary defects. These findings provide fundamental insight into the roles of twin-boundary imperfections on plastic yielding, and offer clues to further extend the extraordinary stability and strength of nanotwinned metals by microalloying.

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,${\mathrm{Sb})}_2$(Se,${\mathrm{Te})}_3$ family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials are on the order of tens of degrees Kelvin, full quantization of the QAH state is achieved below $\sim 100$ mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin films. Starting from singly doped Cr:${\mathrm{Sb}}_2{\mathrm{Te}}_3$ films, free of secondary phases and with a transition temperature of $\sim 72$ K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase ${\mathrm{Cr}}_2{\mathrm{Te}}_3$. No V was detectable in the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our paper highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. A stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ near its second-order FM transition at $T_c\approx 34\mathrm{K}$; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change $-\mathrm{\Delta }{S}_M$ is 5.27 and $16.1\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, and the estimated magnetic refrigerant capacity reaches 68 and $522\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ is mainly attributed to the large saturation moment of $18.9{\mu }_{\mathrm{B}}$ per formula unit owing to a simultaneous FM ordering of the rear-earth ${\mathrm{Er}}^{3+}$ and transition-metal ${\mathrm{Mn}}^{4+}$ localized moments. Our results suggest that $\mathrm{E}{\mathrm{r}}_2\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.