Magnetism-induced negative thermal expansion (NTE) observed in inverse perovskite antiferromagnets ${\mathrm{Mn}}_{3}A\mathrm{N}(A=\mathrm{Zn},\mathrm{Ga}$, etc.) is theoretically studied by a classical spin model with competing bond-length-dependent exchange interactions. We numerically reproduce the crystal-volume expansion upon cooling triggered by a noncoplanar antiferromagnetic order and show that the expansion occurs so as to maximize an energy gain of the nearest-neighbor antiferromagnetic interactions. This mechanism is not specific to inverse perovskite magnets and might also be expected in magnets with other crystal structures. We propose other candidate crystal structures that might exhibit NTE through this mechanism.

We have studied the transport and structural properties of heterostructures based on a conducting transparent oxide and a ferromagnetic metal $(\mathrm{FM}=\mathrm{Co}$, CoFe, CoFeB) layer. An amorphous indium tungsten oxide (IWO) was used to study possible generation of spin current from a conducting transparent oxide. The resistance of the heterostructure was studied as a function of external magnetic field to evaluate the magnetoresistance (MR) in the $yz$ plane that provides information on the spin Hall magnetoresistance (SMR). We found $yz$-plane magnetoresistance of ∼−0.2% in IWO/Co and IWO/FeCo heterostructures whereas the effect was nearly zero for IWO/CoFeB bilayers. The MR vanished when a thin (∼1 nm thick) Cu layer was inserted in between the IWO and Co layers. Cross-sectional transmission electron microscopy images and nanobeam diffraction patterns of the films suggested that the Co and FeCo layers formed crystal grains with preferential crystal orientation whereas the Co layer grown on Cu showed grains with random orientation. The CoFeB layer was found to be amorphous. These results show a strong correlation between the crystalline structure of the FM layer and the appearance of $yz$-plane MR, suggesting that FMs with random and/or amorphous structure are suitable for SMR measurements to avoid contamination of the $yz$-plane MR.

Niobium pentoxide ($\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$) has been investigated extensively for applications such as electrochemical energy storage, memristors, solar cells, light emitting diodes, and electrochromic devices. The thermal properties of $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ play a critical role in device performance of these applications. However, very few studies on the thermal properties of $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ have been reported and a fundamental understanding of heat transport in $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ is still lacking. The present paper closes this gap and provides a study of thermal conductivity of amorphous $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. By using the vibrational density of states of the single-crystal phase obtained from density functional theory simulations as an approximation, a diffuson-mediated minimum thermal conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit, which proves that the diffuson model works better than the amorphous limit model to describe the thermal conduction mechanism in the amorphous $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.

Formation, crystal structure, and macroscopic and microscopic magnetism of the binary $\mathrm{T}{\mathrm{b}}_3\mathrm{N}{\mathrm{i}}_2$ and derivative pseudobinary $\mathrm{T}{\mathrm{b}}_3\mathrm{C}{\mathrm{o}}_x\mathrm{N}{\mathrm{i}}_{2-x}$ phases have been investigated using an array of experimental methods. While $\mathrm{T}{\mathrm{b}}_3\mathrm{N}{\mathrm{i}}_2$ crystallizes in the monoclinic $\mathrm{D}{\mathrm{y}}_3\mathrm{N}{\mathrm{i}}_2$ structure type ($\mathit{mS}20, C2$/m), the substitution of Co for Ni results in a structural transition into the rhombohedral $\mathrm{E}{\mathrm{r}}_3\mathrm{N}{\mathrm{i}}_2$ type ($\mathit{hR}\mathrm{45}, R\overline{3}h$) at $x\left(\mathrm{Co}\right)\approx 0.34$ and beyond in the $\mathrm{T}{\mathrm{b}}_3\mathrm{C}{\mathrm{o}}_x\mathrm{N}{\mathrm{i}}_{2-x}$ system. In both the monoclinic and rhombohedral phases, the addition of Co leads to an anisotropic change of lattice parameters and unexpected reduction of the cell volume. Measurements of bulk properties reveal that these compounds order ferrimagnetically or ferromagnetically at about 100 K. Complex noncollinear ferromagnetic ordering in the Tb sublattices is weakly dependent on composition. For $x$ > 0.34 the long-range magnetic ordering leads to a strong anisotropic magnetostriction accompanied by a symmetry reduction from rhombohedral to triclinic.

Lithium carbide, ${\mathrm{Li}}_2{\mathrm{C}}_2$, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from the ground-state orthorhombic ($\mathrm{I}mmm$) to the high-temperature cubic ($\mathrm{F}m\overline{3}m$) crystal structure is not well understood and here we elucidate it with help of first-principles calculations. We show that stabilization of the cubic phase is a result of a temperature-induced disorientation of the C-C dumbbells and their further thermal rotations. Due to these rotations rather large deviatoric stress, which is associated with the dumbbell alignment along one of the crystallographic axes, averages out making the cubic structure mechanically stable. At high temperature we observe a type-II superionic transition to a state of high Li self-diffusion involving collective ionic motion mediated by the formation of Frenkel pairs.

Using density-functional calculations, we predict the emergence of electrically reversible magnetization at the interface between antiferromagnetic noncollinear antiperovskite $\mathrm{GaNM}{\mathrm{n}}_3$ and ferroelectric perovskite $\mathrm{BaTi}{\mathrm{O}}_3$. We find that Mn magnetic moments are enhanced and reoriented at the $\mathrm{GaNM}{\mathrm{n}}_3/A\mathrm{Ti}{\mathrm{O}}_3$ (001) ($A=\mathrm{Sr}$ and Ba) interface, resulting in a sizable net magnetization along the [110] direction. This magnetization is reversed with ferroelectric polarization of $\mathrm{BaTi}{\mathrm{O}}_3$ through $\sim {20}^{\circ }$ rotation of the noncollinear magnetic moments. The effect is driven by ferroelectric modulation of the antiferromagnetic exchange coupling between the interfacial Mn atoms mediated by the Mn-$3d$ orbital population. Our results open opportunities for controlling the magnetic properties by electric fields by exploiting noncollinear antiferromagnetism at antiperovskite/perovskite interfaces.

Although several approaches have been used in the past to investigate the impact of nitrogen (N) on the electronic structure of ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x$ alloys, there is no agreement between theory and experiments about the importance of the different N interstitial defects in these alloys, and their nature is still unknown. Here we analyze the impact of five different N defects on the electronic structure of ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x$ alloys, using density-functional methods: we calculate electronic states, formation energies, and charge transition levels. The studied defects include ${\mathrm{N}}_{\text{As}}, {\mathrm{As}}_{\text{Ga}}, {\mathrm{As}}_{\text{Ga}}-{\mathrm{N}}_{\text{As}}$ substitutional defects and ${(\mathrm{N}-\mathrm{N})}_{\text{As}}, {(\mathrm{N}-\mathrm{As})}_{\text{As}}$ split-interstitial complex defects. Our calculated defect formation energies agree with those reported by Zhang et al. [Phys. Rev. Lett. 86, 1789 (2001)], who predicted these defects. Among the interstitial defects, we found that ${(\mathrm{N}-\mathrm{As})}_{\text{As}}$ emerges as the lowest energy configuration in comparison with ${(\mathrm{N}-\mathrm{N})}_{\text{As}}$, in agreement with recent experiments [Jen et al., Appl. Phys. Lett. 107, 221904 (2015)]. We also calculated the levels induced in the electronic structure due to each of these defects: defect states may occur as deep levels in the gap, shallow levels close to the band edges, and as levels resonant with bulk states. We find that the largest changes in the band structure are produced by an isolated N atom in GaAs, which is resonant with the conduction band, exhibiting a strong hybridization between N and GaAs states. Deeper levels in the band gap are obtained with ${(\mathrm{N}-\mathrm{N})}_{\text{As}}$ split-interstitial defects. Our results confirm the formation of highly localized states around the N sites, which is convenient for photovoltaics and photoluminescence applications.

The electronic band structures and second harmonic generations (SHGs) of phosphorus nitrides ${\mathrm{LiPN}}_2, {\mathrm{NaPN}}_2$, and ${\mathrm{CuPN}}_2$ are studied by first-principles calculations. ${\mathrm{NaPN}}_2$ has a band gap of $\sim 6.2$ eV (200 nm) and the strongest SHG coefficient among phosphorus-based deep-ultraviolet nonlinear optical (NLO) materials, i.e., $d_{36}=2.35$ pm/V at static limit and $d_{36}=3.77$ pm/V under external optic field with wavelength 1064 nm. Compared with the SHG coefficients of borate NLO materials working in the region of deep ultraviolet, ${\mathrm{NaPN}}_2$ presents an unprecedented high SHG coefficient, which is the manifestation of its enhanced orbital hybridization owing to the shorter bond lengths from high pressure facilitated synthesis.

To reveal the relationship between microstructural disorder and charge localization dynamics in a van-der-Waals-bonded molecular semiconductor, the electrical properties of field-effect transistors with zone-cast pentacene films were characterized by using in situ micro-Raman spectroscopy to monitor the films’ intermolecular and intramolecular vibrations. Transformations from a high-temperature phase with reduced molecular vibration along the long axis that were induced by electron localization in pentacene molecules at the channel interface and transformations from a low-temperature phase with increased molecular vibration that were induced by the electron delocalization were clearly observed under gate-bias stress, photoexcitation, and thermal annealing. Multilayer molecular dynamics simulations revealed that electrons should be localized in each pentacene molecule to stabilize the low-temperature phase and that the intramolecular Raman peak corresponds to the amount of out-of-plane molecular displacement that is most influential to charge localization and mobility reduction induced by fluctuating transfer integrals. We propose a unified picture to describe the relationship between out-of-plane structural disorder and charge localization dynamics in pentacene polymorphs.

Materials containing multiple topological characteristics become more exotic when combined with noncentrosymmetric crystal structures and unusual magnetic phases such as the compensated half-metal state, which is gapped in one spin direction and conducting in the other. First-principles calculations reveal these multiple topological features in the compensated half-metal ${\mathrm{Cr}}_2\mathrm{CoAl}$ having neither time-reversal nor inversion symmetries. In the absence of (minor) spin-orbit coupling (SOC), there are (1) a total of 12 pairs of magnetic Weyl points, (2) three distinct sets of triple nodal points near the Fermi level that are (3) interconnected with six symmetry-related nodal lines. This combination gives rise to fully spin polarized nexus fermions, in a system with broken time-reversal symmetry but negligible macroscopic magnetic field. The observed high Curie temperature of 750 K and calculated SOC hybridization mixing of several meV should make these nexus fermions readily measurable. Unlike topological features discussed for other Heuslers which emphasize their strong ferromagnetism, this compensated half-metal is impervious to typical magnetic fields, thus providing a complementary set of experimental phenomena. Making use of the soft calculated magnetic state, large magnetic fields can be used to rotate the direction of magnetism, during which certain topological features will evolve. Our results suggest that these features may be common in inverse-Heusler systems, particularly the isostructural and isovalent Ga and In analogs.

We report wide-doping-range $(8\times {10}^{17}$ to $4\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ Hall electron density) low-temperature specific heat measurements on single crystal $\mathrm{SrTi}{\mathrm{O}}_3$:Nb, correlated with electronic transport data and tight-binding modeling. Lattice dynamic contributions to specific heat are shown to be well understood, albeit with unusual sensitivity to doping, likely related to the behavior of soft modes. Electronic contributions to specific heat provide effective masses that increase substantially, from 1.8 to $4.8m_{\mathrm{e}}$, across the two $\mathrm{SrTi}{\mathrm{O}}_3$ Lifshitz transitions. It is shown that this behavior can be quantitatively reconciled with quantum oscillation data and calculated band structure, establishing a remarkably doping-independent mass enhancement factor of 2.0. Most importantly, with the doping-dependent $T^2$ resistivity prefactor and Sommerfeld coefficient known, Kadowaki-Woods scaling has been tested over the entire doping range probed. Despite classic Fermi liquid behavior in electronic specific heat, standard Kadowaki-Woods scaling is dramatically violated, highlighting the need for new theoretical descriptions of $T^2$ resistivity in $\mathrm{SrTi}{\mathrm{O}}_3$.

The effect of dopants on the metallic glass forming ability is usually considered based on the analysis of changes in the liquid structure or thermodynamics. What is missing in such considerations is an analysis of how a dopant changes the properties of the crystal phases which can form instead of the glass. In order to illuminate this aspect we performed molecular dynamics simulations to study the effects of Mg and Sm dopants on the crystal nucleation in Al. The simulation data were found to be consistent with the experimental observations that addition of Mg to Al does not lead to vitrification but addition of only 8% Sm does. The significant effect of Sm doping was related to the intolerance of Al to this dopant. This leads to increase in the solid-liquid interfacial free energy, and therefore, to increase in the nucleation barrier and to a dramatic decrease in the nucleation rate. The intolerance mechanism also significantly affects the growth kinetics.

We study the energy level structures of the defective graphane lattice, where a carbon dimer defect is created by removing the hydrogen atoms on two nearest-neighbor carbon sites. Robust defect states emerge inside the bulk insulating gap of graphane. While for the stoichiometric half-filled system there are two doubly degenerate defect levels, there are four nondegenerate and spin-polarized in-gap defect levels in the system with one electron less than half filling. A universal set of quantum gates can be realized in the defective graphane lattice, by triggering resonant transitions among the defect states via optical pulses and ac magnetic fields. The sizable energy separation between the occupied and the empty in-gap states enables precise control at room temperature. The spatial locality of the in-gap states implies a qubit network of extremely high areal density. Based on these results, we propose that graphane as a unique platform could be used to construct the future all-purpose quantum computers.

Quaternary alloys enable the independent optimization of different semiconductor properties, such as the separate tuning of the band gap and the lattice constant. Nanowire core-shell structures should allow a larger range of compositional tuning as strain can be accommodated in a more effective manner than in thin films. Still, the faceted structure of the nanowire may lead to local segregation effects. Here, we explore the incorporation of indium in AlGaAs shells up to $25\%$. In particular, we show the effect of In incorporation on the energy shift of the AlGaInAs single-photon emitters present in the shell. We observe a redshift up to 300 meV as a function of the group-III site fraction of In. We correlate the shift with segregation at the nanoscale. We find evidence of the segregation of the group-III elements at different positions in the nanowire, not observed before. We propose a model that takes into account the strain distribution in the nanowire shell and the adatom diffusion on the nanowire facets to explain the observations. This work provides novel insights on the segregation phenomena necessary to engineer the composition of multidinary alloys.

A long-standing question in condensed matter and material science is understanding how local structural order determines the fate of a liquid on cooling, i.e., whether it crystallizes or experiences dynamic slowing down leading to glass formation. Here we address this issue by focusing on a metallic glass former with concurring crystal and amorphous local order based on icosahedral symmetry. Using atomistic simulations, we study its deep supercooled regime in which we clearly identify spontaneous crystal nucleation close to the glass transition. We find that regions of slow dynamics characterized by a high degree of icosahedral orientational order play a central role in the nucleation process. Our study brings a new perspective into the microscopic mechanism of crystallization from the supercooled phase, which could be of significance for the in silico design of materials with selected glass-forming ability.

Strained epitaxial SiGe on vicinal Si(001) substrates develops a morphological instability perpendicular to the steps unlike the usual growth instabilities on vicinal substrates, eventually leading to planar nanowires. We assess both theoretically and experimentally the effect of strain anisotropy on the 1D elongation of the Asaro-Tiller-Grinfel'd (ATG) instability. The anisotropy of strain relaxation due to the presence of step edges is considered in a continuum model with two different effective strains in the surface plane. We show that the measured in-plane strain anisotropy and the theoretical model are consistent with the experimental morphologies. Nice network of ultrasmall aligned elongations are predicted resulting from a complex interplay of kinetic and energetic phenomena associated with strain anisotropy.

The present paper describes a powerful extension of the x-ray standing-wave technique that combines it with the capability of full-field x-ray fluorescence imaging, so that the impurity depth profiles at all different parts of a nanolayer material can be measured in parallel. The imaging capability improves the robustness and reliability of the x-ray standing-wave technique in investigating inhomogeneous two-dimensional materials and soft interfaces. It can also visualize the three-dimensional element-specific structure at a buried interface with a depth resolution at the nanometer level.