The molybdate oxides ${\mathrm{SrMoO}}_3$, ${\mathrm{PbMoO}}_3$, and ${\mathrm{LaMoO}}_3$ are a class of metallic perovskites that exhibit interesting properties including high mobility, and unusual resistivity behavior. We use first-principles methods based on density functional theory (DFT) to explore the electronic, crystal, and magnetic structure of these materials. To account for the electron correlations in the partially-filled Mo $4d$ shell, a local Hubbard $U$ interaction is included. The value of $U$ is estimated via the constrained random-phase approximation approach, and the dependence of the results on the choice of $U$ are explored. For all materials, generalized-gradient approximation $\mathrm{DFT}+U$ predicts a metal with an orthorhombic, antiferromagnetic structure. For ${\mathrm{LaMoO}}_3$, the $Pnma$ space group is the most stable, while for ${\mathrm{SrMoO}}_3$ and ${\mathrm{PbMoO}}_3$, the $Imma$ and $Pnma$ structures are close in energy. The $R_4^{+}$ octahedral rotations for ${\mathrm{SrMoO}}_3$ and ${\mathrm{PbMoO}}_3$ are found to be overestimated compared to the experimental low-temperature structure.

Ultralow thermal conductivity materials continue to be of great interest for technologically important applications such as thermal insulators and thermoelectrics. Often, such materials possess constituents with extensive dynamic disorder, or “rattlers,” or structural disorder such as metallic glasses. Nevertheless, targeted crystalline bonding order and anharmonicity can provide effective means for the scattering of the phonons in certain materials. Our combined experimental and theoretical investigation of ${\mathrm{Ba}}_6{\mathrm{Sn}}_6{\mathrm{Se}}_{13}$ reveals such a material, with ultralow thermal conductivity measured over a large temperature range. Optic phonon modes hybridize with acoustic modes at relatively low energies resulting in strong acoustic-optic scattering and limited phonon lifetimes. Moreover, our investigations reveal various avoided crossings at low energies that contribute to phonon scattering. These are derived from chiral phase symmetries and may result in nontrivial topological behaviors for phonon band crossings in this chiral material. This work contributes to the ongoing research on low thermal conductivity materials and the underlying mechanisms that affect their thermal behaviors. Manipulation of these effects may provide pathways for enhancement of material properties for targeted thermal applications using similar materials.

We report an analysis of the discontinuous plastic flow of a multiprincipal-element alloy, ${\mathrm{Co}}_{17.5}{\mathrm{Cr}}_{12.5}{\mathrm{Fe}}_{55}{\mathrm{Ni}}_{10}{\mathrm{Mo}}_5$ (atomic percent, at. %), in the temperature range of 0.5–4.2 K showing serrated deformation curves. Using the analytical techniques, we studied the statistics of the stress drops associated with the unstable plastic flow. The analysis showed that the complexity and heterogeneity of a discontinuous plastic flow were reduced when the temperature was lowered. This behavior was associated with the effects of dynamic recovery and adiabatic heating on the dislocation-density evolution.

Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to −30.7% of spin-locked circular dichroism at 0.76 THz and −98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

The $2\mathrm{D}\text{−}\mathrm{W}{\mathrm{S}}_2$ is an emerging material for next-generation electronic and optoelectronic devices. These applications are very sensitive and can be adversely affected by defects incorporated during the sample growth. Using hybrid density functional approach, we carried out a comprehensive study on intrinsic and extrinsic defects in $2\mathrm{D}\text{−}\mathrm{W}{\mathrm{S}}_2$. All the intrinsic defects and their complexes are found to be deep and self-compensating. S vacancy $\left({\mathrm{V}}_{\text{S}}\right)$, which has been previously attributed as the source of $n$-type conductivity, turns out to be an electron trap center. We found that ${\mathrm{V}}_{\text{S}}$ gives rise to a suboptical gap, which can be the source of single-photon emitters. Interestingly, hydrogen interstitial $\left({\mathrm{H}}_i\right)$ makes multicenter bond and acts as a shallow donor. In addition, H as adatom (H-ad) also provides shallow donor levels and is the cause of unintentional $n$-type doping. Among the extrinsic defects, halogens and transition metals are found to be promising dopants. While halogens at the S site act as shallow donors, Nb at the W site provides a reasonable shallow acceptor level with low formation energy.

Graphite is used as an anode material in conventional lithium-ion batteries owing to its ability to form stable Li-intercalated graphite intercalation compounds (Li GICs). Its application to sodium-ion batteries has long been of great interest, but the instability of Na GICs hampers its implementation. First-principles calculations were performed to gain physical insight into the intercalation process in alkali-metal (AM) GICs, where $\mathrm{AM}=\mathrm{Li}$, Na. In this study, the structure, stability, and diffusion properties of AM GICs with various in-plane AM concentrations were systematically investigated using a van der Waals density functional simulation, and the differences between Li and Na GICs were discussed. Li GICs were found to be quite stable over a wide range of in-plane Li concentrations, with a change in the favorable stacking sequence of graphite. In terms of diffusion, the migration energy for Li in graphite increases as the graphite stacking transition occurs, suggesting that hindering the stacking transition could realize fast and uniform Li diffusion. In contrast, Na GICs are less stable than Li GICs because of following two reasons: (1) interaction between Na and carbon is less stable than that between Li and carbon, and (2) a larger amount of deformation in the interlayer distance is necessary. The Na GICs tend to be stabilized by increasing the number of Na-carbon interactions. Namely, fasted Na diffusion is expected in the Na-rich phase. Our systematic simulations of the formation energy and migration energy of Na GICs with different structures and in-plane AM concentrations suggested that the expansion of graphite layers prior to Na intercalation could achieve graphite anodes for Na-ion batteries.

The concept of the elastic dipole tensor of a defect is generalized to enable the treatment of lattice distortions produced by defects at elevated temperatures. Thermodynamic and statistical mechanics treatments show the feasibility of applying the formalism to the evaluation of formation free energies and finite-temperature elastic dipole tensors of $\frac{1}{2}\langle 111\rangle$ prismatic self-interstitial atom dislocation loops. The method exhibits good numerical stability even in the high-temperature limit, and relates discrete atomic and continuum representations of displacement and stress and strain fields of defects.

Understanding the crystallization mechanism in silica-based materials is of paramount importance to comprehend geological phenomena and to design novel materials for a variety of technological and industrial applications. In this work, we show that metadynamics simulations can effectively overcome a large energy barrier to crystallize from viscous oxide glass melts and can be used to identify the melt-to-crystal transition path of the lithium disilicate system. The accelerated atomistic simulation revealed of a two-step mechanism of the nanoscale crystal formation. First, a partially layered silica embryo appeared, and then a more ordered crystalline layer with size larger than the critical nucleus size was formed. Subsequently, lithium ions piled up around the silicate layer and triggered stacking of adjacent silicate layers, which eventually built a perfect crystal. Contrarily to previous hypotheses, no lithium metasilicate crystal was observed as a precursor of the homogeneous crystallization of lithium disilicate.

A crack propagating in a strained rubberlike solid exhibits an abrupt change of the propagation velocity by typically more than ${10}^2$ times at a specific threshold strain energy, which is a phenomenon called the “velocity jump.” Despite its scientific and industrial significance, the mechanism of the velocity jump had been unsolved for more than 30 years. This paper gives a clear answer to the mechanism, showing dynamic glass transition at the crack tip is the true origin of the crack velocity jump. We present concerted investigations involving theoretical analysis, numerical calculation, and experiment to establish an integrated understanding of the mechanism. Our findings indicate that the velocity jump can be found in general viscoelastic materials.

Novel materials with both topological nontrivial states and superconductivity have attracted considerable attention in recent years. Single-crystal ${\mathrm{FeB}}_4$ was recently synthesized and demonstrated to exhibit superconductivity at temperatures lower than 2.9 K, and its nanoindentation hardness was measured to be 65 GPa. In this study, based on first-principles calculations and the low-energy $\mathit{k}·\mathit{p}$ effective Hamiltonian, we found that this $Pnnm$-type superhard ${\mathrm{FeB}}_4$ superconductor hosts topological behaviors with intersecting nodal rings (INRs) in the $k_x=0$ and $k_z=0$ planes and nodal wall states in the $k_y=\pi$ and $k_z=\pi$ planes. The observed surface drum-head-like (D-H-L) states on the [100] and [001] surfaces confirmed the presence of INR states in this system. According to our investigation results, ${\mathrm{FeB}}_4$, with its superconductivity, superior mechanical behaviors, one-dimensional and two-dimensional topological elements, and D-H-L surface states, is an existing single-phase target material that can be used to realize the topological superconducting state in the near future.

Here we investigate the thermodynamic and electronic properties of ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_9$ single crystals. Electrical transport data show that ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_9$ has a semiconducting ground state with a relatively narrow band gap of 320 meV. Magnetic susceptibility data reveal antiferromagnetic order at low temperatures, whereas ferromagnetic interactions dominate at high temperature. Specific heat, magnetic susceptibility, and electrical resistivity measurements reveal three phase transitions at $T_{N1}=9.3$ K, $T_{N2}=8.3$ K, and $T_{N3}=4.3$ K. Unlike ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$, a related europium-containing Zintl compound, no colossal magnetoresistance (CMR) is observed in ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_9$. We attribute the absence of CMR to the smaller carrier density and the larger distance between Eu ions and In-Sb polyhedra in ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_9$. Our results indicate that ${\mathrm{Eu}}_{11}\mathrm{In}{\mathrm{Sb}}_9$ has potential applications as a thermoelectric material through doping or as a long-wavelength detector due to its narrow gap.

It has long remained elusive whether ${\mathrm{CuCo}}_2{\mathrm{S}}_4$ thiospinel shows bulk superconductivity (SC). Here we clarify the issue by studying the samples of sulfur-deficient ${\mathrm{CuCo}}_2{\mathrm{S}}_{3.7}$ and sulfurized ${\mathrm{CuCo}}_2{\mathrm{S}}_4$. The sample ${\mathrm{CuCo}}_2{\mathrm{S}}_{3.7}$ has a smaller lattice constant of $a=9.454$ Å, and it is not superconducting down to 1.8 K. After a full sulfurization, the $a$ axis of the thiospinel phase increases to 9.475 Å, and the thiospinel becomes nearly stoichiometric ${\mathrm{CuCo}}_2{\mathrm{S}}_4$, although a secondary phase of slightly Cu-doped ${\mathrm{CoS}}_2$ forms. Bulk SC at 4.2 K and Pauli paramagnetism have been demonstrated for the sulfurized ${\mathrm{CuCo}}_2{\mathrm{S}}_4$ by the measurements of electrical resistivity, magnetic susceptibility, and specific heat.

High-quality titanium nitride (TiN) film is a prominent plasmonic material in the fields of plasmonics and nanophotonics, which is usually synthesized on several limited substrates (such as MgO, silicon, and sapphire). With the rapid development of hybrid optoelectronic devices composed of plasmonic materials and functional oxides, it is promising to integrate high-quality TiN films with a variety of functional oxides, which requires to accommodate TiN films to a broad spectrum of lattice mismatch. In this work, we synthesized high-quality single-crystalline TiN films on various oxide substrates $[\mathrm{MgO}, {\mathrm{MgAl}}_2{\mathrm{O}}_4,$ $({\mathrm{LaAlO}}_3{)}_{0.30}\text{−}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_3{)}_{0.70}, \mathrm{and} {\mathrm{YAlO}}_3]$ by high-pressure magnetron sputtering. It is surprising that the epitaxial TiN films can accommodate extremely large lattice mismatch up to $-15.39\%$. The crystal and electronic structures of the TiN films were characterized by high-resolution x-ray diffraction, atomic force microscopy, and x-ray photoemission spectroscopy measurements. The optical and electrical properties of the TiN films on various oxide substrates were investigated by spectroscopic ellipsometry and Hall effect measurements. It is revealed that the remarkable optical properties of TiN films are robust even on highly lattice-mismatched oxide substrates. Our work paves a way to integrate plasmonic TiN films with a variety of functional oxides for high-performance hybrid optoelectronic devices.

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge.

Epitaxial growth of ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ (HZO) thin films allows for the stabilization of the metastable orthorhombic phase with robust ferroelectric properties. So far, the ferroelectric phase is most commonly stabilized on perovskite substrates upon insertion of a buffer layer of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO); however, little is known about the role played by the LSMO buffer layer and the interface between HZO and LSMO. Inspection of a $\mathrm{HZO}/\mathrm{LSMO}/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure by scanning transmission electron microscope imaging and electron energy loss spectroscopy shows that, despite the substantial structural mismatch between HZO and LSMO, the interface between them is relatively sharp spanning ∼2 atomic layers. The LSMO surface, expected to be mostly $\mathrm{Mn}{\mathrm{O}}_2$ terminated, undergoes a chemical reconstruction consisting of the substitution of the Mn cations by a mixture of Hf/Zr cations. Density functional theory calculations show that the substitution of Mn by Hf on the $\mathrm{Mn}{\mathrm{O}}_2$-terminated surface of LSMO is energetically favorable, as the higher electronegativity and valence of Hf with respect to Mn balances the surface charge of the $\mathrm{Mn}{\mathrm{O}}_2$ layer.

Dislocations provide fast diffusion pathways for atoms in semiconductors which can alter compositional profiles of finely tuned heterostructures. We show in model lattice-mismatched IV-VI semiconductor heterostructures of SnSe/PbSe on GaAs substrates that threading dislocations are highly enriched with group IV species of the neighboring layer, with artifacts that reflect the dynamic nature of dislocations when growing dissimilar materials as well as altered bonding properties at the dislocation core. Using atom probe tomography, we characterize one-dimensional nanometer-wide Sn-enriched filaments, which extend downward from SnSe through threading dislocations in the PbSe layer and infiltrate the remote PbSe/GaAs interface through the misfit dislocation network in the short time of growth. Local Sn compositions of only 6%–8% around the dislocations are significantly lower than the 30% Sn expected in bulk PbSe. We estimate the diffusivity of Sn atoms along threading dislocations to be ${10}^{–14}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{–1}$ at 300 °C, approximately three orders of magnitude larger than the lattice diffusivity. In contrast, Pb atoms from PbSe either do not diffuse upward into the orthorhombic SnSe layer or do so only a short distance as one-dimensional filaments before abruptly stopping, likely due to the nature of SnSe on PbSe epitaxy involving lattice mismatch. Beyond compositional anomalies, we detect elevated multiple-atom evaporation events that are spatially correlated to the dislocation, over and above an already high baseline in the matrix. In mixed-bonded IV-VI materials, where previous work has linked such events to the nature of bonding, we find that dislocations show up as distinct from the matrix.

Despite much experimental and simulation effort, the existence of a Hall-Petch to inverse Hall-Petch transition in nanocrystalline ceramics remains elusive. By employing molecular dynamics simulations, we unambiguously reveal a transition from strengthening to softening in the shear deformation of nanocrystalline silicon carbide ceramics as a function of grain size. Results show a well-defined maximum in the shear strength for grain sizes in the range 6.2 to 7.7 nm. Further decrease in grain size leads to diminishing strength, consistent with an inverse Hall-Petch behavior. As grain size is reduced the increasing grain boundary (GB) regions lead to homogenization of shear stresses across the microstructure, allowing for lower local shear stress levels at higher macroscopically applied stresses. This delays shear localization within GB regions, preventing cavitation, nanocracking, and premature failure, and is responsible for the observed Hall-Petch behavior. In contrast, at grain sizes <6.2 nm, the rather compliant nature of the structurally disordered GB regions dominates the mechanical response, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. A composite model delineating the transition between Hall-Petch and inverse Hall-Petch behavior is successful at describing the mechanical behavior of nanocrystalline silicon carbide as a function of grain size.

Using first-principles calculations and magnetic resonance experiments, we investigated the physical properties of the negatively charged nitrogen-vacancy (NV) center in 4H-SiC, a promising spin qubit. Our predictive theoretical model in conjunction with experimental measurements reveals a large sensitivity to strain and symmetry. The measured and computed zero-phonon lines (ZPLs) are in agreement and show a consistent trend as a function of the defect location in the crystal. The computed ZPLs are extremely sensitive to the geometrical configurations of the ground and excited states, and large supercells with more than 2 000 atoms are required to obtain accurate numerical results. We find that the computed decoherence time of the basal NV centers at zero magnetic field is substantially larger than that of the axial configurations. Furthermore, at natural nuclear spin abundance and zero field, the Hahn-echo coherence time of one of the basal configurations is similar to that of the axial divacancy in isotopically purified SiC.

${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_1{\mathrm{Cu}}_2{\mathrm{O}}_x$ (Bi-2212) is the only high-temperature superconductor (HTS) available as a round wire with high critical current density $J_c$, which makes it a very compelling candidate for ultrahigh-field magnet applications. By contrast, other copper oxide HTS conductors like $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (where $R$ stands for rare earth) and (Bi,${\mathrm{Pb})}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (Bi-2223) must be made in tape form to minimize the density of current blocking high-angle grain boundaries. Understanding the mechanism enabling high $J_c$ in round wire Bi-2212 is important intellectually because it breaks the paradigm that forces HTS conductors into tape geometries that reproduce their strong crystalline anisotropy. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high $J_c$, even though its $\sim {15}^{\circ }$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range of $\le 5^{\circ }$ misorientation. Its ability to be strongly overdoped should be valuable too since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While $J_c$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature [a hysteretic $J_c\left(H\right)$ characteristic] evident in even the very best Bi-2223 tapes with an $\sim {15}^{\circ }$ FWHM uniaxial texture. We attribute the high $J_c$ and lack of weak-link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212. These features enable $c$-axis brick wall current flow when ab-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.