Millimeter-sized ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ single crystals are grown out of a Bi-Te flux and characterized using magnetic, transport, scanning tunneling microscopy, and spectroscopy measurements. The magnetic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ below $T_N$ is determined by powder and single-crystal neutron diffraction measurements. Below $T_N$ = 24 K, ${\mathrm{Mn}}^{2+}$ moments order ferromagnetically in the $ab$ plane but antiferromagnetically along the crystallographic $c$ axis. The ordered moment is 4.04(13)${\mu }_B$/Mn at 10 K and aligned along the crystallographic $c$ axis in an A-type antiferromagnetic order. Below $T_N$, the electrical resistivity drops upon cooling or when going across the metamagnetic transition in increasing magnetic fields. A critical scattering effect is observed in the vicinity of $T_N$ in the temperature dependence of thermal conductivity, indicating strong spin-lattice coupling in this compound. However, no anomaly is observed in the temperature dependence of thermopower around $T_N$. Fine tuning of the magnetism and/or electronic band structure is needed for the proposed topological properties of this compound. The growth protocol reported in this work might be applied to grow high-quality crystals where the electronic band structure and magnetism can be finely tuned by chemical substitutions.

Nonmonotonic bending-induced changes of fundamental band gaps and quasiparticle energies are observed for realistic nanoscale phosphorene nanosheets. Calculations using stochastic many-body perturbation theory show that even slight curvature causes significant changes in the electronic properties. For small bending radii ($<4$ nm) the band gap changes from direct to indirect. The response of phosphorene to deformation is strongly anisotropic (different for zigzag vs armchair bending) due to an interplay of exchange and correlation effects. Overall, our results show that fundamental band gaps of phosphorene sheets can be manipulated by as much as 0.7 eV depending on the bending direction.

We study here a mesoscopic metasurface made of a randomly distributed set of long vertical metallic rods attached to a thin elastic plate. The $A_0$ Lamb wave propagation is strongly affected by the local change in apparent stiffness of the plate induced by the low-quality factor resonance of the rods. At the resonance, the plate-plus-rods system is allowed to move freely, and plate waves can penetrate into the metamaterial. At the antiresonance, the plate behaves in terms of waves as if it was clamped by the rods in the metamaterial region, which induces large-frequency band gaps. Between the resonant and antiresonant frequencies, the continuous change in effective rigidity results in a continuous change in reflectivity. In the present paper, we aim at measuring the corresponding complex impedance of the metasurface in terms of amplitude and phase. Experimental data are presented to estimate the effective impedance of a locally resonant metasurface, in agreement with theoretical prediction and numerical simulation.

In ionic materials, circularly polarized phonons carry orbital magnetic moments that arise from circular motions of the ions, and which interact with other magnetic moments or fields. Here, we calculate the orbital magnetic moments of phonons in 35 different materials using density functional theory, and we identify the factors that lead to, and materials that show, large responses. We compute the resulting macroscopic orbital magnetic moments that can be induced by the excitation of coherent phonons using mid-infrared laser pulses, and we evaluate the magnitudes of the phonon Zeeman effect in a strong magnetic field. Finally, we apply our formalism to chiral phonons, in which the motions of the ions are intrinsically circular. The zoology presented here may serve as a guide to identifying materials for phonon and spin-phonon driven phenomena.

The nonlocal van der Waals (NL-vdW) functionals [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] are being applied more and more frequently in solid-state physics, since they have shown to be much more reliable than the traditional semilocal functionals for systems in which weak interactions play a major role. However, a certain number of NL-vdW functionals have been proposed during the last few years, such that it is not always clear which one should be used. In this work, an assessment of NL-vdW functionals is presented. Our test set consists of weakly bound solids, namely rare gases, layered systems such as graphite, and molecular solids, but also strongly bound solids in order to provide a more general conclusion about the accuracy of NL-vdW functionals for extended systems. We found that among the tested functionals, rev-vdW-DF2 [I. Hamada, Phys. Rev. B 89, 121103(R) (2014)] is very accurate for weakly bound solids, but also quite reliable for strongly bound solids.

We report a detailed investigation of ${\mathrm{K}}_2{\mathrm{MnS}}_2$ and ${\mathrm{K}}_2{\mathrm{MnSe}}_2$ from the ${\mathrm{K}}_2{\mathrm{MnS}}_2$ structure type and their magnetic solid solution ${\mathrm{K}}_2{\mathrm{MnS}}_{2-x}{\mathrm{Se}}_x$ and find that compounds of this structure type consist of strongly coupled pseudo-one-dimensional antiferromagnetic chains that collectively represent a frustrated two-dimensional triangular antiferromagnet. Bulk samples of ${\mathrm{K}}_2{\mathrm{MnS}}_{2-x}{\mathrm{Se}}_x$ with $0\le x\le 2$ are characterized using x-ray diffraction, neutron diffraction, magnetization, and heat-capacity measurements. An incommensurate cycloid magnetic structure with a magnetic propagation vector $k=\left[0.5801\right]$ is observed for all samples in ${\mathrm{K}}_2{\mathrm{MnS}}_{2-x}{\mathrm{Se}}_x$, and the ordering is robust despite a 12% increase in cell volume. Geometric frustration of chains results in incommensurability along $a$ and a two-step magnetic transition. The varying geometries accessible in compounds of this structure type are presented as promising avenues to tune frustration.

Edge-contacted superconductor-graphene-superconductor Josephson junctions have been utilized to realize topological superconductivity, and have shown superconducting signatures in the quantum Hall regime. We perform first-principles calculations to interpret electronic couplings at the superconductor-graphene edge contacts by investigating various aspects in hybridization of molybdenum $d$ orbitals and graphene π orbitals. We also reveal that interfacial oxygen defects play an important role in determining the doping type of graphene near the interface.

Four oxidation states $\left({\mathrm{V}}^{2+},{\mathrm{V}}^{3+},{\mathrm{V}}^{4+},\mathrm{and}{\mathrm{V}}^{5+}\right)$ in vanadium oxides and the conversion between them have attracted attention for application to batteries and electronics. Compared to single-valence counterparts, however, there have been few reports on the fundamental properties of mixed-valence vanadium oxide films, as their complexity and closeness in thermodynamic phase diagrams hinder the formation of pure phases in film. Here, using an epitaxial growth technique with precise control of oxygen partial pressure (20–100 mTorr) on (111)-oriented Y-stabilized $\mathrm{Zr}{\mathrm{O}}_2$, we selectively stabilize pure phases of $\mathrm{V}{\mathrm{O}}_2\left(\mathrm{B}\right)$ $(m=\infty ), {\mathrm{V}}_6{\mathrm{O}}_{13}$ $\left(m=6\right)$, and ${\mathrm{V}}_2{\mathrm{O}}_5$ $\left(m=2\right)$, so-called Wadsley phases $\left({\mathrm{V}}_m{\mathrm{O}}_{2m+1},m>1\right)$ in which ${\mathrm{V}}^{4+}$ and/or ${\mathrm{V}}^{5+}$ can coexist. Fractional increase of ${\mathrm{V}}^{4+}$ changes the electrical ground state, insulating $\mathrm{V}{\mathrm{O}}_2\left(\mathrm{B}\right)$ and ${\mathrm{V}}_2{\mathrm{O}}_5$, metallic ${\mathrm{V}}_6{\mathrm{O}}_{13}$ transition into insulators below 150 K. While $\mathrm{V}{\mathrm{O}}_2\left(\mathrm{B}\right)$ and ${\mathrm{V}}_6{\mathrm{O}}_{13}$ exhibit strong spectral weights at low photon energy in the room-temperature extinction coefficients, the band-edge absorption shifts toward higher photon energy for smaller $m$, opening an indirect band gap of 2.6 eV in ${\mathrm{V}}_2{\mathrm{O}}_5$. The sharp contrast of electrical and optical properties between vanadium Wadsley phases highlights the importance of precisely controlling the oxidation state of vanadium.

Neutron powder diffraction has been used to investigate the spin structure of the hard-magnetic alloy $\mathrm{F}{\mathrm{e}}_{3+x}\mathrm{C}{\mathrm{o}}_{3-x}\mathrm{T}{\mathrm{i}}_2$ ($x=0,2,3$). The materials are produced by rapid quenching from the melt, they possess a hexagonal crystal structure, and they are nanocrystalline with crystallite sizes $D$ of the order of 40 nm. Projections of the magnetic moment onto both the crystalline $c$ axis and the basal plane were observed. The corresponding misalignment angle exhibits a nonlinear decrease with $x$, which we explain as a micromagnetic effect caused by Fe-Co site disorder. The underlying physics is a special kind of random-anisotropy magnetism that leads to the prediction of $1/D^{1/4}$ power-law dependence of the misalignment angle on the crystallite size.

Vacancy formation and migration control self-diffusion in pure crystalline materials, whereas irradiation produces high concentrations of vacancy and self-interstitial atom defects, exceeding by many orders of magnitude the thermal equilibrium concentrations. The defects themselves, and the extended dislocation microstructure formed under irradiation, generate strongly spatially fluctuating strain and stress fields. These fields alter the local formation and migration enthalpies of defects, and give rise to the anisotropy of diffusion even if in the absence of stress the diffusion tensor is isotropic. We have performed ab initio calculations of formation and migration energies of vacancies in all the commonly occurring body-centered-cubic (bcc) metals, including alkaline, alkaline-earth and transition metals, and computed elastic dipole and relaxation volume tensors of vacancies at the equilibrium lattice positions and along the vacancy migration pathways. We find that in all the bcc metals the dipole tensor of a migrating vacancy at a saddle point exhibits an anticrowdion character. Applied external stresses or the local stresses generated by dislocations may enhance or suppress anisotropic diffusion by altering the energy barriers with respect to the direction of migration of a defect.

Emergent properties of complex oxide interfaces are based on interface reconstruction that is driven by mismatch of electronic bands, valence states, interaction lengths, and even crystal symmetry of the interface. In particular, emergent ferromagnetism at the interface of two materials that do not exhibit ferro- or ferrimagnetism in the bulk has been stabilized as a result of competing exchange interactions. When ${\mathrm{LaNiO}}_3$ and ${\mathrm{CaMnO}}_3$, which are a paramagnetic metal and antiferromagnetic insulator in the bulk, respectively, are brought together, ferromagnetism emerges at the interface. Here we show that in (111)-oriented ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ (LNO/CMO) superlattices, ${\mathrm{Ni}}^{2+}\text{−}{\mathrm{Mn}}^{4+}$ superexchange interactions due to polar mismatch at the LNO/CMO interfaces are responsible for the emergent ferromagnetism. Compared to (001)-oriented LNO/CMO superlattices, (111)-oriented LNO/CMO superlattices exhibit enhanced interfacial ferromagnetism with a $T_C>200$ K, greater than the bulk antiferromagnetic transition temperature of ${\mathrm{CaMnO}}_3$ and a saturated magnetic moment enhanced by up to a factor of 3. Furthermore, we observe exchange bias in (111)-oriented superlattices. The strong exchange interactions along the (111) interface, manifest in the enhanced $T_c$ and exchange bias, make this class of CMO-based materials with (111)-oriented interfaces good candidates for low-dimensional spin-polarized materials in spintronic applications.

The ferromagnetic (FM) nature of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ has been investigated with muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) measurements on an aligned, sintered plate-shaped sample. A clear muon spin precession frequency ($f_{\mathrm{FM}}$) corresponding to the static internal FM field at the muon site showed an order parameter-like temperature dependence and disappeared above around 582 K ($\sim T_{\mathrm{C}}$). This indicated that the implanted muons are static in the ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ lattice even at temperatures above around 600 K. Using the muon site and local spin densities predicted by DFT calculations, the ordered Nd moment ($M_{\mathrm{Nd}}$) was estimated to be 3.31 ${\mu }_{\mathrm{B}}$ at 5 K, when both ${\mathit{M}}_{\mathrm{Fe}}$ and ${\mathit{M}}_{\mathrm{Nd}}$ are parallel to the $\widehat{\mathbf{c}}$ axis and $M_{\mathrm{Fe}}=2.1{\mu }_{\mathrm{B}}$. Furthermore, $M_R$ in $R_2{\mathrm{Fe}}_{14}\mathrm{B}$ with $R=\mathrm{Y}$, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, and Tm was estimated from $f_{\mu }$ values reported in earlier ${\mu }^{+}\mathrm{SR}$ work, using the FM structure proposed by neutron scattering and the same muon site and local spin density as in ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. Such estimations yielded $M_R$ values consistent with those obtained by the other methods.

We show that biexciton formation can be highly efficient in a solar cell whose semiconductor absorber is filled with an array of metallic nanoparticles having plasmonic resonances tuned to the semiconductor gap energy. This process can be viewed as a plasmon-enhanced multiple exciton generation, with the resulting cell efficiency exceeding the Shockley-Queisser limit. We demonstrate via theory and simulations that the efficiency of the process increases with decreasing semiconductor gap size, and illustrate such by considering in detail three systems with gradually decreasing gap size: GaAs, Si, and Ge.

The vibrational properties of glasses remain a topic of intense interest due to several unresolved puzzles, including the origin of the Boson peak and the mechanisms of thermal transport. Inelastic scattering measurements have revealed that amorphous solids support collective acoustic excitations with low THz frequencies despite the atomic disorder, but these frequencies are well below most of the thermal vibrational spectrum. Here, we report the observation of acoustic excitations with frequencies up to 10 THz in amorphous silicon. The excitations have atomic-scale wavelengths as short as 6 Å and exist well into the thermal vibrational frequencies. Simulations indicate that these high-frequency waves are supported due to the high group velocity and monatomic composition of a-Si, suggesting that other glasses with these characteristics may also exhibit such excitations. Our findings demonstrate that a substantial portion of thermal vibrational modes in amorphous materials can still be described as a phonon gas despite the lack of atomic order.

Phase transitions of water molecules are commonly expected to occur only under extreme conditions, such as nanoconfinement, high pressure, or low temperature. We herein report the disordered-ordered phase transition of two-dimensional interfacial water molecules under ambient conditions using molecular-dynamics simulations. This phase transition is greatly dependent on the charge dipole moment, production of both charge values, and the dipole length of the solid surface. The phase transition can be identified by a sharp change in water-water interaction energies and the order parameters of the two-dimensional interfacial water monolayer, under a tiny dipole moment change near the critical dipole moment. The critical dipole moment of the solid material surface can classify a series of materials that can induce distinct ordered phases of surface water, which may also result in surface wetting, friction, and other properties.

Herein, we report on the growth of single crystals of various ${\left({\text{Mo}}_{2/3}{\mathrm{RE}}_{1/3}\right)}_2\text{Al}\text{C} (\mathrm{RE}=\mathrm{Nd},\mathrm{Gd},\mathrm{Dy},\mathrm{Ho},\mathrm{Er}) i$-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing $i$-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.

A mesoscopic constitutive law for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), an ultra-anisotropic energetic molecular crystal, is proposed and validated on molecular dynamics (MD) simulations. The two dominant deformation mechanisms observed at nanometric scale and limited stress (less than 1 GPa) are a buckling instability and a nonsymmorphic twinning (irreversible) transformation. A thermodynamically consistent continuum model is detailed, with nonlinear elasticity in pressure constructed to reproduce a cold equation of state. The twinning-buckling phase transition observed in MD is modeled by using a phase field by reaction pathway (PF-RP) formalism. In order to validate the present constitutive law, we first study the response of the single crystal under constant-strain-rate uniaxial compressions for various directions in the basal plane and present one-to-one comparisons between MD and PF-RP simulations. As an upscale case for the constitutive law, a large polycrystal is subjected to a shock compression at low velocity and the activation of the twinning-buckling mechanism is discussed.

$\mathrm{U}{\mathrm{O}}_{2.07}$ was characterized using neutron total scattering in order to elucidate defect morphology in the low oxygen-to-metal regime $(x<0.125$ for $\mathrm{U}{\mathrm{O}}_{2+x})$. Data were collected at temperatures (600 and 1000 °C) coinciding with the single-phase $\mathrm{U}{\mathrm{O}}_{2+x}$ region of the established phase diagram, and results were compared with data of stoichiometric $\mathrm{U}{\mathrm{O}}_2$ collected at near-identical temperatures. Experimental data were modeled and interpreted using a holistic approach employing a combination of analyses that characterized multiple spatial length scales. Preferential modeling of long-range atomic arrangements with Rietveld refinement suggests the existence of primarily monointerstitials in $\mathrm{U}{\mathrm{O}}_{2.07}$, whereas preferential modeling of short-range atomic structures with small-box pair distribution function (PDF) refinement indicates the presence of defect clusters in $\mathrm{U}{\mathrm{O}}_{2.07}$. Simultaneous modeling of multiple length scales using complementary reverse Monte Carlo (RMC) and molecular dynamics (MD) methods confirms that excess oxygen atoms in $\mathrm{U}{\mathrm{O}}_{2.07}$ exist as small defects, such as monointerstitials and di-interstitials. RMC and MD results agree with diffraction analysis but differ significantly from small-box PDF refinements, which may be related to a lack of intermediate- and long-range structural information gained from the small-box PDF refinement procedure. Employing a combination of analysis methods with varying length-scale sensitivities enabled more accurate assessment of the $\mathrm{U}{\mathrm{O}}_{2+x}$ defect structure. Our findings provide experimental support for previously predicted di-interstitial defect morphologies in $\mathrm{U}{\mathrm{O}}_{2+x}$ that highly influence the accurate prediction of bulk physiochemical properties of $\mathrm{U}{\mathrm{O}}_{2+x}$, such as oxygen diffusivity.

Room-temperature electron-beam deposition of transparent silica produces thin films that are typically under large compressive stresses (a few hundreds of megapascals), yet of smaller density than tempered glass. The physical origin of these stresses remains unknown as all prior experimental studies were performed in manufacturing conditions where it is impossible to disentangle effects arising from vaporization versus residual gases. Here, we tackle this problem using a combination of experimental and numerical approaches. Our key experiment is performed after having achieved a high vacuum that allows us to approach the ideal case where residual gas effects are negligible. It shows that, even in this case, films are strongly compressive, which rules out in situ reactive processes as a possible origin of these stresses. Another test allows us to show that collisional slowing down causes films to grow much less dense and less compressive (even tensile). Our numerical model, designed to represent deposition in the absence of reactive processes, produces films with reasonable stress and density values, yet only when assuming that vaporized particles reach the growing film with kinetic energies of order a few electron volts, which is an order of magnitude higher than literature estimates. We argue that such values are plausible because, silica being an electric insulator, its surface may concentrate charges when irradiated by electrons, and hence its e-beam vaporization, which is a much more rapid process than its evaporation, is likely to involve Coulombian effects. Finally, the microstructural analysis of numerical systems shows that these compressive stresses result primarily from the fact that, in usual deposition conditions, films tend to be oxygen-deficient, and thus contain a significant fraction of coordination defects akin to partial oxygen vacancies, which cause their Si-O covalent network to be less cohesive and tensile than that of relaxed silica.