A comprehensive first-principles calculation has been carried out revealing that at sufficiently high Li concentration and certain well-defined configurations a phase transition from black to blue phosphorene can take place. Blue phosphorene, a newly predicted allotrope of phosphorus, possesses unique crystalline and electronic structure and is a promising candidate, not only for fundamental research but also for electronic and optoelectronic applications. Methods of growing high-quality blue phosphorene layers are highly desirable but challenging. Here, a kinetic pathway to grow blue phosphorene layers from black phosphorene layers via Li intercalation is proposed based on first-principles study. This study pointed out that Li atoms intercalated in black phosphorene could act as a “catalyst” in the “reactive region” of the lone pair of P atoms, leading to a P-P bond breaking and, subsequently, a local structural transformation from an orthorhombic lattice to an assembly of parallel narrow nanoribbons with rhombohedra-like symmetry. During Li deintercalation, these nanoribbons are self-mended and form blue phosphorene layers. The interlayer distance was found to be 4.60 Å for a double layer with AA stacking and 4.13 Å for a multilayer with ABC stacking, indicating monolayer blue phosphorene can be mechanically exfoliated. This study also points out the possibility of new phases in other systems, where intercalation can lead to an unexpected structural phase transition and even a discovery of novel materials.

We report an investigation of the spin-to-charge current conversion in sputter-deposited films of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ onto single crystalline layers of yttrium iron garnet (YIG-${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) and polycrystalline films of permalloy ($\mathrm{Py}\text{−}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$). Pure spin current was injected into the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layer by means of the spin pumping process produced by the spin precession in microwave driven ferromagnetic resonance of the ferromagnetic film. The spin-to-charge current conversion occurring at the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/ferromagnet interface is attributed to the inverse Rashba-Edelstein effect (IREE). From the data, we verified that the voltage generated by the spin-to-charge current conversion process in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has the same polarity as the inverse spin Hall effect in Ta, which is opposite to the one in Pt. Also, from the dependence of the voltage on the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thickness we were able the to calculate the IREE length values and found that $1.2\mathrm{pm}\le |{\lambda }_{\mathrm{IREE}}|\le 2.2\mathrm{pm}$. This result allows us to conclude that indeed the surface states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ have a dominant role in the spin-to-charge current conversion process, and the mechanism based on the spin diffusion process plays a secondary role.

The accurate determination of compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content ${\mathrm{In}}_x{\mathrm{Ga}}_{1–x}\mathrm{N}$ light emitting diodes (LEDs), the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at subnanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in ${\mathrm{In}}_x{\mathrm{Ga}}_{1–x}\mathrm{N}$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in ${\mathrm{In}}_x{\mathrm{Ga}}_{1–x}\mathrm{N}$ QWs at different indium concentrations and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim 18\%$) content ${\mathrm{In}}_x{\mathrm{Ga}}_{1–x}\mathrm{N}$ QWs, there exists a striking deviation from the model in higher In content (≥24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role in the material performance.

Accessing unexplored crystal growth conditions often reveals new regimes of physical behavior. In this work, performing molecular beam epitaxy growth of the technologically important superconductor NbN at temperatures greater than 1000 ${}^{\circ }\mathrm{C}$ reveals a growth mode that has not been accessed before. This mode results in persistent reflection high-energy electron diffraction (RHEED) oscillations through the entire growth, resulting in atomically smooth surfaces, normal metal resistivities of $\sim 37\mu \mathrm{\Omega }$ cm. We find that the superconducting critical temperature depends strongly on growth temperature, and report a maximum superconducting critical temperature of 15.5 K. Electron microscopy studies reveal a rich range of crystalline phases that depend on the growth temperature and correlate to the physical properties. Surprisingly, a reversal of the sign of the Hall coefficient from n-type to p-type is observed as the NbN films are cooled, indicating an electronic structure that has not been observed before in this material. In addition to this observation, the crystallinity of the high-temperature epitaxial NbN allows for an ordered Abrikosov vortex lattice to be imaged for the first time in this superconductor.

We find unusually large cross-polarized (and antisymmetric) Raman signature of ${\mathrm{A}}_{\mathrm{g}}$ phonon mode in ${\mathrm{CrI}}_3$, in agreement with experiments. The signal is present only when the following three effects are considered in concert: ferromagnetism on Cr atoms, spin-orbit interaction, and resonant effects. Somewhat surprisingly, we find that the relevant spin-orbit interaction potential originates from iodine atoms, despite magnetism being mostly on chromium atoms. We analyze the Raman signature as a function of magnetic order, the direction of the magnetic moment, energy, and polarization of light used for Raman scattering, as well as carrier lifetime. In addition to a strong cross-polarized Raman signal, we also find unusually strong phonon modulated magneto-optical Kerr effect (MOKE) in ${\mathrm{CrI}}_3$.

Tuning the heat flow is fundamentally important for the design of advanced functional materials. Here polymers are of particular importance because they provide different pathways for the energy transfer. More specifically, the heat flow between the two covalently bonded monomers is over 100 times faster than between the two nonbonded monomers interacting via the van der Waals (vdW) forces. Therefore, the delicate balance between these two contributions often provides a guiding tool for the tunability in thermal transport coefficient $\kappa$ of the polymeric materials. Traditionally most studies have investigated $\kappa$ in the linear polymeric materials, the recent interests have also been directed towards the highly cross-linked polymers (HCP). In this work, using the generic molecular dynamics simulations, we investigate the factors effecting $\kappa$ of HCP. We emphasize the importance of the cross-linking bond types and their influence on the network microstructure, with a goal of providing a guiding principle for the tunability in $\kappa$. While these simulation results are discussed in the context of the available experimental data, we also make predictions.

We demonstrate the fabrication of an ultrathin gallium film, also known as gallenene, beneath epitaxial graphene on 6H-SiC under ambient conditions triggered by liquid gallium intercalation. Gallenene has been fabricated using liquid metal intercalation, achieving lateral intercalation and diffusion of Ga atoms at room temperature on square centimeter areas limited only by the graphene samples’ size. The stepwise self-propagation of the gallenene film below the epitaxial graphene surface on the macroscopic scale was observed by optical microscopy shortly after the initial processing without further physical or chemical treatment. Directional Ga diffusion of gallenene occurs on SiC terraces since the terrace steps form an energetic barrier (Ehrlich-Schwoebel barrier), retarding the gallenene propagation. The subsequent conversion of the epitaxial graphene into quasi-free-standing bilayer graphene and the graphene-gallenene heterostack interactions have been analyzed by x-ray photoelectron spectroscopy and Raman measurements. The results reveal an alternative approach for the controlled fabrication of wafer-scale gallenene as well as for two-dimensional heterostructures and stacks based on the interaction between liquid metal and epitaxial graphene.

Here we systematically investigate the structure, phase stability, half-metallicity, and topological electronic structure for a topological spintronic material $\mathrm{Na}{\mathrm{V}}_2{\mathrm{O}}_4$. The material has a tetragonal structure with excellent dynamical and thermal stabilities. It shows a half-metallic ground state, where only the spin-up bands are present near the Fermi level. These bands are demonstrated to form a nodal line with the double degeneracy on the $k_z=0$ plane. The nodal line is robust against spin-orbit coupling, under the protection of the mirror symmetry. The nodal line band structure is very clean, thus the drumhead surface states can be clearly identified. Remarkably, the nodal line and drumhead surface states have the $100\%$ spin polarization, which are highly desirable for spintronics applications. In addition, by shifting the magnetic field in-plane, we find that the nodal line can transform into a single pair of Weyl nodes. The nodal-line and Weyl-node fermions in the bulk, as well as the drumhead fermions on the surface are all fully spin-polarized, which may generate interesting physical properties and promising applications.

Research on the remarkable characteristics of topological phases and topological phase transitions in various topological systems has been performed. In this paper, we propose a two-dimensional topological system composed of triangle photonic crystals (PCs) with $C_3$ point group symmetry, which hosts both regularly multiple fourfold-degenerate double Dirac cones and topological transitions in all-dielectric PC systems. By applying the tight-binding model to our system, we reveal the mechanism for topological transitions in the present configuration manufactured of conventional silicon material. Topological edge states at the common boundary of two different topological regions are demonstrated, which indicates that electromagnetic wave can propagate unidirectionally and robustly along the boundary with eximious transmission performance. Further, we manifest that topological edge states characterized by pseudospin-dependent unidirectional edge transport and have robustness against commonly minor defects. This flexible topological system demonstrates more abundant physics principles compared with previous research, which can be used to explore different topological phenomena and be implemented in integrated optical devices.

Zirconium dioxide, or zirconia, is a common and useful ceramic with a wide range of applications, from fuel cells to odontology. Its phase diagram is simple and well understood, having a structure which is monoclinic at temperatures up to 1500 K, tetragonal up to 2700 K and cubic up to 3000 K. Zirconia is rarely used in its pure form, being typically doped with ${\text{Y}}_2{\text{O}}_3$, MgO or ${\text{TiO}}_2$, and in this regime its phase diagram becomes much more complex. In this context, ab initio molecular dynamics (AIMD) can provide a detailed atomistic description of the phase diagram of this system, accurately describing its stable phases and transition regions. In this work, 3 mol-% ${\text{Y}}_2{\text{O}}_3$ (3YSZ) crystals doped with different Ti contents were studied at the density-functional level. For Ti contents varying from 0 to 30 at%, a global search algorithm was first used to explore the 0 K potential-energy surface and determine the most stable sites for the added Ti atoms. It was found that, at low Ti compositions $X_{\text{Ti}}$, small ${\text{TiO}}_2$ clusters form, followed by ${\text{TiO}}_2$ channels and infinite ${\text{TiO}}_2$ planes at larger $X_{\text{Ti}}$ values, and that the highest stability is achieved at 9% Ti. AIMD simulations within the isothermal-isobaric NPT ensemble were then performed to characterize the temperature-dependent phase changes as a function of the Ti content, where it was found that the Ti-doped structures presented considerably smaller volume changes near the phase-change critical temperatures. These findings suggest that YSZ materials doped with a small amount of Ti are both energetically and kinetically more stable than the undoped counterparts, in the ideal proportion of 3% ${\text{TiO}}_2$ for every 1% ${\text{Y}}_2{\text{O}}_3$ doping.

Phase stability and properties of two-dimensional phosphorus carbide, ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ ($0\le x\le 1$), are investigated using the first-principles method in combination with cluster expansion and Monte Carlo simulation. Monolayer ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ is found to be a phase-separating system which indicates difficulty in fabricating monolayer ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ or crystalline ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ thin films. Nevertheless, a bottom-up design approach is used to determine the stable structures of ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ of various compositions which turn out to be superlattices consisting of alternating carbon and black phosphorene nanoribbons along the armchair direction. Results of first-principles calculations indicate that once these structures are produced, they are mechanically and thermodynamically stable. All the ordered structures are predicted to be semiconductors, with band gap (Perdew-Burke-Ernzerhof) ranging from 0.2 to 1.2 eV. In addition, the monolayer ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ are predicted to have high carrier mobility, and high optical absorption in the ultraviolet region which shows a redshift as the P:C ratio increases. These properties make two-dimensional ${\mathrm{P}}_x{\mathrm{C}}_{1-x}$ promising materials for applications in electronics and optoelectronics.

Low-dimensional spin-1/2 transition metal oxides and oxyhalides continue to be at the forefront of research investigating nonclassical phases such as quantum spin liquids. In this study, we examine the magnetic properties of the oxyhalide ${\text{Cu}}_2{\text{(OH)}}_3\text{Br}$ in the botallackite structure using first-principles density functional theory, linear spin-wave theory, and exact diagonalization calculations. This quasi-two-dimensional system consists of ${\text{Cu}}^{2+}S=1/2$ moments arranged on a distorted triangular lattice. Our exact diagonalization calculations, which rely on a first-principles-based magnetic model, generate spectral functions consistent with inelastic neutron scattering data. By performing computational experiments to disentangle the chemical and steric effects of the halide ions, we find that the dominant effect of the halogen ions is steric in the ${\text{Cu}}_2{\text{(OH)}}_{3}X$ series of compounds.

Two-dimensional (2D) magnetic materials with strong magnetism, long magnetic relaxation times, high temperature ferromagnetism, and large magnetic anisotropy energy (MAE) are promising for applications of nanoscale spintronic devices. Using the spin-orbital coupling density functional theory (DFT) calculations, we investigate the structural stability, electronic structures, and magnetic properties of graphenelike carbon-nitride ($\mathrm{gh}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$) sheets with the adsorption of whole series of $4f$-block elemental (lanthanide; Ln) atoms. Our results demonstrate that Ln atoms can be stably embedded into/above the center of the cavity of $\mathrm{gh}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ monolayer and significantly affect the electronic and magnetic properties. Upon single Ln atom adsorption, all Ln@$\mathrm{gh}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ systems show metal character, large spin and orbital magnetic moments, and most of them favor the long-range ferromagnetic ordering. Interestingly, Pr, Nd, Ho adsorbates would possess total magnetic moments of 1.09, 1.47, 6.13 ${\mu }_B$, high Curie temperatures ($T_c$) of 593, 699, 700 K, large MAEs of 4.89, $-17.88$, 21.31 meV/Ln atom, respectively. Based on the electronic structure analyses, we propose that the $4f$ electron hopping between the occupied and unoccupied states around the Fermi level contributes to the large MAE, significant intratomic Ln-$sd$ orbital hybridization with N-$2p$ orbital hybridization gives rise to structural stability, and the coexistence of superexchange and RKKY interactions determines the long-term ferromagnetic coupling between Ln atoms. The present study demonstrates that Ln@$\mathrm{gh}\text{−}{\mathrm{C}}_3{\mathrm{N}}_4$ sheets have significant promise for applications in spintronics such as high density memory devices or for magnetic random access memory.

${\mathrm{Ti}}_2\mathrm{Co}\mathrm{Si}$, ${\mathrm{Ti}}_2\mathrm{Mn}\mathrm{Al},$ and ${\mathrm{Ti}}_2\mathrm{V}\mathrm{As}$ Heusler compounds have been identified as spin-gapless semiconductors when grown in the inverse XA lattice structure of the full-Heusler compounds. Especially ${\mathrm{Ti}}_2\mathrm{Mn}\mathrm{Al}$ and ${\mathrm{Ti}}_2\mathrm{V}\mathrm{As}$ combine this unique property with a zero magnetization being also fully compensated ferrimagnets. All three compounds are usual metals in the ground-state cubic $\mathrm{L}{2}_1$ lattice structure of the Heusler compounds. We present extensive first-principles electronic band structure calculations keeping the unit cell volume constant and varying the $c/a$ ratio and thus both the in-plane and out-of-plane lattice parameters. Our results suggest that while ${\mathrm{Ti}}_2\mathrm{Mn}\mathrm{Al}$ keeps its cubic character, this is not the case for ${\mathrm{Ti}}_2\mathrm{Co}\mathrm{Si}$ and ${\mathrm{Ti}}_2\mathrm{V}\mathrm{As}$ which prefer to crystallize in tetragonal $\mathrm{L}{2}_1$-like lattice structures with sizable $c/a$ ratios. In this tetragonal structure both compounds are usual nonmagnetic metals loosing their unique properties. Our results suggest that the stability of the cubic structure should be confirmed for all novel Heusler compounds under study and should not be considered as given. The exact behavior of each compound is materials specific and cannot be easily predicted.

${\mathrm{ZnGeN}}_2$ is sought as a semiconductor with comparable lattice constant to GaN and tunable band gap for integration in optoelectronic devices. Configurational disorder on the cation sublattice of ${\mathrm{ZnGeN}}_2$ can strongly modify the electronic structure compared to the ordered material, and both ordered and disordered forms of ${\mathrm{ZnGeN}}_2$ are candidates for light-emitting diodes and other emitters. The nonisovalent character of the disordered species (${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ge}}^{4+}$) subjects the cation ordering to strong short-range order effects. To model these effects, we use Monte Carlo (MC) simulations utilizing a cluster expansion to approximate formation enthalpy. Representative disordered configurations in 1024-atom supercells are relaxed in density functional theory calculations. From the MC structures, we extract a short-range order parameter (the N-cation coordination motif), and two long-range order parameters (Bragg-Williams and stretching parameters), and examine their correlations. We perform a thermodynamic integration to determine the mixing entropy and free energy. ${\mathrm{ZnGeN}}_2$ exhibits a first-order phase transition with pronounced discontinuities in enthalpy and entropy, as well as in the structural order parameters. We discuss the relationship between the effective temperature used in the MC simulation and the growth temperatures in experiment in relation to the crossover from the nonequilibrium to the equilibrium growth regime. This work expands on current models of site disorder in ${\mathrm{ZnGeN}}_2$ and provides atomic structure models with a systematic variation of the degree of cation disorder.

Electronic phase separation into charge-ordered (CO) and charge-averaged (CA) phases in $\mathrm{Ca}{\mathrm{Fe}}_{3-x}{M}_x{\mathrm{O}}_5$ samples for dopants $M=\mathrm{Mn}$ and Co has been investigated using powder neutron and x-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. Electronic phase separation is observed in lightly doped $\mathrm{Ca}{\mathrm{Fe}}_3{\mathrm{O}}_5$ samples, where $4–10\%$ Ca is replaced by Mn, Fe, or Co, and the CA ground state is stabilized at higher doping levels. The CO and CA phases are found to emerge below a common magnetic ordering temperature at $300–320\mathrm{K}$ providing strong evidence for a lattice strain-driven mechanism that couples their magnetic transitions.

The efficiencies of ${\mathrm{Cu}}_2\mathrm{ZnSn}{\mathrm{Se}}_4$ (CZTSe) solar cells with a narrower band gap at 1.0 eV are currently higher than those of ${\mathrm{Cu}}_2\mathrm{ZnSn}{\mathrm{S}}_4$ (CZTS), with the optimal band gap according to the Shockley-Queisser model. To understand this abnormal observation, we studied the nonradiative recombination rates induced by the deep levels of the dominant defects in CZTSe, i.e., the ${\mathrm{Sn}}_{\mathrm{Zn}}\left(2+/+\right)$, ${\mathrm{Sn}}_{\mathrm{Zn}}\left(+/0\right)$, and $\left[{\mathrm{Cu}}_{\mathrm{Zn}}\text{−}{\mathrm{Sn}}_{\mathrm{Zn}}\right]\left(+/0\right)$ levels. We found that the effective recombination centers in CZTS, namely, ${{\mathrm{Sn}}_{\mathrm{Zn}}}^{2+}$ and ${\left[{\mathrm{Cu}}_{\mathrm{Zn}}\text{−}{\mathrm{Sn}}_{\mathrm{Zn}}\right]}^{+}$, have much smaller carrier capture rates in CZTSe, and are less detrimental to the minority carrier lifetime and energy conversion efficiency. The smaller carrier capture rates for CZTSe can be attributed to the higher electronic transition energies, lower phonon frequencies, and weaker electron-phonon coupling effects in CZTSe compared to those in CZTS, because the large Se cations give rise to larger lattice constants and a softer lattice in CZTSe.

The physical rules governing the tearing of a packaging or of a piece of paper are not completely elucidated despite being common phenomena in daily life. Here, we investigate how the presence of a straight rigid object, a ruler, guides the fracture of a thin sheet within a wide range of pulling directions. In the case of thin isotropic brittle sheets, a simple geometrical analysis shows that fracture follows the direction of a tearing vector, which differs significantly from the pulling direction. In addition to geometry, bending energy or material anisotropy has to be implemented in the case of thicker or anisotropic sheets, to predict the direction of propagation. A generalization of the Wulff-type construction introduced by Takei et al. [Phys. Rev. Lett. 110, 144301 (2013)] accounts successfully for our experimental results.

Fracture is the ultimate source of failure of amorphous carbon (a-C) films; however, it is challenging to measure fracture properties of a-C from nanoindentation tests, and results of reported experiments are not consistent. Here, we use atomic-scale simulations to make quantitative and mechanistic predictions of fracture of a-C. Systematic large-scale K-field controlled atomic-scale simulations of crack propagation are performed for a-C samples with densities of $\rho =2.5$, 3.0, and $3.5{\text{g/cm}}^3$ created by liquid quenches for a range of quench rates ${\dot{T}}_q=10–1000\text{K/ps}$. The simulations show that the crack propagates by nucleation, growth, and coalescence of voids. Distances of $\approx 1\text{nm}$ between nucleated voids result in a brittlelike fracture toughness. We use a crack growth criterion proposed by Drugan, Rice, and Sham [J. Mech. Phys. Solids 30, 447 (1982)] to estimate steady-state fracture toughness based on our short crack-length fracture simulations. Fracture toughness values of $2.4–6.0\text{MPa}\sqrt{\text{m}}$ for initiation and $3–10\text{MPa}\sqrt{\text{m}}$ for the steady-state crack growth are within the experimentally reported range. These findings demonstrate that atomic-scale simulations can provide quantitatively predictive results even for fracture of materials with a ductile crack propagation mechanism.