Many-body excitonic effect is crucial in two-dimensional (2D) materials and can significantly impact their optoelectronic properties. Because defects are inevitable in 2D materials, understanding how they influence the optical and excitonic properties of the 2D materials is of significant scientific and technological importance. Here we focus on intrinsic point defects in monolayer and bilayer phosphorene and examine whether and how their optoelectronic properties may be modified by the defects. Based on large-scale first-principles calculations, we have systematically explored the optical and excitonic properties of phosphorene in the presence and absence of the point defects. We find that the optical properties of bilayer phosphorene depend on the stacking order of the layers. More importantly, we reveal that the dominant point defects in few-layer phosphorene are optically inactive, which renders phosphorene particularly attractive in optoelectronic applications.

Body-centered cubic (bcc) Fe is known as a typical soft magnetic material with high-saturation magnetization $\left(M_s\right)$ and low magnetocrystalline anisotropy. However, first-principles calculations demonstrate that body-centered tetragonal (bct) Fe has higher magnetocrystalline anisotropy than bcc Fe and comparable $M_s$. In this work, bct Fe nanoparticles (NPs) were successfully fabricated by a gas-phase condensation method for the first time. The bct Fe phase is confirmed by the x-ray diffraction pattern and diffraction images of transmission electron microscopy. An increased magnetocrystalline anisotropy of bct Fe, $\left(2.65\pm 0.67\right)\times 10^5\mathrm{J}/{\mathrm{m}}^3\mathrm{[}\left(21.2\pm 5.3\right)\mu \mathrm{eV}/\mathrm{atom}]$, is observed, which is around seven times higher than that of bcc Fe $4.8\times 10^4\mathrm{J}/{\mathrm{m}}^3\left(3.5\mu \mathrm{eV}/\mathrm{atom}\right)$. The bct Fe NPs sample has coercivity of $3.22\times 10^5$ A/m at 5 K and $1.04\times 10^5$ A/m at 300 K, which are much higher than that of bcc Fe NPs. In addition, the saturation magnetization at 5 K is estimated to be $\left(1.6\pm 0.4\right)\times 10^6$ A/m $\left(2.2\pm 0.5{\mu }_B/\mathrm{atom}\right)$, comparable to that of bcc Fe $1.7\times 10^6$ A/m $\left(2.2{\mu }_B/\mathrm{atom}\right)$.

Determining the types and concentrations of vacancies present in intentionally doped GaN is a notoriously difficult and long-debated problem. Here, we use an unconventional approach, based on thermal transport modeling, to determine the prevalence of vacancies in previous measurements. This allows us to provide conclusive evidence of the recent hypothesis that gallium vacancies in ammonothermally grown samples can be complexed with hydrogen. Our calculations for O-doped and Mg-O codoped samples yield a consistent picture interlinking dopant and vacancy concentration, carrier density, and thermal conductivity, in excellent agreement with experimental measurements. These results also highlight the predictive power of ab initio phonon transport modeling, and its value for understanding and quantifying defects in semiconductors.

We investigate the dynamics of tilted cellular/dendritic array patterns that form during directional solidification of a binary alloy when a preferred-growth crystal axis is misoriented with respect to the temperature gradient. In situ experimental observations and phase-field simulations in thin samples reveal the existence of a propagative source-sink mechanism of array spacing selection that operates on larger space and time scales than the competitive growth at play during the initial solidification transient. For tilted arrays, tertiary branching at the diverging edge of the sample acts as a source of new cells with a spacing that can be significantly larger than the initial average spacing. A spatial domain of large spacing then invades the sample propagatively. It thus yields a uniform spacing everywhere, selected independently of the initial conditions, except in a small region near the converging edge of the sample, which acts as a sink of cells. We propose a discrete geometrical model that describes the large-scale evolution of the spatial spacing profile based on the local dependence of the cell drift velocity on the spacing. We also derive a nonlinear advection equation that predicts the invasion velocity of the large-spacing domain, and sheds light on the fundamental nature of this process. The models also account for more complex spacing modulations produced by an irregular dynamics at the source, in good quantitative agreement with both phase-field simulations and experiments. This basic knowledge provides a theoretical basis to improve the processing of single crystals or textured polycrystals for advanced materials.

Control over the morphology of nanostructured materials is of primary importance in structure-property relationship studies. Although the size of ligaments and pores in dealloyed nanoporous metals can be controlled by thermal and/or (electro)chemical treatments, tuning the shape of those ligaments is much harder. In the present work, we use corroding media with different reactivity to effectively tailor the ligament shape in nanoporous tin (NP-Sn) during dealloying by free corrosion. NP-Sn architectures with nanowire and granular ligament shapes were made by controlling the pH of the corroding solution, and thus the rate of Sn oxidation relative to the etching rate of the sacrificial component. The standard nanowire structure was formed under acidic conditions where oxidation was slow, but a hierarchical granular structure was formed when fusion of the Sn nanocrystals was inhibited by surface oxidation. To demonstrate the advantages of this architectural control, these two materials systems were investigated as electrodes for Na-ion battery anodes. Similar initial Na storage capacities of $\sim 500$ and 550 mAh/g were achieved in the nanowire and granular materials, respectively, but the cycle life of the two materials was quite different. NP-Sn with a granular ligament shape showed enhanced stability with a capacity retention of $\sim 55\%$ over 95 cycles at a specific current of 40 mA/g. By contrast, NP-Sn with a nanowire ligament shape showed very fast capacity fading within the first 10 cycles. This work thus demonstrates the dramatic impact of the nanoscale morphology on the electrochemical performance of nanoporous materials and highlights the need for both shape and size control in dealloyed nanoporous metals.

Elastic strain is used widely to alter the mobility of free electronic carriers in semiconductors, but a predictive relationship between elastic lattice strain and the extent of charge localization of electronic defects is still underdeveloped. Here we considered $\mathrm{SrTi}{\mathrm{O}}_3$, a prototypical perovskite as a model functional oxide for thin film electronic devices and nonvolatile memories. We assessed the effects of biaxial strain on the stability of electronic defects at finite temperature by combining density functional theory (DFT) and quasiharmonic approximation (QHA) calculations. We constructed a predominance diagram for free electrons and small electron polarons in this material, as a function of biaxial strain and temperature. We found that biaxial tensile strain in $\mathrm{SrTi}{\mathrm{O}}_3$ can stabilize the small polaron, leading to a thermally activated and slower electronic transport, consistent with prior experimental observations on $\mathrm{SrTi}{\mathrm{O}}_3$ and distinct from our prior theoretical assessment of the response of $\mathrm{SrTi}{\mathrm{O}}_3$ to hydrostatic stress. These findings also resolved apparent conflicts between prior atomistic simulations and conductivity experiments for biaxially strained $\mathrm{SrTi}{\mathrm{O}}_3$ thin films. Our computational approach can be extended to other functional oxides, and for the case of $\mathrm{SrTi}{\mathrm{O}}_3$ our findings provide concrete guidance for conditions under which strain engineering can shift the electronic defect type and concentration to modulate electronic transport in thin films.

The anisotropic magnetoresistance (AMR) of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy-axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of an applied pressure. At zero applied pressure, the easy axis is along the $c$ direction or perpendicular to the layer. Upon application of a hydrostatic pressure > 1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the $c$ axis to the $ab$ plane at zero magnetic field, which amounts to a giant magnetic anisotropy energy change (> 100%). As the temperature is increased across the Curie temperature, the characteristic AMR effect gradually decreases and disappears. Our first-principles calculations confirm the giant magnetic anisotropy energy change with moderate pressure and assign its origin to the increased off-site spin-orbit interaction of Te atoms due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation transition is very rare in three-dimensional ferromagnets, but it may be common to other layered ferromagnets with similar crystal structures to CGT, and therefore offers a unique way to control magnetic anisotropy.

The crystal structure of $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{u}}_3{\mathrm{O}}_{12}$ is based on triangular planes of elongated $\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{12}$ trimers oriented perpendicular to the plane. We report that it is semiconducting, that its Weiss temperature and effective magnetic moment are −155 K and $2.59{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, respectively, and that the magnetic susceptibility and specific-heat data indicate that it exhibits magnetic ordering near 4 K. The presence of a high density of low energy states is evidenced by a substantial Sommerfeld-like T-linear term $\mathrm{[}\gamma =31\left(2\right)\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}]$ in the specific heat. Electronic-structure calculations reveal that the electronic states at the Fermi energy reside on the $\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{12}$ trimers and that the calculated density of electronic states is high and continuous around the Fermi energy—in other words density functional theory calculates the material to be a metal. The results imply that $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{u}}_3{\mathrm{O}}_{12}$ is a geometrically frustrated trimer-based Mott insulator.

The source of $n$-type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: ${\mathrm{In}}_2{\mathrm{O}}_3,{\mathrm{SnO}}_2$, and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk ${\mathrm{In}}_2{\mathrm{O}}_3$ introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and ${\mathrm{SnO}}_2$, the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic $n$-type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in ${\mathrm{In}}_2{\mathrm{O}}_3$, but also in ${\mathrm{SnO}}_2$ and ZnO.

We report the low-temperature properties of phase-pure single crystals of the half-Heusler compound CuMnSb grown by means of optical float zoning. The magnetization, specific heat, electrical resistivity, and Hall effect of our single crystals exhibit an antiferromagnetic transition at $T_{\mathrm{N}}=55$ K and a second anomaly at a temperature $T^{*}\approx 34$ K. Powder and single-crystal neutron diffraction establish an ordered magnetic moment of $(3.9\pm 0.1){\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, consistent with the effective moment inferred from the Curie-Weiss dependence of the susceptibility. Below $T_{\mathrm{N}}$, the Mn sublattice displays commensurate type-II antiferromagnetic order with propagation vectors and magnetic moments along $\langle 111\rangle$ (magnetic space group $R\left[I\right]3c$). Surprisingly, below $T^{*}$, the moments tilt away from $\langle 111\rangle$ by a finite angle $\delta \approx {11}^{\circ }$, forming a canted antiferromagnetic structure without uniform magnetization consistent with magnetic space group $C\left[B\right]c$. Our results establish that type-II antiferromagnetism is not the zero-temperature magnetic ground state of CuMnSb as may be expected of the face-centered cubic Mn sublattice.

SnSe, a group IV-VI monochalcogenide with layered crystal structure similar to black phosphorus, has recently attracted extensive interest due to its excellent thermoelectric properties and potential device applications. Experimental electronic structure of both the valence and conduction bands is critical for understanding the effects of hole versus electron doping on the thermoelectric properties, and to further reveal possible change of the band gap upon doping. Here, we report the multivalley valence bands with a large effective mass on semiconducting SnSe crystals and reveal single-valley conduction bands through electron doping to provide a complete picture of the thermoelectric physics. Moreover, by electron doping through potassium deposition, the band gap of SnSe can be widely tuned from 1.2 eV to 0.4 eV, providing new opportunities for tunable electronic and optoelectronic devices.

Copper-based ZrCuSiAs-type compounds of LnCuChO ($\mathrm{Ln}=\mathrm{Bi}$ and lanthanides, $\mathrm{Ch}=\mathrm{S}$, Se, Te) with a layered crystal structure continuously attract worldwide attention in recent years. Although their high-temperature (T ≥ 300 K) electrical properties have been intensively studied, their low-temperature electronic transport properties are little known. In this paper, we report the integration of ZrCuSiAs-type copper oxyselenide thin films of $\mathrm{B}{\mathrm{i}}_{0.94}\mathrm{P}{\mathrm{b}}_{0.06}\mathrm{CuSeO}$ (BPCSO) with perovskite-type ferroelectric $\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-PT) single crystals in the form of ferroelectric field effect devices that allow us to control the electronic properties (e.g., carrier density, magnetoconductance, dephasing length, etc.) of BPCSO films in a reversible and nonvolatile manner by polarization switching at room temperature. Combining ferroelectric gating and magnetotransport measurements with the Hikami-Larkin-Nagaoka theory, we demonstrate two-dimensional (2D) electronic transport characteristics and weak antilocalization effect as well as strong carrier-density-mediated competition between weak antilocalization and weak localization in BPCSO films. Our results show that ferroelectric gating using PMN-PT provides an effective and convenient approach to probe the carrier-density-related 2D electronic transport properties of ZrCuSiAs-type copper oxyselenide thin films.

The key feature of glass forming liquids is the super-Arrhenian temperature dependence of the mobility, where the mobility can increase by ten orders of magnitude or more as the temperature is decreased if crystallization does not intervene. A fundamental description of the super-Arrhenian behavior has been developed; specifically, the logarithm of the relaxation time is a linear function of $1/{\overline{U}}_x$, where ${\overline{U}}_x$ is the independently determined excess molar internal energy and $B$ is a material constant. This one-parameter mobility model quantitatively describes data for 21 glass forming materials, which are all the materials where there are sufficient experimental data for analysis. The effect of pressure on the $loga$ mobility is also described using the same ${\overline{U}}_x(T,p)$ function determined from the difference between the liquid and crystalline internal energies. It is also shown that $B$ is well correlated with the heat of fusion. The prediction of the $B/{\overline{U}}_x$ model is compared to the Adam and Gibbs $1/T{\overline{S}}_x$ model, where the $B/{\overline{U}}_x$ model is significantly better in unifying the full complement of mobility data. The implications of the $B/{\overline{U}}_x$ model for the development of a fundamental description of glass are discussed.

This paper reports the experimental determination of the growth exponents and activation enthalpies for the ordering process of standing cylinder-forming all-organic polystyrene-block-poly (methyl methacrylate) block copolymer (BCP) thin films as a function of the BCP degree of polymerization (N). The maximum growth exponent of 1/3 is observed for the BCP with the lowest N at the border of the order-disorder transition. Both the growth exponents and the activation enthalpies exponentially decrease with the BCP segregation strength (χN) following the same path of the diffusivity.

We investigate the vibrational heat transfer mechanisms in amorphous Stillinger-Weber silicon and germanium-based alloys and heterostructures via equilibrium and nonequilibrium molecular dynamics simulations along with lattice dynamics calculations. We find that similar to crystalline alloys, amorphous alloys demonstrate large size effects in thermal conductivity, while layering the constituent materials into superlattice structures leads to length-independent thermal conductivities. The thermal conductivity of an amorphous ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloy reduces by as much as $\sim 53\%$ compared to the thermal conductivity of amorphous silicon; compared to the larger reduction in crystalline phases due to alloying, we show that compositional disorder rather than structural disorder has a larger impact on the thermal conductivity reduction. Our thermal conductivity predictions for a-Si/a-Ge superlattices suggest that the alloy limit in amorphous SiGe-based structures can be surpassed with interface densities above $\sim 0.35{\mathrm{nm}}^{-1}$. We attribute the larger reduction in thermal conductivity of layered Si/Ge heterostructures to greater localization of modes at and around the cutoff frequency of the softer layer as demonstrated via lattice dynamics calculations and diffusivities of individual eigenmodes calculated according to the Allen-Feldman theory [P. B. Allen and J. L. Feldman, Phys. Rev. B 48, 12581 (1993)] for our amorphous SiGe-based alloys and superlattice structures.

The roles of an electric field and electronic doping in insulator-to-metal transitions are still not well understood. Here we formulated a phase-field model of insulator-to-metal transitions by taking into account both structural and electronic instabilities as well as free electrons and holes in ${\mathrm{VO}}_2$, a strongly correlated transition-metal oxide. Our phase-field simulations demonstrate that in a ${\mathrm{VO}}_2$ slab under a uniform electric field, an abrupt universal resistive transition occurs inside the supercooling region, in sharp contrast to the conventional Landau-Zener smooth electric breakdown. We also show that hole doping may decouple the structural and electronic phase transitions in ${\mathrm{VO}}_2$, leading to a metastable metallic monoclinic phase which could be stabilized through a geometrical confinement and the size effect. This work provides a general mesoscale thermodynamic framework for understanding the influences of electric field, electronic doping, and stress and strain on insulator-to-metal transitions and the corresponding mesoscale domain structure evolution in ${\mathrm{VO}}_2$ and related strongly correlated systems.

Heterostructures made by stacking different materials on top of each other are expected to exhibit unusual properties and new phenomena. Interface of the heterostructures plays a vital role in determining their properties. Here, we report the observation of a two-dimensional (2D) spinodal interface in graphene-molybdenum carbide $\left(\alpha \text{−}\mathrm{M}{\mathrm{o}}_2\mathrm{C}\right)$ heterostructures, which arises from spinodal decomposition occurring at the heterointerface, by using scanning tunneling microscopy. Our experiment demonstrates that the 2D spinodal interface modulates graphene into whispering gallery resonant networks filled with quasibound states of massless Dirac fermions. Moreover, below the superconducting transition temperature of the underlying $\alpha \text{−}\mathrm{M}{\mathrm{o}}_2\mathrm{C}$, the 2D spinodal interface behaves as disorders, resulting in the breakdown of the proximity-induced superconductivity in graphene. Our result sheds light on tuning properties of heterostructures based on interface engineering.

The search for a superconductor with non-$s$-wave pairing is important not only for understanding unconventional mechanisms of superconductivity but also for finding new types of quasiparticles such as Majorana bound states. Materials with both topological band structure and superconductivity are promising candidates as $p+ip$ superconducting states can be generated through pairing the spin-polarized topological surface states. In this work, the electronic and phonon properties of the superconductor molybdenum carbide (MoC) are studied with first-principles methods. Our calculations show that nontrivial band topology and $s$-wave Bardeen–Cooper–Schrieffer superconductivity coexist in two structural phases of MoC, namely the cubic $\alpha$ and hexagonal $\gamma$ phases. The $\alpha$ phase is a strong topological insulator and the $\gamma$ phase is a topological nodal-line semimetal with drumhead surface states. In addition, hole doping can stabilize the crystal structure of the $\alpha$ phase and elevate the transition temperature in the $\gamma$ phase. Therefore, MoC in different structural forms can be a practical material platform for studying topological superconductivity.

Based on first-principles density functional calculations, we show that SnS and SnSe layers can form mechanically rigid heterostructures with the constituent puckered or buckled monolayers. Due to the strong interlayer coupling, the electronic wave functions of the conduction and valence band edges are delocalized across the heterostructure. The resultant band gaps of the heterostructures reside in the infrared region. With strain engineering, the heterostructure band gap undergoes a transition from indirect to direct in the puckered phase. Our results show that there is a direct correlation between the electronic wave function and the mechanical rigidity of the layered heterostructure.