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We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multi-mode $E \otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids., Comment: 21 pages, 17 figures

The binary oxides MgO, CaO, ZnO, and CdO have a variety of band gaps which enables band-gap engineering over a wide range. ZnO takes the wurtzite structure, while MgO, CaO, and CdO take the rocksalt structure. We examine the formal polarization in these materials and in particular the formal polarization difference at the interface between ZnO and each of the rocksalt oxides. We find that any rocksalt oxide (including alloys) strained to ZnO will exhibit a very large polarization charge discontinuity of $\ensuremath{-}0.907\phantom{\rule{0.16em}{0ex}}{\mathrm{Cm}}^{\ensuremath{-}2}$. Heterostructures between ZnO and the rocksalt oxides can therefore host a free-carrier gas with sheet densities up to $5.7\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Such large polarization fields and carrier densities may allow applications in tunnel junctions or as contact layers for UV devices.

$\mathrm{BaSn}{\mathrm{O}}_{3}$ (BSO) is a wide-band-gap oxide in which very high levels of $n$-type doping have been demonstrated. Here we perform a detailed investigation of the conduction-band structure of BSO, reporting band velocities and effective masses as a function of energy, and explore whether sufficiently high doping levels can be achieved to fill the conduction band to energies near the inflection point. The presence of carriers near the inflection point can lead to negative differential resistance (NDR) due to the effective mass above the inflection point being negative. Our first-principles calculations, based on density functional theory with a hybrid functional, show that BSO has an inflection point 2.29 eV above the conduction-band minimum and that secondary conduction-band valleys are at least 2 eV higher in energy. We discuss the options available to achieve the required $n$-type doping levels to reach the inflection point in BSO.

The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and complex formation can occur. Using a combination of hybrid density functional theory (DFT) and group theory, we analyze the electronic structure and provide various useful physical properties, such as hyperfine structure, quasi-local vibrational modes, and zero-phonon line, to enable experimental identification of these complexes. We demonstrate that the presence of substitutional silicon adjacent to nitrogen significantly shifts the donor level toward the conduction band, resulting in an activation energy for the SiN center that is comparable to phosphorus. We also find that the neutral SiNV center is of particular interest due to its photon emission at $\sim$1530 nm, which falls within the C band of telecom wavelengths, and its paramagnetic nature. In addition, the optical transition associated with the SiNV$^0$ color center exhibits very small electron--phonon coupling (Huang--Rhys factor~=~0.78) resulting in high quantum efficiency (Debye-Waller factor = 46\%) for single-photon emission. These features render this new center very attractive for potential application in scalable quantum telecommunication networks., Comment: 12 pages, 9 figures, 2 tables

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:${V}_{\text{Ga}}\text{\ensuremath{-}}{\text{Te}}_{\text{As}}$ and GaN:${\mathrm{C}}_{\text{N}}$, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

The growth rate and crystal quality of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films are limited by suboxide (${\mathrm{Ga}}_{2}\mathrm{O}$) desorption. It has been reported that the growth of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ by molecular beam epitaxy and metalorganic chemical vapor deposition can be improved by an additional indium supply. In order to reveal the mechanism by which In impacts growth, we use density functional theory to explore the adsorption of Ga and In adatoms on the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$(010) surface and the effect of In on the growth rate. We find that the most stable configuration under an extreme Ga-rich condition is the surface with four Ga adatoms per unit cell. In the presence of indium, Ga adatoms become more strongly bound to the surface. Coadsorption of In and Ga thus increases the stability of the metal-covered surface and suppresses suboxide desorption; this explains the higher growth rate under Ga and In flux in experiments.

The identification of deep-level defects that act as detrimental nonradiative recombination centers is critical for optimizing the optoelectronic performance of hybrid perovskites. Although extensive studies have been devoted to revealing the nature of deep-level defects in hybrid perovskites, it is still unclear what defects are responsible for the experimentally observed nonradiative recombination rates. Employing first-principles approaches, we quantitatively show that the iodine interstitial is a dominant nonradiative recombination center in methylammonium-lead iodide. This important insight points to a target for defect engineering in order to further improve the performance of hybrid perovskites.

We investigate the transport properties of tungsten trioxide $({\mathrm{WO}}_{3})$ from first principles. The full $\mathbf{k}$ dependence of the scattering rate is included, and electron mobilities as a function of doping are calculated using Boltzmann transport theory. Our predicted room-temperature mobility is as high as $485{\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\ensuremath{-}1}$ ${\mathrm{s}}^{\ensuremath{-}1}$ at ${10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ doping, which makes ${\mathrm{WO}}_{3}$ a promising material for device applications. We elucidate why the mobility is so much higher in ${\mathrm{WO}}_{3}$ compared to the prototype perovskite oxide ${\mathrm{SrTiO}}_{3}$ based on an analysis of the electronic and vibronic structure.

In order for the wide-band-gap Zn-IV-nitrides to find applications in power electronics or optoelectronics, controlled $n$-type doping is required. We systematically explore group-V and group-VI dopants that can act as donors in ${\mathrm{ZnGeN}}_{2}$ and ${\mathrm{ZnSiN}}_{2}$. We address compensation by native acceptors as well as self-compensation by formation of $DX$ centers. For ${\mathrm{ZnGeN}}_{2}$, we find that ${\mathrm{S}}_{\mathrm{N}}$, ${\mathrm{Se}}_{\mathrm{N}}$, ${\mathrm{P}}_{\mathrm{Ge}}$, and ${\mathrm{As}}_{\mathrm{Ge}}$ act as shallow donors. ${\mathrm{P}}_{\mathrm{Ge}}$ has the lowest formation energy and can be used to reach carrier concentrations up to $n=1.3\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. ${\mathrm{P}}_{\mathrm{Si}}$ acts as a shallow donor in ${\mathrm{ZnSiN}}_{2}$; however, because of strong compensation from native acceptors, we find that ${\mathrm{ZnSiN}}_{2}$ cannot be doped $n$ type.

Reentrant melting (in which a substance's melting point starts to decrease beyond a certain pressure) is believed to be an unusual phenomenon. Among the elements, it has so far only been observed in a very limited number of species, e.g., the alkali metals. Our density functional theory calculations reveal that this behavior actually extends beyond alkali metals to include magnesium, which also undergoes reentrant melting, though at the much higher pressure of $\ensuremath{\sim}300$ GPa. We find that the origin of reentrant melting is the faster softening of interatomic interactions in the liquid phase than in the solid, as pressure rises. We propose a simple approach to estimate pressure-volume relations and show that this characteristic softening pattern is widely observed in metallic elements. We verify this prediction in the case of aluminum by finding reentrant melting at $\ensuremath{\sim}4000$ GPa. These results suggest that reentrant melting may be a more universal feature than previously thought.

We extend the methodology for calculating Auger recombination from first principles to include spin-orbit interaction, which allows us to study materials in which the spin-orbit splitting and the band gap are of similar magnitude. We use this methodology to compute the direct and indirect Auger coefficients in InAs and related alloys. The direct process involves only Coulomb interaction, while the indirect process is mediated by phonons. We show how the spin-orbit split-off states allow for efficient direct Auger recombination, greatly enhancing the excitation rate of Auger holes due to a near resonance between the spin-orbit splitting and the band gap of the material. We find that the direct hole-hole-electron Auger recombination coefficient decays exponentially with increasing band gap, while the electron-electron-hole Auger recombination coefficient decays faster than exponentially. Additionally, we modeled the phonon-assisted process in InAs and show that the direct Auger process is dominant.

Iron is a common trace impurity in the group-III nitrides. Iron is also intentionally introduced in III-nitride electronic devices to create semi-insulating substrates and in the context of spintronics and quantum information applications. Despite the wide-ranging consequences of iron's presence in III-nitrides, the properties of iron impurities in the nitrides are not fully established. We investigate the impact of iron impurities on the electrical and optical properties of GaN, AlN, and InN using first-principles calculations based on a hybrid functional. We report formation energies of substitutional and interstitial iron impurities as a function of the Fermi-level position. We also investigate complexes of Fe with substitutional oxygen on the nitrogen site, with nitrogen vacancies and with hydrogen interstitials. In GaN and AlN, iron on the cation site is amphoteric. We discuss the role of the Fe-induced acceptor level and its impact on nonradiative recombination in the context of loss mechanisms in light emitters, and current collapse in high-electron-mobility transistors. In InN, we find the iron interstitial to be the most favorable configuration, where it acts as a shallow double donor.

The topochemical transformation of single crystals of ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ into ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}{\mathrm{F}}_{2}$ is reported via fluorine insertion. Characterization of the newly formed ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}{\mathrm{F}}_{2}$ phase shows a nearly complete oxidation of ${\mathrm{Ir}}^{4+}$ cations into ${\mathrm{Ir}}^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled ${J}_{\mathrm{eff}}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent in the Ir metal ions of this compound.

The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator Sr$_{3}$Ir$_{2}$O$_{7}$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron concentration corresponding to 4.8\% substitution of Sr with La, in excellent agreement with experiment. Additionally, we describe the structural effects of electron doping in Sr$_{3}$Ir$_{2}$O$_{7}$ via a competition between the steric effect from smaller La atoms substituted within the lattice and the dominant doping-driven deformation-potential effect. Curiously, our first-principles calculations fail to capture the low-temperature structural distortion reported in the low-gap phase of Sr$_{3}$Ir$_{2}$O$_{7}$, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material.

Hexagonal boron nitride ($h$-BN) is a technologically important electronic and dielectric material. Recently, research into this material has intensified due to the discovery of bright single-photon-emitting color centers in the form of point defects, with potential applications in advanced quantum technologies. Despite this technological promise, the defect physics of this material is still largely unexplored. Here, the authors have performed detailed first-principles investigations of the defect properties of $h$-BN, finding that the defect physics of this material is dictated by impurities, in particular carbon, oxygen, and hydrogen. They show how the defect properties of this material can be selectively engineered by growth or processing conditions. These insights will be fundamental to controlled generation of single-photon emitters and for general defect engineering of this material.

Materials that are simultaneously transparent and electrically conductive play a key role in optoelectronic devices. Barium stannate has recently come to the forefront as a promising new transparent conductor, but its conductivity has been difficult to control. The authors have performed computational studies to determine the mechanisms that limit doping. They find that atomic-scale defects, such as vacancies in the crystal lattice, are responsible, but their formation can be controlled by tailoring the growth conditions. The authors also investigated related materials, such as strontium stannate and calcium stannate, which are transparent even in the ultraviolet. Calcium stannate is found to always be insulating, but guidelines are provided for maximizing the conductivity of the strontium compound.

In a recent article [Phys. Rev. B 91, 205315 (2015)] Shi, Xu, and Wang presented a comparison of several formalisms to calculate nonradiative recombination rates and concluded the ``one-dimensional (1D) quantum formula'' that was used by Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] is insufficient to accurately describe nonradiative capture rates. Our analysis of the results of Shi, Xu, and Wang indicates that their conclusions about the 1D quantum formula are unfounded and stem from an error in their calculations. Our own calculations demonstrate that the 1D quantum formula approach yields reliable and accurate results for nonradiative recombination rates.

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for $p$-type doping of hexagonal ($h\ensuremath{-}\mathrm{BN}$), wurtzite ($wz\ensuremath{-}\mathrm{BN}$), and cubic ($c\ensuremath{-}\mathrm{BN}$) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in $h\ensuremath{-}\mathrm{BN}$ compared to $wz\ensuremath{-}\mathrm{BN}$ or $c\ensuremath{-}\mathrm{BN}$. Despite forming small hole polarons, Be acceptors enable $p$-type doping, with ionization energies of 0.31 eV for $wz\ensuremath{-}\mathrm{BN}$ and 0.24 eV for $c\ensuremath{-}\mathrm{BN}$; these values are comparable to the Mg ionization energy in GaN.

$\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{Ox}}_{3}$ is a wide-band-gap semiconductor with promising applications in transparent electronics and in power devices. $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ has monoclinic crystal symmetry and does not display a layered structured characteristic of 2D materials in the bulk; nevertheless, monolayer-thin ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layers can be created. We used first-principles techniques to investigate the structural and electronic properties of these nanolayers. Surprisingly, freestanding films do not exhibit any signs of quantum confinement and exhibit the same electronic structure as bulk material. A detailed examination reveals that this can be attributed to the presence of states that are strongly confined near the surface. When the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layers are embedded in a wider band-gap material such as ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, the expected effects of quantum confinement can be observed. The effective mass of electrons in all the nanolayers is small, indicating promising device applications.

The effect of Ti and Hf impurities on the $(111)$ antiphase boundary (APB) energy of ${\mathrm{Ni}}_{3}\mathrm{Al}$ is investigated via ab initio calculations. Cluster expansion is performed to predict supercell total energies sampled in a Monte Carlo approach that accounts for nondilute point defects at finite temperature, obtaining APB energies as a function of impurity concentration and temperature. Of the two ternary elements, Hf is more effective in increasing the APB energy. While the $(111)$ APB energy of a pure ${\mathrm{L}1}_{2}$ material requires at least second-nearest-neighbor interactions, we observe a strong correlation between impurity-induced APB energy enhancement and formation of first-nearest-neighbor Ni-Ni bonds across the APB due to symmetry breaking. Using a linear-chain model and effective bond energies derived from effective cluster interactions, we propose a mechanism that explains why Hf is more effective than Ti.

The task of finding the smallest energy needed to bring a solid to its onset of mechanical instability arises in many problems in materials science, from the determination of the elasticity limit to the consistent assignment of free energies to mechanically unstable phases. However, unless the space of possible deformations is low-dimensional and a priori known, this problem is numerically difficult, as it involves minimizing a function under a constraint on its Hessian, which is computionally prohibitive to obtain in low symmetry systems, especially if electronic structure calculations are used. We propose a method that is inspired by the well-known dimer method for saddle point searches but that adds the necessary ingredients to solve for the lowest onset of mechanical instability. The method consists of two nested optimization problems. The inner one involves a dimer-like construction to find the direction of smallest curvature as well as the gradient of this curvature function. The outer optimization then minimizes energy using the result of the inner optimization problem to constrain the search to the hypersurface enclosing all points of zero minimum curvature. Example applications to both model systems and electronic structure calculations are given., 10 pages, 10 figures