We performed density functional theory calculations with self-consistent van der Waals corrected exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results, which are independent of XC choice: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. Such structural behavior is unlike the one seen in more traditional two-dimensional materials such as graphene or ${\mathrm{MoS}}_2$, in which monolayers keep their structure upon exfoliation. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves and may require corrections beyond PBE XC. This observation is relevant for the prediction of the critical temperature $T_c$ and important for that reason. (b) There exists a hierarchy of independent parameters that uniquely define a ground state ferroelectric unit cell (and square and rectangular paraelectric unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelectric ground state unit cell, while square and rectangular paraelectric structures are defined by only three or two independent optimizable variables, respectively. (c) The reduced number of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelectric unit cell when compared to the elastic energy barrier of a square paraelectric structure. This implies that $T_c$ obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temperature on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) Surprisingly, the dissociation energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and ${\mathrm{MoS}}_2$. (e) There exists a linear relation among the square paraelectric unit cell lattice parameter and the lattice parameters of the rectangular ferroelectric ground state unit cell. These results highlight the subtle atomistic structure and chemical bond of these novel 2D ferroelectrics.

Nonsymmorphic symmetries, such as screw rotations or glide reflections, can enforce band crossings within high-symmetry lines or planes of the Brillouin zone. When these band degeneracies are close to the Fermi energy, they can give rise to a number of unusual phenomena, e.g., anomalous magnetoelectric responses, transverse Hall currents, and exotic surface states. In this paper, we present a comprehensive classification of such nonsymmorphic band crossings in trigonal materials with strong spin-orbit coupling. We find that in trigonal systems there are two different types of nonsymmorphic band degeneracies: (i) Weyl points protected by screw rotations with an accordion-like dispersion, and (ii) Weyl nodal lines protected by glide reflections. We report a number of existing materials, where these band crossings are realized near the Fermi energy. This includes ${\mathrm{Cu}}_2{\mathrm{SrSnS}}_4$ and elemental tellurium (Te), which exhibit accordion Weyl points, and the tellurium-silicon clathrate ${\mathrm{Te}}_{16}{\mathrm{Si}}_{38}$, which shows Weyl nodal lines. The ab initio band structures and surface states of these materials are studied in detail, and implications for experiments are briefly discussed.

The linear magneto-optic Kerr effect (MOKE) is often used to probe magnetism of ferromagnetic materials, but MOKE cannot be applied to collinear antiferromagnets due to the cancellation of sublattice magnetization. Magneto-optic constants that are quadratic in magnetization, however, provide an approach for studying antiferromagnets on picosecond timescales. Here, we combine transient measurements of linear birefringence and optical reflectivity to study the optical response of ${\mathrm{Fe}}_2\mathrm{As}$ to small ultrafast temperature excursions. We performed temperature-dependent pump-probe measurements on crystallographically isotropic (001) and anisotropic (010) faces of ${\mathrm{Fe}}_2\mathrm{As}$ bulk crystals. We find that the largest optical signals arise from changes in the index of refraction along the $z$ axis, perpendicular to the Néel vector. Both real and imaginary parts of the transient optical birefringence signal approximately follow the temperature dependence of the magnetic heat capacity, as expected if the changes in dielectric function are dominated by contributions of exchange interactions to the dielectric function.

The Elliott-Yafet spin relaxation mechanism posits linear proportionality between spin and momentum lifetimes in low spin-orbit coupling nonmagnetic metals, and is widely accepted in spintronics. Accurate experimental determination of the Elliott-Yafet proportionality constants (${\beta }_i$) between the spin and momentum relaxation times for individual scattering sources is challenging, however. This is apparent from the literature on nonlocal spin transport in Cu, for example, where reported phonon (${\beta }_{\mathrm{ph}}$) and defect (${\beta }_{\mathrm{def}}$) Elliott-Yafet constants vary by an order of magnitude. In recent work we discovered that even part-per-million-level magnetic impurity concentrations can substantially influence spin relaxation in Cu, via a spin transport analog of the Kondo effect. To clarify whether this could explain the reported variability in ${\beta }_i$, here we report on a comprehensive study of spin transport in Cu-based lateral nonlocal spin valves, varying the ferromagnetic contact material, interface structure, Cu thickness, and post-fabrication annealing conditions, resulting in widely varied microstructures and magnetic impurity concentrations. Quantifying the effects of magnetic impurities on charge and spin transport we demonstrate the dramatic, even dominant, effect these can have on spin relaxation rates, and thus extracted ${\beta }_i$. Minimization of magnetic impurity effects is achieved via Al interlayer insertion or moderate annealing, restoring the expected temperature dependence for phonon-mediated spin relaxation, and enabling more reliable determination of ${\beta }_i$ for phonons (740 ± 200), and nonmagnetic defects (240 ± 50). The latter contribution is shown to be dominated by grain boundaries in these polycrystalline Cu films. Cross-sectional transmission electron microscopy measurement of grain sizes in actual nonlocal spin valve devices then establishes a useful empirical relationship between average grain size and spin diffusion length. These measurements highlight the importance of magnetic impurities in metallic spin transport, explain the wide variability in reported ${\beta }_{\mathrm{ph}}$ and ${\beta }_{\mathrm{def}}$ in Cu, and elucidate the relationship between metallic spin transport and microstructure.

Multiprincipal element alloys, called high-entropy alloys represent a promising group of materials. They possess unique mechanical or electrical properties, which provide a wide range of potential applications. In general, mechanical or electrical properties of a material are influenced by the magnetic behavior. Therefore, in our work, we are exploring magnetic behavior of so-called “Cantor alloy” CoCrFeMnNi and its molybdenum based derivatives. Mo alloys were studied not only to carefully describe their properties but also to probe the magnetic behavior of the parent alloy by adding a nonmagnetic element. Based on ab initio calculations using the TB-LMTO-ASA (tight-binding linear-muffin-tin-orbital atomic-sphere approximation) method within CPA (coherent-potential approximation), we have found the ground-state magnetic structures of a particular alloy. We deal with various magnetic structures including complex structures beyond the simple FM or DLM phases. We show the influence of the presence of a particular element on the magnetic properties. It includes, e.g., magnitudes of magnetic moments or preferred magnetic phases. The calculations were extended by studying the magnetic exchange interaction employing the Liechtenstein formula. We clearly show the contribution of each element to the magnetism as a function of the composition or crystal structure. We provide a thorough description of magnetic behavior in the mentioned compounds.

We explore how the structure and composition of the terminal layer of $n\text{−}\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ determine key surface electronic properties. We have measured and calculated from first principles the absolute work functions and electron affinities of bulk $\mathrm{SrN}{\mathrm{b}}_{0.01}\mathrm{T}{\mathrm{i}}_{0.99}{\mathrm{O}}_3\left(001\right)$ terminated along the $\mathrm{Ti}{\mathrm{O}}_2$ and SrO planes. The match between theory and experiment is quite satisfactory for the $\mathrm{Ti}{\mathrm{O}}_2$ termination if an ideal, bulk-truncated surface structure is assumed. In contrast, the ideal SrO termination leads to a calculated work function considerably lower than the experimental value. We show that this discrepancy can be associated with defects on the SrO surface that act as electron scavengers. These defects deplete the concentration of itinerant electrons in the subsurface region and increase the negative charge density on the surface, thus increasing the work function. Several different surface defect configurations were modeled; the ones that yield the best agreement with experiment involve Sr vacancies in the terminal layer along with ${\mathrm{O}}^{2-}$, $\mathrm{O}{\mathrm{H}}^{-}/{\mathrm{H}}^{-}$ pairs or ${{\mathrm{O}}_2}^{2-}$ occupying anion sites adjacent to the Sr vacancies.

A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking.

The results of a single-crystal x-ray-diffraction study of the evolution of the crystal structures of $\mathrm{V}{\mathrm{I}}_3$ with temperature with an emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure $R\overline{3}$ (148) has been confirmed. Upon cooling, $\mathrm{V}{\mathrm{I}}_3$ undergoes a structural phase transition to a monoclinic phase at $T_{\mathrm{s}}\sim 79\mathrm{K}$. $T_{\mathrm{s}}$ is reduced in magnetic fields applied along the trigonal $c$ axis. When $\mathrm{V}{\mathrm{I}}_3$ becomes ferromagnetic at $T_{\mathrm{FM}1}\sim 50\mathrm{K}$, magnetostriction-induced changes of the monoclinic-structure parameters are observed. Upon further cooling, the monoclinic structure transforms into a triclinic variant at 32 K which is most likely occurring in conjunction with the previously reported transformation of the ferromagnetic structure. The observed phenomena are preliminarily attributed to strong magnetoelastic interactions.

Coherent diffraction imaging using third generation synchrotron sources and x-ray free-electron lasers has been demonstrated to be an excellent tool for revealing the inner structures of a single particle. The primary challenge in imaging pure, tiny, and isolated biological specimens is obtaining a reliable reconstruction from diffraction data that have a poor signal-to-noise ratio. Thus, we developed a robust method that yields the internal density distribution of liposome vesicles immersed in a solution. By combining the guided hybrid input-output method and a new order parameter defined by the consistency of the reconstruction, we, without prior knowledge, retrieved both pure and drug-containing liposome vesicles from individual diffraction patterns. This result is currently the smallest noncrystalline biological specimen resolved by single-shot coherent diffraction microscopy.

Uncompensated moments (UMs) in antiferromagnets are responsible for exchange bias in antiferromagnet/ ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magnetothermal microscopy to image micron-scale uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a first-order phase transition to a ferromagnetic (FM) state near 400 K. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic (AF) phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (390–420 K) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk Néel order. This exchange coupling leads to the nucleation of unusual configurations in which FM domains are pinned in different in-plane orientations, even in the presence of a nominally saturating magnetic field, before suddenly collapsing into a state uniformly parallel to the field.

Interest in antimony selenide ($\mathrm{S}{\mathrm{b}}_2\mathrm{S}{\mathrm{e}}_3$) has been constantly growing in the recent past, especially for its promising properties for applications in the field of solar energy technologies. Surprisingly, the surface properties of this material, which is built from van der Waals stacked one-dimensional (1D) ribbons, have not been studied in detail yet. Here we demonstrate that $\mathrm{S}{\mathrm{b}}_2\mathrm{S}{\mathrm{e}}_3$ crystals cleave along the (100) planes. The resulting surface shows a pronounced 1D structure, reflecting the stacking of ribbons in the bulk crystal. The cleaving process leads to the formation of slightly tilted surface domains, with the tilt angles oriented invariably along the ribbon directions, suggesting a strong anisotropy of the internal friction forces. Our angle-resolved photoemission data reveal that the 1D character of the crystalline structure of this material is also reflected in its electronic band structure.

Developing the field of quantum information science (QIS) hinges upon designing viable qubits, the smallest unit in quantum computing. One approach to creating qubits is introducing paramagnetic defects into semiconductors or insulators. This class of qubits has seen success in the form of nitrogen-vacancy centers in diamond, divacancy defects in SiC, and P doped into Si. These materials feature paramagnetic defects in a low-nuclear-spin environment to reduce the impact of nuclear spin on electronic spin coherence. In this work, we report single-crystal growth of $\mathrm{B}{\mathrm{a}}_2\mathrm{CaW}{\mathrm{O}}_{6-\delta }$ and the coherence properties of introduced ${\mathrm{W}}^{5+}$ spin centers generated by oxygen vacancies. $\mathrm{B}{\mathrm{a}}_2\mathrm{CaW}{\mathrm{O}}_{6-\delta }$ ($\delta =0$) is a B-site ordered double perovskite with a temperature-dependent octahedral tilting wherein oxygen vacancies generate ${\mathrm{W}}^{5+}$ ($d^1$), $S=1/2$, $I=0$, centers. We characterized these defects by measuring the spin-lattice ($T_1$) and spin-spin relaxation ($T_2$) times from $T=5-150\mathrm{K}$. At $T=5\mathrm{K}$, $T_1=310\mathrm{ms}$ and $T_2=4\mu \mathrm{s}$, establishing the viability of these qubit candidates. With increasing temperature, $T_2$ remains constant up to $T=60\mathrm{K}$ and then decreases to $T_2\sim 1\mu \mathrm{s}$ at $T=90\mathrm{K}$, and remains roughly constant until $T=150\mathrm{K}$, demonstrating the remarkable stability of $T_2$ with increasing temperature. Together, these results demonstrate that systematic defect generation in double-perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for QIS.

We investigated the magnetic properties of the ilmenite-type manganates $\mathrm{MgMn}{\mathrm{O}}_3$ and $\mathrm{ZnMn}{\mathrm{O}}_3$, both of which are composed of a honeycomb lattice of magnetic $\mathrm{M}{\mathrm{n}}^{4+}$ ions. Both compounds show antiferromagnetic order with weak ferromagnetic moments. In particular, $\mathrm{MgMn}{\mathrm{O}}_3$ exhibits a magnetization “reversal” behavior which can be described by the $N$-type ferrimagnetism in the Néel's classification. The relationship between the magnetic properties and the crystal and magnetic structures probed by the neutron diffraction experiments indicates that the two honeycomb lattice magnets have different $J_1-J_2$ parameter sets, placing them in the distinct regions in the phase diagram; both nearest neighbor (NN) and next nearest neighbor (NNN) exchange interactions are antiferromagnetic in $\mathrm{MgMn}{\mathrm{O}}_3$, while NN and NNN interactions become ferromagnetic and antiferromagnet, respectively, in $\mathrm{ZnMn}{\mathrm{O}}_3$.

Nonempirical hybrid functionals are investigated for band-gap predictions of inorganic metal-halide perovskites belonging to the class $\mathrm{Cs}B{X}_3$, with $B$ = Ge, Sn, Pb and $X$ = Cl, Br, I. We consider both global and range-separated hybrid functionals and determine the parameters through two different schemes. The first scheme is based on the static screening response of the material and thus yields dielectric-dependent hybrid functionals. The second scheme defines the hybrid functionals through the enforcement of Koopmans' condition for localized defect states. We also carry out quasiparticle self-consistent $GW$ calculations with vertex corrections to establish state-of-the-art references. For the investigated class of materials, dielectric-dependent functionals and those fulfilling Koopmans' condition yield band gaps of comparable accuracy $(\sim 0.2$ eV), but the former only require calculations for the primitive unit cell and are less subject to the specifics of the material.

Room-temperature ferrimagnetism was discovered for the anti-site-disordered perovskite $\mathrm{C}{\mathrm{a}}_2\mathrm{MnOs}{\mathrm{O}}_6$ with $T_{\mathrm{c}}=305\mathrm{K}$. $\mathrm{C}{\mathrm{a}}_2\mathrm{MnOs}{\mathrm{O}}_6$ crystallizes into an orthorhombic structure with a space group of Pnma, in which Mn and Os share the oxygen-coordinated-octahedral site at an equal ratio without a noticeable ordered arrangement. The material is electrically semiconducting with variable-range-hopping behavior. X-ray absorption spectroscopy confirmed the trivalent state of the Mn and the pentavalent state of the Os. X-ray magnetic circular dichroism spectroscopy reveals that the Mn and Os magnetic moments are aligned antiferromagnetically, thereby classifying the material as a ferrimagnet which is in accordance with band structure calculations. It is intriguing that the magnetic signal of the Os is very weak, and that the observed total magnetic moment is primarily due to the Mn. The $T_{\mathrm{c}}=305\mathrm{K}$ is the second highest in the material category of so-called disordered ferromagnets such as $\mathrm{CaR}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x{\mathrm{O}}_3$, $\mathrm{SrR}{\mathrm{u}}_{1-x}\mathrm{C}{\mathrm{r}}_x{\mathrm{O}}_3$, and $\mathrm{CaI}{\mathrm{r}}_{1-x}\mathrm{M}{\mathrm{n}}_x{\mathrm{O}}_3$, and hence, may support the development of spintronic oxides with relaxed requirements concerning the anti-site disorder of the magnetic ions.

Here we report on $P\text{−}E$ hysteresis loop measurements that unravel the ferroelectric nature of melanothallite ${\mathrm{Cu}}_2{\mathrm{OCl}}_2$, a new multiferroic material with high critical temperature. Its spin structure was investigated by polarized and unpolarized neutron scattering experiments which reveal a cycloidal magnetic structure with vector chirality (magnetic polarity) that can be inverted by opposite poling of the sample with an inverted electric field. This shows that ${\mathrm{Cu}}_2{\mathrm{OCl}}_2$ is a spin-induced ferroelectric material. Finally, we show that the ferroelectric properties of ${\mathrm{Cu}}_2{\mathrm{OCl}}_2$ are driven by the inverse Dzyaloshinskii-Moriya interaction mechanism which is also able to predict the observed direction of the ferroelectric polarization properly. The origin of the noncollinear spin structure in melanothallite are competing AFM-FM exchange couplings which we estimate from a combined ab initio $+$ cluster configuration interaction calculation.

We consider a ground-state band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a nonequilibrium Green's function treatment on the Konstantinov-Perel' contour. We present the first ab initio band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states. We also show that NEQ-EI states can be generated by currently available pump pulses. Peculiar fingerprints of the NEQ-EI phase in time-resolved angle-resolved photoemission spectroscopy spectra are highlighted: (i) during the pump-driving, the system goes through a BEC-BCS crossover and (ii) concomitantly the excitonic spectral structure undergoes a convex-to-concave shape transition; (iii) attosecond pulses shone after the pump-driving at different times $t_{\mathrm{delay}}$ produce a photocurrent that oscillates in $t_{\mathrm{delay}}$ with a pump-tunable frequency—we relate this phenomenon to the ac response of an exotic Josephson junction.

Modification of the carrier dynamics in correlated oxide systems via epitaxial strain is a promising pathway for the practical realization of energy-efficient electronic devices. Here we report on the thermally induced metal-to-insulator transition (MIT) of $\mathrm{Nb}{\mathrm{O}}_2$ films grown on $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ substrates and the modulation of the MIT temperature via epitaxial strain from the substrate. The metal-insulator transition temperature ($T_{\mathrm{MIT}}$) increased from 910 to 1066 K with increasing strain. An ultrathin 3.9-nm film consisting of a single strained layer with minimal structural defects yielded a bulklike sharp transition. The substrate-induced strain offers another degree of freedom to improve device functionality of MIT materials.

Binary Cr-Te compounds are known to exhibit various structures and interesting physical properties, which are very sensitive to Cr content and arrangement. We report the structure and physical properties of a compound $\mathrm{C}{\mathrm{r}}_{12-x}\mathrm{T}{\mathrm{e}}_{16}$ with $x\sim 2.65$. $\mathrm{C}{\mathrm{r}}_{9.35}\mathrm{T}{\mathrm{e}}_{16}$ crystallizes in the trigonal structure with the space group $P\overline{3}m1$ (No. 164), consisting of four different Cr sites (Cr1, Cr2, Cr3, Cr4) with an ordered Cr4 vacant layer. The electrical resistivity, magnetization, specific heat, and neutron diffraction measurements indicate that there is a ferromagnetic transition at $T_{\mathrm{C}1}\sim 160$ K with the first-order nature, and an anomaly at $T_{\mathrm{C}2}\sim 67$ K. At 5 K, the magnetic moments of Cr2 and Cr3 are aligned along the $c$ axis with $2.78{\mu }_{\mathrm{B}}/\mathrm{Cr}$. Detailed neutron diffraction refinement suggests that ordered magnetic moments on Cr1 (fully occupied) and Cr4 (partially occupied) are negligible. The different magnetic response stems from different valence states and different magnetic interactions between Cr2/Cr3 and Cr1/Cr4 as evidenced by the x-ray absorption fine-structure measurements. This creates a unique two-dimensional magnetic system with ordered moments confined in the Cr2/Cr3 layer.

Owing to their exceptional semiconducting properties, hybrid inorganic-organic perovskites show great promise as photovoltaic absorbers. In these materials, long-range diffusion of charge carriers allows for most of the photogenerated carriers to contribute to the photovoltaic efficiency. Here, time-resolved photoluminescence (PL) microscopy is used to directly probe ambipolar carrier diffusion and recombination kinetics in hybrid perovskites. This technique is applied to thin films of methylammonium lead tri-iodide $\left(\mathrm{MAPb}{\mathrm{I}}_3\right)$ obtained with two different fabrication routes, methylammonium lead tribromide $\left(\mathrm{MAPbB}{\mathrm{r}}_3\right)$, and an alloy of formamidinium lead tri-iodide and methylammonium lead bromide $\mathrm{F}{\mathrm{A}}_{0.85}\mathrm{M}{\mathrm{A}}_{0.15}\mathrm{Pb}{\left({\mathrm{I}}_{0.85}\mathrm{B}{\mathrm{r}}_{0.15}\right)}_3$. Average diffusion coefficients in the films leading to the highest device efficiencies and longest lifetimes, i.e., in $\mathrm{F}{\mathrm{A}}_{0.85}\mathrm{M}{\mathrm{A}}_{0.15}\mathrm{Pb}{\left({\mathrm{I}}_{0.85}\mathrm{B}{\mathrm{r}}_{0.15}\right)}_3$ and acetonitrile-processed $\mathrm{MAPb}{\mathrm{I}}_3$, are found to be several orders of magnitude lower than in the other films. Further examination of the time dependence shows strong evidence for nondiffusive transport. In particular, acetonitrile-processed $\mathrm{MAPb}{\mathrm{I}}_3$ shows distinct diffusion regimes on short and long timescales with an effective diffusion constant varying over 2 orders of magnitude. Our results also highlight the fact that increases in carrier lifetime in this class of materials are not necessarily concomitant with increased diffusion lengths and that the PL quantum efficiency under solar cell operating conditions is a greater indication of material, and ultimately device, quality.