Substitutional doping of two-dimensional semiconducting transition metal dichalcogenides such as $\mathrm{Mo}{\mathrm{S}}_2$ offers a stable and promising route for tailoring their electrical, optical, and magnetic properties. We perform density functional theory calculations for two promising transition metal dopants, Re and Nb, and their defect complexes with intrinsic S vacancies in $\mathrm{Mo}{\mathrm{S}}_2$. We compute the formation energy of each dopant and complex in different charge states utilizing a charge correction scheme that enables us to accurately predict the charge transition levels and complex binding energies, as well as characterize their electronic properties. We predict remarkably different behavior between Re and Nb dopants and their defect complexes: Re dopants can form complexes with S vacancies which quench the $n$-type doping of ${\mathrm{Re}}_{\mathrm{Mo}}$, while Nb dopants are unlikely to form such complexes and their $p$-type doping properties appear to be less sensitive to the presence of S vacancies.

The realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, ${\mathrm{Al}}_{2}X{X}^{\prime }$ $(X/X^{\prime }=\mathrm{O},\mathrm{S},\mathrm{Se},\mathrm{Te})$ by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all ${\mathrm{Al}}_{2}X{X}^{\prime }$ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of ${\mathrm{Al}}_{2}X{X}^{\prime }$ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, ${\mathrm{Al}}_{2}X\mathrm{O}$ monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary ${\mathrm{Al}}_{2}X{X}^{\prime }$ structures but also point out them as promising materials for optoelectronic applications.

Cross-sectional scanning tunneling microscopy (X-STM) was employed to characterize the InAs submonolayer quantum dots (SMLQDs) grown on top of a Si-doped GaAs(001) substrate in the presence of ($2\times 4$) and c($4\times 4$) surface reconstructions. Multiple layers were grown under different conditions to study their effects on the formation, morphology, and local composition of the SMLQDs. The morphological and compositional variations in the SMLQDs were observed by both filled and empty-state imaging. A detailed analysis of indium segregation in the SMLQD layers was described by fitting the local indium-concentration profile with a standard segregation model. We observed a strong influence of the arsenic flux over the indium incorporation and formation of the SMLQDs. We investigated the well-width fluctuations of the InGaAs quantum well in which the SMLQDs were formed. The well-width fluctuations were small compared to the more pronounced composition fluctuations in all the layers. The lateral compositional variations lead to the formation of indium-rich clusters which can act as quantum dots and yield charge-carrier confinement.

Although it is widely recognized that even small amounts of hydrogen (H) can cause embrittlement in iron (Fe) and in (high-strength) steels, the fundamental mechanisms that cause it are not yet completely understood. To contribute to a better understanding of the behavior of H in metals, we use parallel replica dynamics (PRD) to study the effect that different amounts of atomic H have in the diffusion of a single vacancy in body-centered cubic (BCC) Fe. Using PRD we calculate the diffusion of H-vacancy complexes at temperatures and timescales that are not reachable using classical methods. Additionally, based on the PRD results and calculations of the minimum energy paths of migration, we propose a mechanism for the migration of vacancy-H complexes and formulate an analytical model for the calculation of their joint diffusivity.

Concentrated solid solutions including the class of high entropy alloys (HEAs) have attracted enormous attention recently. Among these alloys a recently developed face-centered cubic (fcc) equiatomic VCoNi alloy revealed extraordinary high yield strength, exceeding previous high-strength fcc CrCoNi and FeCoNiCrMn alloys. Significant lattice distortions had been reported in the VCoNi solid solution. There is, however, a lack of knowledge about potential short-range order (SRO) and its implications for most of these alloys. We performed first-principles calculations and Monte Carlo simulations to compute the degree of SRO for fcc VCoNi, namely, by utilizing the coherent-potential approximation in combination with the generalized perturbation method as well as the supercell method in combination with recently developed machine-learned potentials. We analyze the chemical SRO parameters as well as the impact on other properties such as relaxation energies and lattice distortions.

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates in hosting novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability characterized by Curie temperature $\left(T_{\mathrm{C}}\right)$, remains to be achieved in such films. Here we present self-intercalated epitaxial ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular-beam epitaxy based technique, we have realized epitaxial ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ films with smoothly tunable δ over a wide range (0.33–0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of $T_{\mathrm{C}}$ from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy-axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. This demonstration of tunable $T_{\mathrm{C}}$ and magnetic anisotropy across room temperature in TMD films paves the way for engineering different magnetic phases for spintronic applications.

A theoretical study is performed on the relation between magnetic properties and lattice structures, that is, lattice constants and local atom displacement, for rhombhedral (rh-) ${\mathrm{Y}}_2{\mathrm{Fe}}_{17}$ compounds. We use real-space full-orbital tight-binding formalism to calculate electronic states. Magnetic anisotropy (MA) is calculated with high numerical accuracy by adopting a second-order perturbation for spin-orbit interaction. It is shown that the local magnetic moments of Fe atoms on 9d and 18h sites increase with increasing lattice volume. The result is attributed to the high atomic area density of these sites on the hexagonal planes. Those of Fe atoms on the other sites are found to be nearly independent of the volume. We calculate local MA energy of Fe atoms on each nonequivalent site and find that the magnitude of the local MA is strongly affected by the local atom displacement. As a result, the MA of ${\mathrm{Y}}_2{\mathrm{Fe}}_{17}$ is shown to be sensitive to volume.

The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films—${\mathrm{Fe}}_x{\mathrm{Si}}_{1–x}, {\mathrm{Fe}}_x{\mathrm{Ge}}_{1–x}, {\mathrm{Co}}_x{\mathrm{Ge}}_{1–x}, {\mathrm{Co}}_x{\mathrm{Si}}_{1–x}$, and ${\mathrm{Fe}}_{1–y}{\mathrm{Co}}_y\mathrm{Si}$. The experimental results are compared with density functional theory calculations of the density of Berry curvature and intrinsic anomalous Hall conductivity. In all samples, the longitudinal conductivity (${\sigma }_{xx}$), magnetization ($M$), and Hall resistivity (${\rho }_{xy}$) increase with increasing transition metal concentration; due to the structural disorder ${\sigma }_{xx}$ is lower in all samples than a typical crystalline metal. In the systems with Fe as the transition metal (including ${\mathrm{Fe}}_{1–y}{\mathrm{Co}}_y\mathrm{Si}$), the magnetization and AHE are large and in some cases greater than the crystalline analog. In all samples, the AHE is dominated by the intrinsic mechanism, arising from a nonzero, locally derived Berry curvature. The anomalous Hall angle (AHA) $(={\sigma }_{xy}/{\sigma }_{xx})$ is as large as 5% at low temperature. These results are compared with the AHAs reported in a broad range of crystalline and amorphous materials. Previous work has shown that in a typical crystalline ferromagnet the Hall conductivity (${\sigma }_{xy}$) and ${\sigma }_{xx}$ are correlated and are usually either both large or both small, resulting in an AHA that decreases with increasing ${\sigma }_{xy}$. By contrast, the AHA increases linearly with increasing ${\sigma }_{xy}$ in the amorphous systems. This trend is attributed to a generally low ${\sigma }_{xx}$, while ${\sigma }_{xy}$ varies and can be large. In the amorphous systems, ${\sigma }_{xx}$ and ${\sigma }_{xy}$ are not coupled, and there may thus exist the potential to further increase the AHA by increasing ${\sigma }_{xy}$.

One branch of the La-214 family of cuprate superconductors, ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ (Nd-LSCO), has been of significant and sustained interest, in large part because it displays the full complexity of the phase diagram for canonical hole-doped, high-$T_{\mathrm{C}}$ superconductivity, while also displaying relatively low superconducting critical temperatures. The low-superconducting $T_{\mathrm{C}}\mathrm{s}$ imply that experimentally accessible magnetic fields can suppress the superconductivity to zero temperature. In particular, this has enabled various transport and thermodynamic studies of the $T=0$ ground state in Nd-LSCO, free of superconductivity, across the critical doping $p^{*}=0.23$ where the pseudogap phase ends. The strong dependence of its superconducting properties on its crystal symmetry has itself motivated careful studies of the Nd-LSCO structural phase diagram. This paper provides a systematic study and summary of the materials preparation and characterization of both single-crystal and polycrystalline samples of Nd-LSCO. Single-phase polycrystalline samples with $x$ spanning the range from 0.01 to 0.40 have been synthesized, and large single crystals of ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ for select $x$ across the region (0.07, 0.12, 0.17, 0.19, 0.225, 0.24, and 0.26) were grown by the optical floating-zone method. Systematic neutron and x-ray-diffraction studies on these samples were performed at both low and room temperatures, 10 and 300 K, respectively. These studies allowed us to follow the various structural phase transitions and propose an updated structural phase diagram for Nd-LSCO. In particular, we found that the low-temperature tetragonal (LTT) phase ends at a critical doping $p_{\mathrm{LTT}}=0.255\pm 0.005$, clearly separated from $p^{*}$.

We present a model of pair interactions on rigid lattice to study the thermodynamic properties of iron-nickel alloys. The pair interactions are fitted at 0 K on ab initio calculations of formation enthalpies of ordered and disordered (special quasirandom) structures. They are also systematically fitted on the Gibbs free energy of the $\gamma$ Fe-Ni solid solution as described in a CALPHAD (CALculation of PHAse Diagrams) study by Cacciamani et al. This allows the effects of finite temperature, especially those of magnetic transitions, to be accurately described. We show that the ab initio and CALPHAD data for the $\gamma$ solid solution and for the $\mathrm{Fe}{\mathrm{Ni}}_3\text{−}\mathrm{L}{1}_2$ ordered phase can be well reproduced, in a large domain of composition and temperature, using first and second neighbor pair interactions which depend on temperature and local alloy composition. The procedure makes it possible to distinguish and separately compare magnetic, chemical, and configuration enthalpies and entropies. We discuss the remaining differences between the pair interaction model and CALPHAD, which are mainly due to the treatment of the short-range order and configurational entropy of the solid solution. The FCC phase diagram of the Fe-Ni system is determined by Monte Carlo simulations in the semigrand canonical ensemble and is compared with experimental studies and other models. We especially discuss the stability of the $\mathrm{Fe}\mathrm{Ni}\text{−}\mathrm{L}{1}_0$ phase at low temperature.

We use irreversible thermodynamics to formulate a model for the formation of a cation-deficient, $p$-type oxide on the surface of a pure metal in which local equilibrium is not necessarily maintained at the oxide interfaces and no a priori assumption regarding a rate limiting step is made. The dissipation rate derived here accounts for most of the conceivable processes that can occur during the oxidation of a metal, including bulk diffusion of cations and surface diffusion along the metal-oxide and gas-oxide interfaces. By including capillary effects in the formulation, shape changes in the oxide, which are usually neglected in most theories, can be described. We examine the steady-state solution of the model in one dimension for planar interfaces, and show that the growth law naturally captures the transition from linear to parabolic kinetics as the film thickens. A characteristic lengthscale for the transition is derived and can be obtained from oxide growth curves as a fitting parameter. Wagner's classical oxidation model is obtained as a limiting case of our more general model. We obtain expressions for the interfacial defect concentrations as a function of the departure from local equilibrium at the interfaces. From these expressions, we show that the exponent for the oxygen pressure dependence of the cation vacancy concentration can deviate from the usual values obtained from equilibrium considerations, and thus measuring nonstandard exponents can indicate a nonequilibrium effect. Our analysis serves to clarify the meaning and validity of the local equilibrium assumption in the context of metal oxidation.

We report detailed magnetic, transport, heat-capacity, and neutron-diffraction measurements of ${\mathrm{Ba}}_5{\mathrm{Ru}}_3{\mathrm{O}}_{12}$, a compound consisting of isolated ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimers. We show that this system develops long-range antiferromagnetic ordering at $T_{\mathrm{N}}\sim 60\mathrm{K}$ without structural distortion or metal-insulator type transition, which is in sharp contrast to other barium ruthenate trimer systems such as $9\mathrm{R}\text{−}{\mathrm{BaRuO}}_3$ and ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$. A complex magnetic structure is revealed which is attributable to the magnetic frustration due to competing exchange interactions between Ru ions on different crystallographic sites within the ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ trimer and different degree of orbital hybridization on different Ru sites. We have also investigated the magnetic structure of ${\mathrm{Ba}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ for comparisons.

We report the growth of CuMnSb thin films by molecular beam epitaxy on InAs (001) substrates. The CuMnSb layers are compressively strained (0.6 %) due to lattice mismatch. The thin films have a $\omega$ full width at half-maximum of 7.7 arcsec according to high resolution x-ray diffraction, and a root-mean-square roughness of 0.14 nm as determined by atomic force microscopy. Magnetic and electrical properties are found to be consistent with reported values from bulk samples. We find a Néel temperature of 62 K, a Curie-Weiss temperature of −65 K, and an effective moment of $5.9{\mu }_B/\mathrm{f}.\mathrm{u}.$ Transport measurements confirm the antiferromagnetic transition and show a residual resistivity at 4 K of $35\mu$Ω cm.

Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first-order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural, and magnetic properties of the ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ Heusler alloys. The alloys present a thermal hysteresis reduction of ∼60% when the Cu content in the compound varies from $x=0.10$ to $x=0.25$, with a minimum hysteresis width of 6 K being achieved. We applied the geometric nonlinear theory of martensite to address the phase compatibility, quantified by the parameter ${\lambda }_2$, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (${\lambda }_2$ closer to 1) between the austenite and martensite phases. In addition, we show that the valleylike properties of hysteresis found in the ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ compounds is present in several other alloys in the literature. These results provide pathways to understand as well as to master the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys.

The Kitaev spin model offers an exact quantum spin liquid in the ground state, which has stimulated exploration of its material realization over the last decade. Thus far, most of the candidates are found in $4d$- and $5d$-electron compounds, in which the low-spin $d^5$ electron configuration subject to strong spin-orbit coupling comprises a Kramers doublet with the effective angular momentum $j_{\mathrm{eff}}=1/2$ and gives rise to the bond-dependent anisotropic interactions in the Kitaev model. Extending the recent work on another candidates in $4f$-electron compounds [S.-H. Jang et al., Phys. Rev. B 99, 241106(R) (2019)], here we systematically study the Pr-based materials $A_2{\mathrm{PrO}}_3$ ($A=$ alkali metals) on both quasi-two-dimensional honeycomb and three-dimensional hyperhoneycomb structures. By using the ab initio-based scheme used in the previous study, we show that the low-energy magnetic properties of $A_2{\mathrm{PrO}}_3$ are well described by an effective spin model with the isotropic Heisenberg, anisotropic Kitaev, and symmetric off-diagonal interactions, dubbed the $J\text{−}K\text{−}{\mathrm{\Gamma }}^{\prime }$ model, with systematic modulations of the exchange coupling constants for the $A$-site substitution; while increasing the $A$-site ionic radii, $J$ is not largely modulated but the antiferromagnetic $K$ is reduced and ${\mathrm{\Gamma }}^{\prime }$ is slightly increased. We analyze the systematic changes by decomposing each interaction into the contributions from different perturbation processes in terms of both virtual hoppings and intermediate states. In addition, by computing the ground states of the $J\text{−}K\text{−}{\mathrm{\Gamma }}^{\prime }$ model by using the exact diagonalization, we map out the systematic evolution of the model parameters in the phase diagram. Our results will stimulate material exploration of the antiferromagnetic Kitaev interaction in $f$-electron compounds, including the previously-synthesized honeycomb and hyperhoneycomb compounds, ${\mathrm{Na}}_2{\mathrm{PrO}}_3$.

The in situ biasing transmission electron microscopy technique is employed to investigate the nucleation and growth of the ferroelectric phase during the electric field-induced phase transition in ${\mathrm{Pb}}_{0.99}\left\{{\mathrm{Nb}}_{0.02}{\left[{\left({\mathrm{Zr}}_{0.57}{\mathrm{Sn}}_{0.43}\right)}_{0.94}{\mathrm{Ti}}_{0.06}\right]}_{0.98}\right\}{\mathrm{O}}_3$, a ${\mathrm{PbZrO}}_3$-based antiferroelectric ceramic. The first-order displacive phase transition is found to be highly reversible with the initial antiferroelectric domain configuration almost completely recovered upon removal of the applied field. In the forward transition from the antiferroelectric to ferroelectric phase, ${\left\{100\right\}}_{\mathrm{c}}$ facets are dominant on the phase boundary; while in the reverse transition from the ferroelectric to antiferroelectric phase during bias unloading, the phase boundary is segmented into ${\left\{101\right\}}_{\mathrm{c}}$ and ${\left\{121\right\}}_{\mathrm{c}}$ facets. The motion of the phase boundary is nonuniform, taking the form of sequential sweeping of facet segments. The elastic distortion energy and the depolarization energy at the antiferroelectric/ferroelectric phase boundary is suggested to dictate the facet motion.

Amorphous ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ magnetic alloys exhibit a list of intriguing properties, such as perpendicular magnetic anisotropy, high magneto-optical activity, and magnetization switching using ultrashort optical pulses. Varying the Tb:Co ratio in these alloys allows for tuning properties such as the saturation magnetic moment, coercive field, and the performance of light-induced magnetization switching. In this paper, we investigate the magnetic, optical, and magneto-optical properties of various ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ thin-film alloy compositions. We report on the effect the choice of different seeding layers has on the structural and magnetic properties of ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ layers. We also demonstrate that for a range of alloys, deposited on fused silica substrates, with Tb content of 24–30 at. $\%$, helicity-dependent all-optical switching of magnetization can be achieved, albeit in a multishot framework. We explain this property to arise from the helicity-dependent laser-induced magnetization on the Co sublattice due to the inverse Faraday effect. Our paper provides an insight into material aspects for future potential hybrid magnetoplasmonic TbCo-based architectures.

Antiferroelectricity (AFE) and its robust dielectric response under a high electric field were experimentally demonstrated in tetragonal tungsten bronze ${\mathrm{K}}_{2}R{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ (R: rare-earth). Electrical resistivity and density of ceramics samples were sufficiently improved by optimizing chemical compositions and processes, and the phase transition temperatures are widely controlled by changing R ions. Typical features of AFE, i.e., a double-hysteresis loop and a dielectric peak under a DC electric field, were demonstrated at room temperature in ${\mathrm{K}}_2{\mathrm{Pr}}_{0.75}{\mathrm{La}}_{0.25}{\mathrm{Nb}}_5{\mathrm{O}}_{15}$. Notably robust relative permittivity against DC fields, which might be applicable to ceramics capacitors for high-voltage usage, is realized owing to AFE. Its dielectric constant under a high DC electric field, ε ∼ 800 at 10 MV/m, exceeds that of conventional ${\mathrm{BaTiO}}_3$, ε ∼ 620 at 10 MV/m. First-principles calculations suggested competing polar and antipolar instabilities underlying the successive phase transitions in this system. Two different types of antipolar displacement patterns $({{\mathrm{\Gamma }}_2}^{-} \mathrm{and} {M_1}^{+}{M_4}^{+})$ were found and are considered to be the structural origin of AFE. These results clearly demonstrate that ${\mathrm{K}}_{2}R{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ offers a new antiferroelectric materials platform that operates around room temperature.

Thin layers of near-surface nitrogen-vacancy (NV) defects in diamond substrates are the workhorse of NV-based wide-field magnetic microscopy, which has applications in physics, geology, and biology. Several methods exist to create such NV layers, which generally involve incorporating nitrogen atoms (N) and vacancies (V) into the diamond through growth and/or irradiation. While there have been detailed studies of individual methods, a direct side-by-side experimental comparison of the resulting magnetic sensitivities is still missing. Here we characterize, at room and cryogenic temperatures, $\approx 100\text{−}\mathrm{nm}$-thick NV layers fabricated via three different methods: (1) low-energy carbon irradiation of N-rich high-pressure high-temperature (HPHT) diamond, (2) carbon irradiation of $\delta$-doped chemical vapor deposition (CVD) diamond, (3) low-energy ${\mathrm{N}}^{+}$ or ${\mathrm{CN}}^{-}$ implantation into N-free CVD diamond. Despite significant variability within each method, we find that the best HPHT samples yield similar magnetic sensitivities (within a factor 2 on average) to our $\delta$-doped samples, of $<2\mu \mathrm{T}{\mathrm{Hz}}^{-1/2}$ for dc magnetic fields and $<100\mathrm{nT}{\mathrm{Hz}}^{-1/2}$ for ac fields (for a $400\mathrm{nm}$ $\times 400\mathrm{nm}$ pixel), while the ${\mathrm{N}}^{+}$ and ${\mathrm{CN}}^{-}$ implanted samples exhibit an inferior sensitivity by a factor 2–5, at both room and low temperatures. We also examine the crystal lattice strain caused by the respective methods and discuss the implications this has for wide-field NV imaging. The pros and cons of each method, and potential future improvements, are discussed. This study highlights that low-energy irradiation of HPHT diamond, despite its relative simplicity and low cost, is a competitive method to create thin NV layers for wide-field magnetic imaging.

Although phenomenologically simple to define, the yield stress is often difficult to quantify unambiguously in practice, especially for thixotropic materials with complex shear histories. Here, we identify a stress-controlled bifurcation in the yielding response of cellulose nanofibril gels, which we show can rigorously localize the yield stress in disordered materials with time-dependent behavior. After an initial yielding event, the fibrillar networks subsequently yield faster and at lower magnitudes of stress. For low stresses, the time to yielding increases with waiting time $t_w$ and diverges once the network has restored sufficient entanglement density to support the stress. For higher stresses, the yield time instead plateaus at a finite value because the developed network density is insufficient to support the applied stress. We quantitatively relate the yielding and aging behavior of the network to the competition between stress-induced disentanglement and dynamic fluctuations of the fibrils rebuilding the network. The critical stress ${\sigma }_c$ that bifurcates the response of the material between these two states identifies the intrinsic yield stress in these disordered materials, independent of aging, thixotropic effects, or shear history.