With advanced synthetic techniques, a wide variety of well-defined graphene nanoribbons (GNRs) can be produced with atomic precision. Hence, finding the relation between their structures and properties becomes important for the rational design of GNRs. In this work, we explore the complete chemical space of gulf-edged zigzag graphene nanoribbons (ZGNR-Gs), a subclass of zigzag GNRs in which the zigzag edges miss carbon atoms in a regular sequence. We demonstrate that the electronic properties of ZGNR-Gs depend on four structural parameters: ribbon width, gulf edge size, unit length, and gulf offset. Using tight-binding calculations and the Hubbard model, we find that all ZGNR-Gs are semiconductors with varying band gaps; there are no metals in this class of materials. Notably, when spin polarization is considered, most ZGNR-Gs exhibit antiferromagnetic behavior, with the spin moments and spin-induced band gap opening being stabilized by longer zigzag segments at the edges. Furthermore, we provide simple empirical rules that describe the ${\mathbb{Z}}_2$ topological invariant based on the aforementioned structural parameters. By analyzing the full chemical space of ZGNR-Gs, we offer insights into the design of GNRs with desired electronic, magnetic, and topological properties for nanoelectronic applications.

The electronic density of states (DOS) provides information regarding the distribution of electronic energy levels in a material, and can be used to approximate its optical and electronic properties and therefore guide computational materials design. Given its usefulness and relative simplicity, it has been one of the first electronic properties used as a target for machine-learning approaches going beyond interatomic potentials. A subtle but important point, well appreciated in the condensed matter community but usually overlooked in the construction of data-driven models, is that for bulk configurations the absolute energy reference of single-particle energy levels is ill-defined. Only energy differences matter, and quantities derived from the DOS are typically independent of the absolute alignment. We introduce an adaptive scheme that optimizes the energy reference of each structure as part of the training process and show that it consistently improves the quality of machine-learning models compared to traditional choices of energy reference for different classes of materials and different model architectures. On a practical level, we trace the improved performance to the ability of this self-aligning scheme to match the most prominent features in the DOS. More broadly, we believe that this paper highlights the importance of incorporating insights on the nature of the physical target into the definition of the architecture and of the appropriate figures of merit for machine-learning models, translating into better transferability and overall performance.

We explore the optical dipole orientation of single photon emitters in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ as produced by a focused helium ion beam. The single photon emitters can be understood as single-sulfur vacancies. The corresponding far-field luminescence spectra reveal several photoluminescence lines below the dominating luminescence of the exciton in $\mathrm{Mo}{\mathrm{S}}_2$. These sub-bandgap emission lines were predicted by ab initio theory, but they have never been resolved in luminescence experiments because of their small amplitude. We reveal the lines by their dependence as a function of the photon energy and momentum as measured in the back focal plane of the optical circuitry. The agreement between theory and experiment suggests that the defect states interact strongly within the Brillouin zone.

Antiferromagnets with exotic spin textures promise low-power spintronic devices with extremely high operating frequencies and resistance to external perturbations. In particular, the combination of highly tunable correlated electron physics, as in complex oxides, with metallicity and antiferromagnetism is desirable but exceedingly rare. ${\mathrm{LaNiO}}_3$, the lone example of a perovskite nickelate which is metallic across all temperatures, has long been a promising candidate, but the antiferromagnetic metallic state has remained elusive. We demonstrate the emergence of this state in ultrathin films of (111)-oriented ${\mathrm{LaNiO}}_3$ using a combination of polarized neutron reflectometry, low-energy muon spectroscopy and anomalous Hall effect measurements, and density functional theory calculations with a Hubbard U term. Further, we find a highly strained symmetry-breaking interfacial region which may support canting of the AFM moments leading to a vanishingly small net magnetization at the film-substrate interface, providing a convenient route toward control of the Néel order. Evidence of antiferromagnetic metallic behavior in (111)LNO films highlights the role of crystal symmetry in tuning the novel quantum states in complex oxides.

The non-local spin valve (NLSV) is a useful device for studying spin transport at nanoscopic dimensions, with potential technological applications. Despite this appeal, background signals, unrelated to spin diffusion, often hinder the interpretation of spin signals in NLSVs and could compromise performance in future devices. In this paper, we comprehensively investigate these background signals in all-metallic NLSVs fabricated from a variety of ferromagnetic (FM; $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$, Fe, Co) and nonmagnetic (NM; Al, Cu) metals. We demonstrate that a background signal emerges in AC measurements, with contributions from both current spreading and thermoelectric effects, with a complex dependence on both temperature and FM injector-detector separation. Despite the complexity of these dependencies, we demonstrate excellent agreement with three-dimensional finite-element modelling that accounts for current-spreading and thermoelectric effects, across a wide range of temperatures, FM separations, and FM/NM pairings. This approach additionally offers a means to estimate the Seebeck coefficients for the tested FM/NM pairings, providing further insight into the charge and heat flow in such nanoscopic spintronic devices.

Refractory high-entropy alloys show promising resistance to irradiation, yet little is known about the fundamental nature of radiation-induced defect formation. Here, we simulate threshold displacement energies in equiatomic MoNbTaVW using an accurate machine-learned interatomic potential, covering the full angular space of crystal directions. The effects of local chemical ordering are assessed by comparing results in random and short-range-ordered MoNbTaVW. The average threshold displacement energy in the random alloy is $44.3\pm 0.15\mathrm{eV}$, and slightly higher, $48.6\pm 0.15\mathrm{eV}$, in the short-range-ordered alloy. Both are significantly lower than in any of the constituent pure metals. We identify the mechanisms of defect creation and find they are mainly dependent on the masses of the recoiling and colliding elements. Low thresholds are generally found when heavy atoms (W, Ta) displace and replace the lightest atoms (V). The average threshold energies, when separated by recoiling element, are consequently ordered inversely according to their mass, opposite to the trend in the pure metals where W has by far the highest thresholds. However, the trend in the alloy is reversed when considering the cross sections for defect formation in electron irradiation, due to the mass-dependent recoil energies from the electrons.

${\mathrm{Li}}_6{\mathrm{PS}}_5\mathrm{Cl}$ is a promising candidate for the solid electrolyte in all-solid-state Li-ion batteries due to its high ionic conductivity. In applications, this material is in a polycrystalline state with grain boundaries (GBs) that can affect ionic conductivity. While atomistic modeling provides valuable information on the impact of GBs on Li diffusion, such studies face either high computational cost (when using ab initio methods) or accuracy limitations (when using classical potentials) as challenges. Here, we develop a quality-level-based active learning scheme for efficient and systematic development of ab initio–based machine-learning interatomic potentials, specifically moment tensor potentials (MTPs), for large-scale, long-time, and high-accuracy simulations of complex atomic structures and diffusion mechanisms as encountered in solid electrolytes. Based on this scheme, we obtain MTPs for ${\mathrm{Li}}_6{\mathrm{PS}}_5\mathrm{Cl}$ and investigate two tilt GBs, $\mathrm{\Sigma }3\left(1\overline{1}2\right)\left[110\right]$ and $\mathrm{\Sigma }3\left(\overline{1}11\right)\left[110\right]$, and one twist GB, $\mathrm{\Sigma }5\left(001\right)\left[001\right]$. All three GBs exhibit low formation energies of less than 20 meV$/{\AA }^2$, indicating their high stability in polycrystalline ${\mathrm{Li}}_6{\mathrm{PS}}_5\mathrm{Cl}$. Using the MTPs, diffusion coefficients of the anion-ordered and anion-disordered bulk, as well as the three GBs, are obtained from molecular dynamics simulations of atomistic models with more than $16000$ atoms for 5 ns. At 300 K, the GB diffusion coefficients fall between the ones of the anion-ordered bulk structure ($1.2\times {10}^{-9}{\mathrm{cm}}^2/\mathrm{s}$, corresponding to ionic conductivity about 0.2 mS/cm) and the anion-disordered bulk structure ($50\%$ Cl/S-anion disorder; $2.2\times {10}^{-7}{\mathrm{cm}}^2/\mathrm{s}$, about 29.8 mS/cm) of ${\mathrm{Li}}_6{\mathrm{PS}}_5\mathrm{Cl}$. Experimental data fall between the Arrhenius-extrapolated diffusion coefficients of the investigated atomic structures, supporting our quantitative in silico predictions.

A detailed understanding of the irradiation-induced damage evolution is valuable when considering materials for extreme environments, such as in fusion applications. In this work, irradiation-induced pointlike defects and their recovery kinetics in vanadium are studied with positron annihilation spectroscopy. The defects were created with 10 MeV proton irradiation at cryogenic temperatures. Several distinct recovery stages are found, at 60 K, 100 K, 150 to 250 K, and 375 to 475 K. These are interpreted as the motion of self-interstitial atoms, monovacancies, clusters of vacancies and multivacancy-hydrogen complexes. These observations allow for estimating migration barriers for the elementary defects.

The thermodynamics of point defects is crucial for determining the functional properties of various materials. Typically, defect stability is assessed using grand-canonical defect formation energy, which requires deducing the equilibrium chemical potential or Fermi level that governs atom and electron exchange with the environment. This process is complicated by the interplay of chemical potential and Fermi level and their dependence on composition and temperature. Typically, the grand-canonical formation energy is incorporated as an additional term to the bulk Gibbs energy, creating a defect-centric framework where each new defect state necessitates a distinct Gibbs energy formulation. The calculation of phase diagrams (CALPHAD) method offers a more flexible alternative by integrating defect energies into the total Gibbs energy model, allowing for easier extrapolation to more complex compositions. Additionally, CALPHAD unifies the analysis of chemically and electronically driven defects using chemical composition as the primary variable. However, the compound energy formalism (CEF) used in CALPHAD has limitations, including a lack of clear connections between defect formation energies and Gibbs energy parameters and an exponential increase in complexity with added chemical or charge variations. We present the theoretical derivation of the defect energy formalism (DEF), which we have recently proposed. DEF overcomes the limitations of CEF by establishing explicit relationships between the absolute defect energies—independent of chemical potential or Fermi level—and the Gibbs energy parameters of defective compounds. This results in a first-principles model for dilute defects, eliminating the need for model fitting to experimental or simulation data. Additionally, DEF reduces the inherent complexity of CEF by applying the superposition of absolute defect energies, making it feasible for modeling multicomponent and chemically complex compounds. This paper presents a formal, general derivation of DEF and offers guidelines for its application, promising more accurate and efficient thermodynamic modeling of defective materials.

Fluctuations of quantum spins play a crucial role in the emergence of exotic magnetic phases and excitations. The lack of the charge degree of freedom in insulating quantum magnets, however, precludes such fluctuations from mediating electronic transport. Here, we show that the quantum fluctuations of a localized frustrated magnet induce strong proximitized charge transport of the conduction electrons in a synthetic heterostructure comprising an epitaxial ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ultrathin film on a single crystal of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. The proximity effects are evidenced by the scaling behavior of the ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ resistance in correspondence with the dynamic scaling of the dynamic spin-correlation function of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, which is a result of quantum fluctuations near a multiphase quantum critical point. The proximitized transport in ${\mathrm{Bi}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ can be effectively tuned by a magnetic field through suppressing the quantum spin fluctuations (QSFs) as well as inducing transitions via magnetic anisotropy in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. In this paper, we establish a pathway for harnessing QSFs in magnetic insulators with electric transport, offering exciting prospects for potential applications in the realm of quantum spintronics.

Elastomeric materials display a complicated set of stretchability and fracture properties that strongly depend on the flaw size, which has long been of interest to engineers and material scientists. Here, we combine experiments and numerical simulations for a comprehensive understanding of the nonlocal, size-dependent features of fracture in elastomers. We quantitatively describe the size-dependent fracture behavior using a nonlocal continuum model. The key ingredient of the nonlocal model is the use of an intrinsic length scale associated with a finite fracture process zone, which is inferred from experiments. Of particular importance, our experimental and theoretical approach passes the critical set of capturing the key aspects of the size-dependent fracture in elastomers. Applications to a wide range of synthetic elastomers that exhibit moderate ($\sim 100\%$) to extreme stretchability ($\sim 1000\%$) are presented, which is also used to demonstrate the applicability of our approach in elastomeric specimens with complex geometries.

The mode of intermolecular arrangements is crucial in determining organic semiconductor (OSC) device performance, although the method for controlling the arrangements has not yet been established. Here it is demonstrated that mixing OSC molecules with different lengths of substituted alkyl chains allows the emergence of a distinct high-mobility polymorphic phase in alkylated layered crystalline OSCs. The pristine OSC molecules of $para$-tolyl/alkylated-[1]benzothieno[3,2-$b]$[1]benzothiophene ($p\mathrm{Tol}\text{−}\mathrm{BTBT}\text{−}{\mathrm{C}}_n$) at $n$ $=6$ and 8 both exhibit a low-mobility isomorphous phase in the crystalline film states. By contrast, a distinct polymorphic phase showing much higher mobility emerges as a result of equimolar mixing of $p\mathrm{Tol}\text{−}\mathrm{BTBT}\text{−}{\mathrm{C}}_n$ at $n$ $=6$ and 8 during the film-coating process. The mixing-induced phase presents tenfold enhanced mobility compared with pure compounds, accompanied by a drastic polymorphic transformation. The emerging technique of electron diffraction structure analysis reveals that interlayer alkyl chain interdigitation involving close tail-to-tail packing plays a significant role in stabilizing the mixing-induced phase. The analysis of polarized absorption spectra indicates that a metastable liquid-crystalline (LC) phase is formed prior to the crystallization, implying that the formation of the mixing-induced phase is promoted by the intralayer molecular diffusion motion within the metastable LC phase. The finding of the controllable nature of layered crystallinity should lead to high-end printed electronics.

We use scanning tunneling microscopy and spectroscopy under ultrahigh vacuum and down to 1.7 K to study the local variations of the chemical potential on the surface of the topological insulator ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ thin films (thickness $7–30\mathrm{nm}$) with varying Sb concentration $x$, to gain insight into the charge puddles formed in thin films of a compensated topological insulator. We found that the amplitude of the potential fluctuations, $\mathrm{\Gamma }$, is between 5 and 14 meV for quasi-bulk-insulating films and about $30–40\mathrm{meV}$ for bulk-insulating films. The length scale of the fluctuations, $\lambda$, was found to span the range of $13–54\mathrm{nm}$, with no clear correlation with $\mathrm{\Gamma }$. Applying a magnetic field normal to the surface, we observe the condensation of the two-dimensional topological surface state into Landau levels and find a weak but positive correlation between $\mathrm{\Gamma }$ and the spectral width of the Landau-level peaks, which suggests that quantum smearing from drift motion is the source of the Landau-level broadening. Our systematic measurements give useful guidelines for realizing ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ thin films with an acceptable level of potential fluctuations. In particular, we found that $x\approx 0.65$ realizes the situation where $\mathrm{\Gamma }$ shows a comparatively small value of 14 meV and the Dirac point lies within $\sim 10\mathrm{meV}$ of the Fermi energy.

Layered transition metal dichalcogenides (TMDs) stabilize in multiple structural forms with profoundly distinct and exotic electronic phases. Interfacing different layer types is a promising route to manipulate TMDs' properties, not only as a means to engineer quantum devices but also as a route to explore fundamental physics in complex matter. Here we use angle-resolved photoemission (ARPES) to investigate a strong layering-dependent enhancement of superconductivity in ${\mathrm{TaS}}_2$, in which the superconducting transition temperature, $T_c$, of its $2H$ structural phase is nearly tripled when insulating $1T$ layers are inserted into the system. The study is facilitated by a vanadium-intercalation approach to synthesizing various ${\mathrm{TaS}}_2$ polytypes, which improves the quality of the ARPES data while leaving key aspects of the electronic structure and properties intact. The spectra show the clear opening of the charge density wave gap in the pure $2H$ phase and its suppression when $1T$ layers are introduced to the system. Moreover, in the mixed-layer $4Hb$ system, we observe a strongly momentum-anisotropic increase in electron-phonon coupling near the Fermi level relative to the $2H$ phase. Both phenomena help to account for the increased $T_c$ in mixed $2H/1T$ layer structures.

The chromium-trihalides ($\mathrm{Cr}{X}_3$; $X=\mathrm{Cl}$, Br, I) have long served as model systems for understanding magnetism and magneto-optics in ionic crystals and they have recently also emerged as archetypes of magnetic two-dimensional (2D) van der Waals materials. Although the physical properties of $\mathrm{Cr}{X}_3$ compounds have now been explored in great depth, the use of impurity doping to generate new properties remains surprisingly underexplored. Here we report on the magnetic and magneto-optical properties of lanthanide-doped ${\mathrm{CrBr}}_3$, in which ${\mathrm{Yb}}^{3+}$ is introduced as a spin-bearing optical point defect. Narrow-line near-infrared dopant photoluminescence is efficiently sensitized by ${\mathrm{CrBr}}_3$-to-${\mathrm{Yb}}^{3+}$ energy transfer. Magneto-optical measurements show facile spin manipulation of the paramagnetic ${\mathrm{Yb}}^{3+}$ impurities by the surrounding ${\mathrm{CrBr}}_3$ lattice. Photoluminescence data reveal that the Kramers spin degeneracies of the ground- and excited-state ${\mathrm{Yb}}^{3+}$ doublets are spontaneously lifted by strong magnetic exchange with neighboring ${\mathrm{Cr}}^{3+}$ ions, such that individual ${\mathrm{Yb}}^{3+}$ spin transitions are easily resolved even at zero external magnetic field. The temperature and field dependence of these splittings can be used to probe both local and long-range spin correlations. These results highlight the use of extrinsic optical impurities to add new spin-photonic functionality to this classic 2D van der Waals magnet.

In biology, mechanosensitive ion channels facilitate the conversion of mechanical stimuli, such as sound and touch, into electrical signals. Similar functionality in artificial systems was recently predicted in the form of stretch-activated transport through porous 2D membranes. Here we describe an opposite behavior, in which ion transport is inactivated upon stretching a subnanoporous 2D membrane. We explore electrophoretic ion transport through several subnanoporous membranes using molecular dynamics simulations. We demonstrate that aqueous ${\mathrm{K}}^{+}$ transport decreases by a factor of 3–8 under stretching of order 3%. In contrast, ${\mathrm{Na}}^{+}$ ions exhibit transport activation by stretching, suggesting ion-dependent activation and inactivation in a single membrane. Our analysis shows that inactivation of ${\mathrm{K}}^{+}$ transport is caused by a strain-induced repulsive-to-attractive transition in the ${\mathrm{K}}^{+}$-pore interactions due to alignment of the local energy minima.

Two-dimensional semiconductors present unique opportunities to intertwine optical and magnetic functionalities and to tune these performances through defects and dopants. Here, we integrate exciton pumping into a quantum sensing protocol on nitrogen-vacancy centers in diamond to image the optically induced transient stray fields in few-layer, antiferromagnetic ${\mathrm{CrCl}}_3$. We discover that exciton recombination enhances the in-plane magnetization of the ${\mathrm{CrCl}}_3$ layers, with a predominant effect in the surface monolayers. Concomitantly, time-resolved photoluminescence measurements reveal that nonradiative exciton recombination intensifies in atomically thin ${\mathrm{CrCl}}_3$ with tightly localized, nearly dipole-forbidden excitons and amplified surface-to-volume ratio. Supported by experiments under controlled surface exposure and density functional theory calculations, we interpret the magnetically enhanced state to result from a defect-assisted Auger recombination that optically activates electron transfer between water vapor related surface impurities and the spin-polarized conduction band. Our work validates defect engineering as a route to enhance intrinsic magnetism in single magnetic layers and opens an experimental platform for studying optically induced, transient magnetism in condensed matter systems.

We present that $\mathrm{M}{\mathrm{n}}_{4-x}\mathrm{C}{\mathrm{u}}_x\mathrm{N}$ films grown by molecular beam epitaxy on $\mathrm{SrTi}{\mathrm{O}}_3$(001) have a magnetic compensation (MC) composition with noncollinear magnetic components at room temperature. Due to the high crystalline quality and perpendicular magnetic anisotropy, the remanent magnetization ratio was almost 100% for $x\le 0.6$. At 0.10 ≤ $x\le 0.15$, the saturation magnetization was the smallest, while the coercivity was the largest. The hysteresis curves obtained by both the magneto-optical Kerr effect and the anomalous Hall effect showed sign reversal between $x=0.1$ and 0.15. X-ray magnetic circular dichroism (XMCD) spectra at the Cu $L_{2,3}$ edges also showed sign reversal over the entire energy range. These results provide evidence that the MC composition is in the vicinity of $x\sim 0.1$. However, the XMCD spectra at the Mn $L_{2,3}$ edges showed a different magnetic structure change from that of Ni-doped $\mathrm{M}{\mathrm{n}}_4\mathrm{N}$, which also exhibits MC. A magnetic structure model for $\mathrm{M}{\mathrm{n}}_{4-x}\mathrm{C}{\mathrm{u}}_x\mathrm{N}$ is discussed in comparison with the experimental results.

Topological Hall effect (THE) is a hallmark of scalar spin chirality, which is found in static skyrmion lattices. Recent theoretical works have shown that scalar spin chirality could also emerge dynamically from thermal spin fluctuations. Evidence of such a mechanism was found in the kagome magnet $\mathrm{Y}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ where fluctuations arise from frustrated exchange interactions between Mn kagome layers. In $\mathrm{Y}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$, the rare-earth ion ${\mathrm{Y}}^{3+}$ is nonmagnetic. When it is replaced by a magnetic ion $({\mathrm{Gd}}^{3+}-{\mathrm{Ho}}^{3+})$, the intrinsically antiferromagnetic Mn-Mn interlayer coupling is overwhelmed by the indirect ferromagnetic $\mathrm{Mn}\text{−}R\text{−}\mathrm{Mn}$ one, relieving frustration. This generates interesting anomalous Hall conductivity, but not THE. Here we show that Er lies in an intermediate regime where direct and indirect interactions closely compete, so that $\mathrm{Er}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ can switch from one regime to the other by temperature, i.e., from a collinear ferrimagnetic ground state to a spiral antiferromagnet at 78 K. The antiferromagenetic  phase forms a dome in the temperature-field phase diagram. Close to the boundary of this dome, we find a sizable fluctuations-driven THE, thus underscoring the universality of this chiral fluctuation mechanism for generating nonzero scalar spin chirality.

A threshold dynamics model of grain growth that accounts for the anisotropy in the grain boundary energy has been used to simulate experimentally observed grain growth of polycrystalline Ni. The simulation reproduces several aspects of the observed microstructural evolution that are not found in the results of simulations assuming isotropic properties. For example, the relative areas of the lowest-energy twin boundaries increase as the grains grow and the average grain boundary energy decreases with grain growth. This decrease in energy occurs because the population of higher-energy grain boundaries decreases while the population of lower-energy boundaries increases as the total grain boundary area decreases. This phenomenon emerges from the assumption of anisotropic grain boundary energies without modification of the energy minimizing algorithm. These findings are consistent with the observation that, in addition to the decrease in grain boundary area, additional energy is dissipated during grain growth by a decrease in the average grain boundary energy.