We report spin-singlet formation in an organic-inorganic hybrid identified as ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Cu}}_2{\mathrm{Cl}}_5$. The hybrid adopts a layered structure in which ${{\mathrm{Cu}}_2{\mathrm{Cl}}_5}^{-}$ layers are separated by ${{\mathrm{CH}}_3{\mathrm{NH}}_3}^{+}$ layers. The inorganic layers consist of corner- and edge-sharing ${\mathrm{CuCl}}_6$ octahedra, forming edge-sharing tetramers. Magnetic susceptibility measurements indicate strong antiferromagnetic interactions within the tetramers, whereas the coupling between the tetramers is weak. Low-temperature magnetic susceptibility data as well as electron spin resonance measurements suggest a nonmagnetic ground state with a large spin gap of $\sim 130\mathrm{K}$, in apparent contradiction with ferromagnetic interactions between nearest-neighbor spins. We discuss a spin-tetramer model in which antiferromagnetic next-nearest-neighbor interactions lead to a spin-singlet state.

We report on experimental and computational investigations of the domain structure of single-crystal Ni nanowires (NWs). The $\sim 200\times 200\times 8000\mathrm{n}{\mathrm{m}}^3$ Ni NWs were grown by a thermal chemical vapor deposition technique that results in single-crystal structures. Magnetoresistance measurements of individual NWs suggest the average magnetization points largely off the NW long axis at zero field. X-ray photoemission electron microscopy images obtained at room temperature show a well-defined periodic magnetization pattern along the surface of the nanowires with a period of ${\lambda }_{\mathrm{avg}}=239\pm 37\mathrm{nm}$. Finite element micromagnetic simulations reveal that when the material parameters of the modeled system match those of nickel crystal at $T=10\mathrm{K}$, an oscillatory magnetization configuration with a period closely matching experimental observation ($\lambda =222\mathrm{nm}$) is obtainable at remanence. This magnetization configuration involves a periodic array of alternating chirality vortex domains distributed along the length of the NW. Vortex formation is attributable to the relatively high cubic anisotropy of the single crystal Ni NW system at $T=10\mathrm{K}$ and its reduced structural dimensions. The periodic alternating chirality vortex state is a topologically protected metastable state, analogous to an array of 360° domain walls in a thin strip. Simulations show that other remanent states are also possible, depending on the field history. At room temperature ($T=273\mathrm{K}$), simulations show vortices are no longer stable due to the expected reduced cubic anisotropy of the system, suggesting a disparity between the fabricated and modeled nanowires. Negative uniaxial anisotropy and magnetoelastic effects in the presence of compressive biaxial strain are shown to promote and restore formation of vortices at room temperature.

An increase of static friction during stationary contacts of two solids due to interfacial chemical bonding has been reported in multiple experiments. However, the physics underlying such frictional aging is still not fully understood because it involves multiple physical and chemical effects coupled with each other, making direct interpretation of experimental results difficult. Here, we develop a multiphysics chemical aging model that combines contact mechanics, mechanochemistry, and interfacial chemical reaction kinetics. Our model predicts that aging is proportional to normal loads in a low-load regime and becomes nonlinear at higher loads. We also discovered a nonmonotonic temperature dependence of aging with a peak near room temperature. In addition, our simulations provide insights into contributions from specific physical/chemical effects on the overall aging. Our model shows quantitative agreement with available single-asperity experiments on silica-silica interfaces, and it provides a framework for building a chemical aging model for other material systems with arbitrary types of physical and chemical effects involved.

We investigated initial steps of oxidation of the Si(557)-Au system by plasmon spectroscopy and first-principles calculations. The measurements, performed using an electron energy loss instrument with simultaneous high resolution in energy and momentum, reveal that metallicity is preserved under all oxidation conditions that are experimentally accessible in UHV. Corresponding simulations, performed within density functional theory, confirm this finding: Only the oxidation of the Si environment of the Au chains turned out to be strongly exothermic, with similar binding energy for adsorption on different structural elements. While large and site specific changes of the band structure were observed, the upper edge of the excitation spectrum of electron-hole pairs, to which plasmon dispersion is most sensitive, remains almost unchanged during the various steps of oxidation, due to the opposite and largely compensating contributions of different adsorption configurations. This investigation not only proves the robustness of metallicity of the gold chains upon oxidation of the surrounding environment of Si atoms, but also demonstrates the usefulness of plasmon spectroscopy in characterizing the electronic excitation spectrum of quasi-one-dimensional systems and unoccupied band structure.

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.

Antidot materials, i.e., two-dimensional nanostructures with a periodic array of nanopores, are of great scientific interest due to their unique nanomagnetic properties and their potential application in storage devices. It is well known that physical properties of antidots are directly linked to the diameter of nanopores and their spacing, as well as to the morphology and the localizations of chemical species. However, due to their nanoscale size, their characterization remains challenging. Here, we present a detailed investigation of the morphology and presence of oxide species in antidots as a function of the pore diameter using polarized x-ray absorption spectroscopy. For this study we synthesized and characterized Cu(10 nm)/Co(12 nm)/Cu(10 nm) sputtered antidots, fabricated by the double-anodization technique assisted by atomic layer deposition. The pore size ranged from 20 to 80 nm, with a fixed interpore distance (105 nm). An unholed multilayer deposited on Si/${\mathrm{SiO}}_2$ was also investigated for comparison. We observed a clear correlation between the increase of pore diameter and the enhancement of oxide content from three different x-ray absorption near edge structure analysis methods. Polarized XAS allowed us to localize the CoO nanorings inside the pores. We propose that the CoO formation is directly related to the crescent shape of the multilayer deposit inside the pores. The coercivity of antidots is enhanced by increasing the magnetic atomic proportion in the periphery of nanopores. The structural observations were also used to develop a simple model in order to estimate the proportion of atoms inside the pores and on the top of the antidots as a function of the interpore distance, the hole diameter, and the penetration length of deposition inside the pores. This model can be easily used in the literature for estimating the atomic species deposited on antidots.

The origins of the metal-insulator and magnetic transitions exhibited by perovskite rare-earth nickelates, R${\mathrm{NiO}}_3$ (where $R$ is a rare-earth element), remain open issues, with charge disproportionation, magnetic interactions, and lattice response across multiple length scales being among the many possible origins. Recently, growth of single crystals of ${\mathrm{LaNiO}}_3$, which is the only member of these compounds that remains metallic in its ground state, has been reported, opening a new chapter in the investigation of the perovskite nickelates. Here, using a combination of magnetometry, heat capacity, and neutron scattering on as-grown and purposely reduced $\mathrm{LaNi}{\mathrm{O}}_{3-\delta }$ crystals, we show that both antiferromagnetic and ferromagnetic phases with a Néel temperature of ∼152 K and a Curie temperature of ∼225 K can be induced by reduction of the oxygen content. Transmission electron microscopy shows that these phases are characterized by ordered oxygen vacancy defect structures that exist as dilute secondary phases in as-grown crystals despite growth in partial oxygen pressures up to at least 130 bar. The demonstration of antiferromagnetism resulting from oxygen vacancy ordered structures implies that stoichiometry must be explicitly considered when interpreting the bulk properties of $\mathrm{LaNi}{\mathrm{O}}_{3-\delta }$ single crystals; accordingly, the implications of our results for putative oxygen-stoichiometric ${\mathrm{LaNiO}}_3$ are discussed.

In an ideal InAs/GaSb bilayer of appropriate dimension, in-plane electron and hole bands overlap and hybridize, and a topologically nontrivial, or quantum spin Hall (QSH) insulator, phase is predicted to exist. The in-plane dispersion's potential landscape, however, is subject to microscopic perturbations originating from material imperfections. In this work, the effect of disorder on the electronic structure of InAs/GaSb (001) bilayers was studied by observing the temperature and magnetic-field dependence of the resistance of a dual-gated heterostructure gate-tuned through the inverted to normal gap regimes. Conduction with the electronic structure tuned to the inverted (predicted topological) regime and the Fermi level in the hybridization gap was qualitatively similar to behavior in a disordered two-dimensional system. The impact of charged impurities and interface roughness on the formation of topologically protected edge states and an insulating bulk was estimated. The experimental evidence and estimates of disorder in the potential landscape indicated that the potential fluctuations in state-of-the-art films are sufficiently strong such that conduction with the electronic structure tuned to the predicted topological insulator (TI) regime and the Fermi level in the hybridization gap was dominated by a symplectic metal phase rather than a TI phase. The implications are that future efforts must address disorder in this system, and focus must be placed on the reduction of defects and disorder in these heterostructures if a TI regime is to be achieved.

Half-Heusler (HH) alloys have shown promising thermoelectric properties in the medium- and high-temperature range. To harness these material properties for thermoelectric applications, it is important to realize electrical contacts with low electrical contact resistivity. However, little is known about the detailed structural and electronic properties of such contacts and the expected values of contact resistivity. Here, we employ atomistic ab initio calculations to study electrical contacts in a subclass of HH alloys consisting of the compounds HfCoSb, HfZrCoSb, and HfZrNiSn. By using Ag as a prototypical metal, we show that the termination of the HH material critically determines the presence or absence of strong deformations at the interface. Our study includes contacts to doped materials, and the results indicate that the $p$-type materials generally form ohmic contacts while the $n$-type materials have a small Schottky barrier. We calculate the temperature dependence of the contact resistivity in the low- to medium-temperature range and provide quantitative values that set lower limits for these systems.

Defect-tolerant semiconductors have the ability to retain the electronic properties of their pristine form even in the presence of defects. Currently, the presence of antibonding states at the valence band edges induced by a lone pair of $6s^2$ or $5s^2$ electrons is used as a descriptor to predict defect-tolerant semiconductors. Based on this descriptor, bismuth triiodide ($\mathrm{Bi}{\mathrm{I}}_3$) has been proposed as a defect-tolerant semiconductor with promise for photovoltaic applications. However, clear demonstration of the defect tolerance of $\mathrm{Bi}{\mathrm{I}}_3$ including a comprehensive study of the type of defects and their effect on the electronic structure has not been reported so far. Here, we present an atomic-scale landscape of point defects and intergrowths in $\mathrm{Bi}{\mathrm{I}}_3$ using a combination of density-functional-theory calculations and aberration-corrected scanning transmission electron microscope imaging. We show that $\mathrm{Bi}{\mathrm{I}}_3$ is not a defect-tolerant semiconductor as intrinsic point defects have low formation energy and show transition levels that are deep within the band gap and can act as nonradiative recombination centers. We show that Bi-rich growth conditions lead to higher carrier concentration over I-rich conditions. We also show the presence of intergrowths that are made up of a bilayer of bismuth atoms sandwiched within $\mathrm{Bi}{\mathrm{I}}_3$ sheets with a missing layer of iodine atoms. These intergrowths result in metallic behavior within the semiconducting matrix of $\mathrm{Bi}{\mathrm{I}}_3$. We propose that atomic-scale control of the intergrowths can be beneficial to avoid carrier trapping and to enhance photon absorbance. Overall, this work highlights the need to go beyond heuristic descriptors based on band-edge characteristics to predict defect-tolerant semiconductors.

Precise manipulation of the electronic phases in strongly correlated oxides offers an avenue to control the macroscopic functionalities, thereby sparking enormous research interests in condensed matter physics. In the present paper, phase-separated ${\mathrm{La}}_{0.33}{\mathrm{Pr}}_{0.34}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ (LPCMO) thin films with a fraction of the ferromagnetic metallic phase close to the percolation threshold are successfully prepared, in which the nonvolatile and erasable switching between different electronic states is realized through cooperative effects of Au-nanodots capping and laser illumination. The deposition of Au nanodots on LPCMO thin films leads to the occurrence of a thermally inaccessible nonpercolating state at low temperatures, manifested as the absence of insulator-metal transition as temperature decreases. Such a nonpercolating state can be substantially tuned back to a percolating state by laser illumination in a nonvolatile and erasable way, accompanied by gigantic resistance drops in a wide temperature range. The formation of local oxygen vacancies near Au nanodots and thereby the modulation of mesoscopic electronic texture should be the key factor for the realization of flexible modulation of global transport properties in LPCMO thin films. Our findings pave a way toward the manipulation of physical properties of the electronically phase-separated systems and the design of optically controlled electronic devices.

We study the effects of molecular ordering on charge transport at the mesoscale level in a layer of $\approx 9000$ hexa-octyl-thio-triphenylene discotic mesogens with dimensions of $\approx 20\times 20\times 60{\mathrm{nm}}^3$. Ordered (columnar) and disordered isotropic morphologies are obtained from a combination of atomistic and coarse-grained molecular-dynamics simulations. Electronic structure codes are used to find charge hopping rates at the microscopic level. Energetic disorder is included through the Thole model. Kinetic Monte Carlo simulations then predict charge mobilities. We reproduce the large increase in mobility in going from an isotropic to a columnar morphology. To understand how these mobilities depend on the morphology and hopping rates, we employ graph theory to analyze charge trajectories by representing the film as a charge-transport network. This approach allows us to identify spatial correlations of molecule pairs with high transfer rates. These pairs must be linked to ensure good transport characteristics or may otherwise act as traps. Our analysis is straightforward to implement and will be a useful tool in linking materials to device performance, for example, to investigate the influence of local inhomogeneities in the current density. Our mobility-field curves show an increasing mobility with field, as would be expected for an organic semiconductor.

Using a combination of first-principles and magnetization-dynamics calculations, we study the effect of the intense optical excitation of phonons on the magnetic behavior in insulating magnetic materials. Taking the prototypical magnetoelectric ${\mathrm{Cr}}_2{\mathrm{O}}_3$ as our model system, we show that excitation of a polar mode at 17 THz causes a pronounced modification of the magnetic exchange interactions through a change in the average Cr-Cr distance. In particular, the quasistatic deformation induced by nonlinear phononic coupling yields a structure with a modified magnetic state, which persists for the duration of the phonon excitation. In addition, our time-dependent magnetization dynamics computations show that systematic modulation of the magnetic exchange interaction by the phonon excitation modifies the magnetization dynamics. This temporal modulation of the magnetic exchange interaction strengths using phonons provides a route to creating nonequilibrium magnetic states and suggests avenues for fast manipulation of spin arrangements and dynamics.

We demonstrate and investigate the coupling of contributions from both in-plane (IP) polarization and out-of-plane (OP) components in ${\mathrm{BiFeO}}_3$ (BFO) thin-film polarization probed by piezoresponse force microscopy (PFM). Such coupling leads to image artifacts which prevent the correct determination of OP polarization vector directions and the corresponding piezoelectric coefficient $d_{33}$. Using material strength theory with a one-dimensional modeling of the cantilever oscillation amplitude under electrostatic and elastic forces as a function of the tip length, we have evidenced the impact of IP piezoresponse to the OP signal for tip length longer than 4 μm. The IP polarization vector induces a significant longitudinal bending of the cantilever, due to the small spring constant of long tips, which provokes a normal deviation superimposed to the OP piezoresponse. These artifacts can be reduced by increasing the longitudinal spring constant of the cantilever by shortening the tip length. Standard cantilevers with 15-μm-long tips were modified to reach the desired tip length, using focused ion-beam techniques and tested using PFM on the same BFO thin film. Tip length shortening has strongly reduced IP artifacts as expected, while the impact of nonlocal electrostatic forces, becoming predominant for tips shorter than 1 μm, has led to a non-negligible deflection offset. For shorter tips, a strong electric field from a cantilever beam can induce polarization switching as observed for a 0.5-μm-long tip. Tip length ranging from 1 to 4 μm allowed minimizing both artifacts to probe unambiguously OP piezoresponse and quantify the $d_{33}$ piezoelectric coefficient.

The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, $\mathrm{Ir}{\mathrm{O}}_2$, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of $\mathrm{Ir}{\mathrm{O}}_2$ band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.

The soft magnetic properties of Fe-based metallic glasses are reduced significantly by external and residual stresses, e.g., the susceptibility decreases and coercivity increases, which limits their application severely. Unraveling the micromechanism of how the stress influences the soft magnetic properties is of great help for enhancing the performance of Fe-based metallic glasses. In this work, we investigate the effect of viscoelastic heterogeneity on the motion of magnetic domain wall surrounding nanoindentations. Compared to the matrix, dissipation of the viscoelastic heterogeneity increases toward the nanoindentation. Meanwhile, the motion of domain wall under external magnetic field becomes more difficult toward the nanoindentations. A correlation between the viscoelastic dissipation and the moving ability of magnetic domain walls is observed, which can be well fitted using magnetoelastic coupling theory. This suggests that manipulating the microscale viscoelastic heterogeneity is probably a helpful strategy for enhancing the soft magnetic properties of metallic glasses.

From electronic-structure-based thermodynamic linear response, we establish chemical ordering behavior in complex solid solutions versus how Gibbs' space is traversed—applying it on prototype refractory A2 Ta-Nb-Mo-W high-entropy alloys. Near ideal stoichiometry, this alloy has anomalous, intricate chemical ordering tendencies, with long-ranged chemical interactions that produce competing short-range order (SRO) with a crossover to spinodal segregation. This atypical SRO arises from canonical band behavior that, with alloying, creates features near the Fermi surface (well defined even with disorder) that change to simple commensurate SRO with (un)filling of these states. Our results reveal how complexity and competing electronic effects control ordering in these alloys.

Cesium-137 is a common radioactive byproduct found in nuclear spent fuel. Given its 30 year half life, its interactions with potential storage materials—such as cement paste—is of crucial importance. In this paper, simulations are used to establish the interaction of calcium silicate hydrates (C-S-H)—the main binding phase of cement paste—with Cs at the nano- and mesoscale. Different C-S-H compositions are explored, including a range of Ca/Si ratios from 1.0 to 2.0. These calculations are based on a set of 150 atomistic models, which qualitatively and quantitatively reproduce a number of experimentally measured features of C-S-H—within limits intrinsic to the approximations imposed by classical molecular dynamics and the steps followed when building the models. A procedure where hydrated ${\mathrm{Ca}}^{2+}$ ions are swapped for ${\mathrm{Cs}}^{1+}$ ions shows that Cs adsorption in the C-S-H interlayer is preferred to Cs adsorption at the nanopore surface when Cs concentrations are lower than 0.19 Mol/kg. Interlayer sorption decreases as the Ca/Si ratio increases. The activation relaxation technique nouveau is used to access timescales out of the reach of traditional molecular dynamics (MD). It indicates that characteristic diffusion time for ${\mathrm{Cs}}^{1+}$ in the C-S-H interlayer is on the order of a few hours. Cs uptake in the interlayer has little impact on the elastic response of C-S-H. It leads to swelling of the C-S-H grains, but mesoscale calculations that access length scales out of the range of MD indicate that this leads to practically negligible expansive pressures for Cs concentrations relevant to nuclear waste repositories.

We address a three-dimensional, coarse-grained description of dislocation networks at grain boundaries between rotated crystals. The so-called amplitude expansion of the phase-field crystal model is exploited with the aid of finite element method calculations. This approach allows for the description of microscopic features, such as dislocations, while simultaneously being able to describe length scales that are orders of magnitude larger than the lattice spacing. Moreover, it allows for the direct description of extended defects by means of a scalar order parameter. The versatility of this framework is shown by considering both fcc and bcc lattice symmetries and different rotation axes. First, the specific case of planar, twist grain boundaries is illustrated. The details of the method are reported and the consistency of the results with literature is discussed. Then, the dislocation networks forming at the interface between a spherical, rotated crystal embedded in an unrotated crystalline structure, are shown. Although explicitly accounting for dislocations which lead to an anisotropic shrinkage of the rotated grain, the extension of the spherical grain boundary is found to decrease linearly over time in agreement with the classical theory of grain growth and recent atomistic investigations. It is shown that the results obtained for a system with bcc symmetry agree very well with existing results, validating the methodology. Furthermore, fully original results are shown for fcc lattice symmetry, revealing the generality of the reported observations.