The interaction of energetic electrons with the specimen during imaging in a transmission electron microscope (TEM) can give rise to the formation of defects or even complete destruction of the sample. This is particularly relevant to atomically thin two-dimensional (2D) materials. Depending on electron energy and material type, different mechanisms such as knock-on (ballistic) damage, inelastic interactions including ionization and excitations, as well as beam-mediated chemical etching can govern defect production. Using first-principles calculations combined with the McKinley-Feshbach formalism, we investigate damage creation in two representative 2D materials, ${\mathrm{MoS}}_2$ and hexagonal boron nitride (hBN) with adsorbed single adatoms (H, C, N, O, etc.), which can originate from molecules always present in the TEM column. We assess the ballistic displacement threshold energies $T$ for the host atoms in 2D materials when adatoms are present and demonstrate that $T$ can be reduced, as chemical bonds are locally weakened due to the formation of new bonds with the adatom. We further calculate the partial and total cross sections for atom displacement from ${\mathrm{MoS}}_2$ and hBN, compare our results to the available experimental data, and conclude that adatoms should play a role in damage creation in ${\mathrm{MoS}}_2$ and hBN sheets at electron energies below the knock-on threshold of the pristine system, thus mediating the buildup of electron beam-induced damage.

Twisted bilayer graphene (tBLG) has emerged as an exciting platform for novel condensed matter physics. However, electron-phonon ($e$-ph) interactions in tBLG and their effects on electronic transport are not completely understood. Here we show first-principles calculations of $e$-ph interactions and resistivity in commensurate tBLG with large twist angles of 13.2 and 21.8 degrees. These calculations overcome key technical barriers, including large unit cells of up to 76 atoms, Brillouin-zone folding of the $e$-ph interactions, and unstable lattice vibrations due to the AA-stacked domains. We show that $e$-ph interactions due to layer-breathing phonons enhance intervalley scattering in large-angle tBLG. This interaction effectively couples the two layers, which are otherwise electronically decoupled at such large twist angles. As a result, the phonon-limited resistivity in tBLG deviates from the temperature-linear trend characteristic of monolayer graphene and tBLG near the magic angle. Taken together, our work quantifies $e$-ph interactions and scattering mechanisms in tBLG, revealing subtle interlayer coupling effects at large twist angles.

Two-dimensional materials with coexisting multifunctional properties offer platforms for revealing multifield coupling physics in both lower dimension and symmetry, and enable the potential application demand for multifunctional devices in nanoelectronics. Preceding theoretical and experimental investigations show that layered metal oxyhalides (MOX) have diverse functional properties such as the visible-light-active photocatalyst in Bi-based system as well as the magnetic and electron-correlated properties in transition-metal-based regime. Here, applying first-principles calculations and time-dependent perturbation theory, we identify that GaOCl monolayer has intrinsically multiple functional characteristics including high-temperature ferroelectricity, superferroelasticity, and strong deep-ultraviolet nonlinear optical response. Our calculations indicate that GaOCl monolayer has a ultrawide direct band gap comparable with the third-generation semiconductors, a moderate energy barrier of ferroelectric polarization reversal but with a high Curie temperature, a superferroelasticity attributed to the largely tunable ionic radius ration of ${\mathrm{Ga}}^{3+}$ and ${\mathrm{O}}^{2-}$, and remarkable nonlinear shift and injection currents inside deep-ultraviolet frequencies. These intrinsical multifunctional properties render GaOCl monolayer possible applications on ultrathin multifunctional nanoelectronics such as integrated multiferroic devices and ultraviolet optoelectronic devices.

The magnetic order and phase transitions in the normal spinel system ${\mathrm{Ni}}_{1-x}{\mathrm{Cu}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ are studied by powder-neutron and x-ray diffraction, as well as by magnetization measurements to get a complete magnetic phase diagram. For chromites with $x$(Cu) > 0.5 a canted antiferromagnetic phase of Cr appears first, followed by the onset of ferromagnetism in the Cu sublattice at lower temperature forming a ferrimagnetic lattice. Conversely, with $x$(Cu) < 0.5 the ferrimagnetic order between the Cr and Cu spins occurs at the higher ordering temperature followed by the onset of antiferromagnetic order in the Cr sublattice. Apart from the crossing of the two boundary lines of the transition temperatures at $x$(Cu) = 0.50 a compensation point of the ferrimagnetic moments is determined at $x$(Cu) = 0.60, where the spontaneous magnetization has almost completely vanished. Most remarkable is the antiferromagnetic Cr ordering on the orthorhombic distorted pyrochlore lattice for samples in the $x$(Cu) range from 0 to 0.12 due to the large variety of coexisting magnetic phases. In the magnetic ground state of ${\mathrm{NiCr}}_2{\mathrm{O}}_4$ two commensurate antiferromagnetic structures with the propagation vectors $k$ = (0,0,1) and (½,½,½) coexist. With increasing Cu content from $x$(Cu) = 0 to 0.09 these phases undergo a transition to another commensurate structure with $k$ = 0 via two coexisting incommensurate magnetic phases with the vectors $k$ = (0,0,$k_z$) and $k$ = ($0,k_y,k_z$). The different magnetic phases are discussed qualitatively based on the lattice dimensions depending on the concentration ratio of two Jahn-Teller ions at the tetrahedral $A$ site, where ${\mathrm{Ni}}^{2+}$ causes elongated and ${\mathrm{Cu}}^{2+}$ compressed tetragonal lattice distortions. Further, magnetoelasticity studies on selected samples indicate that the magnetically induced lattice strains follow the symmetry of the underlying Jahn-Teller distorted lattices.

Recent studies have shown that the magnetic properties of the ferromagnetic perovskite oxide $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) grown on an $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrate, such as its magnetic moment and Curie temperature, can be improved by releasing the film from the substrate. However, the microscopic origin of this enhancement is not yet well understood. In this study, we use synchrotron radiation measurements to investigate the mechanism of ferromagnetism enhancement in an LSMO membrane released from an STO substrate by dissolving a water-soluble $\mathrm{S}{\mathrm{r}}_4\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_7$ buffer layer. Using resonant photoemission spectroscopy on the as-grown LSMO film and LSMO membrane, we elucidate that the strain release from the STO substrate enhances the itineracy of the $\text{Mn-}3d$ electrons via $p\text{−}d$ hybridization, and this strengthens the double-exchange interaction. The reinforcement of the double-exchange interaction, in turn, improves the ferromagnetism of LSMO.

The controlled switching of spontaneous polarization in ferroelectrics by applying an external electric field is essential for many device operations. Oxygen vacancy defects commonly found in oxide ferroelectrics offer a tempting means for polarization switching regulation at an atomic scale. This study presents a method involving a series of annealing cycles under various environments to modulate the amount of oxygen vacancies in ferroelectric ${\mathrm{Bi}}_2{\mathrm{FeCrO}}_6$ thin films. By performing a piezoresponse force microscopy study after each annealing cycle, it is shown that a high content of oxygen vacancies induces specific fingerprints on polarization hysteresis loops. Reducing oxygen vacancy density by annealing in oxygen-rich environments results in hysteresis loops being wider and displaying less imprint, while increasing oxygen vacancy density by annealing in oxygen-poor environments results in thinner and more imprinted hysteresis loops. The process is largely reversible, constituting an alternative method for polarization switching control at specific electric fields. Ferroelectric phase-field calculations and energy level diagrams obtained through ellipsometry, Kelvin probe, and photoemission spectroscopy indicate a combined switching mechanism based on a doping effect due to oxygen vacancies and a built-in electric field originating from the interface. Our study is helpful for designing ferroelectric films with a precise imprint or, more generally, for understanding the impact of charged defects on the polarization states in ferroelectric films.

Interlayers are known to enhance the performance of organic devices by reducing contact resistance, however, the details of the mechanism are uncertain. Models have correlated properties of interlayers to their extent of reduction of contact resistance, but a universal parameter correlating the microscopic phenomenon to device characteristics is yet to be established. Here, we demonstrate that the energy-level modulation at the interface of interlayer functionalized electrode and organic semiconductor, combined with the charge transfer integral between them, determines the extent of the reduction of contact resistance. Moreover, the rate of charge transfer calculated from these quantities is demonstrated to be a universal parameter predicting the characteristics of devices with functionalized electrodes, regardless of the nature of the semiconductor ($p$- or $n$-type). These observations explain the mechanism of interlayers and provide a computational model capable of selecting interlayers leading to high-performing devices.

We report scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) measurements on uncapped and ${\mathrm{SrTiO}}_3$ (STO) capped ${\mathrm{NdNiO}}_2$ realized by pulsed-laser deposition and ex situ topotactic reduction process. We find that untreated ${\mathrm{NdNiO}}_2$ surfaces are insulating and contain Ni mostly in a nominal ${\mathrm{Ni}}^{2+}$ oxidation state. Room temperature STM shows signatures of a striped-like topographic pattern, possibly compatible with recent reports of ordered oxygen vacancies in uncapped nickelates due to incomplete oxygen deintercalation of the upper layers. A metallic surface and a full ${\mathrm{Ni}}^{1+}$ oxidation state are recovered by ultra–high vacuum annealing at 250 ${}^{\circ }\mathrm{C}$, as shown by STM and XPS. STO-capped ${\mathrm{NdNiO}}_2$ films, on the other hand, show Ni mostly in ${\mathrm{Ni}}^{1+}$ configuration, but Nd $3d_{5/2}$ core-level spectra have a relevant contribution from ligand $4f^4$ states, suggesting the formation of a ${\mathrm{NdTiNiO}}_x$ layer at the interface with the STO. By in situ unit cell by unit cell Ar-ion sputtering removal of the STO capping and of the nonstoichiometric interface layer, we were able to address the surface electronic properties of these samples, as shown by high-resolution valence-band photoemission spectroscopy. The results provide insights into the properties of infinite-layer ${\mathrm{NdNiO}}_2$ thin films prepared by the ex situ ${\mathrm{CaH}}_2$ topotactic reduction of perovskite ${\mathrm{NdNiO}}_3$ and suggest methods to improve their surface quality.

We investigated the electron-phonon (e-ph) coupling and vibrational thermal conductivity in the representative MAB compounds, namely MoAlB, WAlB, ${\mathrm{Tc}}_2{\mathrm{AlB}}_2$, and ${\mathrm{Cr}}_2{\mathrm{AlB}}_2$. The spectral distribution functions of e-ph interaction, obtained through ab initio linear-response calculations, reveal that the electron-phonon coupling values range from low (0.15) to moderate (0.58). With such e-ph coupling, out of the considered compounds, only ${\mathrm{Tc}}_2{\mathrm{AlB}}_2$ exhibits a superconducting transition, at 4 K. We further evaluated the thermal conductivity and associated properties like scattering rates, obtained using ab initio and other methodologies. The latter included the iterative solution of the Peierls-Boltzmann transport equation, using hiphive package for advanced optimization and machine learning techniques, and employing maximum likelihood estimation to approximate scattering rates from a limited set of scattering processes. We found that these methods yield nearly identical predictions for thermal conductivity values, with a significant decrease in the computational cost compared to the first-principles methods. We examined interactions arising from both three-phonon ($3ph$) and four-phonon ($4ph$) scattering processes. The $4ph$ interactions demonstrated a smaller yet significant impact on the overall vibrational thermal conductivity, most notably in ${\mathrm{Tc}}_2{\mathrm{AlB}}_2$. Our findings indicate that ${\mathrm{Cr}}_2{\mathrm{AlB}}_2$ has the highest thermal conductivity across all considered crystal directions, with the thermal conductivity being spatially anisotropic, most pronouncedly in ${\mathrm{Tc}}_2{\mathrm{AlB}}_2$. Finally, we show that empirical expressions based on Slack models are well suited for screening the thermal conductivity properties of MAB phases, and can be employed to establish upper and lower limits of their thermal conductivity.

In organic proton-transfer ferroelectrics (OPTFe), molecules are linked to form hydrogen-bonded networks, and proton transfer (PT) between molecules is the dominant mechanism of ferroelectric switching. The fast-switching frequencies of OPTFes make them attractive alternatives to conventional ceramic ferroelectrics, which contain rare and/or toxic elements and require high processing temperatures. In this study, we mined the Cambridge Structural Database (CSD) for potential OPTFes, uncovering most of the previously reported ones, both tautomeric compounds and acid-base salts, in addition to 12 new compounds, nine of which were tautomeric salts. The identification of PT tautomeric salts highlights a potential new avenue for engineering novel OPTFes. The CSD mining was based on identifying polar crystal structures with pseudoinversion symmetry and viable PT paths. The spontaneous polarization ${\mathit{P}}_{\mathrm{s}}$ and the PT barriers $U_{\mathrm{PT}}$ were calculated for a selection of the compounds using density-functional theory.

There is great interest in developing new materials with Rashba split bands near the Fermi level for spintronics. Using first-principles calculations, we predict BiAs as a semiconductor with large Rashba splitting in bulk and monolayer forms. Bulk BiAs has a layered crystal structure with two atoms in a rhombohedral primitive cell, derived from the structure of the parent Bi and As elemental phases. It is a narrow band gap semiconductor, and it shows a combination of Rashba and Dresselhaus spin splitting with a characteristic spin texture around the L point in the Brillouin zone of the hexagonal conventional unit cell. It has sizable Rashba energies and Rashba coupling constants in the valence and conduction bands at the band edges. The 2D monolayer of BiAs has a much larger band gap at $\mathrm{\Gamma }$, with a circular spin texture characteristic of a pure Rashba effect. The Rashba energy and Rashba coupling constant of monolayer BiAs are large compared to other known 2D materials and rapidly increase under biaxial tensile strain.

Introducing only a few atomic percent of Bi or N in GaAs has a large effect on the band gap of the material. Specifically Bi doped GaAs shows potential for local band gap engineering in optoelectronic applications. The incorporation of Bi and N into GaAs is difficult due to strain effects. In this work we study the ordering of these dopants at the atomic scale in order to get a better understanding of the behavior of these dopants in the host lattice. Cross-sectional scanning tunneling microscopy is used to find the exact position of Bi and N dopants in the GaAs matrix, allowing us to study both their nearest neighbor pair occurrences and pair correlation functions. An attractive interaction between Bi dopants at short ranges (1–2 nm) is found and a similar effect is observed between N dopants. We find a repulsive interaction with a similar length scale between Bi and N dopants. A similar repulsion is found in the Bi-N nearest neighbor pairs. Density functional theory is used to calculate the different nearest neighbor pair energies and test these results to the experimental pair occurrences. It is concluded from the experimental and theoretical results that the growth conditions and N inclusion greatly affects the Bi distribution in GaAs.

While many piezoelectric materials are known, there is still great potential to improve on the figures of merit of existing materials through compositional doping and forming solid solutions. Specifically, it has been shown that doping and alloying wurtzite-structured materials can improve the piezoelectric response; however, a vast compositional space has remained unexplored. In this work, we apply a multilevel screening protocol combining machine learning, chemical intuition, and thermodynamics to systematically discover dopant combinations in the wurtzite material space that improve the desired piezoelectric response. Through our protocol, we use computationally inexpensive screening calculations to consider more than 3000 possible ternary wurtzite solid solutions from nine different wurtzite base systems: AlN, BeO, CdS, CdSe, GaN, ZnO, ZnS, ZnSe, and AgI. Finally, based on thermodynamic analysis and explicit piezoelectric response calculations, we predict 11 materials with improved piezoelectric response, due to the incorporation of electropositive dopants.

A theory and mechanistic understanding of the thermal properties of solids under nanoscale confinement are currently missing. We develop a theoretical quantum confinement description of thin films which predicts a quantitative physical law for the heat capacity. In particular, due to the suppression of vibrational modes caused by the thin-film confinement, the vibrational density of states deviates from the Debye quadratic law in frequency and is, instead, cubic in frequency. This leads to a temperature dependence of the heat capacity which is $\sim T^4$ instead of Debye's $\sim T^3$ law. Furthermore, this theory predicts a linear increase of the heat capacity upon increasing the nanometric film thickness. Both dependencies are found in excellent agreement with recent experimental data on NbTiN thin films.

Spin-active defects in silicon carbide (SiC) are promising quantum light sources for realizing scalable quantum technologies. In different applications, these photoluminescent defects are often placed in a nanostructured host or close to surfaces in order to enhance the signal from the defects. However, proximity to the surface not only modifies the frequencies of the quantum emission from the defect, but also adversely affects their photostability, resulting in blinking and/or photobleaching of the defect. These effects can be ameliorated by passivating surfaces with optimal adsorbates. In this work, we explore different passivation schemes using density-functional-theory-based calculations. We show that a uniform surface passivation with either hydrogen or with mixed hydrogen/hydroxyl groups completely removes surface states from the SiC band gap, restoring the optical properties of the defects.

In this work, we envisage an evolution paradigm in a promising noncentrosymmetric one-dimensional zigzag chain structure, $\left(3\mathrm{AMP}\right){\mathrm{BiI}}_5$ [where AMP indicates (aminomethyl)piperidinium], through rigorous electronic structure calculations based on density functional theory (DFT). The electronic and optical properties along with the Rashba splitting and spin texture are systematically observed within the thermodynamic limit under compression equivalent to 9.6 GPa in this promising halide perovskite. Our study successfully reveals the intriguing transition of the electronic band structure from an indirect to a direct band gap phenomenon under compression in addition to an interesting redshift in the optical absorption spectra. To accurately describe the spin polarization both in plane and out of plane, we explore a three-dimensional Rashba model. The in-plane spin texture is found to arise from the octahedral distortion along the $b$ direction. The fundamental interplay between structural distortions and the Rashba splitting in the considered one-dimensional system under the influence of compression along with the evolution of spin texture could hold great potential for the pursuit of sustainable energy.

Cottrell atmospheres form when solute atoms segregate to dislocations, and is one of the most basic processes by which solutes alter mechanical properties of materials. In the case of hydrogen, Cottrell atmospheres are believed to contribute to hydrogen embrittlement. Here, we use direct molecular dynamics simulations to study the formation of hydrogen Cottrell atmospheres around edge dislocations in aluminum. Using coarse-graining techniques, we resolve the temporal and spatial evolution of the hydrogen concentration field. By comparing our results with theories of Cottrell atmosphere formation and thermodynamics, we show that a repulsive, concentration-dependent H-H interaction reduces the peak atmosphere concentration at the dislocation core. Furthermore, this interaction differs significantly from the H-H interaction that is experienced in the bulk away from a dislocation.