Currently, the formation of a two-dimensional electron gas (2DEG) at the ${\mathrm{TiO}}_2/\mathrm{LaO}$-terminated interface between ${\mathrm{LaAlO}}_3$ (LAO) grown on ${\mathrm{SrTiO}}_3$(001) (STO) is well understood. The role of the polarity of LAO and its critical thickness for the formation of 2DEG have been established. Here, we show how 2DEG can be tuned externally by changing electronic balance at the LAO(001) surface or, alternatively, at the intrinsically hole-doped ${\mathrm{AlO}}_2/\mathrm{SrO}$ interface of LAO/STO heterostructures. The effects of liquid gating at LAO(001) and imperfected ${\mathrm{AlO}}_2/\mathrm{SrO}$ were simulated in the framework of the first-principles Green function method within the coherent-potential approximation. We evaluated the Fermi surface cross sections, which were computed varying the degree of chemical disorder. These findings may be extremely important for potential application of 2DEG.

We report on the observation of a field-induced magnetic bias effect in a $\mathrm{G}{\mathrm{a}}_{0.94}\mathrm{M}{\mathrm{n}}_{0.06}\mathrm{A}{\mathrm{s}}_{1-y}{\mathrm{P}}_y$ thin film with digitally graded phosphorus content. Although phosphorus concentration in the sample is changed in steps from $y\approx 0.03$ to $y\approx 0.28$, the magnetometry and magnetotransport data display a coherent magnetic response typical for single-layer in-plane magnetized films with cubic and weak [110] uniaxial anisotropy. Unexpectedly, low-temperature planar Hall resistance loops exhibit remarkable asymmetry tunable by application of strong in-plane initial magnetic field $H_{\mathrm{Ini}}$. We discuss this unusual memory effect, resembling the magnetization asymmetry in the exchange biased magnetic bilayers, in terms of a unidirectional bias field $H_b$ induced by $H_{\mathrm{Ini}}$. We show that such bias field defines the delayed or accelerated nucleation of domains with chiral domain walls performing the magnetization reversal.

The renowned two-dimensional electron gas at the interface of insulating ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ poses a grand challenge in engineering the hole counterpart and its coexistence with the electron gas. The room-temperature realization of such coexisting states was recently achieved in the ${\mathrm{SrTiO}}_3$-capped ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$, and the electron and hole gases in the ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ reside at the respective bottom and top interfaces, which are separated by a few tens of the interlayer-${\mathrm{LaAlO}}_3$ unit cells. What happens if the two interfaces are brought to the extreme proximity of just a few lattices away? Using atomic-resolution electron microscopy and spectroscopy, we tackled this problem in ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ with ${\mathrm{LaAlO}}_3$ of 3 and 5 unit cells, respectively. Notably, both ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures are dominated by electron conductivity at room temperature. The microscopic evaluation at picometer accuracy unveils head-to-head (tail-to-head) ferroelectriclike polarizations across the bottom (top) interface, in comparison to the established notion of characteristic strain-induced head-to-head polarizations in primitive metallic ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ that form a potential well and confine the accompanied screening electrons into two-dimensional electron gas. The atomic-scale spectral investigation of the ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ reveals the unexpected depletion (resurrection) of electrons at the head-to-head bottom (tail-to-head top) interface and the ${\mathrm{LaAlO}}_3$ hosts coexisting localized electrons and holes, casting an electric field with tens of meV/Å and serving as the depolarization field to the ${\mathrm{LaAlO}}_3$ dipole. The landscape of the ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ turns out to be the polarity-neutral ${\mathrm{LaAlO}}_3$ sandwiched by the head-to-head ${\mathrm{SrTiO}}_3$ polarizations, with the potential gradient due to the unbalanced polarizations pointing to the top interface and resurrecting the screening electrons thereby. The dominant electron conductivity in the thin ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ is profoundly related to this polarization pattern and the intricate strain-field interference across the two closely spaced interfaces, accounting for the pattern onset, was atomically revealed.

The presence of impurities and disorder has an important impact on the optical response of monolayer transition metal dichalcogenides (TMDs). Here, we investigate elastic exciton-impurity scattering and its influence on the linewidth of excitonic resonances in different TMD materials. We derive an analytic expression for the linewidth broadening within the density matrix formalism. We find that the exciton linewidth increases for states up to the $3s$ exciton due to the scattering with impurities. For higher states, the impurity contribution decreases, reflecting the reduced scattering cross section. Furthermore, we reveal that the scattering efficiency is the largest for transitions between $s$ and $p$ exciton states. Finally, different TMDs show generally a similar behavior. The quantitatively smaller broadening in tungsten-based TMDs can be ascribed to their smaller effective masses resulting in a less efficient scattering.

Perovskite solar cells based on lead-halide perovskites have attracted significant attention as prime candidates for next-generation solar cells because of their high-power conversion efficiency. To avoid the toxicity of lead-based perovskites, alternatives such as tin-halide perovskite have been investigated. However, the photovoltaic performance of these alternatives is relatively low, and novel perovskites with low cost, low toxicity, and high performance have not yet been discovered. In this study, to investigate whether promising alternative perovskites exist, a high-throughput material search scheme based on materials informatics was developed and performed for perovskite solar cell materials. Using this scheme, over 28 million $A{A}^{\prime }B{B}^{\prime }{X}_3{X}_3^{\prime }$ double-perovskite-like compositions were screened. Among the 24 most promising candidates identified, 5 were well-known organic-inorganic tin-halide perovskites and 17 were sodium-, potassium-, and ammonium-based tin-halide perovskites. Interestingly, two transition-metal-based perovskites were also identified as promising solar cell materials. The pioneering material search scheme reported is expected to find use in the identification of practically feasible materials for a number of real-world applications.

The formation and ageing of gels is a complex issue that has to be resolved to investigate manifold synthetic materials, among them: porous materials such as cement, high-quality glass fiber, and geomaterials for radioactive waste sealing. Herein, a coupling between a grand canonical Monte Carlo and the parallel tempering methods is developed. The gain in simulation time is of, at least, two orders of magnitude; therefore, we are able to move at will on the water to silicon ratio axis and to observe the restructuring of gels during dilution and drying. At high water to silicon ratio, a colloidal-like structure is obtained, mostly constituted of silicate chains. As humidity is an essential aspect of gels, affecting their physical and mechanical properties, the effect of drying is herein investigated. In agreement with experiments, the structure becomes denser, crosslinks between silicate chains increase and glasslike structures are observed locally.

The dendrite growth velocity in $\mathrm{A}{\mathrm{l}}_{75}\mathrm{N}{\mathrm{i}}_{25}$ melts has been measured in a containerless procedure as a function of undercooling using an electromagnetic levitation technique both in the Earth laboratory and in Space on board the International Space Station. The growth shows an anomalous behavior inasmuch as the growth velocity decreases with increasing undercooling, confirming previous experiments on Earth. Within the scatter of experimental data, results obtained on Earth and in Space do not show significant differences. Thus, convection effects as the origin of the anomalous growth characteristics can be excluded. However, high-speed video recording exhibits multiple nucleation events in front of the growing solid-liquid interface. This effect is identified as the origin of the anomalous dendrite growth characteristics in undercooled melts of Al-rich Al-Ni melts.

Both simulations and experiments have suggested that Cu/CuZr nanolaminates are stronger and more ductile than their individual constituents due to interface-mediated interactions between plasticity carriers. In this work, we use the effective-temperature theories of dislocation and amorphous shear-transformation-zone (STZ) plasticity to study amorphous-crystalline interface (ACI)-mediated plasticity in Cu/CuZr nanolaminates under mechanical straining. The model is shown to capture reasonably well the measured deformation response when strained either in tension parallel to or in compression normal to the amorphous-crystalline interface. Our analysis indicates that increasing CuZr or decreasing Cu layer thickness increases the maximum flow stress for both perpendicular and parallel loading cases. For the cases of parallel and perpendicular loading, the maximum flow stress values are 3.4 and $2.5\mathrm{GPa}$, respectively. Furthermore, increasing the strain rate for the parallel loading case decreases the slip strain in the amorphous and crystalline layers. For the perpendicular loading case, an increase in strain rate decreases the amorphous layer slip but increases the crystalline layer slip. In all slip strain analyses, maximum slip strain occurs at the ACI, thus indicating that plasticity carriers accumulate at the interface and are absorbed there. These findings indicate a significant anisotropy in strength with greater sensitivity to layer thickness for the case of tensile loading parallel to the ACI. Further findings signify that slip strain is more sensitive when the nanolaminate is compressed perpendicular to the ACI.

Second harmonic generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has been recently reported [2D Mater. 5, 025019 (2018); J. Am. Chem. Soc. 137, 7994 (2015)] to be extremely strong with respect to bulk and multilayer forms. To clarify the origin of this strong SHG signal, we perform first-principles real-time simulations of linear and nonlinear optical properties of these two-dimensional semiconducting materials. The simulations, based on ab initio many-body theory, accurately treat the electron-hole correlation and capture excitonic effects that are deemed important to correctly predict the optical properties of such systems. We find indeed that, as observed for other 2D systems, the SHG intensity is redistributed at excitonic resonances. The obtained theoretical SHG intensity is an order of magnitude smaller than that reported at the experimental level. This result is in substantial agreement with previously published simulations which neglected the electron-hole correlation, demonstrating that many-body interactions are not at the origin of the strong SHG measured. We then show that the experimental data can be reconciled with the theoretical prediction when a single-layer model, rather than a bulk one, is used to extract the SHG coefficient from the experimental data.

The single helical Fermi surface on the surface state of three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is constrained by the time-reversal invariant bulk topology to possess a spin-singlet superconducting pairing symmetry. In fact, the Cu-doped and pressure-tuned superconducting ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ show no evidence of the time-reversal symmetry (TRS) breaking. We report on the detection of the TRS breaking in the topological superconductor ${\mathrm{Sr}}_{0.1}{\mathrm{Bi}}_2{\mathrm{Se}}_3$, probed by zero-field $\mu \mathrm{SR}$ measurements. The TRS breaking provides strong evidence for the existence of a spin-triplet pairing state. The existence of TRS breaking is also verified by longitudinal-field $\mu \mathrm{SR}$ measurements, which negates the possibility of magnetic impurities as the source of TRS breaking. The temperature-dependent superfluid density deduced from transverse-field $\mu \mathrm{SR}$ measurements yields nodeless superconductivity with low superconducting carrier density and penetration depth $\lambda =1622\left(134\right)\mathrm{nm}$. From the microscopic theory of unconventional pairing, we find that such a fully gapped spin-triplet pairing channel is promoted by the complex interplay between the structural hexagonal warping and higher order Dresselhaus spin-orbit-coupling terms. Based on Ginzburg-Landau analysis, we delineate the mixing of singlet- to triplet-pairing symmetry as the chemical potential is tuned far above from the Dirac cone. Our observation of such spontaneous TRS breaking chiral superconductivity on a helical surface state, protected by the TRS invariant bulk topology, can open avenues for interesting research and applications.

The enigma of the emergent ferromagnetic state in tensile-strained $\mathrm{LaCo}{\mathrm{O}}_3$ thin films remains to be explored because of the lack of an agreed-upon explanation. The direct magnetic imaging technique using a low-temperature magnetic force microscope (MFM) is critical to reveal new aspects of the ferromagnetism by investigating the lateral magnetic phase distribution. Here we show the experimental demonstration of the rare halved occupation of the ferromagnetic state in tensile-strained $\mathrm{LaCo}{\mathrm{O}}_3$ thin films on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates using the MFM. The films have a uniformly strained lattice structure and minimal oxygen vacancies (less than 2%) beyond the measurement limit. It is found that percolated ferromagnetic regions with typical sizes between 100 and 200 nm occupy about 50% of the entire film, even down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Preformed ferromagnetic droplets were still observed when the temperature is 20 K above the Curie temperature, indicating the existence of a possible Griffiths phase. Our study demonstrates a submicron-level phase separation in high-quality $\mathrm{LaCo}{\mathrm{O}}_3$ thin films, which has substantial implications in revealing the intrinsic nature of the emergent ferromagnetism.

The metal-insulator transition of ${\mathrm{NbO}}_2$ is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed ${\mathrm{NbO}}_x$ that may be inherently different to crystalline ${\mathrm{NbO}}_2$. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial ${\mathrm{NbO}}_2$ thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at $\sim 810{}^{\circ }\mathrm{C}$. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline ${\mathrm{NbO}}_2$ as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.

In ordinary solids, material disorder is known to increase the size of the process zone in which stress concentrates at the crack tip, causing a transition from localized to diffuse failure. Here, we report experiments on disordered 2D lattices, derived from frictional particle packings, in which the mean coordination number $\langle z\rangle$ of the underlying network provides a similar control. Our experiments show that tuning the connectivity of the network provides access to a range of behaviors from brittle to ductile failure. We elucidate the cooperative origins of this transition using a frictional pebble game algorithm on the original, intact lattices. We find that the transition corresponds to the isostatic value $\langle z\rangle =3$ in the large-friction limit, with brittle failure occurring for structures vertically spanned by a rigid cluster, and ductile failure for floppy networks containing nonspanning rigid clusters. Furthermore, we find that individual failure events typically occur within the floppy regions separated by the rigid clusters.

The mechanical and electronic properties of transition metal dichalcogenide (TMD) monolayers corresponding to transition groups IV, VI, and X are explored under mechanical bending from first principles calculations using the strongly constrained and appropriately normed (SCAN) meta-GGA. SCAN provides an accurate description of the phase stability of the TMD monolayers. Our calculated lattice parameters and other structural parameters agree well with experiment. We find that bending stiffness (or flexural rigidity) increases as the transition metal group goes from IV to X to VI, with the exception of ${\mathrm{PdTe}}_2$. Variation in mechanical properties (local strain, physical thickness) and electronic properties (local charge density, band structure) with bending curvature is discussed. The local strain profile of these TMD monolayers under mechanical bending is highly nonuniform. The mechanical bending tunes not only the thickness of the TMD monolayers, but also the local charge distribution as well as the band structures, adding more functionalization options to these materials.

We have synthesized single crystals of ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ and characterized the structure and magnetic order by measuring anisotropic magnetic properties, heat capacity, and x-ray and neutron single crystal diffraction. Magnetic properties and specific heat of polycrystalline ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ were also measured for comparison. ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ crystallizes in a monoclinic structure (space group $C2/m)$ with ${\left[{\mathrm{Co}}_2{\mathrm{SbO}}_6\right]}^{3-}$ layers separated by ${\mathrm{Na}}^{+}$ ions. The temperature dependence of magnetic susceptibility shows significant anisotropic behavior in the whole temperature range 2–350 K investigated in this work. An effective moment of about $5.5{\mu }_B/{\mathrm{Co}}^{2+}$ from a Curie-Weiss fitting of the magnetic susceptibility is larger than the spin only value and signals significant orbital contribution. ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ single crystal undergoes a transition into a long-range antiferromagnetically ordered state below $T_N=5\mathrm{K}$. Neutron single crystal diffraction confirmed the zigzag magnetic structure with a propagation vector $k=(0.5,0.5,0)$. The ordered moment is found to be $0.9{\mu }_B$ at 4 K and align along the crystallographic $b$ axis. Density functional theory calculations suggest that the experimentally observed zigzag order is energetically competing with the Néel order. It is also found that the covalency between Co $d$ and O $p$ is quite strong and competes with the local spin-orbit coupling, suggesting a $J_{\mathrm{eff}}=1/2$ ground state may not be realized in this compound.

Motivated by environmental reasons, ${\mathrm{In}}_2{\mathrm{S}}_3$ is a promising candidate for a Cd-free buffer layer in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){(\mathrm{S},\mathrm{Se})}_2$ (CIGSSe)-based thin-film solar cells. For an impactful optimization of the ${\mathrm{In}}_2{\mathrm{S}}_3$ alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and ${\mathrm{In}}_2{\mathrm{S}}_3$. Furthermore, we consider two chemically modified indium sulfide layers, ${\mathrm{NaIn}}_5{\mathrm{S}}_8$ and ${\mathrm{CuIn}}_5{\mathrm{S}}_8$, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with ${\mathrm{NaIn}}_5{\mathrm{S}}_8$ and ${\mathrm{CuIn}}_5{\mathrm{S}}_8$.

${\mathrm{Li}}_2{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ and its derivatives are promising solid electrolytes for solid-state batteries. In this work, a potential model is proposed, and an extensive classical molecular dynamics study is performed to understand the origin of the fast ion conduction in ${\mathrm{Li}}_2{\mathrm{B}}_{12}{\mathrm{H}}_{12}$. The proposed potential model reveals structural and dynamical properties of ${\mathrm{Li}}_2{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ that are consistent with first-principles molecular dynamics simulation and experimental results. The mechanism of ${\mathrm{Li}}^{+}$-ion transport is studied systematically. The low-temperature $\alpha$ phase exhibits negligible diffusivity within a timescale of a few nanoseconds, whereas the high-temperature $\beta$ phase with a similar crystal structure and larger lattice parameter exhibits entropy-driven high ${\mathrm{Li}}^{+}$-ion diffusion assisted by anionic reorientation. We explicitly demonstrate the role of closo-borane anionic reorientational motion in the ${\mathrm{Li}}^{+}$-ion diffusion and explain how the cell parameter facilitates the anionic reorientational motion. In addition, enhancing the degree of H freedom (by changing the B-B-H angular force parameters) results in significantly high cationic diffusivity at low temperature. Further insight into ${\mathrm{Li}}^{+}$-ion transport is obtained by constructing a three-dimensional density map and determining the free-energy barrier, and the factors affecting cationic diffusion are thoroughly investigated with high precision using long simulations (5 ns).

We have synthesized the high-pressure form of $\mathrm{CrS}{\mathrm{b}}_2$ with $\mathrm{CuA}{\mathrm{l}}_2$-type structure under 7 GPa and 700 °C, and characterized its structural, transport, and magnetic properties by a suite of measurement techniques over a broad range of temperature, magnetic field, and pressure. In addition to previously reported ferromagnetic (FM) transition at $T_{\mathrm{C}}\approx 160$ K, we discover another antiferromagnetic spin-density-wave (SDW) transition at $T_{\mathrm{s}}\approx 90$ K, which is characterized by FM sheets of spins in the $ab$ plane that vary along the $c$ axis as determined by neutron powered diffraction. Pronounced anomalies around these two magnetic transitions are visible only in the lattice parameter $c$, signaling a strong spin-lattice coupling along the -Cr-Cr-Cr- infinite linear chain. We find that the application of magnetic field can suppress the SDW phase and stabilize the FM state down to the lowest temperature above ${\mu }_0{H}_{\mathrm{c}}\approx 3$ T, around which a peculiar non-Fermi-liquid behavior with reduced effective mass emerges. On the other hand, the application of high pressure induces complex evolution of the magnetic states, i.e., the FM order is lowered while the SDW order is enhanced quickly until they merge together into a single antiferromagnetic transition, which is suppressed completely at $P_{\mathrm{c}}\approx 9$ GPa. We observe near $P_{\mathrm{c}}$ non-Fermi-liquid behavior and enhancement of effective mass, which indicates the possible occurrence of magnetic quantum critical point. No superconductivity was observed down to 2 K around $P_{\mathrm{c}}$.

We have performed angle-resolved photoemission spectroscopy on transition-metal dichalcogenide $1T\text{−}{\mathrm{HfTe}}_2$ to elucidate the evolution of electronic states upon potassium (K) deposition. In pristine ${\mathrm{HfTe}}_2$, an in-plane hole pocket and electron pockets are observed at the Brillouin-zone center and corner, respectively, indicating the semimetallic nature of bulk ${\mathrm{HfTe}}_2$, with dispersion perpendicular to the plane. In contrast, the band structure of heavily K-dosed ${\mathrm{HfTe}}_2$ is obviously different from that of bulk and resembles the band structure calculated for monolayer ${\mathrm{HfTe}}_2$. It was also observed that lightly K-dosed ${\mathrm{HfTe}}_2$ is characterized by quantized bands originating from bilayer and trilayer ${\mathrm{HfTe}}_2$, indicative of staging. The results suggest that the dimensionality crossover from 3D (dimensional) to 2D electronic states due to systematic K intercalation takes place via staging in a single sample. The study provides a strategy for controlling the dimensionality and functionality of novel quantum materials.