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The intrinsic properties of two-dimensional (2D) SiO2 were revealed by forming the material on inert Au(111). Growth by SiO deposition enabled formation of a crystalline phase consisting of two linked sheets of six-membered rings of tetrahedral [SiO4] building units. The bases of the tetrahedra form an isostatic 2D kagome lattice. The weak interaction with Au allowed a new corrugation of the 2D material to be detected: ripples with a similar to 4 nm periodicity that help stabilize the 2D crystalline layer. On the atomic scale, substantial distortions from ideal hexagonal rings were observed even though crystalline defects were extremely rare. The ring distortions were reproduced in theoretical models that showed that they were associated with a twisting of the 2D kagome lattice, which stabilizes the material when subjected to disturbances. The twisting and rippling impart 2D silica with the flexibility to adapt to strain and changes in the crystallographic direction without introducing defects.

Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported. Micrometre-scale single-crystal domains formed as isolated faceted islands or merged together to achieve full monolayer coverage. The crystal structure of this phase has ten boron atoms and two hexagonal vacancies in its unit cell. First-principles calculations indicate that charge transfer, rather than covalent bonding, binds this two-dimensional boron to the Cu(100) surface. The electronic band structure of this material features multiple anisotropic tilted Dirac cones.

Efficient and accurate computational methods for dealing with interacting electron problems on a lattice are of broad interest to the condensed matter community. For interacting Hubbard models, we introduce a cluster slave-particle approach that provides significant computational savings with high accuracy for total energies, site occupancies, and interaction energies. Compared to exact benchmarks using density matrix renormalization group for $d$-$p$ Hubbard models, our approach delivers accurate results using two to three orders of magnitude lower computational cost. Our method is based on a novel slave-particle decomposition with an improved description of particle hoppings, and a new density matrix expansion method where the interacting lattice slave-particle problem is then turned into a set of overlapping real-space clusters which are solved self-consistently with appropriate physical matching constraints at shared lattice sites between clusters., 21 pages, 13 figures

This Perspective addresses the design, creation, characterization and control of synthetic quantum materials with strong electronic correlations. We show how emerging synergies between theoretical/computational approaches and materials design/experimental probes are driving recent advances in the discovery, understanding and control of new electronic behaviour in materials systems with interesting and potentially technologically important properties. The focus here is on transition metal oxides, where electronic correlations lead to a myriad of functional properties including superconductivity, magnetism, Mott transitions, multiferroicity and emergent behaviour at picoscale-designed interfaces. Current opportunities and challenges are also addressed, including possible new discoveries of non-equilibrium phenomena and optical control of correlated quantum phases of transition metal oxides. This Perspective addresses the properties of strongly correlated materials, with a particular focus on computational, synthetic and spectroscopic approaches.

The ability to control the dispersion of RuO2 monolayers on a range of perovskite- and corundum-structured oxide substrates was investigated using complementary experiment and theory. Controlled deposition under UHV conditions and subsequent in situ characterization by x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) was used to follow the chemical state, dispersion and structure of the RuO2 layers as a function of growth and post-growth processing conditions. The experimental results reveal that monolayer RuO2 on ferroelectric PbxZr1-xTiO3(001) and non-polar SrTiO3(001) α-Al2O3 ( 1 1 ¯ 02 ) tends to dewet the surface to form three-dimensional clusters, though some kinetic stability of dispersed RuO2 monolayers was achieved on the SrTiO3 surface. Although an RuO2 termination is not favored for SrRuO3(001), deposition onto SrRuO3(001) yields a stable RuO2 surface which ISS suggests is capped by chemisorbed oxygen. The complementary density functional theory modeling and ab initio thermodynamics indicate that Ru oxide monolayers on the perovskite substrates tend to be unstable under the low oxygen pressure experimental conditions; however, either lower temperatures or higher oxygen pressures can lead to a kinetic and even thermodynamic stability. The theoretical findings also provide guidance on how the instability of dispersed RuO2 monolayers can be overcome by inserting buffer layers and increasing the partial pressure of molecules that selectively, strongly adsorb at coordinatively unsaturated Ru sites on the oxide surface.

The ability to synthesize two-dimensional (2D) oxide layers with nonbulk structures on transition-metal surfaces has attracted attention as a method to expand the range of 2D structures and propert...

Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on NixPd1-x(111) alloy substrates was systematically investigated experimentally as a function of Si surface coverage, annealing time and temperature, O2 partial pressure, and substrate composition and the results were compared with thermodynamic predictions based on density functional theory (DFT) calculations and thermochemical data for O2. Experimentally, 2D Ni silicate was exclusively observed at higher O2 pressures (∼10-6 Torr), higher annealing temperatures (1000 K), and more prolonged annealing (10 min) if the substrate contained any Ni and for initial Si coverages up to 2 monolayers. In contrast, decreasing the O2 pressure to ∼10-8 Torr and restricting the annealing temperature and time enabled 2D VDW silica formation. Amorphous 2D VDW silica was observed even when the substrate composition was tuned to lattice match crystalline 2D VDW silica. The trend of decreased O2 pressure favoring 2D VDW silica was consistent with the theoretical predictions; however, theory also suggested that sufficient Si coverage could avoid Ni silicate formation. The effect of epitaxial strain on 2D Ni silicate was investigated by modifying the solid solution alloy substrate composition. It was found that 2D Ni silicate will stretch to match the substrate lattice constant up to 1.12% tensile strain. When the lattice mismatch was over 1.40%, incommensurate crystalline domains were observed, indicating relaxation of the overlayer to its favored lattice constant. The limited epitaxial strain that could be applied was attributed to a combination of the 2D silicate stiffness, the insensitivity of its bonding to the substrate to its alignment with the substrate, and its lack of accessible structural rearrangements that can reduce the strain energy. The results demonstrate how the resulting 2D material can be manipulated through the growth conditions and how a solid solution alloy substrate can be used to maximize the epitaxial strain imparted to the 2D system.

We report on the synthesis and electronic properties of epitaxial perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}\mathrm{Rh}{\mathrm{O}}_{3}$ thin films. Thin films with a Sr content ranging from $x=0$ to 0.5 have been grown using molecular beam epitaxy. Transport and x-ray photoemission spectroscopy data reveal an insulator-metal-insulator transition, accompanied by a $p$- to $n$-type carrier change observed in Hall measurements. Combined with theoretical calculations, we find that the addition of Sr does not directly dope carriers into the conduction band, but rather induces localized Rh $4d$ states within the $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ band gap. The bandwidth of the impurity band increases with Sr content, eventually causing the valence band (VB) and the localized Rh $4d$ band to overlap, which explains the first insulator-to-metal transition occurring at $x=0.35$. For Sr content $xg0.4$, possible cation ordering results in an increase of the gap between the VB and the Rh $4d$ band, leading to the second metal-to-insulator transition. We map out the electronic phase diagram of the Sr-doped $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ system and suggest strategies to engineer the electronic states in rhodate systems via delocalizing the ${\mathrm{Rh}}^{3+}$ states.

The unequal electronic occupation of localized orbitals (orbital polarization), and associated lowering of symmetry and degeneracy, play an important role in the properties of transition metal oxides. Here, we examine systematically the underlying origin of orbital polarization, taking as exemplar the $3d$ manifold of ${\mathrm{Co}}^{2+}$ in a variety of spin, orbital, and structural phases in the double perovskite ${\mathrm{La}}_{2}{\mathrm{CoTiO}}_{6}$ and the (001) superlattice ${({\mathrm{LaCoO}}_{3})}_{1}+{({\mathrm{LaTiO}}_{3})}_{1}$ systems. Superlattices are of specific interest due to the large experimentally observed orbital polarization of their Co cations. Based on first principles calculations, we find that robust and observable orbital polarization requires symmetry reduction through the lattice structure; the role of local electronic interactions is to greatly enhance the orbital polarization.

${\mathrm{LaCoO}}_{3}$ provides a poignant example of a transition metal oxide where the cobalt cations display multiple spin states and spin transitions and which continues to garner substantial attention. In this work, we describe first principles studies, based on $\mathrm{DFT}+U$ theory, of superlattices containing ${\mathrm{LaCoO}}_{3}$, specifically ${({\mathrm{LaCoO}}_{3})}_{n}+{({\mathrm{LaTiO}}_{3})}_{n}$ for $n=1,2$. The superlattices show strong electron transfer from Ti to Co resulting in ${\mathrm{Co}}^{2+}$, significant structural distortions and a robust orbital polarization of the ${\mathrm{Co}}^{2+}$. We predict high-spin ${\mathrm{Co}}^{2+}$ and a checkerboard (G-type) antiferromagnetic ground state. We provide a detailed analysis of the magnetic interactions and phases in the superlattices. We predict that ferromagnetic order on the ${\mathrm{Co}}^{2+}$ can be stabilized by hole doping (e.g., replacing La by Sr), which is rather unusual for ${\mathrm{Co}}^{2+}$ cations.

Two-dimensional van der Waals (2D vdW) single layered materials with ferromagnetism and piezoelectricity have been a subject of recent attention. Despite numerous reports of 2D ferromagnetic materials, developing an air stable and transferable vdW material has been challenging. To address this problem, we studied layered transition metal silicates that are derivatives of kaolinites and lizardites with transition metals substituting on ${\mathrm{Al}}^{3+}$ and ${\mathrm{Mg}}^{2+}$ sites using ab initio density functional theory calculations. This class of materials is appealing because they meet the symmetry requirements for piezoelectricity and can host a range of transition metal cations. As oxides, these materials are inherently stable in air. Following our previous experimental work, we predict that these compounds are stable under varying ${\mathrm{O}}_{2}$ partial pressure and can be synthesized using a surface-assisted method. We also show that the oxidation states of the substituted transition metal ions can be tuned through the level of hydrogenation.

A single layer of Fe silicate was grown on Pd(111) and analyzed experimentally and theoretically. Following sequential deposition of SiO and Fe and annealing above 900 K in O2, an incommensurate but well-ordered, low-defect density layer was observed with low-energy electron diffraction and scanning tunneling microscopy (STM). The STM images revealed a moire pattern due to the lattice mismatch between the relaxed oxide layer and the substrate, while high-resolution images showed a honeycomb structure consistent with a silicate layer with six-membered rings of corner-sharing SiO4 tetrahedra at its surface. Reflection-absorption infrared spectroscopy revealed a single peak at 1050 cm−1 due to Si–O–Fe linkages, while x-ray photoelectron spectroscopy data indicated a Si/Fe ratio of one, that the Fe were all 3+, and that the Si atoms were closest to the surface. Consistent with these experimental observations, first principles theory identified a layer with an overall stoichiometry of Fe2Si2O9 with the six-membered rings of SiO4 tetrahedra at the surface. One of the oxygen atoms appears as a chemisorbed atom on the Pd surface, and, thus, the layer is better described as Fe2Si2O8 atop an oxygen-covered Pd surface. The Fe2Si2O8 is chemically bound to the Pd surface through its oxygen atoms; and the passivation of these bonds by hydrogen was investigated theoretically. Upon hydrogenation, the adsorbed O atom joins the Fe silicate layer and thermodynamic analysis indicates that, at room temperature and H2 pressures below 1 atm, Fe2Si2O9H4 becomes favored. The hydrogenation is accompanied by a substantial increase in the equilibrium distance between the oxide layer and the Pd surface and a drop in the adhesion energy to the surface. Together the results indicate that a highly ordered 2D Fe silicate can be grown on Pd(111) and that subsequent hydrogenation of this layer offers potential to release the 2D material from the growth substrate.

Coherent and epitaxial interfaces permit the realization of electric field driven devices controlled by atomic-scale structural and electronic effects at interfaces. Compared to conventional field effect devices where channel conductivity is modulated by carrier density modification, the propagation of atomic-scale distortions across an interface can control the atomic scale bonding, interatomic electron tunneling rates and thus the mobility of the channel material. We use first-principles theory to design an atomically abrupt epitaxial perovskite heterostructure involving an oxide ferroelectric (PbZr0.2Ti0.8O3) and conducting oxide channel (LaNiO3) where coupling of polar atomic motions to structural distortions can induce large, reversible changes in the channel mobility. We fabricate and characterize the heterostructure and measure record values, larger than 1000%, for the conductivity modulation. Our results describe how purely interfacial effects can be engineered to deliver unique electronic device properties and large responses to external fields.

A single atomic layer of ZrO2 exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance–voltage measurements of a ZrO2 gate stack demonstrate that a reversible polarization of the ZrO2 interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states. This monolayer ferroelectric represents a new class of materials for achieving devices that transcend conventional complementary metal oxide semiconductor (CMOS) technology. Significantly, a single atomic layer ferroelectric allows for more aggressively scaled devices than bulk ferroelectrics, which currently need to be thicker than 5–10 nm to exhibit significant hysteretic behavior (Park, et al. Adv. Mater. 2015, 27, 1811).

The elasto-optic effect, or photoelasticity, describes the linear change of dielectric tensor with applied strain and is a universal material property of insulators and semiconductors. Though the elasto-optic responses for solids can be computed directly from first principles (e.g., by using density functional perturbation theory) and measured experimentally, these methods do not provide sufficient insight into the governing microscopic physical principles of photoelasticity. In this work, we describe a microscopic first-principles analysis of photoelasticity in real space and apply it to investigate the elasto-optic responses of Si, diamond, NaCl, and MgO. By writing the random phase approximation (RPA) dielectric constant in the basis of maximally localized Wannier functions, we show that the strain-dependent change of dipole transitions between occupied and unoccupied Wannier functions are the main determinants of photoelasticity. By organizing the dipole transitions into spatially localized shells, we develop a ``constrained sum'' method that converges both the dielectric and photoelastic responses systematically and reveals a relatively long-ranged nature to these responses: one needs to sum up contributions of up to third neighbor shells to converge the elasto-optic coefficient with reasonable precision.

Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on Ni

Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an ${({\mathrm{LaTiO}}_{3})}_{2}/{({\mathrm{LaCoO}}_{3})}_{2}$ oxide heterostructure. X-ray absorption spectra and ab initio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone. The asymmetry between in-plane and out-of-plane orbital occupancies in the high-spin cobaltate layer is predicted by theory and observed through x-ray linear dichroism experiments. Manipulating orbital configurations using interfacial coupling within heterostructures promises exciting ground-state engineering for realizing new emergent electronic phases in metal oxide superlattices.

The authors provide a first-principles analysis of and methodology for easily estimating the surface piezoelectric response of materials that are not piezoelectric in the bulk. This study shows that the surface of sapphire ($\ensuremath{\alpha}$-Al${}_{2}$O${}_{3}$) is piezoelectric, in both Al-terminated and hydroxylated forms. If confirmed by experiment, this effect would be of high technological significance. For example, it would lead to a loss mechanism in the sapphire-based Josephson junctions used in a variety of quantum devices, and would allow for the fine tuning of $e.g.$ lasers and chemical catalysts. Furthermore, many technologically relevant materials are grown using sapphire as a substrate.

We present a comprehensive first principles study of doped hafnia in order to understand the formation of ferroelectric orthorhombic[001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal[100] grains can transform into orthorhombic[001] during thermal annealing when they are laterally confined by other grains. We show that among 0%, 2% and 4% Si doping, 4% doping provides the best conditions for the tetragonal[100] → orthorhombic[001] transformation. This also holds for Al doping. We also show that for Hf

The GW method is a many-body electronic structure technique capable of generating accurate quasiparticle properties for realistic systems spanning physics, chemistry, and materials science. Despite its power, GW is not routinely applied to large complex assemblies due to its large computational overhead and quartic scaling with particle number. Here, the GW equations are recast, exactly, as Fourier-Laplace time integrals over complex time propagators. The propagators are then "shredded" via energy partitioning and the time integrals approximated in a controlled manner using generalized Gaussian quadrature(s) while discrete variable methods are employed to represent the required propagators in real-space. The resulting cubic scaling GW method has a sufficiently small prefactor to outperform standard quartic scaling methods on small systems ($\gtrapprox$ 10 atoms) and also represents a substantial improvement over other cubic methods tested for all system sizes studied. The approach can be applied to any theoretical framework containing large sums of terms with energy differences in the denominator., 24 pages, 18 figures

We use density functional theory and Monte Carlo lattice simulations to investigate the structure of ZrO$_{2}$ monolayers on Si(001). Recently, we have reported on the experimental growth of amorphous ZrO$_{2}$ monolayers on silicon and their ferroelectric properties, marking the achievement of the thinnest possible ferroelectric oxide [M. Dogan et al. Nano Lett., 18 (1) (2018)]. Here, we first describe the rich landscape of atomic configurations of monocrystalline ZrO$_{2}$ monolayers on Si and determine the local energy minima. Because of the multitude of low-energy configurations we find, we consider the coexistence of finite-sized regions of different configurations. We create a simple nearest-neighbor lattice model with parameters extracted from DFT calculations, and solve it numerically using a cluster Monte Carlo algorithm. Our results suggest that up to room temperature, the ZrO$_{2}$ monolayer consists of small domains of two low-energy configurations with opposite ferroelectric polarization. This explains the observed ferroelectric behavior in the experimental films as a collection of crystalline regions, which are a few nanometers in size, being switched with the application of an external electric field.

A monolayer of ZrO$_{2}$ has recently been grown on the Si(001) surface and shown to have ferroelectric properties, which signifies the realization of the lowest possible thickness in ferroelectric oxides [M. Dogan et al., Nano Lett., 18 (1) (2018)]. In our previous computational study, we reported on the multiple (meta)stable configurations of ZrO$_{2}$ monolayers on Si, and how switching between a pair of differently polarized configurations may explain the observed ferroelectric behavior of these films [M. Dogan and S. Ismail-Beigi, arXiv:1902.01022 (2019)]. In the current study, we conduct a DFT-based investigation of (i) the effect of oxygen content on the ionic polarization of the oxide, and (ii) the role of zirconia monolayers as buffer layers between silicon and a thicker oxide film that is normally paraelectric on silicon, e.g. SrTiO$_{3}$. We find that (i) total energy-vs-polarization behavior of the monolayers, as well as interface chemistry, is highly dependent on the oxygen content; and (ii) SrTiO$_{3}$/ZrO$_{2}$/Si stacks exhibit multiple (meta)stable configurations and polarization profiles, i.e. zirconia monolayers can induce ferroelectricity in oxides such as SrTiO$_{3}$ when used as a buffer layer. This may enable a robust non-volatile device architecture where the thickness of the gate oxide (here strontium titanate) can be chosen according to the desired properties.

We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal [100] to orthorhombic [001] transformation. This also holds for Al doping. We also show that for Hf$_{x}$Zr$_{1-x}$O$_{2}$, where we have studied ${x}=1.00,0.75,0.50,0.25,0.00$, the value ${x}=0.50$ provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] to monoclinic [100] transformation, out-of-plane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an explanation for common experimental observations for the optimal ranges of doping in Si:HfO$_{2}$, Al:HfO$_{2}$ and Hf$_{x}$Zr$_{1-x}$O$_{2}$. We also present model thin film heterostructure computations of Ir/HfO$_{2}$/Ir stacks in order to isolate the interface effects, which we show to be significant.

Density functional theory was used to assess the viability of approaches to controlling the structure of a recently discovered two-dimensional form of SiO2. In accord with prior work, a hexagonal bilayer of mirror image planes of corner-sharing SiO4 tetrahedra in six-membered rings yielded only a slightly higher energy than α-quartz. Structures including four- through eight-membered rings were evaluated and in certain cases found to be as little as 17 meV/Si higher in energy than the hexagonal bilayer. When either biaxial or uniaxial tensile strain was applied, combinations of eight-, six-, and four-membered rings became favored due to the lower density of structures with larger rings. These findings, together with experiments that reveal expansion of silica bilayers to match the lattice of metal substrates, suggest that epitaxial strain may be used to control the bilayer structure. Replacement of Si with Ge and Al as prototypical tetravalent and trivalent dopants was also investigated. Substituting Ge fo...

It has been known for more than six decades that ferroelectricity can affect a material's surface physics and chemistry thereby potentially enhancing its catalytic properties. Ferroelectrics are a class of materials with a switchable electrical polarization that can affect surface stoichiometry and electronic structure and thus adsorption energies and modes; e.g., molecular versus dissociative. Therefore, ferroelectrics may be utilized to achieve switchable surface chemistry whereby surface properties are not fixed but can be dynamically controlled by, for example, applying an external electric field or modulating the temperature. Several important examples of applications of ferroelectric and polar materials in photocatalysis and heterogeneous catalysis are discussed. In photocatalysis, the polarization direction can control band bending at water/ferroelectric and ferroelectric/semiconductor interfaces, thereby facilitating charge separation and transfer to the electrolyte and enhancing photocatalytic activity. For gas–surface interactions, available results suggest that using ferroelectrics to support catalytically active transition metals and oxides is another way to enhance catalytic activity. Finally, the possibility of incorporating ferroelectric switching into the catalytic cycle itself is described. In this scenario, a dynamic collaboration of two polarization states can be used to drive reactions that have been historically challenging to achieve on surfaces with fixed chemical properties (e.g., direct NOx decomposition and the selective partial oxidation of methane). These predictions show that dynamic modulation of the polarization can help overcome some of the fundamental limitations on catalytic activity imposed by the Sabatier principle.

There is widespread interest in finding sustainable replacements for fossil fuels, and hence hydrogen production via water splitting has received much attention. However, finding efficient water splitting methods remains a challenge. We use first principles theory to design a catalytic cycle for water splitting that employs the pyroelectric effect in ferroelectrics. Taking PbTiO3 as an example, we show that actively controlling the ferroelectric polarization via cyclic temperature modulations can catalyze the splitting of H2O into O2 and H2. In practice, the energy needed to drive this cycle may be provided by low/intermediate grade heat. Since no precious metals are used in this catalytic cycle, this approach may lead to viable water-splitting methods that employ earth-abundant elements.

Synthesizing monolayers and heterostructures is an enabling approach to extract new physical phenomena from bulk materials. Among the structures amenable to this approach is stanene, which is a monolayer of tin, similar to graphene, and has been predicted to host one-dimensional topological states at its edges. Stanene can be tuned by decorating with different adatoms, which makes it a promising platform on which to engineer topological devices. Here, we deposit Sn on Bi2Te3 and characterize the growth using anomalous synchrotron x-ray scattering and x-ray photoelectron spectroscopy (XPS). X-ray diffraction data reveal the formation of epitaxial Sn-based structures, along with penetration of Sn into the Bi2Te3, with Sn intercalating between the upper 10 Bi2Te3 quintuple layers. Additionally, XPS data show deposited Sn reacting to form SnTe and Bi at the Bi2Te3 surface. The calculated heat of reaction for Sn and Bi2Te3 is consistent with an exothermic reaction to SnTe and Bi. Using thermodynamic calculations as a guide, we identify several candidate substrates that can stabilize the stanene phase.

We demonstrate the integration of a two-dimensional electron gas (2DEG) oxide structure composed of $\mathrm{LaTi}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LTO/STO) on undoped Si(001) that possesses the attractive attributes of high charge density from the oxide and high mobility of silicon. Key to this approach is modification of the oxygen content at the STO-Si interface, which tunes the band alignment and induces electron carriers to move from the oxide 2DEG to form a 2DEG in the silicon substrate. As a consequence, the overall mobility of the heterostructure increases by two orders of magnitude compared to that in the oxide 2DEG to up to $100\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at room temperature, with a carrier density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. This approach can be applied to technologies that require both high carrier and high carrier mobility for applications in plasmonics and high power electronics.

The GW method, which can describe accurately electronic excitations, is one of the most widely used ab initio electronic structure technique and allows the physics of both molecular and condensed phase materials to be studied. However, the applications of the GW method to large systems require supercomputers and highly parallelized software to overcome the high computational complexity of the method scaling as $O(N^4)$. Here, we develop efficient massively-parallel GW software for the plane-wave basis set by revisiting the standard GW formulae in order to discern the optimal approaches for each phase of the GW calculation for massively parallel computation. These best numerical practices are implemented into the OpenAtom software which is written on top of charm++ parallel framework. We then evaluate the performance of our new software using range of system sizes. Our GW software shows significantly improved parallel scaling compared to publically available GW software on the Mira and Blue Waters supercomputers, two of largest most powerful platforms in the world., 48 pages, 10 figures

We investigate the (surface) bonding of a class of industrially and biologically important molecules in which the chemically active orbital is a 2 p electron lone pair located on an N or O atom bound via single bonds to H or alkyl groups. This class includes water, ammonia, alcohols, ethers, and amines. Using extensive density functional theory (DFT) calculations, we discover scaling relations (correlations) among molecular binding energies of different members of this class: the bonding energetics of a single member can be used as a descriptor for other members. We investigate the bonding mechanism for a representative (H2O) and find the most important physical surface properties that dictate the strength and nature of the bonding through a combination of covalent and noncovalent electrostatic effects. We describe the importance of surface intrinsic electrostatic, geometric, and mechanical properties in determining the extent of the lone-pair-surface interactions. We study systems including ionic materials in which the surface positive and negative centers create strong local surface electric fields, which polarize the dangling lone pair and lead to a strong "electrostatically driven bond". We emphasize the importance of noncovalent electrostatic effects and discuss why a fully covalent picture, common in the current first-principles literature on surface bonding of these molecules, is not adequate to correctly describe the bonding mechanism and energy trends. By pointing out a completely different mechanism (charge transfer) as the major factor for binding N- and O-containing unsaturated (radical) adsorbates, we explain why their binding energies can be tuned independently from those of the aforementioned species, having potential implications in scaling-driven catalyst discovery.

In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogeneous, single-phase material. We show, using scanning tunneling microscopy/spectroscopy, that defects that break local symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The finding highlights the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of topological surface states and can operate at practical temperatures.

The maximum efficiency of a fixed catalytic surface occurs when the adsorbate–surface interaction strength is optimal as per the Sabatier principle: strong enough to drive the reactions forward but weak enough to permit the products’ desorption. Such a compromise can fundamentally limit catalytic activity. One solution is to create a surface with switchable activity between strong binding (rapid dissociation) and weak binding (easy desorption). On the basis of first-principles theory, we describe a class of catalysts comprising an epitaxial monolayer of a transition metal oxide on an oxide ferroelectric substrate in which reversing the ferroelectric polarization state switches the surface activity between these two limits. As an example, a CrO2 monolayer on ferroelectric PbTiO3 permits direct NOx decomposition and CO oxidation while circumventing oxygen and sulfur poisoning. Our computed binding energy trends are explained by a generalization of the canonical d-band model for transition metals to metal ox...

We describe a theoretical approach for finding spontaneously symmetry-broken electronic phases due to strong electronic interactions when using recently developed slave-particle (slave-boson) approaches based on occupation numbers. We describe why, to date, spontaneous symmetry breaking has proven difficult to achieve in such approaches. We then provide a total-energy based approach for introducing auxiliary symmetry breaking fields into the solution of the slave-particle problem that leads to lowered total energies for symmetry broken phases. We point out that not all slave-particle approaches yield to energy lowering: the slave-particle model being used must explicitly describe the degrees of freedom that break symmetry. Finally, our total energy approach permits us to greatly simplify the formalism used to achieve a self-consistent solution between spinon and slave modes while increasing numerical stability and greatly speeding up the calculations.

The way in which atoms bond to form a material — in particular the pattern of bond lengths and angles — is the fundamental determinant of the properties of the resulting material. Functional materials often derive their properties from alterable or reversible bond distortions at the picometre length scale that modify the electronic configuration. By considering several examples, we discuss how picoscale bond perturbations can be used to achieve specific materials properties. In particular, we examine the orbital engineering demonstrated in nickelates, the functional properties obtained in perovskite superlattices and the influence of interfacial effects on the high superconductive transition temperature of iron selenide. Moreover, we emphasize the relation between band topology and picoscale distortions in transition metal dichalcogenides and the effect of the excitation of lattice modes on materials properties. We use these examples to highlight how the combination of first-principles methods, materials growth techniques that allow control of the composition of individual atomic layers and state-of-the-art methods to characterize or dynamically excite picoscale bond distortions provides a powerful approach for discovering rules and concepts for picoscale materials engineering. Picoscale engineering of bonds is a powerful method to tune materials properties. Transcending nanoscience, picoscience offers the ultimate length scale to manipulate materials through subtle bond distortions between individual atoms, as this Review discusses through several examples ranging from perovskites to superconductors and to topological materials.

SnTe is a topological crystalline insulator that exhibits crystal symmetry protected topological surface states (SS), which are useful for the development of novel devices, such as low-dissipation transistors. However, major obstacles remain to probe the SS and realize the application of SnTe and other topological insulators. Due to unintentional doping by Sn vacancies, bulk conduction may overwhelm the transport through SS in SnTe. Synthesis of SnTe films thin enough to suppress bulk conduction has proven difficult due to the formation of discontinuous domain structures. By introducing a novel deposition method that builds upon molecular beam epitaxy, we achieve ultrathin continuous films of single-orientation SnTe (001) on SrTiO3 (STO) (001) substrates. We separate the carrier concentrations in the bulk and in the SS and discover that conduction through the SS dominates (a majority of hole carriers occupy the SS) in films thinner than 40 unit cells, with a large temperature independent hole density of SS nS = 5 × 1014 cm−2. Unlike the depletion of SS carriers observed at the vacuum/SnTe interface that inhibits topological behavior, we show that SS carriers are buried and protected from depletion at the SnTe/STO interface, which is enabled by the relatively large bandgap of STO and its favorable band alignment with SnTe. This work provides an important pathway for probing and realizing SS transport in SnTe and other TIs even when bulk conduction coexists.