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Cleavable metallic antiferromagnets may be of use for low-dissipation spintronic devices; however, few are currently known. Here we present orthorhombic MnRhBi3 as one such compound and present a thorough study of its physical properties. Exfoliation is demonstrated experimentally, and the cleavage energy and electronic structure are examined by density functional theory calculations. It is concluded that MnRhBi3 is a van der Waals layered material that cleaves easily between neighboring Bi layers, and that the Bi atoms have lone pairs extending into the van der Waals gaps. A series of four phase transitions are observed below room temperature, and neutron diffraction shows that at least two of the transitions involve the formation of antiferromagnetic order. Anomalous thermal expansion points to a crystallographic phase transition and/or strong magnetoelastic coupling. This work reveals a complex phase evolution in MnRhBi3 and establishes this cleavable antiferromagnetic metal as an interesting material for studying the interplay of structure, magnetism, and transport in the bulk and ultrathin limits as well as the role of lone pair electrons in interface chemistry and proximity effects in van der Waals heterostructures.

Understanding band alignment and charge transfer at complex oxide interfaces is critical to tailoring and utilizing their diverse functionality. Towards this goal, we design and experimentally validate both Ohmic- and Schottky-like charge transfers at oxide/oxide semiconductor/metal interfaces. We utilize a method for predicting band alignment and charge transfer in ABO3 perovskites, where previously established rules for simple semiconductors fail. The prototypical systems chosen are the rare class of oxide metals, SrBO3 with B=V-Ta, when interfaced with the multifaceted semiconducting oxide, SrTiO3. For B=Nb and Ta, we confirm that a large accumulation of charge occurs in SrTiO3 due to higher energy Nb and Ta d-states relative to Ti; this gives rise to a high mobility metallic interface, which is an ideal epitaxial oxide/oxide Ohmic contact. On the other hand, for B=V, there is no charge transfer into the SrTiO3 interface, which serves as a highly conductive epitaxial gate metal. Going beyond these specific cases, this work opens the door to integrating the vast phenomena of ABO3 perovskites into a wide range of practical devices., Comment: Accepted in Small Science

Relaxor ferrolectrics are important in technological applications due to a strong electromechanical response, energy storage capacity, electrocaloric effect, and pyroelectric energy conversion properties. Current efforts to discover and design new materials in this class generally rely on substitutional doping of known ferroelectrics, as slight changes to local compositional order can significantly affect the Curie temperature, morphotropic phase boundary, and electromechanical responses. In this work, we demonstrate that moving to the strong limit of compositional complexity in an ABO3 perovskite allows stabilization of novel relaxor responses that do not rely on a single narrow phase transition region. Entropy-assisted synthesis approaches are used to create single crystal Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3 [Ba(5B)O] films. The high levels of configurational disorder present in this system is found to influence dielectric relaxation, phase transitions, nano-polar domain formation, and Curie temperature. Temperature-dependent dielectric, Raman spectroscopy and second-harmonic generation measurements reveal multiple phase transitions, a high Curie temperature of 570 K, and the relaxor ferroelectric nature of Ba(5B)O films. The first principles theory calculations are used to predict possible combinations of cations to quantify the relative feasibility of formation of highly disordered single-phase perovskite systems. The ability to stabilize single-phase perovskites with such a large number of different cations on the B-sites offers new possibilities for designing high-performance materials for piezoelectric, pyroelectric and tunable dielectric applications.

MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, we show by density functional theory calculations that the strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4; such strain is further enhanced in MnBi4Te7, giving rise to even higher BiMn density. The abundance of intrinsic BiMn donors results in degenerate n-type conductivity under the Te-poor growth condition. Our calculations suggest that growths in a Te-rich condition can lower the Fermi level, which is supported by our transport measurements. We further show that the internal strain can also enable efficient doping by large-size-mismatched substitutional NaMn acceptors, which can compensate BiMn donors and lower the Fermi level. Na doping may pin the Fermi level inside the bulk band gap even at the Te-poor limit in MnBi2Te4. Furthermore, facile defect formation in MnSb2Te4 and its implication in Sb doping in MnBi2Te4 as well as the defect segregation in MnBi4Te7 are discussed. The defect engineering and doping strategies proposed in this paper will stimulate further studies for improving synthesis and for manipulating magnetic and topological properties in MnBi2Te4, MnBi4Te7, and related compounds.

A generic method to estimate the relative feasibility of formation of high entropy compounds in a single phase, directly from first principles, is developed. As a first step, the relative formation abilities of 56 multi-component, AO, oxides were evaluated. These were constructed from 5 cation combinations chosen from A={Ca, Co, Cu, Fe, Mg, Mn, Ni, Zn}. Candidates for multi-component oxides are predicted from descriptors related to the enthalpy and configurational entropy obtained from the mixing enthalpies of two component oxides. The utility of this approach is evaluated by comparing the predicted combinations with the experimentally realized entropy stabilized oxide, (MgCoCuNiZn)O. In the second step, Monte Carlo simulations are utilized to investigate the phase composition and local ionic segregation as a function of temperature. This approach allows for the evaluation of potential secondary phases, thereby making realistic predictions of novel multi-component compounds that can be synthesized., Comment: 43 pages, 5 main figures and 8 supplemental figures. Being considered for Chemistry of Materials

Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temperatures, $T_C$ ~ 310K. In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to 30% enhances $T_C$ and changes the magnetic anisotropy, while approximately 50% cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for ~46% cobalt content relative to iron. First principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe5-xGeTe2 to composition and structure, and emphasize the important role of structural order/disorder and layer stacking in cleavable magnetic materials., Comment: This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices

In this paper, we study the elastic properties of the entropy-stabilized oxide (Mg, Co, Ni, Cu, Zn)O, using experimental and first principles techniques. Our measurements of the indentation modulus on grains with a wide range of crystallographic orientations of the entropy-stabilized oxide revealed a high degree of elastic isotropy at ambient conditions. First principles calculations predict mild elastic anisotropy for the paramagnetic structure, which decreases when the system is considered to be non-magnetic. When the antiferromagnetic state of CoO, CuO and NiO is accounted for in the calculations, a slight increase in the elastic anisotropy is observed, suggesting a coupling between magnetic ordering and the orientation dependent elastic properties. Furthermore, an examination of the local structure reveals that the isotropy is favored through local ionic distortions of Cu and Zn - due to their tendency to form tenorite and wurtzite phases. The relationships between the elastic properties of the multicomponent oxide and those of its constituent binary oxides are reviewed. These insights open up new avenues for controlling isotropy for technological applications through tuning composition and structure in the entropy-stabilized oxide or the high entropy compounds in general., Comment: 52 pages, includes 29 pages supplemental materials, 5 figures

Designing and understanding functional electronic and magnetic properties in perovskite oxides requires controlling and tuning the underlying crystal lattice. Here we report the structure, including oxygen and cation positions, of a single-crystal, entropy stabilized perovskite oxide film of La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 grown on SrTiO3 (001). The parent materials range from orthorhombic (LaCrO3, LaMnO3 and LaFeO3) to rhombohedral (LaCoO3 and LaNiO3), and first principles calculations indicate that these structural motifs are nearly degenerate in energy and should be highly distorted site-to-site. Despite this extraordinary local configurational disorder on the B-site sublattice, we find a structure with unexpected macroscopic crystalline homogeneity with a clear orthorhombic unit cell, whose orientation is demonstrated to be controlled by the strain and crystal structure of the substrate for films grown on (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) and NdGaO3 (110). Furthermore, quantification of the atom positions within the unit cell reveal that the orthorhombic distortions are small, close to LaCrO3, which may be driven by a combination of disorder averaging and the average ionic radii. This is the first step towards understanding the rules for designing new crystal motifs and tuning functional properties through controlled configurational complexity., Comment: To appear in Physical Review Materials

Multiferroic materials have driven significant research interest due to their promising technological potential. Developing new room-temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel LiFe5O8 (LFO) and a ferroelectric perovskite BiFeO3 (BFO) is presented. We observed that lithium (Li)-doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of LixBi1-xFeO3 nanoceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping-induced phase separation and local ferroic properties in both the BFO-LFO composite ceramics and self-assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO-LFO composites is supported by first principles calculations. These findings shed light on Li-ion role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites.

Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.

Two-dimensional charge carrier accumulation at oxide heterointerfaces presents a paradigm shift for oxide electronics. Like a capacitor, interfacial charge buildup couples to an electric field across the dielectric medium. To prevent the so-called polar catastrophe, several charge screening mechanisms emerge, including polar distortions and interfacial intermixing which reduce the sharpness of the interface. Here, we examine how atomic intermixing at oxide interfaces affect the balance between polar distortions and electric potential across the dielectric medium. We find that intermixing moves the peak charge distribution away from the oxide/oxide interface; thereby changing the direction of polar distortions away from this boundary with minimal effect on the electric field. This opposing electric field and polar distortions is equivalent to the transient phase transition tipping point observed in double well ferroelectrics; resulting in an anomalous dielectric response -- a possible signature of local negative differential capacitance, with implications for designing dissipationless oxide electronics.

We used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca2Sr2IrO6. The crystal structure is determined to be $R\bar3$ with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below $T_N = 13.1$ K. The magnetic wave vector is identified as $(0,0.5,1)$ with ferromagnetic coupling along the $a$ axis and antiferromagnetic correlation along the $b$ axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34$^\circ$ towards the $c$ axis. The ordered moment is 0.66(3) $\mu_B$/Ir, larger than other iridates where iridium ions form corner- or edge-sharing $\rm IrO_6$ octahedral networks. The tilting angle is reduced to $\approx19^\circ$ when a magnetic field of 4.9 Tesla is applied along the $c$ axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap $\sim0.5$~eV.

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO$_3$ and PbTiO$_3$ (STO/PTO) is determined using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies E$_v$ in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on the E$_v$ are examined. The formation energies vary layer-by-layer in the superlattices, with E$_v$ being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, E$_v$ of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a$^0$a$^0$c$^-$) in the FE superlattice have a significant effect on the E$_v$, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties via controlling the constituent materials of superlattices and interface engineering., Comment: 20 pages, 5 figures, accepted at Physical Review Materials

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting critical temperature T$_C$ than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial Ti$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning transmission electron microscopy and density functional theory. We find Ti$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer electron-dopes the FeSe monolayer in agreement with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that favor a T$_C$ increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers., Comment: 10 pages, 4 figures, supplemental materials available upon request

The crystallographic and magnetic properties of the cleavable 4d3 transition metal compound a-MoCl3 are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

Strong interplay of fundamental order parameters in complex oxides are known to give rise to exotic physical phenomena. The 4d transition metal oxide SrRhO3 has generated much interest, but advances have been hindered by difficulties in preparing single crystalline phases. Here, we have epitaxially stabilized high quality single crystalline SrRhO3 films and investigated their structural, electronic, and magnetic properties. We determine that their properties significantly differ from the paramagnetic metallic ground state that governs bulk samples and are strongly related to rotations of the RhO6 octahedra.

CrCl3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl3, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Finally, we demonstrate that monolayer and few-layer CrCl3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

The crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe3 have been investigated on single crystal and polycrystalline materials. The crystal structure contains layers made up of lozenge shaped Cr_4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3eV. The magnetic susceptibility exhibits a broad maximum near 300K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55K, and single crystal neutron diffraction reveals the onset of long range antiferromagnetic order at this temperature. Strongly dispersive spin-waves are observed in the ordered state. Significant magneto-elastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is apparent in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first principles calculations, which predict a cleavage energy 0.5J/m^2, similar to graphite. Based on these results, CrTe3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.

Strain is known to greatly influence low temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition metal oxide thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO3 to systematically determine its influence on both the oxygen reduction and oxygen evolution reaction. Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to strain-induced splitting of the eg orbitals, which can customize orbital asymmetry at the surface. Analogous to strain-induced shifts in the d-band center of noble metals relative to Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides.

The chemical bonding of adsorbate molecules on transition-metal surfaces is strongly influenced by the hybridization between the molecular orbitals and the metal d-band. The strength of this interaction is often correlated with the location of the metal d-band center relative to the Fermi level. Here, we exploit finite size effects in the electronic structure of ultrathin Pd(111) films grown on Ru(0001) to tune their reactivity by changing the film thickness one atom layer at a time, while keeping all other variables unchanged. Interestingly, while bulk Pd(111) is reactive towards oxygen, Pd(111) films below five monolayers are surprisingly inert. This observation is fully in line with the d-band model prediction when applied to the orbitals involved in the bonding. The shift of the d-band center with film thickness is primarily attributed to shifts in the partial density of states associated with the 4dxz and 4dyz orbitals. This study gives an in-depth look into the orbital specific contributions to the surface chemical reactivity, providing new insights that could be useful in surface catalysis.

A density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology is reviewed. The insights that led to the construction of the Rutgers-Chalmers van der Waals Density Functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasising more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only for dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.

The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory (DFT). One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO$_3$, the adsorption of small molecules within metal-organic frameworks (MOFs), the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general purpose functional that could be applied to a range of materials problems with a variety of competing interactions.

We investigate the surface electronic structure and thermodynamic stability of the SrTiO3 (111) slabs using density functional theory. We observe that, for Ti-terminated slabs it is indeed possible to create a two-dimensional electron gas (2DEG). However, the carrier density of the 2DEG displays a strong thickness dependence due to the competition between electronic reconstruction and polar distortions. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, thereby making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications., Comment: 7 pages, 8 figures

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoOx (x = 2.5 - 3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases, i.e. the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originates from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides., Comment: 18 pages, 5 figures, to be published in Phys. Rev. Lett

We demonstrate from molecular dynamics simulations that windowed carbon nanotubes can efficiently separate CO2 from the CO2/CH4 mixture, resembling polymeric hollow fibers for gas separation. Three CO2/CH4 mixtures with 30%, 50% and 80% CO2 are investigated as a function of applied pressure from 80 to 180 bar. In all simulated conditions, only CO2 permeation is observed; CH4 is completely rejected by the nitrogen-functionalized windows or pores on the nanotube wall in the accessible timescale, while maintaining a fast diffusion rate along the tube. The estimated time-dependent CO2 permeance ranges from 107 to 105 GPU (gas permeation unit), compared with ~100 GPU for typical polymeric membranes. CO2/CH4 selectivity is estimated to be ~108 from the difference in free-energy barriers of permeation. This work suggests that a windowed carbon nanotube can be used as a highly efficient medium, configurable in hollow-fiber-like modules, for removing CO2 from natural gas., Comment: 18 pages, 7 figures

A 2D electron gas system in an oxide heterostructure serves as an important playground for novel phenomena. Here, we show that, by using fractional delta-doping to control the interface's composition in LaxSr1-xTiO3/SrTiO3 artificial oxide superlattices, the filling-controlled 2D insulator-metal transition can be realized. The atomic-scale control of d-electron band filling, which in turn contributes to the tuning of effective mass and density of the charge carriers, is found to be a fascinating route to substantially enhanced carrier mobilities., Comment: 14 pages, 4 figures, Nano Letters published online (2012)

In this paper, density functional theory calculations are used to explore the electronic and atomic reconstruction at interfaces between III-III/I-V oxides. In particular, at these interfaces, two dimensional electron gases (2DEGs) with twice the interfacial charge densities of the prototypical LaTiO3/SrTiO3 heterostructure are observed. Furthermore, a significant decrease in the band effective masses of the conduction electrons is shown, suggesting that possible enhancements in electron mobilities may be achievable. These findings represent a framework for chemically modulating 2DEGs, thereby providing a platform through which the underlying physics of electron confinement can be explored with implications for modern microelectronic devices.

The adsorption of 1,4-benzenediamine (BDA) on the Au(111) surface and azobenzene on the Ag(111) surface is investigated using density functional theory (DFT) with a non-local density functional (vdW-DF) and a semi-local Perdew-Burke-Ernzerhof (PBE) functional. For BDA on Au(111), the inclusion of London dispersion interactions not only dramatically enhances the molecule-substrate binding, resulting in adsorption energies consistent with experimental results, but also significantly alters the BDA binding geometry. For azobenzene on Ag(111), the vdW-DF produces superior adsorption energies compared to those obtained with other dispersion corrected DFT approaches. These results provide evidence for the applicability of the vdW-DF method and serves as a practical benchmark for the investigation of molecules adsorbed on noble metal surfaces.

A strategy for obtaining low band gap oxide ferroelectrics based on charge imbalance is described and illustrated by first principles studies of the hypothetical compound Bi$_6$Ti$_4$O$_{17}$, which is an alternate stacking of the ferroelectric Bi$_4$Ti$_3$O$_{12}$. We find that this compound is ferroelectric, similar to Bi$_4$Ti$_3$O$_{12}$ although with a reduced polarization. Importantly, calculations of the electronic structure with the recently developed functional of Tran and Blaha yield a much reduced band gap of 1.83 eV for this material compared to Bi$_4$Ti$_3$O$_{12}$. Therefore, Bi$_6$Ti$_4$O$_{17}$ is predicted to be a low band gap ferroelectric material.

The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional (vdW-DF) [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similar-sized naphthalene. Based on the single molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption., Comment: 5 figures, 2 tables

In this paper, an exchange functional which is compatible with the non-local Rutgers-Chalmers correlation functional (vdW-DF) is presented. This functional, when employed with vdW-DF, demonstrates remarkable improvements on intermolecular separation distances while further improving the accuracy of vdW-DF interaction energies. The key to the success of this three parameter functional is its reduction of short range exchange repulsion through matching to the gradient exchange approximation in the slowly varying/high density limit while recovering the large reduced gradient, s, limit set in the revised PBE exchange functional. This augmented exchange functional could be a solution to long-standing issues of vdW-DF lending to further applicability of density functional theory to the study of relatively large, dispersion bound (van der Waals) complexes.

Short period ferroelectric/ferroelectric BaTiO3 (BTO)/PbTiO3 (PTO) superlattices are studied using density functional theory. Contrary to the trends in paraelectric/ferroelectric superlattices the polarization remains nearly constant for PTO concentrations below 50%. In addition, a significant decrease in the c/a ratio below the PTO values were observed. Using a superlattice effective Hamiltonian we predict an enhancement in the d33 piezoelectric coefficient peaking at ~75% PTO concentration due to the different polarization-strain coupling in PTO and BTO layers. Further analysis reveals that these trends are bulk properties which are a consequence of the reduced $P$ brought about by the polarization saturation in the BTO layers.

Hydrogen adsorption by the metal organic framework (MOF) structure Zn2(BDC)2(TED) is investigated using a combination of experimental and theoretical methods. By use of the nonempirical van der Waals density-functional (vdW-DF) approach, it is found that the locus of deepest H2 binding positions lies within two types of narrow channel. The energies of the most stable binding sites, as well as the number of such binding sites, are consistent with the values obtained from experimental adsorption isotherms and heat of adsorption data. Calculations of the shift of the H-H stretch frequency when adsorbed in the MOF give a value of approximately -30 cm-1 at the strongest binding point in each of the two channels. Ambient temperature infrared absorption spectroscopy measurements give a hydrogen peak centered at 4120 cm-1, implying a shift consistent with the theoretical calculations., Comment: 4 pages, 4 figures

We examine the performance of a recently developed nonlocal density functional in predicting a model noncovalent interaction, the weak bond between an aromatic $\pi$ system and an aliphatic C-H group. The new functional is a significant improvement over traditional density functionals, providing results which compare favorably to high-level quantum-chemistry techniques but at considerably lower computational cost. Interaction energies in several model C-H/$\pi$ systems are in generally good agreement with coupled-cluster calculations, though equilibrium distances are consistently overpredicted when using the revPBE functional for exchange. The new functional correctly predicts changes in energy upon addition of halogen substituents., Comment: 5 pages, 4 figures

The effect of intermixing at the interface of short period PbTiO$_3$/SrTiO$_3$ superlattices is studied using first-principles density functional theory. The results indicate that interfacial intermixing significantly enhances the polarization within the superlattice. This enhancement is directly related to the off-centering of Pb and Sr cations and can be explained through a discussion of interacting dipoles. This picture should be general for a wide range of multicomponent superlattices and may have important consequences for the design of ferroelectric devices., Comment: 4 pages, 6 figures

We derive the exchange-correlation potential corresponding to the nonlocal van der Waals density functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)]. We use this potential for a self-consistent calculation of the ground state properties of a number of van der Waals complexes as well as crystalline silicon. For the latter, where little or no van der Waals interaction is expected, we find that the results are mostly determined by semilocal exchange and correlation as in standard generalized gradient approximations (GGA), with the fully nonlocal term giving little effect. On the other hand, our results for the van der Waals complexes show that the self-consistency has little effect at equilibrium separations. This finding validates previous calculations with the same functional that treated the fully nonlocal term as a post GGA perturbation. A comparison of our results with wave-function calculations demonstrates the usefulness of our approach. The exchange-correlation potential also allows us to calculate Hellmann-Feynman forces, hence providing the means for efficient geometry relaxations as well as unleashing the potential use of other standard techniques that depend on the self-consistent charge distribution. The nature of the van der Waals bond is discussed in terms of the self-consistent bonding charge., Comment: submitted to Phys. Rev. B

The adsorption of Ag$_3$ and Ag$_4$ clusters on the $\alpha$-Al$_2$O$_3$(0001) surface is explored with density functional theory. Within each adsorbed cluster, two different cluster-surface interactions are present. We find that silver clusters simultaneously form both ionic bonds with surface oxygen and intermetallic bonds with surface aluminum. The simultaneous formation of disparate electronic structure motifs within a single metal nanoparticle is termed a "dipolar nanocluster". This coexistence is ascribed to the similar bond enthalpies of Ag--Al and Ag--O bonds, and its importance for nanoparticle catalysis is highlighted.