The crystal structure of layered metal IrTe 2 is determined using single-crystal x-ray diffraction. At T=220 K, it exhibits Ir and Te dimers forming a valence-bond crystal. Electronic structure calculations reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the Ir and Te dimers, which exhibit a signature of covalent bonding character development. Evidence for significant charge disproportionation among the dimerized and non-dimerized Ir (charge order) is also presented.

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--Srn+1TinO3n+1 phases--in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.

The recent observation of a ferroelectric-like structural transition in metallic LiOsO3 has generated a flurry of interest in the properties of polar metals. Such materials are thought to be rare because free electrons screen out the long-range electrostatic forces that favor a polar structure with a dipole moment in every unit cell. In this work, we question whether long-range electrostatic forces are always the most important ingredient in driving polar distortions. We use crystal chemical models, in combination with first-principles Density Functional Theory calculations, to explore the mechanisms of inversion-symmetry breaking in LiOsO3 and both insulating and electron-doped ATiO3 perovskites, A = Ba, Sr, Ca. Although electrostatic forces do play a significant role in driving the polar instability of BaTiO3 (which is suppressed under electron doping), the polar phases of CaTiO3 and LiOsO3 emerge through a mechanism driven by local bonding preferences and this mechanism is 'resistant' to the presence of charge carriers. Hence, our results suggest that there is no fundamental incompatibility between metallicity and polar distortions. We use the insights gained from our calculations to suggest design principles for new polar metals and promising avenues for further research.

Transformational catalytic performance in rate and selectivity is obtainable through catalysts that change on the time scale of catalytic turnover frequency. In this work, dynamic catalysts are defined in the context and history of forced and passive dynamic chemical systems, with classification of unique catalyst behaviors based on temporally-relevant linear scaling parameters. The conditions leading to catalytic rate and selectivity enhancement are described as modifying the local electronic or steric environment of the active site to independently accelerate sequential elementary steps of an overall catalytic cycle. These concepts are related to physical systems and devices that stimulate a catalyst using light, vibrations, strain, and electronic manipulations including electrocatalysis, back-gating of catalyst surfaces, and introduction of surface electric fields via solid electrolytes and ferroelectrics. These catalytic stimuli are then compared for capability to improve catalysis across some of the most important chemical challenges for energy, materials, and sustainability. File list (2) download file view on ChemRxiv Perspective_Manuscript_ChemRxiv.pdf (3.88 MiB) download file view on ChemRxiv Perspective_Supporting_Information_ChemRxiv.pdf (149.75 KiB)

The stationary functional of the all-electron density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform free energy calculations and structural relaxations is implemented for the first time. Here, the first order error in the density leads to a much smaller, second order error in the free energy. The method is applied to several well known correlated materials; metallic SrVO3, Mott insulating FeO, and elemental Cerium, to show that it predicts the lattice constants with very high accuracy. In Cerium, we show that our method predicts the iso-structural transition between the α and γ phases, and resolve the long standing controversy in the driving mechanism of this transiton. PACS numbers: 71.27.+a,71.30.+h Prediction of the crystal structures of solids by large scale quantum mechanical simulations is one of the fundamental problems of condensed matter physics, and occupies a central place in materials design. The workhorse of the field is the Density Functional Theory (DFT) [1] at the level of Local Density Approximation (LDA) or Generalized Gradient Approximations (GGAs), which predict lattice constants of weakly correlated materials typically within ∼1% relative error [2].These errors of DFT in LDA/GGA implementations are an order of magnitude larger in the so called correlated materials: For example, the lattice constant of δ-Pu is underestimated by 11% [3] or non-magnetic FeO by 7% [4]. While GGAs and hybrid functionals can sometimes improve upon conventional LDA, these functionals many times degrade the agreement between predicted and experimentally determined bulk moduli and lattice constants, in particular in materials containing heavy elements. [2] To account for the correlation effects, more sophisticated many body methods have been developed. Among them, one of the most successful algorithms is the dynamical mean-field theory (DMFT) [5]. It replaces the problem of describing correlation effects in a periodic lattice by a strongly interacting impurity coupled to a self-consistent bath [6]. To become material specific, DMFT was soon developed into an electronic structure tool (LDA+DMFT) [7, 8], which achieved great success in numerous correlated materials (for a review see [9]). The LDA+DMFT method has mainly been used for the calculation of spectroscopic quantities, and only a few dozens of studies managed to compute energetics of correlated solids, and only a handful of them used exact solvers and charge self-consistency [18,19,24,25,28,29]. This is not only because of the very high computational cost, but also because previous implementations of LDA+DMFT were not stationary, and hence it was hard to achieve precision of free energies needed for structure optimization and study of phase transitions in solids.Here we implemented the LDA+DMFT functional, which delivers stationary free energies at finite temperatures. This stationarity is crucial for practical implementation and precision of computed energies, since the first order error in the density ρ (or the Green's fu...

Catalytic reactions on surfaces with forced oscillations in physical or electronic properties undergo controlled acceleration consistent with the selected parameters of frequency, amplitude, and external stimulus waveform.

A combination of dynamical mean field theory and density functional theory, as implemented in Phys. Rev. B 81, 195107 (2010), is applied to both the early and late transition metal oxides. For a fixed value of the local Coulomb repulsion, without fine tuning, we obtain the main features of these series, such as the metallic character of SrVO3 and the the insulating gaps of LaVO3, LaTiO3 and La2CO4 which are in good agreement with experiment. This study highlights the importance of local physics and high energy hybridization in the screening of the Hubbard interaction and how different low energy behaviors can emerge from the a unified treatment of the transition metal series. arXiv:1310.1158v2 [cond-mat.str-el]

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