Negative capacitance in multidomain ferroelectric superlattices.

The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors. However, experimental demonstrations of this phenomenon remain contentious. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation, but the effect of domains on negative capacitance has received little attention. Here we report negative capacitance in a model system of multidomain ferroelectric-dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.

Emergent chirality in the electric polarization texture of titanate superlattices.

Chirality is a geometrical property by which an object is not superimposable onto its mirror image, thereby imparting a handedness. Chirality determines many important properties in nature-from the strength of the weak interactions according to the electroweak theory in particle physics to the binding of enzymes with naturally occurring amino acids or sugars, reactions that are fundamental for life. In condensed matter physics, the prediction of topologically protected magnetic skyrmions and related spin textures in chiral magnets has stimulated significant research. If the magnetic dipoles were replaced by their electrical counterparts, then electrically controllable chiral devices could be designed. Complex oxide BaTiO3/SrTiO3 nanocomposites and PbTiO3/SrTiO3 superlattices are perfect candidates, since "polar vortices," in which a continuous rotation of ferroelectric polarization spontaneously forms, have been recently discovered. Using resonant soft X-ray diffraction, we report the observation of a strong circular dichroism from the interaction between circularly polarized light and the chiral electric polarization texture that emerges in PbTiO3/SrTiO3 superlattices. This hallmark of chirality is explained by a helical rotation of electric polarization that second-principles simulations predict to reside within complex 3D polarization textures comprising ordered topological line defects. The handedness of the texture can be topologically characterized by the sign of the helicity number of the chiral line defects. This coupling between the optical and novel polar properties could be exploited to encode chiral signatures into photon or electron beams for information processing.

Ferroelectric transitions at ferroelectric domain walls found from first principles.

We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO(3). At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems.

Ab Initio indications for giant magnetoelectric effects driven by structural softness.

We show that inducing structural softness in regular magnetoelectric (ME) multiferroics-i.e., tuning the materials to make their structure strongly reactive to applied fields-makes it possible to obtain very large ME effects. We present illustrative first-principles results for BiFeO(3) thin films.

Chemical bonding and electronic and magnetic structure in LaOFeAs.

Periodic density functional calculations using hybrid exchange-correlation functionals predict that LaOFeAs is a strongly frustrated antiferromagnetic insulator with important covalence between Fe and As, with evident similarities with cuprates.

Magnetoelectric response of multiferroic BiFeO3 and related materials from first-principles calculations.

We present a first-principles scheme for computing the magnetoelectric response of multiferroics. We apply our method to BiFeO3 (BFO) and related compounds in which Fe is substituted by other magnetic species. We show that under certain relevant conditions--i.e., in the absence of incommensurate spin modulation, as in BFO thin films and some BFO-based solid solutions--these materials display a large linear magnetoelectric response. Our calculations reveal the atomistic origin of the coupling and allow us to identify the most promising strategies to enhance it.

Periodic density functional theory study of spin crossover in the cesium iron hexacyanochromate prussian blue analog.

This paper presents a detailed theoretical analysis of the electronic structure of the CsFe[Cr(CN)(6)] prussian blue analog with emphasis on the structural origin of the experimentally observed spin crossover transition in this material. Periodic density functional calculations using generalized gradient approximation (GGA)+U and nonlocal hybrid exchange-correlation potentials show that, for the experimental low temperature crystal structure, the t(2g)(6) e(g)(0) low spin configuration of Fe(II) is the most stable and Cr(III) (S = 3/2, t(2g)(3) e(g)(0)) remains the same in all cases. This is also found to be the case for the low spin GGA+U fully relaxed structure with the optimized unit cell. A completely different situation emerges when calculations are carried out using the experimental high temperature structure. Here, GGA+U and hybrid density functional theory calculations consistently predict that the t(2g)(4) e(g)(2) Fe(II) high spin configuration is the ground state. However, the two spin configurations appear to be nearly degenerate when calculations are carried out for the geometries arising from a GGA+U full relaxation of the atomic structure carried out at experimental high temperature lattice constant. A detailed analysis of the energy difference between the two spin configurations as a function of the lattice constant strongly suggests that the observed spin crossover transition has a structural origin with non-negligible entropic contributions of the high spin state.

Band gap variation in Prussian Blue via cation-induced structural distortion.

The charge-transfer band gap of the iron cyanide framework material Prussian Blue and its dependence on the type and location of the charge-compensating interstitial cations (K(+), Rb(+), Cs(+)) are investigated via periodic density functional (DF) calculations. The calculated variation in the band gap magnitude with respect to cation type confirms recent experimental results on cation-induced spectral shifts. The role of both the cation interaction with the framework and the cation-induced lattice expansion are examined with respect to their influence on the band gap. The gap magnitude is related to the cation type but is found to be more strongly affected by cation-induced lattice distortion as the cation passes through the material. Our results support the possibility of engineering the electronic structure of Prussian Blue type materials through guest-induced host-framework distortion.

Interaction of SiO2 with single-walled carbon nanotubes.

The effects of coating of a single-walled carbon nanotube (SWNT) with a nonbonded layer of silica are investigated via model system employing fully coordinated silica clusters. The geometric and electronic structures of the SWNT@SiO(2) composite system are calculated using periodic density functional (DF) calculations for a range of confining silica coatings. We show that silica can provide a protective bound coating to a single walled nanotube, which, importantly, only weakly perturbs the underlying properties of both components. Detailed analysis of the charge redistribution and changes in electronic structure upon coating the SWNT are performed to support this conclusion. Furthermore, as allowed by our versatile model system, the energetics of rotating a silica "bearing" around a nanotube "spindle" is also calculated to indicate the possibilities for SWNT@SiO(2)-based nanomechanical devices.

First-principles model potentials for lattice-dynamical studies: general methodology and example of application to ferroic perovskite oxides.

We present a scheme to construct model potentials, with parameters computed from first principles, for large-scale lattice-dynamical simulations of materials. We mimic the traditional solid-state approach to the investigation of vibrational spectra, i.e., we start from a suitably chosen reference configuration of the compound and describe its energy as a function of arbitrary atomic distortions by means of a Taylor series. Such a form of the potential-energy surface is general, trivial to formulate for any material, and physically transparent. Further, such models involve clear-cut approximations, their precision can be improved in a systematic fashion, and their simplicity allows for convenient and practical strategies to compute/fit the potential parameters. We illustrate our scheme with two challenging cases in which the model potential is strongly anharmonic, namely, the ferroic perovskite oxides PbTiO3 and SrTiO3. Studying these compounds allows us to better describe the connection between the so-called effective-Hamiltonian method and ours (which may be seen as an extension of the former), and to show the physical insight and predictive power provided by our approach-e.g., we present new results regarding the factors controlling phase-transition temperatures, novel phase transitions under elastic constraints, an improved treatment of thermal expansion, etc.

First principles calculations on the influence of water-filled cavities on the electronic structure of Prussian Blue.

Prussian Blue is a paradigmatic mixed valence material and a parent compound to a broad family of electronically, optically, and magnetically active materials. Its exact composition varies greatly depending on the preparation route, leading to large variations in its electronic properties. The influence of water molecules on the structural and electronic properties of Prussian Blue were studied using state-of-the-art first principles calculations. Water-filled cavities were found to have a profound influence on the band gap and density of states of this material while simultaneously leaving many of its properties largely unchanged. The resulting model of an almost independent superimposition of dehydrated material and hydrated sites is briefly discussed.

On the prediction of the crystal and electronic structure of mixed-valence materials by periodic density functional calculations: the case of Prussian Blue.

The consistency of periodic density functional approaches to properly describe the crystal and electronic structure of mixed-valence materials is investigated by taking Prussian Blue as prototypical example. Hybrid B3LYP, GGA, and, GGA+U exchange-correlation potentials have been explored. Localized Gaussian-type orbitals or plane waves have been chosen to expand the valence electron density, and the effect of the core electrons on the electronic structure was accounted for either (i) explicitly by including all electrons in the calculations, (ii) by making use of ultrasoft pseudopotentials, or (iii) by the use of the projected augmented wave method. Comparison to available experimental data shows that all-electron calculations within the hybrid exchange-correlation potential can be taken as appropriate benchmarks. It is also concluded that a proper description of the complex magnetic ground state of Prussian Blue can be reached by using a plane-wave basis set and nonhybrid density functional potentials but only if the electronically distinct iron centers in the material are treated in an independent manner. Physical reasons for such rather unexpected results are given and implications for the description of mixed-valence materials by means of density functional approaches are discussed.

Development of realistic models for Double Metal Cyanide catalyst active sites.

Realistic molecular models of one and two-centre catalytic active sites originating from the cleavage of a precursor material known to give rise to an active double metal cyanide catalyst are described. Via periodic density functional calculations the structure of the proposed catalytic sites are shown to be dependent on electrostatic and structural relaxation processes occurring at the surfaces of the precursor material. It is shown how these effects may be adequately captured by small molecular models of the active sites. The general methodology proposed should provide a computationally efficient basis for detailed future studies into catalytic reactions over double metal cyanide materials.

From cluster calculations to molecular materials: a mixed pseudopotential approach to modeling mixed-valence systems.

In this paper we present a technique for finding an appropriate parameterization of ultrasoft pseudopotentials for modeling mixed-valence materials. For the example of hexacyanometallate molecular building blocks, we show how ionic cluster calculations can be used to determine a set of parameters for the metal centers. Pseudopotentials chosen in such a way are then shown to be suitable for periodic calculations of the corresponding mixed-valence materials (e.g., Prussian Blue).