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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.