WIEN2k: An APW+lo program for calculating the properties of solids.

The WIEN2k program is based on the augmented plane wave plus local orbitals (APW+lo) method to solve the Kohn-Sham equations of density functional theory. The APW+lo method, which considers all electrons (core and valence) self-consistently in a full-potential treatment, is implemented very efficiently in WIEN2k, since various types of parallelization are available and many optimized numerical libraries can be used. Many properties can be calculated, ranging from the basic ones, such as the electronic band structure or the optimized atomic structure, to more specialized ones such as the nuclear magnetic resonance shielding tensor or the electric polarization. After a brief presentation of the APW+lo method, we review the usage, capabilities, and features of WIEN2k (version 19) in detail. The various options, properties, and available approximations for the exchange-correlation functional, as well as the external libraries or programs that can be used with WIEN2k, are mentioned. References to relevant applications and some examples are also given.

Quantum Crystallography: Current Developments and Future Perspectives.

Crystallography and quantum mechanics have always been tightly connected because reliable quantum mechanical models are needed to determine crystal structures. Due to this natural synergy, nowadays accurate distributions of electrons in space can be obtained from diffraction and scattering experiments. In the original definition of quantum crystallography (QCr) given by Massa, Karle and Huang, direct extraction of wavefunctions or density matrices from measured intensities of reflections or, conversely, ad hoc quantum mechanical calculations to enhance the accuracy of the crystallographic refinement are implicated. Nevertheless, many other active and emerging research areas involving quantum mechanics and scattering experiments are not covered by the original definition although they enable to observe and explain quantum phenomena as accurately and successfully as the original strategies. Therefore, we give an overview over current research that is related to a broader notion of QCr, and discuss options how QCr can evolve to become a complete and independent domain of natural sciences. The goal of this paper is to initiate discussions around QCr, but not to find a final definition of the field.

The interface between silicon and a high-k oxide.

The ability of the semiconductor industry to continue scaling microelectronic devices to ever smaller dimensions (a trend known as Moore's Law) is limited by quantum mechanical effects: as the thickness of conventional silicon dioxide (SiO(2)) gate insulators is reduced to just a few atomic layers, electrons can tunnel directly through the films. Continued device scaling will therefore probably require the replacement of the insulator with high-dielectric-constant (high-k) oxides, to increase its thickness, thus preventing tunnelling currents while retaining the electronic properties of an ultrathin SiO(2) film. Ultimately, such insulators will require an atomically defined interface with silicon without an interfacial SiO(2) layer for optimal performance. Following the first reports of epitaxial growth of AO and ABO(3) compounds on silicon, the formation of an atomically abrupt crystalline interface between strontium titanate and silicon was demonstrated. However, the atomic structure proposed for this interface is questionable because it requires silicon atoms that have coordinations rarely found elsewhere in nature. Here we describe first-principles calculations of the formation of the interface between silicon and strontium titanate and its atomic structure. Our study shows that atomic control of the interfacial structure by altering the chemical environment can dramatically improve the electronic properties of the interface to meet technological requirements. The interface structure and its chemistry may provide guidance for the selection process of other high-k gate oxides and for controlling their growth.

How close are the Slater and Becke-Roussel potentials in solids?

The Becke­Roussel (BR) potential [Phys. Rev. A 1989, 39, 3761] was proposed as an approximation to the Slater potential, which is the Coulomb potential generated by the exact exchange hole. In the present work, a detailed comparison between the Slater and BR potentials in solids is presented. It is shown that the two potentials usually lead to very similar results for the electronic structure; however, in a few cases, e.g., Si, Ge, or strongly correlated systems like NiO, the fundamental band gap or magnetic properties can differ markedly. Such differences should not be neglected when the computationally expensive Slater potential is replaced by the cheap semilocal BR potential in approximations to the exact-exchange Kohn­Sham potential, such as the one proposed by Becke and Johnson [J. Chem. Phys. 2006, 124, 221101].

DFT Study of the Role of Al3+ in the Fast Ion-Conductor Li7-3x Al3+x La3Zr2O12 Garnet.

We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7-3x Al3+x La3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.

Room-temperature spin-spiral multiferroicity in high-pressure cupric oxide.

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic readout operation. The great challenge is to create multiferroic materials that operate at room temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because it exhibits a significant polarization, that is, P~0.1 µC cm(-2), for a spin-spiral multiferroic. Unfortunately, CuO is only ferroelectric in a temperature range of 20 K, from 210 to 230 K. Here, by using a combination of density functional theory and Monte Carlo calculations, we establish that pressure-driven phase competition induces a giant stabilization of the multiferroic phase of CuO, which at 20-40 GPa becomes stable in a domain larger than 300 K, from 0 to T>300 K. Thus, under high pressure, CuO is predicted to be a room-temperature multiferroic with large polarization.

Theoretical investigation of the magnetic exchange interactions in copper(II) oxides under chemical and physical pressures.

It remains a challenge to understand the unconventional mechanisms that cause high-T(C) superconductivity in cuprate superconductors, high-T(C) multiferroicity in CuO, or low-dimensional magnetism in the spin-Peierls transition compounds such as CuGeO(3). A common feature of all these copper oxide compounds (containing Cu(2+) ions) is the presence of large magnetic superexchange interactions J. It is a general strategy to apply chemical and/or physical pressure in order to tune these exotic properties. Here we show theoretically, for the first time, the impact of physical pressure on J on CuO, for which we predict a strong enhancement of the low-dimensionality of the magnetic interactions and the spin-frustration at high-pressures. Such modifications are expected to strongly influence the multiferroic properties of CuO. We finally demonstrate that PBE0 hybrid DFT calculations provide reliable J values for a wide range of copper(II) oxides compounds, i.e. CuGeO(3), BaCu(2)Si(2)O(7), BaCu(2)Ge(2)O(7), and La(2)CuO(4).

Ligand dynamics on the surface of zirconium oxo clusters.

The dynamics of carboxylate ligands on the surface of zirconium oxo clusters was investigated in two case studies. Zr4O2(methacrylate)12 was investigated by one- and two dimensional NMR spectra both in the solid state and in solution. In solution, the cluster is C2h symmetric; stepwise intramolecular exchange of the four non-equivalent ligands was observed when the temperature was raised from -80 degrees C to -50 degrees C. The individual exchange processes were assigned to different ligand positions. Ab initio molecular dynamics simulations were performed for Zr6(OH)4O4(formate)12 to study the trajectory for the rearrangement of three chelating ligands into bridging positions, i.e. the conversion of the C3-symmetric into an Oh-symmetric cluster. The observation of a dip in the energy barrier along the reaction coordinate was related to the intermediate formation of hydrogen bonds between the moving oxygen atom of the rearranging ligand and a micro3-OH group of the cluster. Thus, the motion of the ligand requires a concerted motion in three dimensions.

Single-layer model of the hexagonal boron nitride nanomesh on the Rh(111) surface.

An alternative model of the hexagonal boron nitride (h-BN) on nanomesh on the Rh(111) surface is presented. It explains the observed ultraviolet photoelectron spectroscopy spectra and reproduces experimental STM images introducing, instead of two, only one strongly corrugated layer of h-BN covering the whole Rh surface. In order to optimize the geometry of the BN layer we calculate the forces by density functional theory and analyze the interactions in the system. The final geometry is a result of a competition between BN-metal attraction or repulsion and elastic properties of the isolated h-BN layer. The calculated bonding energy is around 0.33 eV per BN molecule with a corrugation close to 0.55 A.

Structural and electronic properties of the interface between the high-k Oxide LaAlO3 and Si(001).

The structural and electronic properties of the LaAlO(3)/Si(001) interface are determined using state-of-the-art electronic structure calculations. The atomic structure differs from previous proposals, but is reminiscent of La adsorption structures on silicon. A phase diagram of the interface stability is calculated as a function of oxygen and Al chemical potentials. We find that an electronically saturated interface is obtained only if Al atoms substitute some of the interfacial Si atoms. These findings raise serious doubts whether LaAlO3 can be used as an epitaxial gate dielectric.