We present an implementation of an interface between the full-potential linearized augmented plane wave package Wien2k and the wannier90 code for the construction of maximally localized Wannier functions. The FORTRAN code and a documentation is made available and results are discussed for SrVO 3 , Sr 2 IrO 4 (including spin-orbit coupling), LaFeAsO, and FeSb 2 .

We provide a MATLAB package p1afem for an adaptive P1-finite element method (AFEM). This includes functions for the assembly of the data, different error estimators, and an indicator-based adaptive meshrefining algorithm. Throughout, the focus is on an efficient realization by use of MATLAB built-in functions and vectorization. Numerical experiments underline the efficiency of the code which is observed to be of almost linear complexity with respect to the runtime. Although the scope of this paper is on AFEM, the general ideas can be understood as a guideline for writing efficient MATLAB code.

Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu4Sn6 employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the a − a plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin.Correlated materials with gapped or pseudo-gapped ground states continue to be of great interest. The gap in the electronic density of states (DOS) either opens gradually with decreasing temperature, as the pseudogap of high-temperature superconductors [1], or emerges at a continuous or first order phase transition [2][3][4]. In heavy fermion compounds [5] -systems in which f and conduction electrons strongly interact -a narrow hybridization gap is known to emerge gradually [6][7][8][9]. Generically, the Fermi energy is situated in one of the hybridized bands and a metallic heavy fermion ground state arises. Only for special cases the Fermi energy lies within the gap and the ground state is Kondo insulating. Metallic heavy fermion systems have been intensively investigated over the past decades and are now, at least away from quantum criticality [10], well understood [11] within the framework of Landau Fermi liquid theory. Hence, a very few parameters, most notably the effective mass, allow us to describe thermodynamic and transport properties at the lowest temperatures. In comparison, the physics of Kondo insulators has proven to be much less tractable. This is at least in part due to the fact that the gapped ground state inhibits a characterization via the above properties. Many experimental efforts have therefore focussed on the determination of the gap width from temperature dependencies, which has frequently led to conflicting results, in particular for anisotropic Kondo insulators such as CeNiSn [12]. Here the strongly anisotropic transport and magnetic properties have been interpreted phenomenologically on the basis of a V-shaped DOS [13] or by invoking a hybridization gap with nodes [14][15][16] or extrinsic effects such as impurities, off stoichiometry or strain [17,18]. To advance the field it appears mandatory to model a number of carefully chosen materials ab initio, taking all essential ingredients into account.Here we investigate a new material, CeRu 4 Sn 6 , which due to its tetragonal crystal structure is simpler than the previously studied orthorhombic materials. In a combined experimental and theoretical effort we provide direct spectroscopic evidence for the development of an anisotropic pseudogap: While weak metallicity prevails in the optical conductivity along the c axis, insulatorlike behavior without a Drude peak is observed in the a − a plane. We trace this back to a correlated band structure whic...

Using the merger of local density approximation and dynamical mean field theory, we show how electronic correlations increase the thermopower of Na0.7CoO2 by 200%. The newly revealed mechanism is an asymmetric shift of (quasi) electrons and holes away from the Fermi level, concurrent with an asymmetry of the respective (group) velocities. Exploiting this effect in bandstructure and correlation engineering may lead to a substantial increase of the thermoelectric figure of merit.PACS numbers: 71.27.+a,71.1.FdClimate change and the prospective oil peak necessitate the discovery of new green energy sources. One possibility in this context is to convert hithero unused excess heat into electrical energy using thermoelectrics [1]. For a real breakthrough and a widespread application of this technology however, thermoelectric figures of merit ZT 3 are needed [2,3]. Recently, some advance was brought about through phonon [4] and bandstructure engineering [5] so that ZT 1 could be achieved. Pushing ZT much higher requires however a thermoelectric leap, which is most likely brought about by discovering a new class of materials -at least in the case of (high-T) superconductivity scientific progress went along this line with the discovery of novel correlated materials. At commercially available ZT 1, thermoelectrics are already applied in niche markets such as radioisotope power systems for satellites [6] where reliability is more important than efficiency; and the car industry is taking encouraging first steps to use thermoelectrical generators [7]. The materials presently used in industry are mainly Te-based and have been investigated most thoroughly [8] with respect to optimizing their thermoelectric properties, including the aforementioned phonon [4] and bandstructure engineering [5].The good thermoelectric properties of Te-based materials are related to their physics of slightly doped semiconductors with a low effective mass and high mobility of the charge carriers [2,8]. A possible improvement are compounds with a pudding-mold type of bandstructure as those of Na x CoO 2 [9] and LiRh 2 O 4 [10] -two oxides with remarkably good thermoelectric properties [11][12][13][14].Another route to good thermoelectric properties is through electronic correlations [15,16] which can renormalize bands most substantially, so that narrow quasiparticle bands with a high density of states emerge. The renormalization factor is most dramatic in heavy Fermion systems which have particularly narrow Kondo resonances, modeled e.g. by a periodic Anderson model [17]. While one might expect the narrowest resonances to be most suitable for thermoelectrics, there is a trade-off since increasing the temperature T above the Kondo temperature (essentially the resonance width) will eventually FIG. 1: (Color online) Scheme of a NaxCoO2 thermoelectric module and its crystal structure. The charge carriers are confined to the hexagonal layers of Co atoms, which are inequivalent because of the (disorderly distributed) Na ions above and below. If we apply a h...

By means of a Wannier projection within the framework of density functional theory, we are able to identify the modified c-axis hopping and the energy mismatch between the cation bands as the main source of the t2g splitting around the Γ point for oxide heterostructures, excluding previously proposed mechanisms such as Jahn-Teller distortions or electric field asymmetries. Interfacing LaAlO3, LaVO3, SrVO3 and SrNbO3 with SrTiO3 we show how to tune this orbital splitting, designing heterostructures with more dxy electrons at the interface. Such an "orbital engineering" is the key for controlling the physical properties at the interface of oxide heterostructures. 79.60.Jv When an atom is part of a periodic arrangement, such as a solid, the spherical symmetry of its potential is lowered with respect to the case of a free atom. Typical is the example of transition metal ions surrounded by oxygen octahedra in cubic perovskites. Consequently, e.g., the five d orbitals split: In bulk SrTiO 3 one has three t 2g orbitals (d xy , d xz and d yz ) and two e g ones (d x 2 −y 2 and d 3z 2 −r 2 ). The physics of transition metal oxides is deeply influenced by the further (finer) splittings of the t 2g or e g orbitals, for instance when distortions of the octahedra are energetically favored [1]. Both from the point of view of basic materials research and from that of technological development and device fabrication, it would be fascinating if one could control such deviations from the perfect cubic perovskite structure by means of some external adjustable parameters. Yet, this is something not easy to do in a flexible and controlled way in bulk materials.The recent breakthrough in growing oxide heterostructures, such as LaAlO 3 grown on SrTiO 3 , offers a new possibility to tune the orbital degrees of freedom. The interface breaks the translational, and hence the cubic, symmetry. As a consequence, the three t 2g orbitals of the Ti atoms close to the interface split [2]. Evidences for similar effects come also from experiments on a bare SrTiO 3 (001) surface upon cleavage [3,4]. The physical properties of the entire heterostructure, such as superconductivity [5], phase separation [6], magnetism [7-9], etc., depend on the t 2g electrons at the interface [10, 11]. These effects have been revealed in a series of stunning experiments [5,12,13], which also demonstrated that interfaces in layered oxide heterostructures can not only be seen as a way of tuning orbital splittings for specific desired purposes, but also for engendering new physical effects that are absent in the constituent bulk materials.A sine qua non condition for a successful "orbital engineering" in these layered systems is therefore the understanding of the mechanism behind the above-mentioned lifting of the t 2g degeneracy. Several density functional theory (DFT) calculations have been applied to LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures [14][15][16][17][18][19][20][21]. They clearly show that the d xy band with Ti character close to the interface is no longer...

The Kondo system CeRu4Sn6 shows a strong anisotropy in its electric, optic and magnetic properties. We employ density functional theory plus dynamical mean field theory and show that the predominant Ce-f state has total angular moment J = 5/2 and z-component mJ = ±1/2 in agreement with recent X-ray absorption experiments. Even though CeRu4Sn6 has the direct gap of a Kondo insulator through most of the Brillouin zone it remains weakly metallic. This is because of (i) a band crossing in the z-direction and (ii) a negative indirect gap.PACS numbers: 71.27.+a, 31.15.V-

For the thermoelectric properties of NaxCoO2, we analyze the effect of local Coulomb interaction and (disordered) potential differences for Co-sites with adjacent Na-ion or vacancy. The disorder potential alone increases the resistivity and reduces the thermopower, while the Coulomb interaction alone leads only to minor changes compared to the one-particle picture of the local density approximation. Only combined, these two terms give rise to a substantial increase of the thermopower: the number of (quasi-)electrons around the Fermi level is much more suppressed than that of the (quasi-)holes. Hence, there is a particle-hole imbalance acting in the same direction as a similar imbalance for the group velocities. Together, this interplay results in a large positive thermopower. Introducing a thermoelectric spectral density, we located the energies and momenta regions most relevant for the thermopower and changes thereof.

We develop a computational approach for calculating the optical conductivity in the augmented plane wave basis set of Wien2K and apply it for thoroughly comparing the full dipole matrix element calculation and the Peierls approximation. The results for SrVO3 and V2O3 show that the Peierls approximation, which is commonly used in model calculations, works well for optical transitions between the d orbitals. In a typical transition metal oxide, these transitions are solely responsible for the optical conductivity at low frequencies. The Peierls approximation does not work, on the other hand, for optical transitions between p-and d-orbitals which usually became important at frequencies of a few eVs.