Theoretical study of nitride short period superlattices.

Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

Calculated high-pressure structural properties, lattice dynamics and quasi particle band structures of perovskite fluorides KZnF3, CsCaF3 and BaLiF3.

A detailed study of the high-pressure structural properties, lattice dynamics and band structures of perovskite structured fluorides KZnF3, CsCaF3 and BaLiF3 has been carried out by means of density functional theory. The calculated structural properties including elastic constants and equation of state agree well with available experimental information. The phonon dispersion curves are in good agreement with available experimental inelastic neutron scattering data. The electronic structures of these fluorides have been calculated using the quasi particle self-consistent [Formula: see text] approximation. The [Formula: see text] calculations reveal that all the fluorides studied are wide band gap insulators, and the band gaps are significantly larger than those obtained by the standard local density approximation, thus emphasizing the importance of quasi particle corrections in perovskite fluorides.

Rare-earth pnictides and chalcogenides from first-principles.

This review tries to establish what is the current understanding of the rare-earth monopnictides and monochalcogenides from first principles. The rock salt structure is assumed for all the compounds in the calculations and wherever possible the electronic structure/properties of these compounds, as obtained from different ab initio methods, are compared and their relation to the experimental evidence is discussed. The established findings are summarised in a set of conclusions and provide outlook for future study and possible design of new materials.

Nuclear magnetic resonance metabolomic profiling of Day 3 and 5 embryo culture medium does not predict pregnancy outcome in good prognosis patients: a prospective cohort study on single transferred embryos.

STUDY QUESTION: Does the metabolomic profile, obtained with nuclear magnetic resonance (NMR), of spent culture media from human embryos correlate with reproductive potential in a cohort of good prognosis patients? SUMMARY ANSWER: In a large cohort of single transferred blastocysts from a homogeneous group of good prognosis patients, we find a high degree of individual variation in the metabolome that, however, has no relation to pregnancy outcome. WHAT IS KNOWN ALREADY: Differences among various specific metabolites have been linked to reproductive potential. Although results from retrospective near infrared (NIR) spectroscopy analyses of spent culture medias from transferred embryos were promising, randomized controlled trials were unable to demonstrate that NIR analysis improved pregnancy rates. Therefore, a more detailed investigation of the relation between embryo metabolism and reproductive potential is required. NMR is a powerful technique that provides detailed structural and dynamic information. STUDY DESIGN, SIZE, DURATION: A prospective cohort study was conducted at the Fertility Clinic, Aarhus University Hospital between February 2011 and July 2012. Infertile patients aged <38 years without endometriosis were offered participation and their embryos were included if greater than or equal to eight oocytes were retrieved. In total, 161 infertile patients were included in the cohort. PARTICIPANTS/MATERIALS, SETTING, METHODS: Spent culture media was collected on Days 3 and 5 after oocyte retrieval from 148 single transferred embryos. NMR spectra were obtained from 12 µl of spent media. Data were quantitatively analysed using multivariate analysis with respect to pregnancy outcome, defined as a live fetus by ultrasound in gestational Week 8, along with patient and treatment related variables such as embryo score, age, BMI, fertilization method and cause of infertility. MAIN RESULTS AND THE ROLE OF CHANCE: A total of 148 cycles were included in the analysis [embryo transfer cancelled (n = 12), no media collected (n = 1)]. Clinical pregnancy was confirmed in 47 patients (32%). We obtained high quality NMR spectra for 141 Day 3 and 137 Day 5 samples. Our spectra show a high degree of individual variation. Multivariate data analysis was performed on spectral data with several different pre-processing combinations, i.e. binning, alignment, normalization and scaling in the attempt to develop a valid prediction model. Different strategies of multivariate analysis showed, however, no correlation between the NMR profiles and pregnancy outcome, patient or treatment characteristics. No model could therefore be developed for prediction of pregnancy outcome. We conclude that within this group of good prognosis patients, large-scale metabolic variations between embryos detected with NMR have no apparent association with pregnancy outcome. LIMITATIONS, REASONS FOR CAUTION: Although this study is the largest we know of using NMR to investigate metabolomic profiles of single-transferred embryos, there may be differences that would be detected with a larger study. When analysing such a small sample volume, even small variations in the amount of media and dilution may introduce a large uncertainty in the results. WIDER IMPLICATIONS OF THE FINDINGS: Our study questions the usefulness of the entire metabolome for embryo selection, which should direct the search for viability markers in the culture media towards individual components. STUDY FUNDING/COMPETING INTERESTS: Funding was provided by Aarhus University, the Lippert Foundation, the Toyota Foundation, the Aase og Einar Danielsen foundation. Research at the Fertility Clinic, Aarhus Universtity Hospital is supported by an unrestricted grant from MSD and Ferring. The authors declare no competing interest. TRIAL REGISTRATION NUMBER: NCT01139268.

High-pressure study of binary thorium compounds from first principles theory and comparisons with experiment.

The high-pressure structural behaviour of a series of binary thorium compounds ThX (X = C, N, P, As, Sb, Bi, S, Se, Te) is studied using the all-electron full potential linear muffin-tin orbital (FP-LMTO) method within the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated equlibrium lattice parameters and bulk moduli, as well as the equations of state agree well with experimental results. New experiments are reported for ThBi and ThN. Calculations are performed for the ThX compounds in the NaCl- and CsCl-type crystal structures, and structural phase transitions from NaCl to CsCl are found in ThP, ThAs, ThSb and ThSe at pressures of 26.1, 22.1, 8.1 and 23.2 GPa, respectively, in excellent agreement with experimental results. ThC, ThN and ThS are found to be stable in the NaCl structure, and ThBi and ThTe in the CsCl structure, for pressures below 50 GPa. The electronic structures of the ThX compounds are studied using the quasiparticle self-consistent GW method (G: Green function, W: dynamically screened interaction).

Phase transitions in rare earth tellurides under pressure.

Using first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R(3+)) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R(2+)) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 â†’ B2 transition that occurs first, followed by the R(2+) â†’ R(3+) valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

Structural, vibrational, elastic and topological properties of PaN under pressure.

The electronic, structural, vibrational and elastic properties of PaN have been studied at both ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. The high pressure behavior of the acoustic phonon branch along the [1, 0, 0] and [1, 1, 0] directions and the C44 elastic constant are anomalous, which signals a structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition, which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, here it is found that the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). This constitutes an example of GGA and LDA leading to qualitatively different predictions, and therefore it is of great interest to examine experimentally whether this topological transition occurs.

Fermi surface properties of AB3 (A = Y, La; B = Pb, In, Tl) intermetallic compounds under pressure.

The electronic structures, densities of states, Fermi surfaces and elastic properties of AB3 (A =La, Y; B =Pb, In, Tl) compounds are studied under pressure using the full-potential linear augmented plane wave (FP-LAPW) method within the local density approximation for the exchange-correlation functional and including spin-orbit coupling. Fermi surface topology changes are found for all the isostructural AB3 compounds under compression (at V/V0 = 0.90 for LaPb3 (pressure = 8 GPa), at V/V0 = 0.98 for AIn3 (pressure = 1.5 GPa), at V/V0 = 0.80 for ATl3 (pressure in excess of 18 GPa)) apart from YPb3, although its electronic structure at zero pressure is very similar to that of LaPb3. For LaPb3 a softening of the C44 elastic constant under pressure (equivalent to 8 GPa) may be related to the appearance of a new hole pocket around the X point. From the calculated elastic properties and other mechanical properties, all the compounds investigated are found to be ductile in nature with elastic anisotropy. The states at the Fermi level (EF) are dominated by B p states with significant contributions from the A d states. For the La compounds, small hybridizations of the La f states also occur around EF.

Pressure-induced valence and structural changes in YbMn2Ge2-inelastic X-ray spectroscopy and theoretical investigations.

The pressure-induced valence change of Yb in YbMn(2)Ge(2) has been studied by high pressure inelastic X-ray emission and absorption spectroscopy in the partial fluorescence yield mode up to 30 GPa. The crystal structure of YbMn(2)Ge(2) has been investigated by high pressure powder X-ray diffraction experiments up to 40 GPa. The experimental investigations have been complemented by first principles density functional theoretical calculations using the generalized gradient approximation with an evolutionary algorithm for structural determination. The Yb valence and magnetic structures have been calculated using the self-interaction corrected local spin density approximation. The X-ray emission results indicate a sharp increase of Yb valence from v = 2.42(2) to v = 2.75(3) around 1.35 GPa, and Yb reaches a near trivalent state (v = 2.95(3)) around 30 GPa. Further, a new monoclinic P1 type high pressure phase is found above 35 GPa; this structure is characterized by the Mn layer of the ambient (I4/mmm) structure transforming into a double layer. The theoretical calculations yield an effective valence of v = 2.48 at ambient pressure in agreement with experiment, although the pure trivalent state is attained theoretically at significantly higher pressures (above 40 GPa).

Lattice instability and martensitic transformation in LaAg predicted from first-principles theory.

The electronic structure, elastic constants and lattice dynamics of the B(2) type intermetallic compound LaAg are studied by means of density functional theory calculations with the generalized gradient approximation for exchange and correlation. The calculated equilibrium properties and elastic constants agree well with available experimental data. From the ratio between the bulk and shear moduli, LaAg is found to be ductile, which is unusual for B(2) type intermetallics. The computed band structure shows a dominant contribution from La 5d states near the Fermi level. The phonon dispersion relations, calculated using density functional perturbation theory, are in good agreement with available inelastic neutron scattering data. Under pressure, the phonon dispersions develop imaginary frequencies, starting at around 2.3 GPa, in good accordance with the martensitic instability observed above 3.4 GPa. By structural optimization the high pressure phase is identified as orthorhombic B(19).

Superconductivity in SnO: a nonmagnetic analog to Fe-based superconductors?

We discovered that under pressure SnO with α-PbO structure, the same structure as in many Fe-based superconductors, e.g., ß-FeSe, undergoes a transition to a superconducting state for p≳6 GPa with a maximum Tc of 1.4 K at p=9.3 GPa. The pressure dependence of Tc reveals a domelike shape and superconductivity disappears for p≳16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.

Hubbard-U band-structure methods.

The last decade has seen a large increase in the number of electronic-structure calculations that involve adding a Hubbard term to the local-density approximation band-structure Hamiltonian. The Hubbard term is then determined either at the mean-field level or with sophisticated many-body techniques such as using dynamical mean-field theory. We review the physics underlying these approaches and discuss their strengths and weaknesses in terms of the larger issues of electronic structure that they involve. In particular, we argue that the common assumptions made to justify such calculations are inconsistent with what the calculations actually do. Although many of these calculations are often treated as essentially first-principles calculations, in fact, we argue that they should be viewed from an entirely different point of view, namely, as based on phenomenological many-body corrections to band-structure theory. Alternatively, it may also be considered that they are just based on a Hubbard model that is more complex than the simple one- or few-band models traditionally used in many-body theories of solids.

Lanthanide contraction and magnetism in the heavy rare earth elements.

The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.

First-principles calculations of PuO(2+/-x).

The electronic structure of PuO(2+/-x) was studied using first-principles quantum mechanics, realized with the self-interaction corrected local spin density method. In the stoichiometric PuO2 compound, Pu occurs in the Pu(IV) oxidation state, corresponding to a localized f4 shell. If oxygen is introduced onto the octahedral interstitial site, the nearby Pu atoms turn into Pu(V) (f3) by transferring electrons to the oxygen. Oxygen vacancies cause Pu(III) (f5) to form by taking up electrons released by oxygen. At T = 0, the PuO2 compound is stable with respect to free oxygen, but the delicate energy balance suggests the possible deterioration of the material during long-term storage.

5f electron localization-delocalization transition from UPd3 to UPt3.

The electronic structures of URh (3), UPd (3), UPt (3), and UAu (3) are calculated with the self-interaction corrected local-spin-density approximation. We find that only in URh (3) the f electrons are fully delocalized. UPt (3) has one f electron localized at each U site, while a localized f(2) configuration of the U ion is found for UPd (3). It is predicted that, upon application of a pressure of 25 GPa, UPd (3) will acquire the f(1) configuration and possibly exhibit heavy-fermion behavior. We find that UAu (3) is characterized by the same mixed localized-delocalized f-electron manifold as UPd (3).

Cu valency change induced by O doping in YBCPO.

An ab initio local spin density study of YBa2Cu3O6, YBa2Cu3O6.5, and YBa2Cu3O7 is presented. The method includes self-interaction corrections for the Cu d states, which enables a description of various valency configurations of both planar and chain Cu atoms. For YBa2Cu3O6 the antiferromagnetic and insulating state is described with planar (chain) Cu occurring in a divalent (trivalent) state. The evolution in the CuO2 plane from insulating to metallic behavior upon oxygenation is accomplished by the delocalization of the majority Cu d(x2-y2)-O2 p(x)-O3 p(y) band.