Calculation of valence electron momentum densities using the projector augmented-wave method

We present valence electron Compton profiles calculated within the density-functional theory using the all-electron full-potential projector augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al) and hydrogen-bonded ((H_2O)_2) systems agree well with experiments and computational results obtained with other band-structure and basis set schemes. The PAW basis set describes the high-momentum Fourier components of the valence wave functions accurately when compared with other basis set schemes and previous all-electron calculations.Comment: Submitted to Journal of Physics and Chemistry of Solids on September 17 2004. Revised version submitted on December 13 200

Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study

We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2×1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at −0.6eV. Using DFT, we identify this structure as a surface resonance of the (2×1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states

Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCr2_2S4_4 from first principles

We have studied the dielectric properties of the ferromagnetic spinel CdCr$_2$S$_4$ from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr$_2$S$_4$ is not due to the softening of a polar mode. We suggest further experiments to clarify this point

An anomalous alloy: Y_x Si_{1-x}

We study via density functional-based molecular dynamics the structural and dynamical properties of the rare earth silicon amorphous alloy Y_xSi_{1-x} for x=0.093 and x=0.156. The Si network forms cavities in which a Y^{3+} cation is entrapped. Its electrons are transferred to the Si network and are located in the dangling bonds of the Si atoms that line the Y cavities. This leads to the presence of low coordinated Si atoms that can be described as monovalent or divalent anions. For x=0.156, the cavities touch each other and share Si atoms that have two dangling bonds. The vibrational spectrum is similar to that of amorphous Si. However, doping induces a shoulder at 70 cm^{-1} and a pronounced peak at 180 cm^{-1} due to low coordinated Si.Comment: 4 pages, 4 figure

Mediation of the association between vascular risk factors and depressive symptoms by c-reactive protein: Longitudinal evidence from the UK Biobank

People with vascular risk factors (VRFs) are at higher risk for depressive symptoms. Given recent findings implicating low-grade systemic inflammation in both vascular and mental health, this study examined the extent to which the VRF–depressive symptom association might be mediated by low-grade systemic inflammation. To this end, we analysed longitudinal data of 9,034 participants from the UK Biobank (mean age = 56.54 years), who took part in three consecutive assessments over the course of about 8 years. Cumulative VRF burden at baseline was defined as the presence of 5 VRFs (hypertension, obesity, hypercholesterolemia, diabetes, and smoking). Low-grade systemic inflammation was assessed using serum-derived C-reactive protein (CRP) and depressive symptoms were measured using the Patient Health Questionnaire-9 (PHQ-9). We performed mediation models using longitudinal data and a path analytic framework, while controlling for age, gender, racial-ethnic background, socioeconomic status, and baseline mood. VRFs at baseline showed a small association with higher depressive symptoms at follow-up (total effect = 0.014, 95% CI [0.007; 0.021]). CRP mediated this association (indirect effect = 0.003, 95% CI [0.001; 0.005]) and accounted for 20.10% of the total effect of VRF burden on depressive symptoms. Exploratory analyses taking a symptom-based approach revealed that mediating pathways pertained to specific depressive symptoms: tiredness and changes in appetite. These results suggest that the small association between VRF burden and depressive symptoms may be partly explained by the inflammation-promoting effects of VRFs, which might promote a specific symptom-profile of depression

Stability of Ge-related point defects and complexes in Ge-doped SiO_2

We analyze Ge-related defects in Ge-doped SiO_2 using first-principles density functional techniques. Ge is incorporated at the level of ~ 1 mol % and above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency, with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and O vacancies binding strongly to Ge impurities. All the centers considered exhibit potentially EPR-active states, candidates for the identification of the Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure

{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure

A mixed ultrasoft/normconserved pseudopotential scheme

A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the normconservation is released for only one or a few angular channels, is presented. Within this scheme some difficulties of the truly ultrasoft pseudopotentials are overcome without sacrificing the pseudopotential softness. i) Ghost states are easily avoided without including semicore shells. ii) The ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The number of nonlocal operators is reduced. The scheme will be most useful for transition metals, and the feasibility and accuracy of the scheme is demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure

The interface between silicon and a high-k oxide

The ability to follow Moore's Law has been the basis of the tremendous success of the semiconductor industry in the past decades. To date, the greatest challenge for device scaling is the required replacement of silicon dioxide-based gate oxides by high-k oxides in transistors. Around 2010 high-k oxides are required to have an atomically defined interface with silicon without any interfacial SiO2 layer. The first clean interface between silicon and a high-K oxide has been demonstrated by McKee et al. Nevertheless, the interfacial structure is still under debate. Here we report on first-principles calculations of the formation of the interface between silicon and SrTiO3 and its atomic structure. Based on insights into how the chemical environment affects the interface, a way to engineer seemingly intangible electrical properties to meet technological requirements is outlined. The interface structure and its chemistry provide guidance for the selection process of other high-k gate oxides and for controlling their growth. Our study also shows that atomic control of the interfacial structure can dramatically improve the electronic properties of the interface. The interface presented here serves as a model for a variety of other interfaces between high-k oxides and silicon.Comment: 10 pages, 2 figures (one color

First-principles study of spontaneous polarization in multiferroic BiFeO3_3

The ground-state structural and electronic properties of ferroelectric BiFeO$_3$ are calculated using density functional theory within the local spin-density approximation and the LSDA+U method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of 90-100 $\mu$C/cm$^2$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables