3,049 research outputs found
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS 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 CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
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 VRFâdepressive 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
{\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
Anisotropy of the Mobility of Pentacene from Frustration
The bandstructure of pentacene is calculated using first-principles density
functional theory. A large anisotropy of the hole and electron effective masses
within the molecular planes is found. The band dispersion of the HOMO and the
LUMO is analyzed with the help of a tight-binding fit. The anisotropy is shown
to be intimately related to the herringbone structure.Comment: Accepted for publication in Synthetic Metal
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
Neutral-ionic phase transition : a thorough ab-initio study of TTF-CA
The prototype compound for the neutral-ionic phase transition, namely TTF-CA,
is theoretically investigated by first-principles density functional theory
calculations. The study is based on three neutron diffraction structures
collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)).
By means of a topological analysis of the total charge densities, we provide a
very precise picture of intra and inter-chain interactions. Moreover, our
calculations reveal that the thermal lattice contraction reduces the indirect
band gap of this organic semi-conductor in the neutral phase, and nearly closes
it in the vicinity of the transition temperature. A possible mechanism of the
neutral-ionic phase transition is discussed. The charge transfer from TTF to CA
is also derived by using three different technics.Comment: 11 pages, 9 figures, 7 table
First-principles study of spontaneous polarization in multiferroic BiFeO
The ground-state structural and electronic properties of ferroelectric
BiFeO 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 , 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 C/cm 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
Structural and magnetic properties of Fe/ZnSe(001) interfaces
We have performed first principles electronic structure calculations to
investigate the structural and magnetic properties of Fe/ZnSe(001) interfaces.
Calculations involving full geometry optimizations have been carried out for a
broad range of thickness of Fe layers(0.5 monolayer to 10 monolayers) on top of
a ZnSe(001) substrate. Both Zn and Se terminated interfaces have been explored.
Total energy calculations show that Se segregates at the surface which is in
agreement with recent experiments.
For both Zn and Se terminations, the interface Fe magnetic moments are higher
than the bulk bcc Fe moment.
We have also investigated the effect of adding Fe atoms on top of a
reconstructed ZnSe surface to explore the role of reconstruction of
semiconductor surfaces in determining properties of metal-semiconductor
interfaces. Fe breaks the Se dimer bond formed for a Se-rich (2x1)
reconstructed surface. Finally, we looked at the reverse growth i.e. growth of
Zn and Se atoms on a bcc Fe(001) substrate to investigate the properties of the
second interface of a magnetotunnel junction. The results are in good agreement
with the theoretical and experimental results, wherever available.Comment: 7 pages, 8 figures, accepted for publication in PR
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