Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2).

Synthetic bulk and natural pyrite from the hydrothermal mine in Schönbrunn (Saxony, Germany) are confirmed to be stoichiometric FeS2 compounds and stable (for thermoelectric applications) up to ∼600 K by combined thermal, chemical, spectroscopic and X-ray diffraction analyses. Natural pyrite with a small amount (<0.6 wt%) of well-defined transition metal carbonates revealed characteristics of a nondegenerate semiconductor and is suitable as a model system for the investigation of thermoelectric performance. In the temperature range 50-600 K both natural and synthetic high quality bulk FeS2 samples show electrical resistivity and Seebeck coefficients varying within 220-5 × 10-3 Ω m and 4 - (-450) µV K-1, respectively. The large thermal conductivity (∼40 W m-1 K-1 at 300 K) is exclusively due to phononic contribution, showing a well pronounced maximum centered at ∼75 K for natural pyrite (grain size ≤5 mm). It becomes almost completely suppressed in the sintered bulk samples due to the increase of point defect concentration and additional scattering on the grain boundaries (grain size ≤100 µm). The thermoelectric performance of pure pyrite with ZT ∼ 10-6 at 600 K is indeed by a factor of ∼1000 worse than those reported earlier for some minerals and synthetic samples.

Fermi orbital self-interaction corrected electronic structure of molecules beyond local density approximation.

The correction of the self-interaction error that is inherent to all standard density functional theory calculations is an object of increasing interest. In this article, we apply the very recently developed Fermi-orbital based approach for the self-interaction correction [M. R. Pederson et al., J. Chem. Phys. 140, 121103 (2014) and M. R. Pederson, J. Chem. Phys. 142, 064112 (2015)] to a set of different molecular systems. Our study covers systems ranging from simple diatomic to large organic molecules. We focus our analysis on the direct estimation of the ionization potential from orbital eigenvalues. Further, we show that the Fermi orbital positions in structurally similar molecules appear to be transferable.

Pressure-dependent structural and electronic properties of quasi-one-dimensional (TMTTF)2PF6.

We have performed detailed x-ray investigations of the quasi-one-dimensional organic conductor (TMTTF)(2)PF(6) at room temperature and hydrostatic pressures up to 27 kbar. Based on the pressure-dependent crystal structure, the electronic band structure was calculated by density functional theory (DFT). Our systematic study provides important information on the coupling among the organic molecules but also to the anions. We discuss the consequences for the electronic properties and compare them with optical investigations under pressure. The increasing plasma frequency observed perpendicular to the stacks corresponds to a widening of the bands for the b-direction. Around 20 kbar a dimensional crossover occurs from a one-dimensional Mott insulator to a two-dimensional metal.

Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine.

The effects of electronic states and air exposure on the spectroscopic properties of manganese phthalocyanine (MnPc) have been examined. The observed features of the Q-band in the absorption spectra can be explained by intrinsic electronic properties of MnPc, i.e., the formation of singly charged molecules by charge transfer excitations. However, the reaction of MnPc with atmospheric molecular oxygen leads to deviations in peak intensities but does not change the fundamental characteristics of the spectra. Nevertheless, the reaction with oxygen changes the spin state from S = 3/2 to S = 1/2. X-ray diffraction measurements also indicate a slow diffusion process of the oxygen into the MnPc crystal. We discuss both influences to explain the behaviour of MnPc in various spectroscopic methods (EELS, ellipsometry, PES). Furthermore, we support the experimental investigations by detailed ab-initio calculations of spectroscopic properties using methods of the density functional theory framework.

Visualizing the spin of individual cobalt-phthalocyanine molecules.

Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead exchange interaction.

Electron spin precession upon reflecting from ferromagnetic surfaces.

Electrons with the polarization vector perpendicular to the magnetization of Fe, Co, and Ni films are spin analyzed after reflecting from the ferromagnet. At low primary electron energies a strong spin motion is found, namely, a precession of the polarization around the magnetization and a change of the angle between the polarization and the magnetization. This observation can be accounted for by the existence of spin-dependent gaps in the electronic band structure of the ferromagnets.

Dynamical identification of a threshold instability in Si-doped heterofullerenes.

We rationalize the origins of a threshold instability and the mechanism of finite temperature fragmentation in highly Si-doped C(60-m)Si(m) heterofullerenes via a first-principles approach. Cage disruption is driven by enhanced fluctuations of the most internal Si atoms. These are located within fully segregated Si regions neighboring the C-populated part of the cage. The predominance of inner Si atoms over those involved in Si-C bonds marks the transition from thermally stable to unstable C(60-m)Si(m) systems at m = 20.

[Personal experience with treatment of acetabular fractures]. / Nase zkusenosti s lécením zlomenin acetabula.

PURPOSE OF THE STUDY: The article presents a retrospective evaluation of a group of 119 patients treated for acetabular fractures between 1996 and 2002. MATERIAL: In the monitored period 119 patients, (89 men, 30 women) average age 35 years, were hospitalized with an acetabular fracture. Surgically treated were 91 patients (70 men, 21 women), average age 29 years. Prevailing in this group were Type A fractures which accounted for 45%, Type B was represented by 35% and Type C included 19%. Twenty-eight patients were treated conservatively. METHOD: Indication for surgical treatment was instability, hip incongruence or combination of both. Conservative treatment was chosen in the fractures without displacement, in fractures with secondary congruency and in case of contraindication of surgical treatment. The indication was always based on CT examination with a three-dimensional reconstruction. The surgery was performed within 24 hours in 42%, within 48 hours in 64%, within 72 hours in 87%. Posterior Kocher-Langenbeck approach was used in 54%, anterior ilioinguinal approach in 22% and the combination of both approaches in 24%. Posterior approach was indicated in A1, A2, B1 fractures, ilioinguinal approach in A3, B3, C1 Types, the combination of both approaches in B2, C2, C3 fractures. Osteosynthetic material was chosen according to the fracture type, bone quality and fragment size. RESULTS: The evaluation was based on subjective complaints, radiograph and clinical examination using Harris Hip Score. In the group of 91 patients treated surgically we achieved excellent result in 41 cases (45%), very good result in 28 cases (31%), fair result in 7 cases (9%) and poor result in 13 cases (15%). We recorded 17 peroperative complications--11 times inadequate reduction, 5 times nerve injury, once vascular injury. Early complications included infect--twice and loosening of implant--once. Late complications occurred in 26 cases--paraarticular ossification in 17 cases, aseptic necrosis of the femoral head in 8 cases. DISCUSSION: The treatment of fractures of the acetabulum as any intraarticular fracture is based on a precise anatomical reduction, stable fixation and early mobilization. Difficulties of the treatment of the acetabulum arise from the anatomical relations. Decisive for the indication, the method of treatment and prognosis is the type of the injury of the weight-bearing area of the acetabulum. In order to understand the type of the fracture it is necessary to make a careful preoperative examination based particularly on CT examination with a three-dimensional reconstruction. The indication and treatment strategy--surgical approach, method of reduction, type of fixation--is selected accordingly. Each surgical approach has its limits and also the possibilities of fixation are limited by the type of the fracture. The outcomes depend directly on the quality of the reconstruction of the articular surface and the incidence of complications, caused either by the injury or preoperatively. CONCLUSION: The treatment of displaced fractures of the acetabulum is on principle surgical. Satisfactory results are produced only by anatomical reconstruction of the weight-bearing area of the acetabulum. Surgical treatment is highly demanding and the long-term results not always correspond to the efforts made.

[Osteosynthesis of proximal femoral fractures using short proximal femoral nails]. / Osteosyntéza zlomenin proximálního femuru krátkým proximálním femorálním hrebem.

PURPOSE OF THE STUDY: A group of 147 patients with proximal femoral fractures treated with the use of the proximal femoral nail, manufactured by the Synthes company, in the period between October 1997 and June 2001 were retrospectively evaluated. MATERIAL: The group of 147 patients whose average age was 69 years consisted of 59 men (average age, 59 years) and 88 women (average age, 75 years). The most frequent fracture occurring in 54% of the patients was an unstable pertrochanteric fracture (Kyle III); persubtrochanteric and subtrochanteric fractures were treated in 27% and 19% of the patients, respectively. METHODS: In 1997, a new implant, the proximal femoral nail manufactured by the Synthes company (PFN), was introduced in the Czech Republic. In addition to all advantages of a nail to be implanted intramedullary, it has several other favorable characteristics: its length is 240 mm, pre-drillings is not necessary, it can be dynamically locked, it has a high rotation stability, and mechanical stress concentration on the implant-bone interface is low. This evaluation is based on the type of fracture, the duration of healing, the duration of surgery and X-ray exposure. RESULTS: The patients were followed up at 6 months, 3, 6 and 12 months, and once a year. The fracture healed in the anatomical position by 6 months in 95% of them. The average operative time was 56 min and X-ray exposure lasted on average 1 min. We recorded 14 intra-operative complications in nine patients, which included incomplete reduction in four cases, fixation in distraction in two, incorrect length of screws in one, fracture at the site of distal locking in two and incorrect insertion of femoral neck screws in five cases. Early post-operative complication involved seven cases of hematoma in the operation wound with the necessity to re-operate in five cases. Late complications occurred in two patients and included pseudoarthrosis and necrosis of the femoral head after healing. Most of the complications were found in subtrochanteric fractures. DISCUSSION: The treatment of unstable fractures of the proximal femur is still associated with some failures. The reasons are: disregard for biomechanics, overestimation of the potentials of new surgical techniques or new implants or poor adherence to established procedures. PFN is a novel, modern implant based on experience with the gamma nail. Since relevant literature data are very few, any comparison of our results is very difficult. The lower average age of our group was due to the intentional selection of our patients for this type of surgery. CONCLUSIONS: PFN is an excellent implant for the treatment of unstable fractures of the proximal femur. The terms of successful outcome include a good understanding of fracture biomechanics, correct indication and exactly performed osteosynthesis.

Superconductivity in MgB2: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB2. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport.

Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps.

Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB (2) are presented. The E(2g) phonons, which involve in-plane B displacements, couple strongly to the p(x,y) electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting lambda in the clean limit is calculated to be significantly larger. The E(2g) phonons are strongly anharmonic, and the nonlinear contribution to the coupling between the E(2g) modes and the p(x,y) bands is significant.

Superconductivity of metallic boron in MgB2.

Boron in MgB2 forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm(-1), this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.

Hamiltonian of the V15 spin system from first-principles density-functional calculations.

We report first-principles all-electron density-functional-based studies of the electronic structure, magnetic ordering, and anisotropy for the V15 molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with five antiferromagnetic and one ferromagnetic exchange couplings. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin S = 1/2 ground state and low-lying S = 3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.