Spin exchange dynamics in 4H SiC monocrystals with different nitrogen donor concentrations

4H silicon carbide (SiC) polytype is preferred over other SiC polytypes for high-power, high-voltage, and high-frequency applications due to its superior electrical, thermal, and structural characteristics. In this manuscript, we provide a comprehensive study of the spin coupling dynamic between conduction electrons and nitrogen (N) donors in monocrystalline 4H SiC with various concentrations of uncompensated N donors from 10^17 cm^-3 to 5x10^19 cm^-3 by continuous wave, pulsed EPR, and microwave perturbation techniques at T=4.2-300 K. At low temperatures two triplets due to N donors in cubic (Nk) hexagonal (Nh) positions and triplet arisen from spin-interaction between Nh and Nk were observed in 4H SiC having Nd-Na=10^17 cm^-3. A single S-line (S=1/2) dominates the EPR spectra in all investigated 4H SiC monocrystals at high temperatures. It was established that this line occurs due to the exchange coupling of localized electrons (dominate at low temperatures) and non-localized electrons (dominate at high temperatures). The localized electrons were attributed to Nh for Nd-Na=10^17 cm^-3 and Nk donors for Nd-Na>=5x10^18 cm^-3. We have concluded that the conduction electrons in 4H SiC monocrystals are characterized by gpar=2.0053(3) gper=2.0011(3) for Nd-Na<=5x10^18 cm^-3 and gpar=2.0057(3) and gper=2.0019(3) for Nd-Na=5x10^19 cm^-3. Using the theoretical fitting of the temperature variation of S-line EPR linewidth in 4H SiC having Nd-Na<5x10^18 cm-^3, the energy levels of 57-65 meV that correlate with the valley-orbit splitting values for Nk donors in 4H SiC monocrystals were obtained

The dynamical behavior of the s-trioxane radical cation - A low-temperature EPR and theoretical study

The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10-140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely "rigid" half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the "rigid" half-boat conformer can be observed only at very low ( 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation

Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal–Organic Frameworks

Metal–organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed‐metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed‐metal metal–organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed‐metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X‐ray diffraction, X‐ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed‐metal FeCuBTC material with HKUST‐1 structure, which contained bimetallic Fe−Cu paddlewheels as well as monometallic Cu−Cu and Fe−Fe units under optimized synthesis conditions. The in‐depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed‐metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed‐metal metal–organic framework materials and the thorough characterization thereof is particularly important to derive structure–property or structure–activity correlations

Study of the Negative Magneto-Resistance of Single Proton-Implanted Lithium-Doped ZnO Microwires

The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non magnetic heterostructure in the wire after proton implantation.Comment: 6 pages with 5 figure

EPR Evidence of Unusual Dopant Valency States in Nanocrystalline Er-doped CeO<inf>2</inf>

© 2015, Springer-Verlag Wien. The structure and oxidation state of the Er dopant cation in CeO2 single crystal and nanocrystals with size ranging from 22 to 300 nm are studied using electron paramagnetic resonance (EPR) spectroscopy at X- and at Q-band near liquid-He temperatures. Besides the expected EPR line due to Er3+ in cubic sites in the lattice, unusual EPR lines with g values around 14 and 20 are observed in nanocrystalline CeO2. The appearance of these lines suggests the formation of non-Kramers Er2+, Er4+ ions, which becomes increasing favorable with decreasing nanoparticle size. Formation of rare earth ions with such unusual oxidation states in nanoparticles can be exploited in tuning their catalytic activity and optical properties

Direct spectroscopic observation of the atomic-scale mechanisms of clustering and homogenization of rare-earth dopant ions in vitreous silica

Structural aspects of clustering of rare earth ions in oxide glasses have been studied for the last several years in relation to their applications in photonics. However, the mechanism of homogenization of the spatial distribution of rare earth ions by codoping, typically with Al or P, is still not well understood. In this work we report direct experimental determination of the homogenization mechanism of Yb3+ ion clusters in silica glasses doped with 0.1 wt. % Yb2 O3 and codoped with Al or P, using two-dimensional HYSCORE-EPR spectroscopy. The results lead us to conclude that Yb creates its coordination environment via formation of Yb-O-Si and Yb-O-Yb bonds in a Yb-doped silica glass and even the light codoping with Al starts replacing these bonds with Yb-O-Al linkages. Heavy codoping with P replaces all Yb-O-Si Yb linkages with Yb-O-P linkages. The formation of a next-nearest neighbor shell of Al or P creates suitable structural pockets, which ultimately leads to homogenization. © 2006 The American Physical Society

The Jahn-Teller effect in Cr5+-doped PbTiO3: A multi-frequency electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of Cr5+ defects incorporated on Ti4+ sites in powdered ceramics of PbTiO3 were investigated in the temperature range 50-400K at 9GHz (X), 34GHz (Q) and 94GHz (W band). The Jahn-Teller effect stabilizes the vibronic ground state of the 3d1 electron of the Cr5+ ion and leads to a tetragonally distorted defect- O6 octahedron with the point symmetry D4h. The spontaneous electrical polarization present in the ferroelectric phase of PbTiO3 appears as a further perturbation producing an additional g-tensor contribution by the quadratic field effect. Its symmetry is dependent on the orientation of the electrical polarization with respect to the Jahn-Teller distortion axis, the tetragonal axis of the defect- O6 octahedron. If the polarization of a domain is anti-or parallel to this axis, the local tetragonal symmetry of the Cr5+ ion persists whereas it is reduced by a perpendicular orientation. Anisotropic EPR spectra of tetragonally and orthorhombic distorted Cr5+O6 12- are detected at low temperatures. Increasing the temperature, the peaks of the two spectra are broadened and a motionally averaged isotropic spectrum appears at 200K. © 2010 IOP Publishing Ltd

Cardiac troponin I, fructosamine, and cardiovascular parameters in dogs with diabetes mellitus

Diabetes mellitus (DM) is a common endocrinopathy in dogs, however, the relationship between canine DM and cardiomyopathy is still unclear. The aims of this study were: to evaluate serum troponin I concentrations in diabetic dogs under treatment with insulin; to evaluate the hypothesis that the time of DM diagnosis could influence the troponin I concentration; and to evaluate the relationship between biochemical and cardiovascular parameters in diabetic dogs. This is a cross-sectional study including diabetic canine patients under NPH insulin treatment. Troponin I concentration, systolic blood pressure (SBP), complete blood count, serum fructosamine concentration, biochemical profile, and Doppler echocardiography assesments were carried out in each patient, as well as in age- and size- matched healthy controls. All diabetic dogs had concentrations of troponin I below the limits of detection of the assay. There was no difference between cardiovascular parameters between diabetic and healthy control dogs. There was no correlation between time of DM diagnosis and cardiovascular parameters. There was a significant positive correlation between SBP and fructosamine in diabetic dogs (r=0.54; P&lt;0.01). Diabetic dogs receiving insulin treatment, regardless of the time of diagnosis, do not have significantly elevated serum troponin I. The results also suggest that fructosamine levels can be associated with high blood pressure, suggesting a possible correlation between fructosamine levels and vascular complications

Electron spin relaxation of the PO3 2- radical in ferroelectric betaine phosphite and in the proton glass betaine phosphate/betaine phosphite

Measurements of the electron spin-lattice relaxation time T1 and the phase memory time TM of the PO3 2- radical in γ-irradiated betaine phosphite and betaine phosphate/betaine phosphite are presented. The temperature dependence of T1 indicates the interaction of the electron spin with two groups of optical branches via Raman processes in both crystals. An additional relaxation path due to the interaction with two-level local tunneling states has been observed in the mixed crystal confirming glassy behaviour. The TM temperature dependence reflects thermally activated local motional effects of the PO3 group in both crystals. © 1994