RESUMEN
Superconductivity in high-temperature superconductors such as cuprates or iron pnictides is typically achieved by hole or electron doping and it is of great interest to understand how doping affects their properties leading to superconductivity. To study it we conducted Fe and As K edge x-ray absorption spectroscopy measurements on several electron doped compounds from the 112 and 122 family of Eu-based iron pnictides. XANES and EXAFS results confirm that dopants are located at expected sites. For both families we found an electron charge redistribution between As and Fe occurring with doping. The changes it caused are stronger in the 112 family and they are bigger at As sites, which indicates that doped charges are predominantly localized on the dopant site. However, the results obtained do not provide clues why Ni doping in 122 family does not lead to occurrence of superconductivity.
RESUMEN
Compounds containing Eu show a vast range of unique physical properties due to the interplay of electronic and magnetic properties, which can lead to a nontrivial electronic topology combined with magnetic order. We report on the growth of trigonal ([Formula: see text] space group) EuZn2As2 single crystals and on the studies of their structural, electronic and magnetic properties. A range of experimental techniques was applied including X-ray diffraction, electron microscopy, magnetic susceptibility, magnetization, heat capacity and Mössbauer spectroscopy in the study. We found that Eu has solely a 2+ valence state and its magnetic moments below TN = 19.2 K form a canted antiferromagnetic structure, tilted from the basal plane.
RESUMEN
In this article, the results of a study of the magnetic dynamics of superparamagnetic iron oxide nanoparticles (SPIONs) with chitosan and polyethylene glycol (PEG) coatings are reported. The materials were prepared by the co-precipitation method and characterized by X-ray diffraction, dynamic light scattering and scanning transmission electron microscopy. It was shown that the cores contain maghemite, and their hydrodynamic diameters vary from 49 nm for PEG-coated to 200 nm for chitosan-coated particles. The magnetic dynamics of the nanoparticles in terms of the function of temperature was studied with magnetic susceptometry and Mössbauer spectroscopy. Their superparamagnetic fluctuations frequencies, determined from the fits of Mössbauer spectra, range from tens to hundreds of megahertz at room temperature and mostly decrease in the applied magnetic field. For water suspensions of nanoparticles, maxima are observed in the absorption part of magnetic susceptibility and they shift to higher temperatures with increasing excitation frequency. A step-like decrease of the susceptibility occurs at freezing, and from that, the Brown's and Néel's contributions are extracted and compared for nanoparticles differing in core sizes and types of coating. The results are analyzed and discussed with respect to the tailoring of the dynamic properties of these nanoparticle materials for requirements related to the characteristic frequency ranges of MRI and electromagnetic field hyperthermia.
RESUMEN
Universal scaling laws can guide the understanding of new phenomena, and for cuprate high-temperature superconductivity the influential Uemura relation showed, early on, that the maximum critical temperature of superconductivity correlates with the density of the superfluid measured at low temperatures. Here we show that the charge content of the bonding orbitals of copper and oxygen in the ubiquitous CuO2 plane, measured with nuclear magnetic resonance, reproduces this scaling. The charge transfer of the nominal copper hole to planar oxygen sets the maximum critical temperature. A three-dimensional phase diagram in terms of the charge content at copper as well as oxygen is introduced, which has the different cuprate families sorted with respect to their maximum critical temperature. We suggest that the critical temperature could be raised substantially if one were able to synthesize materials that lead to an increased planar oxygen hole content at the expense of that of planar copper.
RESUMEN
We report on the results from a (75)As nuclear magnetic resonance (NMR) study of the overdoped iron pnictide superconductor CeFeAsO0.8F0.2. We find two As sites with different shifts at temperatures as high as 100 K, which is above the superconducting transition temperature of 39 K, and hence they cannot be attributed to the effect of vortices in the superconducting state as previously suggested (Ghoshray et al 2009 Phys. Rev. B 79 144512). The much larger spin-lattice relaxation rate compared with that found in other pnictides without magnetic rare earth ions, and the temperature dependence of the (75)As NMR shifts for the two central lines, are consistent with the hyperfine coupling from magnetic Ce to As. The low temperature spectra indicate that there are As ions with two different quadrupole splittings. Our findings appear to be consistent with an electronic phase segregation into regions with two different F dopings or the presence of a correlated spatial charge and spin density variations.
RESUMEN
While the highest pressures can be achieved with diamond anvil cells, limited sample size and anvil geometry have hampered their application in nuclear magnetic resonance (NMR) experiments due to weak signal-to-noise. Here we report a new probe design that is based on having the resonant radio frequency coil that encloses the sample within the anvil cell inside the gasket hole. This increases the filling factor tremendously and results in greatly enhanced NMR sensitivity. The setup is described together with room temperature Na and Al NMR experiments.
