1,721,181 research outputs found
Thermoelectric properties of iron-based superconductors and parent compounds
No full textHerewith, we review the available experimental data of thermoelectric transport properties of iron-based superconductors and parent compounds. We discuss possible physical mechanisms into play in determining the Seebeck effect, from whence one can extract information about Fermi surface reconstruction and Lifshitz transitions, multiband character, coupling of charge carriers with spin excitations and its relevance in the unconventional superconducting pairing mechanism, nematicity, quantum critical fluctuations close to the optimal doping for superconductivity, correlation. Additional information is obtained from the analysis of the Nernst effect, whose enhancement in parent compounds must be related partially to multiband transport and low Fermi level, but mainly to the presence of Dirac cone bands at the Fermi level. In the superconducting compounds, large Nernst effect in the normal state is explained in terms of fluctuating precursors of the spin density wave state, while in the superconducting state it mirrors the usual vortex liquid dissipative regime. A comparison between the phenomenology of thermoelectric behavior of different families of iron-based superconductors and parent compounds allows to evidence the key differences and analogies, thus providing clues on the rich and complex physics of these fascinating unconventional superconductors
Microstructural evolution throughout the structural transition in 1111 oxypnictides
No full textThe microstructural evolution throughout the first-order tetragonal-to-orthorhombic structural transition is analyzed by powder diffraction analysis for two different systems belonging to the class of compounds referred to as 1111 oxypnictides: (La1-yYy) FeAsO and SmFeAs(O1-xFx). Both systems are characterized by similar behavior: On cooling, microstrain along the tetragonal hh0 direction takes place and increases as the temperature is decreased. Just above the structural transition, microstrain reaches its maximum value and then is abruptly suppressed by symmetry breaking. No volume discontinuity throughout the first-order transition is observed, and a group-subgroup relationship holds between the tetragonal and the orthorhombic structures, thus suggesting that orbital ordering drives symmetry breaking. Microstrain reflects a distribution of lattice parameters in the tetragonal phase and explains the occurrence of anisotropic properties commonly attributed to nematic correlations; in this scenario, the nematic behavior is induced by the tendency towards ordering of Fe orbitals
Thermopower measurements of high-temperature superconductors: Experimental artifacts due to applied thermal gradient and a technique for avoiding them
No full textWe discuss thermoelectric measurements of high-temperature superconductors around the superconducting transition. The Seebeck effect coefficient has been widely used to study the dissipative phenomena of the vortex lattice and its interaction with quasiparticles, in conjunction with the resistivity to which it is correlated in the framework of all existing theories. As is well known, Seebeck effect measurements are far from the accuracy of resistivity measurements because the presence of a thermal gradient across the sample may alter the shape of the curve. We make a quantitative analysis of the errors involved in ac measurement techniques; the error percentage grows considerably in the low-dissipation region where the Seebeck effect vanishes. We propose a method that drastically reduces the experimental artifacts; moreover, we present the Seebeck effect measurements of a Y-Ba-Cu-O thin film, performed with the standard steady-flux ac technique and with the one proposed here, and we discuss the results in light of our calculations. [S0163-1829(98)06842-8]
Evidence of the isoelectronic character of F doping in SmFeAsO1−x F x : a first-principles investigation
No full textWe study the electronic structure of the SmFeAsO1−x F x alloy by means of first-principle calculations. We find that, contrary to common believe, F-doping does not change the charge balance between electrons and holes free-carriers in SmFeAsO1−x F x . For energies within a narrow energy range accross , the effect of F-doping on the band structure dispersion is tiny in both the paramagnetic and stripe antiferromagnetic phase. The charge balance between the conducting FeAs-layer and the SmO1−x F x charge reservoir layer is not influenced by the compositional change. The additional charge carried by fluorine, with respect to the oxygen, is compensated by a change in the oxidation state of the Sm ion from 3+ to 2+. A comparison with the SmFe1−x Co x AsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site
Interband and intraband effects in the upper critical field of disordered<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>MgB</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
No full textIn this work the superconducting properties of disordered MgB2 in applied magnetic field are studied within the lambda(theta theta) model, by taking into account the presence of both interband and intraband scattering with impurities. This approach allows to extract the suppression of the critical temperature T-c and the enhancement of the upper critical field H-c2, as a consequence of the introduction of impurities in the samples. We analyze the dependence of H-c2 on temperature, anisotropy of the electronic structure, and intraband sigma and pi band scattering rates. Comparing our numerical calculations with experimental data on irradiated samples, we find that irradiation defects mainly affect the mobility of sigma carriers. These results rationalize why the H-c2 anisotropy of irradiated samples is quickly reduced with increasing doses and full suppression of superconductivity occurs at rather low-resistivity values. Moreover, our calculations point out that disorder in the pi bands affects only weakly the coupling constants and thus it could yield a significant enhancement of H-c2 without severe suppression of T-c
Application potential of Fe-based superconductors
No full textIn this paper we report basic properties of iron-based superconductors and review the latest achievements in the fabrication of conductors based on these materials. We compare state-of-the-art results with performances obtained with low-T c and high-T c technical superconductors, evidencing in particular the most significant differences with respect to high-T c cuprate coated conductors. Although the optimization of preparation procedures is yet to be established, a potential range of applications for iron-based superconductors in the high field low temperature regime can be envisaged, where they may become competitors to RE-123 coated conductors
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