Propriedades físicas de compostos intermetálicos à base de FeAs

Author(s)

Priscila Ferrari Silveira Rosa

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin
• Defense date: 2013-07-26

Examining board members

Pascoal José Giglio Pagliuso; Michael Josef Nicklas; Raimundo Rocha dos Santos; Rickson Coelho Mesquita; Alex Antonelli

• Advisor: Pascoal José Giglio Pagliuso

Abstract

The microscopic understanding of the intricate interplay between magnetism and unconventional superconductivity is currently one of the great open questions in condensed matter physics. In particular, compounds with a tetragonal crystal structure seems to be favorable to the emergence of such phenomena. The intermetallic compounds BaFe2As2 and EuFe2As2 crystallize in the tetragonal ThCr2Si2-type structure (I 4/mmm) with FeAs sheets separated by barium/europium layers. Both compounds exhibit a structural distortion accompanied by a magnetic spin-density wave (SDW) phase transition at TSDW = 140 K and TSDW = 190 K, respectively. Remarkably, this SDW phase can be tuned toward a superconducting state by substitution and applied pressure. In this thesis, we will present a systematic study of the intermetallic tetragonal compound BaFe2As2 as a function of three parameters: Eu substitution in the Ba crystallographic site, transition metal (TM) substitution (TM = Mn, Co, Ni, Cu, and Ru) in the Fe site, and/or applied hydrostatic pressure. For this purpose, we have grown high-quality single crystals by the alternative In-ux method. The macroscopic characterization has been made by measurements of magnetic susceptibility, specic heat and electrical resistivity at ambient pressure and under hydrostatic pressure. Concerning the microscopic investigation, the experimental approach consists in using electron spin resonance (ESR) technique employing paramagnetic ions of Eu2+ and Mn2+/Cu2+ as probes in the Ba and FeAs planes, respectively and X-ray absorption spectroscopy (XANES and EXAFS) in both As and Fe K edges. In this manner, it was possible to study the site specific spin dynamics and its relation with local distortions in the material. Our results evidentiate that the decrease in the Fe-As distance is intimately related to the SDW phase suppression and to a localization of the Fe 3d bands in the FeAs plane. This increase in the planar xy/x2 - y2 orbital character at the Fermi surface appears to be a propitious ingredient to the emergence of superconductivity in this class of materials (AU)

• FAPESP's process: 10/11949-3 - Electron spin resonance (ESR) in superconducting Fe-based pnictides
• Grantee: Priscila Ferrari Silveira Rosa
• Support Opportunities: Scholarships in Brazil - Doctorate