Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation

The role of orbital differentiation on the emergence of superconductivity in the Fe-based superconductors remains an open question to the scientific community. In this investigation, we employ a suitable microscopic spin probe technique, namely Electron Spin Resonance (ESR), to investigate this issue on selected chemically substituted BaFe$_{2}$As$_{2}$ single crystals. As the spin-density wave (SDW) phase is suppressed, we observe a clear increase of the Fe 3$d$ bands anisotropy along with their localization at the FeAs plane. Such an increase of the planar orbital content interestingly occurs independently on the chemical substitution responsible for suppressing the SDW phase. As a consequence, the magnetic fluctuations combined with the resultant particular symmetry of the Fe 3$d$ bands are propitious ingredients to the emergence of superconductivity in this class of materials.Comment: 6 pages, 5 figure

Possible unconventional superconductivity in substituted BaFe2_{2}As2_{2} revealed by magnetic pair-breaking studies

The possible existence of a sign-changing gap symmetry in BaFe$_{2}$As$_{2}$-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe$_{1.9}M_{0.1}$As$_{2}$ ($M=$ Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for $M=$ Cu and Mn compounds, which display very low SC transition temperature ($T_{c}$) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of $T_{c}$ cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials

Distinct high-T transitions in underdoped Ba1−x_{1-x}Kx_{x}Fe2_{2}As2_{2}

In contrast to the simultaneous structural and magnetic first order phase transition $T_{0}$ previously reported, our detailed investigation on an underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal ($\tau$: I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at $T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $\tau$ and $\vartheta$ phases over a finite temperature range observed in our NMR experiments confirm the first order character of the structural transition and provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our data also show that superconductivity (SC) develops in the $\vartheta$ phase below $T_{c}$ = 20 K and coexists with long range AFM. This new observation, $T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide superconductors.Comment: 4 pages, 3 figure

Pressure effects on magnetic pair-breaking in Mn- and Eu-substituted BaFe2As2

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPWe report a combined study of hydrostatic pressure (P <= 25 kbar) and chemical substitution on the magnetic pair-breaking effect in Eu- and Mn-substituted BaFe2As2 single crystals. At ambient pressure, both substitutions suppress the superconducting (SC) transition temperature (T-c) of BaFe2-xCoxAs2 samples slightly under the optimally doped region, indicating the presence of a pair-breaking effect. At low pressures, an increase of T-c is observed for all studied compounds followed by an expected decrease at higher pressures. However, in the Eu dilute system, T-c further increases at higher pressure along with a narrowing of the SC transition, suggesting that a pair-breaking mechanism reminiscent of the Eu Kondo single impurity regime is being suppressed by pressure. Furthermore, Electron Spin Resonance (ESR) measurements indicate the presence of Mn2+ and Eu2+ local moments and the microscopic parameters extracted from the ESR analysis reveal that the Abrikosov-Gor'kov expression for magnetic pair-breaking in a conventional sign-preserving superconducting state cannot describe the observed reduction of T-c.1151714FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPSem informaçãoSem informaçãoSem informaçãoAnnual Conference on Magnetism and Magnetic Materials4 a 8 de Novembro de 2013Denver, COThis work was supported by FAPESP-SP, AFOSR MURI, CNPq, and FINEP-Brazil

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2 and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (k = 3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.Comment: 7 pages, 6 figure

Electron spin resonance of GD3+ in three dimensional topological insulator BI2SE3

Bi2Se3 has been claimed to be a three dimensional topological insulator (TI) with topologically protected metallic surface states with exotic properties. We have performed electron spin resonance (ESR) measurements on Gd3+ doped (x approximate to 0.01) Bi2Se3 single crystal grown from stoichiometric melt. For the studied crystals, our preliminary results revealed a partly resolved Gd3+ fine structure spectrum with Dysonian (metallic character) lines. At room temperature, the central line has a g approximate to 1.98, a linewidth Delta H approximate to 95 G and the spectra have a overall splitting of roughly 1300 Oe. As the temperature is decreased, the Gd3+ ESR Delta H of the central line presents a very small Korringa-like behavior b = Delta H/Delta T approximate to 0.013 Oe/K and nearly T-independent g-value. However, for T less than or similar to 40 K, Delta H shows a stronger narrowing effect evolving to Korringa-like behavior (b approximate to 0.15 Oe/K) for T less than or similar to 30 K. Concomitantly with the change in Delta H behavior, the Gd3+ central line g value starts to decrease reaching a value of 1.976 at T less than or similar to 4.2 K. The ESR results are discussed in terms of possible effects of protected topological surface states enlightened by complementary data from macroscopic measurements592International Conference on Strongly Correlated Electron Systems (SCES)2014-07FrançaUniv Grenoble, Grenobl

Co-substitution effects on the Fe-valence in the BaFe2As2 superconducting compound: A study of hard x-ray absorption spectroscopy

The Fe K X-ray absorption near edge structure (XANES) of BaFe2-xCoxAs2 superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.Comment: 4+ pages, 4 figures. Published in Phys. Rev. Let

Physical properties and magnetic structure of the intermetallic CeCuBi2 compound

In this work we combine magnetization, pressure dependent electrical resistivity, heat capacity, Cu63 nuclear magnetic resonance (NMR), and x-ray resonant magnetic scattering experiments to investigate the physical properties of the intermetallic CeCuBi2 compound. Our single crystals show an antiferromagnetic ordering at TN≃16 K and the magnetic properties indicate that this compound is an Ising antiferromagnet. In particular, the low temperature magnetization data revealed a spin-flop transition at T=5 K when magnetic fields of about 5.5 T are applied along the c axis. Moreover, the x-ray magnetic diffraction data below TN revealed a commensurate antiferromagnetic structure with propagation wave vector (0012) with the Ce3+ moments oriented along the c axis. Furthermore, our heat capacity, pressure dependent resistivity, and temperature dependent Cu63 NMR data suggest that CeCuBi2 exhibits a weak heavy fermion behavior with strongly localized Ce3+ 4f electrons. We thus discuss a scenario in which both the anisotropic magnetic interactions between the Ce3+ ions and the tetragonal crystalline electric field effects are taking into account in CeCuBi2.Fil: Adriano, C.. Universidade Estadual de Campinas; BrasilFil: Rosa, P.F.S.. Universidade Estadual de Campinas; Brasil. University of California at Irvine; Estados UnidosFil: Jesus, Camilo B. R.. Universidade Estadual de Campinas; BrasilFil: Mardegan, J. R. L.. Universidade Estadual de Campinas; BrasilFil: Garitezi, T. M.. Universidade Estadual de Campinas; BrasilFil: Grant, Taran. California State University; Estados UnidosFil: Fisk, Z.. California State University; Estados UnidosFil: Garcia, Daniel Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Reyes, A. P.. National High Magnetic Field Laboratory; Estados UnidosFil: Kuhns, P. L.. National High Magnetic Field Laboratory; Estados UnidosFil: Urbano, R. R.. Universidade Estadual de Campinas; BrasilFil: Giles, C.. Universidade Estadual de Campinas; BrasilFil: Pagliuso, P. G.. Universidade Estadual de Campinas; Brasi