Quantum Oscillations in EuFe2As2 single crystals

Quantum oscillation measurements can provide important information about the Fermi surface (FS) properties of strongly correlated metals. Here, we report a Shubnikov-de Haas (SdH) effect study on the pnictide parent compounds EuFe$_{2}$As$_{2}$ (Eu122) and BaFe$_{2}$As$_{2}$ (Ba122) grown by In-flux. Although both members are isovalent compounds with approximately the same density of states at the Fermi level, our results reveal subtle changes in their fermiology. Eu122 displays a complex pattern in the Fourier spectrum, with band splitting, magnetic breakdown orbits, and effective masses sistematically larger when compared to Ba122, indicating that the former is a more correlated metal. Moreover, the observed pockets in Eu122 are more isotropic and 3D-like, suggesting an equal contribution from the Fe $3d$ orbitals to the FS. We speculate that these FS changes may be responsible for the higher spin-density wave ordering temperature in Eu122.Comment: 5 pages, 4 figure

Evolution of Eu2+ spin dynamics in Ba1-xEuxFe2As2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Single crystals of Ba1-xEuxFe2As2 were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at T-SDW) and antiferromagnetic (at T-N) phase transitions were mapped as a function of x. For x >= 0.2, we found a single Eu2+ ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth (Delta H) above T-SDW which systematically decreases with decreasing x. In contrast, for a critical concentration x(c) (0.10 < x(c) < 0.20), Delta H decreases with increasing T, suggesting a distinct relaxation process that we associate with a Eu2+ Kondo single impurity regime. The Korringa rate suppression towards the Ba-rich compounds is claimed to be due to the reduction of the q-dependent exchange interaction between the Eu2+ f electrons and the conduction electrons, which is likely associated with an increasing of localization of Fe d electrons. This result may help the understanding of the SDW phase suppression (that can lead to superconductivity) in this class of materials.8616Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FINEP-BrazilFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2006/60440-0, 2009/09247-3, 2010/11949-3, 2011/01564-0, 2011/23650-5

3d magnetism in ThCo2Sn2 single crystals

Intense efforts have been made in order to synthesize new tetragonal ternary intermetallic compounds with low-temperature ground states on the border of magnetism. Here we report the synthesis of ThCo Sn single crystals, which crystallize in the tetragonal CaBe Ge -type structure, using the flux technique. Further investigation of the physical properties by magnetic susceptibility and pressure-dependent electrical resistivity measurements confirm the presence of antiferromagnetic ordering associated with the Co ions, but with a higher ordering temperature of T 78 K when compared to T 65 K of the polycrystalline counterpart. Our results show that both Cu-substitution and applied pressure up to 22 kbar are not able to induce superconductivity in this system. Nevertheless, our analyses point out to a putative quantum critical point at higher pressures. 2 2 2 2 N

Electron spin resonance of the intermetallic antiferromagnet EuIn 2As 2

We present electron spin resonance (ESR) measurements in single-crystalline samples of EuIn 2As 2 grown using the In-flux method. This compound crystallizes in a hexagonal P6(3)/mmc structure and presents antiferromagnetic (AFM) ordering below T N=16 K. In the paramagnetic state, a single Eu2 + Dysonian ESR line with nearly temperature-independent g-factor and linewidth is observed, indicating the absence of Korringa-like relaxation of the Eu2 + ions. Approaching the AFM transition, we observe an anisotropic g-shift and a linewidth broadening which has a maximum at T N, suggesting that the short-range AFM correlation dominates the spin dynamics of the Eu2 + spins in this temperature range. Our results are discussed based on complementary data (magnetic susceptibility, heat capacity, and electrical resistivity measurements) that provide further details about the global macroscopic physical properties of the EuIn 2As 2 intermetallic compound. © 2012 American Physical Society

Electron spin resonance of the intermetallic antiferromagnet EuIn2As2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We present electron spin resonance (ESR) measurements in single-crystalline samples of EuIn2As2 grown using the In-flux method. This compound crystallizes in a hexagonal P6(3)/mmc structure and presents antiferromagnetic (AFM) ordering below T-N = 16 K. In the paramagnetic state, a single Eu2+ Dysonian ESR line with nearly temperature-independent g-factor and linewidth is observed, indicating the absence of Korringa-like relaxation of the Eu2+ ions. Approaching the AFM transition, we observe an anisotropic g-shift and a linewidth broadening which has a maximum at T-N, suggesting that the short- range AFM correlation dominates the spin dynamics of the Eu2+ spins in this temperature range. Our results are discussed based on complementary data (magnetic susceptibility, heat capacity, and electrical resistivity measurements) that provide further details about the global macroscopic physical properties of the EuIn2As2 intermetallic compound.869Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)National Council for Scientific and Technological DevelopmentFinanciadora de Estudos e Projetos, BrazilFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [2006/60440-0, 2009/09247-3, 2010/11949-3, 2011/01564-0, 2011/23650-5

Evolution of Eu2 + spin dynamics in Ba 1-xEu xFe 2As 2

Single crystals of Ba 1-xEu xFe 2As 2 were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at T SDW) and antiferromagnetic (at T N) phase transitions were mapped as a function of x. For 0.2, we found a single Eu2 + ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth (ΔH) above T SDW which systematically decreases with decreasing x. In contrast, for a critical concentration x c (0.1

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

We 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. (C) 2014 AIP Publishing LLC

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 ≈ 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 ≈ 1.98, a linewidth ΔH ≈ 95 G and the spectra have a overall splitting of roughly 1300 Oe. As the temperature is decreased, the Gd3+ ESR ΔH of the central line presents a very small Korringa-like behavior b =ΔH/ΔT ≈ 0.013 Oe/K and nearly T-independent g-value. However, for T ≲ 40 K, ΔH shows a stronger narrowing effect evolving to Korringa-like behavior (b ≈ 0.15 Oe/K) for T ≲ 30 K. Concomitantly with the change in ΔH behavior, the Gd3+ central line g value starts to decrease reaching a value of 1.976 at T =4.2 K. The ESR results are discussed in terms of possible effects of protected topological surface states enlightened by complementary data from macroscopic measurements

High field nuclear magnetic resonance in transition metal substituted BaFe2As2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)We report high field As-75 nuclear magnetic resonance (NMR) measurements on Co and Cu substituted BaFe2As2 single crystals displaying same structural/magnetic transition T-0 similar or equal to 128 K. From our anisotropy studies in the paramagnetic state, we strikingly found virtually identical quadrupolar splitting and consequently the quadrupole frequency nu(Q) similar or equal to 2: 57(1) MHz for both compounds, despite the claim that each Cu delivers 2 extra 3d electrons in BaFe2As2 compared to Co substitution. These results allow us to conclude that a subtle change in the crystallographic structure, particularly in the Fe-As tetrahedra, must be the most probable tuning parameter to determine T-0 in this class of superconductors rather than electronic doping. Furthermore, our NMR data around T-0 suggest coexistence of tetragonal/paramagnetic and orthorhombic/antiferromagnetic phases between the structural and the spin density wave magnetic phase transitions, similarly to what was reported for K-doped BaFe2As2 [Urbano et al., Phys. Rev. Lett. 105, 107001 (2010)]. (C) 2014 AIP Publishing LLC.11517NSF [DMR-0654118]State of FloridaFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)NSF [DMR-0654118