Importance of diminished local structural distortions and magnetism in causing iron-based superconductivity

By analyzing the role of structural variation and magnetism of Cu dopants in FeAs planes, researchers demonstrated that orthorhombic distortions that give strong spin-density-wave spin (SDW) fluctuations are detrimental to superconductivity in BaFe

As

. The results provide new information about the interplay between local composition, magnetism and superconductivity.

Undoped BaFe₂As₂ has a simultaneous tetragonal-to-orthorhombic and spin-density-wave transition below T_s = T_N = 132 K. Although dopants of Co and Ni diminish this transition and give a superconducting dome, the reasons of (almost) no superconductivity in Cu-doped crystals was not understood until now. Figure 1 shows that spin fluctuations remain high for Cu doping, and in fact increase with reducing temperature. In comparison, the spin fluctuations are reduced with Co doping and drop dramatically below superconducting transition temperature.

This study used a combination of crystal synthesis, temperature dependence of single-crystal x-ray diffraction, electrical resistance, magnetic susceptibility, and nuclear-magnetic resonance (NMR) investigations. These results extend our understanding of bulk vs. local structure effects, finding that the orthorhombic distortion below T_s leads to magnetically ordered state of FeAs planes, hence no superconductivity.

H. Takeda, T. Imai, M. Tachibana, J. Gaudet, B. D. Gaulin, B. I. Saparov, and A. S. Sefat, “Cu substitution effects on the local magnetic properties of Ba(Fe_1-xCu_x)₂As₂: a site-selective ⁷⁵As and ⁶³Cu NMR study,” Phys. Rev. Lett. 113, 117001 (2014).

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