Most unconventional superconductors are in the vicinity of certain magnetically ordered states. The CAF state in iron-based superconductors breaks the four-fold symmetry, entering a nematic or two-fold symmetric phase, and it was suggested to drive the tetragonal-to-orthorhombic structural transition. There are usually twinned domains in the orthorhombic states which makes the experimental result an average effect of different domains but not an intrinsic one. However, it has been shown that the twinning could be removed with a uniaxial pressure in which the pressurized direction would be the ferromagnetic direction and the other be the anti-ferromagnetic direction. This mechanically detwinning method has been successfully obtained in many experiments.

Previous studies are focused on the 111, 122, and 1111 systems of iron-based superconductors, however whether the nematicity is an intrinsic property or just caused by anisotropic impurity scattering is still under debate. In order to get a universal picture of the resistivity anisotropy behavior in iron-based superconductors, we firstly studied the parent compound of 11 series, namely FeTe single crystals. Montgomery method is used so that the resistances along both orthorhombic directions can be measured at the same condition, compared with the usual four-lead method. Both as-grown samples and annealed ones with higher qualities are studied to exclude the influence of impurity scattering. Most strikingly, the observed resistivity along AFM direction is larger than that along the FM direction in detwinned FeTe single crystals. We show that these two opposite resistivity anisotropy behaviors can both be explained by the strong Hund’s rule coupling effect: while the iron pnictides are on the itinerant side, where the Hund’s rule coupling causes strong reconstruction and nematicity of the electronic structure; the FeTe is on the localized side, which mimics the double exchange interactions in manganites that makes hopping along the FM direction easier than along the AFM direction. Our result gives a universal understanding of the in-plane resistivity anisotropy behaviors in iron-based superconductors which might be helpful to better understand the nematicity of iron-based superconductors. This result has been published on Phy. Rev. B 88, 115130(2013).

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Figure 1: Resistivity anisotropy of both as-grown and annealed FeTe single crystals and its microcosmic explanation