A very fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of {\it two} other instabilities. Apart from a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here we report a very clear splitting of NMR resonance lines in FeSe at Tnem = 91K, far above superconducting Tc of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe-planes and has the temperature dependence of a Landau-type order-parameter. Spin-lattice relaxation rates are not affected at Tnem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.
@article{arxiv.1408.1875,
title = {Orbital-driven nematicity in FeSe},
author = {S. -H. Baek and D. V. Efremov and J. M. Ok and J. S. Kim and Jeroen van den Brink and B. Büchner},
journal= {arXiv preprint arXiv:1408.1875},
year = {2015}
}
Comments
4 figures; supplementary material is included; Final version, Nature Materials, advance online publication, 10 Nov 2014