Orbital-driven nematicity in FeSe

TL;DR

This study reveals that in FeSe, nematic order is driven by orbital degrees of freedom, with clear NMR evidence of symmetry breaking at 91K, well above the superconducting transition, and shows competition between nematicity and superconductivity.

Contribution

The paper provides direct NMR evidence that orbital degrees of freedom drive nematic order in FeSe, clarifying the mechanism behind nematicity in iron-based superconductors.

Findings

NMR line splitting at 91K indicates nematic order onset.

Spin-lattice relaxation rates remain unaffected at nematic transition.

Superconductivity competes with nematic order.

Abstract

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 $T_{nem}$ = 91K, far above superconducting $T_c$ 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 $T_{nem}$, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.