Singular magnetic anisotropy in the nematic phase of FeSe

Abstract

FeSe is arguably the simplest, yet the most enigmatic, iron-based superconductor. Its nematic but non-magnetic ground state is unprecedented in this class of materials and stands out as a current puzzle. Here, our nuclear magnetic resonance measurements in the nematic state of mechanically detwinned FeSe reveal that both the Knight-shift and the spin–lattice relaxation rate 1/T1 possess an in-plane anisotropy opposite to that of the iron pnictides LaFeAsO and BaFe2As2. Using a microscopic electron model that includes spin–orbit coupling, our calculations show that an opposite quasiparticle weight ratio between the dxz and dyz orbitals leads to an opposite anisotropy of the orbital magnetic susceptibility, which explains our Knight-shift results. We attribute this property to a different nature of nematic order in the two compounds, predominantly bond type in FeSe and onsite ferro-orbital in pnictides. The T1 anisotropy is found to be inconsistent with existing neutron scattering data in FeSe, showing that the spin fluctuation spectrum reveals surprises at low energy, possibly from fluctuations that do not break C4 symmetry. Therefore, our results reveal that important information is hidden in these anisotropies and they place stringent constraints on the low-energy spin correlations as well as on the nature of nematicity in FeSe.

Edwards-Anderson parameter and local Ising-nematicity in FeSe revealed via NMR spectral broadening

“…As illustrated in Fig. 1(a [70,71], the hyperfine field at the nucleus will be different in the two domains, causing the NMR spectrum to split into two peaks in the nematic ordered state. If the field is instead applied along H [110] O (Fig.…”

“…1(b)), then both types of domain see a symmetry-equivalent magnetic field, and there is only a single NMR peak in the nematic state, even though there are still domains. Recent NMR measurements under mechanical strain have revealed that the higher frequency NMR peak comes from the domains which experience H a O , while the lower frequency peak comes from the H b O domains [71].…”

“…The FWHM of the low-frequency NMR peak (open symbols) is greater than that of the high-frequency peak (hatched symbols). The different FWHM of the two NMR peaks in the nematic state of FeSe has been noted in independent measurements and is currently not understood [71].…”

Inhomogeneous Knight shift in vortex cores of superconducting FeSe

“…1) were collected for field aligned along the tetragonal [110], [100] and [001] directions. The [110] ([1 10]) direction corresponds to the Fe-Fe bond, and is the a (b) direction in the nematic phase [26]. The spectra were measured at several temperatures down to 2.1 K using low-power rf pulses (π/2-pulse widths up to 80µs), sweeping frequency and summing the Fourier transforms.…”

“…, where χ spin αα , χ orb αα , and χ mixed αα are the static spin, orbital, and mixed susceptibilities at zero wavevector [26,28]. In the absence of spin-orbit coupling, the mixed term vanishes and the Knight shift is usually decomposed as K α = K α0 + A S αα χ spin αα .…”