NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

Abstract

We present77 Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, Ts. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe. PACS numbers:Much attention in recent research on iron-based superconductivity (SC) has been paid to understanding the nature of the electronic nematic phase, which breaks rotational symmetry while preserving time-reversal symmetry [1,2]. In the archetypical "122" compounds AFe 2 As 2 (A=Ca, Sr, Ba) [3,4], the nematic phase is closely tied to the stripe-type antiferromagnetic (AFM) phase in the phase diagram, suggesting a magnetic origin for the nematic state. Among the Fe-based SCs, FeSe is known to be an exception. At ambient pressure, FeSe undergoes a transition to the nematic phase at a bulk structural phase transition temperature T s ∼ 90 K, as well as to SC below T c ∼ 8 K, but has no stripe-type AFM ordered phase. Under pressure (p), T s is suppressed to ∼20 K at p ∼1.7 GPa [5][6][7] and an AFM ordered state emerges above ∼0.8 GPa [8][9][10][11]. In addition, T c is enhanced from 8 K at ambient pressure to ∼37 K at p ∼ 6 GPa [12]. The decrease of T s (p) and increase of T N (p) under pressure suggests competition between nematic and magnetic orders. Furthermore, NMR measurements [13,14] showed Korringa behavior above T s , consistent with an uncorrelated Fermi liquid, while AFM spin fluctuations (SFs) were found to be strongly enhanced only below T s . These observations suggested that SFs are not the driver for nematic order and therefore pointed to an orbital mechanism for the nematicity [14]. An orbital mechanism was also suggested by Raman spectroscopy [15].In contrast, several recent studies have suggested cooperation between nematicity and magnetism in FeSe. High-energy x-ray diffraction measurements [7] found that the orthorhombic distortion is enhanced in the magnetic state at p = 1.5 GPa. Furthermore, above 1.7 GPa T s (p) and T N (p) were found to coincide as a simultaneous first-order magneto-structural transition. These observations are consistent with a spin-driven mechanism for nematic order in FeSe. Similarly, inelastic neutron scattering (INS) measurements at ambient pressure [16,17] showed that commensurate stripe-type AFM SFs are in fact present well above T s , which could possibly drive the nematic transition. These SFs were not seen by NMR [13,14] due to a spin gap above ∼ 90 K. In addition, 77 Se-NMR data under pressure [18] revealed a first-order transition to a stripe-type magnetic ordered state, and suggested a magnetic driven nematicity. Therefore, the o...

Dynamical susceptibility of a near-critical nonconserved order parameter and quadrupole Raman response in Fe-based superconductors

“…For FeSe and FeSe 1−x S x , we find good agreement with the Pomeranchuk scenario, consistent with the fact that magnetic order in these systems does not develop down to T = 0 (although some magnetic fluctuations have been detected [21][22][23]). For NaFe 1−x Co x As and BaFe 2 As 2 we find a better agreement with the composite magnetic scenario of nematicity, consistent with the fact that nematic and magnetic ordering temperatures nearly coincide and in NaFe 1−x Co x As follow each other as functions of doping.…”

Persistent correlation between superconductivity and antiferromagnetic fluctuations near a nematic quantum critical point in FeSe1−xSx

“…2 shows the T and x dependence of the H||ab spectral splitting ∆K (the difference of the NMR shifts of the two peaks), which is a measure of the local nematic order parameter [34]. For FeSe, ∆K increases sharply below T s and shows a broad maximum near ∼ 50 K, as reported previously [34,36,37]. In contrast to FeSe, ∆K for x = 9% and x = 15% does not exhibit this maximum.…”

“…In FeSe, the single peak observed in the H||ab NMR spectrum at high T splits into two peaks below T s due to nematic order, where the two peaks originate from twinned nematic domains [34][35][36]. Representative NMR spectra at 20 K for both field directions are shown in transition was seen by resistivity [30].…”

“…1) by microscopic disorder from dopants and/or small variations in the local S composition. Due to the broad spectra relative to FeSe, no clear evidence for the local nematicity above T s , observed in FeSe from FWHM measurements [36,38], could be found. We now discuss the behavior of the low-energy magnetic fluctuations from NMR spin-lattice relaxation rate (1/T 1 ) data.…”

Nematicity, magnetism and superconductivity in FeSe

“…In contrast to the structural distortion, the in-plane anisotropy of the NMR spectra at ambient pressure actually decreases slightly below T c [83,142]. This was proposed to be a sign of competition between electronic nematicity and superconductivity [83].…”