Se77NMR investigation of theKxFe

David Torchetti

¹

,

Morgan Fu²,

D.C. Christensen³

et al.

Abstract

We report comprehensive 77 Se NMR measurements on a single crystalline sample of the newly discovered FeSe-based high temperature superconductor KxFe2−ySe2 (Tc = 33 K) in a broad temperature range up to 290 K. Despite deviations from the stoichiometric KFe2Se2 composition, we observed 77 Se NMR lineshapes as narrow as 4.5 kHz under a magnetic field applied along the crystal c-axis, and found no evidence for co-existence of magnetic order with superconductivity. On the other hand, the 77 Se NMR lineshape splits into two peaks with equal intensities at all temperatures when we apply the magnetic field along the ab-plane. This suggests that K vacancies may have a superstructure, and that the local symmetry of the Se sites is lower than the tetragonal four-fold symmetry of the average structure. This effect might be a prerequisite for stabilizing the s± symmetry of superconductivity in the absence of the hole bands at the Brillouin zone center. From the increase of NMR linewidth below Tc induced by the Abrikosov lattice of superconducting vortices, we estimate the in-plane penetration depth λ ab ∼ 290 nm, and the carrier concentration ne ∼ 1 × 10 +21 cm −3 . Our Knight shift 77 K data indicate that the uniform spin susceptibility decreases progressively with temperature, in analogy with the case of FeSe (Tc ∼ 9 K) as well as other FeAs high Tc systems. The strong suppression of 77 K observed immediately below Tc for all crystal orientations is consistent with a singlet pairing of Cooper pairs. We don't, however, observe the Hebel-Slichter coherence peak of the nuclear spin-lattice relaxation rate 1/T1 immediately below Tc, expected for conventional BCS s-wave superconductors. In contrast with the case of FeSe, we do not observe evidence for an enhancement of low frequency antiferromagnetic spin fluctuations (AFSF) near Tc in 1/T1T . Instead, 1/T1T exhibits qualitatively the same behavior as overdoped non-superconducting Ba(Fe1−xCox)2As2 with x ∼ 0.14 or greater, where hole bands are missing in the Brillouin zone center. We will discuss the implications of our results on the hitherto unknown mechanism of high temperature superconductivity in FeSe and FeAs systems.

NMR Study in the Iron-SelenideRb0.74Fe1.6Se2: Determination of the Superconducting Phase as Iron Vacancy-FreeRb0.3

“…2(a) which were already observed in previous reports [12,14,15]. As 87 Rb is a spin 3/2 nucleus, the spectrum consists of a central line (nuclear transition 1/2↔-1/2) and two quadrupolar satellites (nuclear transitions ±3/2↔±1/2) due to the local electric field gradient (EFG).…”

“…However, the 77 Se hyperfine coupling to Fe spins is twice as large as that of 87 Rb, which implies that this AF Se signal would be 2 times broader and with a transverse relaxation T 2 which is 4 times shorter. Such a short T 2 of only a few s makes the signal impossible to detect in standard NMR pulse experiments, which explains why it has not been observed, neither by us nor by other groups [12,14,15].…”

“…SR [8] and Mössbauer [9] studies found that the 245 Rb or K materials segregate into spatially separated SC and AF regions, as also suggested by TEM experiments [13]. But surprisingly, 87 Rb and 77 Se [14,15] NMR studies both reported the observation of just one single NMR line which displays all the characteristics of a pure metallic-SC phase, and did not observe any experimental evidence for phase separation. Two main questions arise from this puzzling body of evidence: Why is NMR at odds with other local probes?…”

Pairing symmetries of several iron-based superconductor families and some similarities with cuprates and heavy-fermions

“…The singlet, [91] nodeless, and isotropic nature of the pairing has been observed by numerous experiments. [64][65][66] Again, based on the sign-reversal properties of this d-wave gap, random-phase approximation (RPA)-based susceptibility calculation has predicted the presence of a spin-resonance mode, [22] which is observed later by inelastic neutron scattering measurement. [67] The unitary transformation of the FS and the pairing symmetry to the 2FUC is more interesting.…”

“…[4][5][6] In this case, the electron-pocket at M point is eliminated by increasing number of iron-vacancies in the non-stoichiometric K y Fe 2-x Se 2 crystal. [61][62][63] Both theories [22,50,52] and experiments [64][65][66] have demonstrated that isotropic and nodeless d-wave pairing is energetically favourable in this case, originating from intra-electron-pockets nesting. Our theoretical prediction of the spin-resonance [22] as a result of such nesting, has been observed recently by inelastic neutron scattering measurement.…”