Similar zone-center gaps in the low-energy spin-wave spectra of Na1−δFeAs and BaFe2As

Abstract

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na 1−δ FeAs and BaFe 2 As 2 . Despite their very different values of the ordered magnetic moment and Néel temperatures, T N , in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T N , with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na 1−δ FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe 2 As 2 along the out-of-plane momentum component, q z . At the magnetic zone boundary (q z = 0), spin excitations in the ordered state persist down to ∼ 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J c , as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model. The discovery of unconventional superconductivity in iron-pnictide compounds [1] with critical temperatures, T c , as high as 56 K has fostered a tremendous interest in these materials in recent years [2]. There are several structurally distinct families of iron-based superconductors with similar phase diagrams [2], governed by an interplay of antiferromagnetism, persistent in pure compounds under ambient pressure, and superconductivity that can be induced by pressure or chemical doping [3]. Although the highest values of T c are usually found in doped compounds with a nonstoichiometric composition, our physical understanding of these systems undoubtedly depends on the detailed knowledge of magnetic properties in the respective parent compounds, which are also easier to treat theoretically due to their simple crystal structure with no substitutional disorder.Among the variety of such stoichiometric materials serving as parent phases for numerous iron-based superconductors, only a few have so far received proper experimental treatment, especially by inelastic neutron scattering (INS), due to miscellaneous reasons related to the availability of sizeable single crystals or their chemical stability. For instance, to the best of our knowledge, direct measurements of spin-wave excitations in the antiferromagnetic (AFM) state of iron pnictides have so far remained limited to a few members of the so-called '122' family with the ThCr 2 Si 2 -type structure, whose single crystals are typically stable in air and are readily available in the large sizes necessary for INS experiments. In particular, high-energy spin-wave modes have been mapped out in CaFe 2 As 2 [4, 5] and BaFe 2 As 2 [6] using time-of-flight (TOF) neutron spectroscopy, which enabled estimations of the effective magnetic exchange interactions in the framework of a localized Heisenberg-type J 1a -J 1b -J 2 -J c model. These results are complemented by INS measurements at lower energies, performed on polycrystalline BaFe 2 As 2 [7] and on single c...

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…Consistent with unpolarized measurements (Matan et al, 2009;Park et al, 2012), there are large spin gaps at (1, 0, 0) and (1, 0, 1). However, the gap value for σ SF z is significantly larger than that for σ SF y ].…”

“…Figure 29 summarizes the outcome from polarized neutron scattering experiments on BaFe 2 As 2 . From unpolarized neutron scattering experiments, it is well known that spin waves in the AF ordered state are gapped below about ∼15 meV at the magnetic Brillouin zone center Q AF (Matan et al, 2009;Park et al, 2012). Figure 29 (Brand et al, 2014).…”

Two-dimensional magnetic interactions in LaFeAsO

“…The common energy scale of the 122 and NaFeAs spin gaps was recently discussed in Ref. 23, as it does not follow from the expectations of simple single-ion anisotropy due to substantial differences in the magnitude of the ordered moments in the two systems. The similar spin gap observed in LaFeAsO, along with its relatively small ordered moment (0.4 μ B ), 27 would seem to add some strength to this argument.…”

“…9, 12, 13, and 23) and NaFeAs (Ref. 23) compounds. Low-energy INS measurements on polycrystalline LaFeAsO observe a similar-sized spin gap and report a 2D-like response.…”

Magnetic anisotropic energy gap and low-energy spin wave excitation in the antiferromagnetic block phase of K2Fe4Se5<

“…It is also noteworthy that the gap energy values obtained in K 2 Fe 4 Se 5 are of the same order of magnitude as the ones of various iron arsenides, in spite of quite large differences in ordering temperature and magnitude of Fe moments. 29 Similar fitting procedures are performed to fit energy scan data observed at different temperatures [ Fig. 2(c)].…”