Soft striped magnetic fluctuations competing with superconductivity inFe1+xTe

Abstract

Neutron spectroscopy is used to investigate the magnetic fluctuations in Fe1+xTe -a parent compound of chalcogenide superconductors. Incommensurate "stripe-like" excitations soften with increased interstitial iron concentration. The energy crossover from incommensurate to stripy fluctuations defines an apparent hour-glass dispersion. Application of sum rules of neutron scattering find that the integrated intensity is inconsistent with an S=1 Fe 2+ ground state and significantly less than S=2 predicted from weak crystal field arguments pointing towards the Fe 2+ being in a superposition of orbital states. The results suggest that a highly anisotropic order competes with superconductivity in chalcogenide systems.Pnictide and chalcogenide superconductors have altered the view of what provides the basis for high temperature superconductivity. While the cuprate superconductors universally derive from Mott insulators which can, at least qualitatively, be understood in terms of a single electronic band, the parent phase of iron based superconductors has been less clear: Fe-based parent phases are either poorly metallic or semimetallic resulting in a debate over whether a localized or itinerant/spin density wave picture is more appropriate. [1, 2] Towards this goal, it is important to understand the magnetic excitation spectrum in starting materials as superconducting variants consist of fluctuating versions of this ground state.[3] Here we study Fe 1+x Te which is arguably the structurally simplest of the iron superconductors based upon single layers of tetrahedrally coordinated Fe 2+ ions. [4, 5] While the iron superconductors have been shown to display both localized [6, 7] and itinerant properties [8, 9], Fe 1+x Te hosts one of the most localized responses of all iron based superconductors evidenced by large ordered magnetic moments and calculated heavy band masses. [10] In this study, we combine neutron scattering data from spectrometers with overlapping dynamic ranges on two samples of Fe 1+x Te to understand the magnetic fluctuations. We report a one dimensional incommensurate excitation that softens with increased charge doping with interstitial iron and hence competes with unconventional chalcogenide superconductivity. We apply sum rules of neutron scattering to evaluate the spin and orbital ground state of the iron cations. The results represent a dynamical signature of a highly anisotropic striped order which competes with superconductivity in the chalcogenides. Superconductivity in Fe 1+xTe 1−y Ch y (where Ch is a chalcogenide ion) has been most commonly achieved through anion substitution on the Te site y with either sulfur or selenium. [11, 12] However, the cation concentration (interstitial iron x) in Fe 1+x Te 1−y Ch y is directly correlated with the anion concentration (y) and chemical techniques have been developed to independently tune x and y.[13] Several studies have found that changing the concentration of interstitial iron has analogous effects to anion doping for a fixed selenium concentrat...

Competing spin density wave, collinear, and helical magnetism inFe1+xTe

“…Unlike the case of Se-doped Fe 1+x Te 1−y Se y , where the magnetic fluctuations are peaked near the (π , π ) position, in parent Fe 1+x Te, the magnetic correlations are peaked near (π , 0) [65]. As previously published, the low-temperature magnetic fluctuations are strongly correlated along both the a and b directions and become one -dimensional at higher-energy transfers in excess of ∼30 meV [61]. This is confirmed in the constantmomentum slices in Figs at high temperatures of 100 K; however, the fluctuations remain anisotropic in momentum at this temperature, as illustrated in constant-momentum slices in Figs.…”

“…[61], with the c axis parallel to the incident beam k i , the H and K axes are projected onto the MAPS detectors, providing a good experimental configuration to measure the momentum dependence in this plane. However, the value of L, or the projection along c, changes as a function of energy transfer and also coordinates (H,K).…”

“…Initial studies on superconducting Fe 1+x Te 0.6 Se 0.4 suggested the importance of itinerant effects [57,58]; however, neutron inelastic scattering has more recently been interpreted in terms of a localized model [59] with competing spin states [60]. Other high-energy neutron inelastic scattering work has observed an hourglass type of dispersion with spectral weight extending up to ∼200 meV and the total integrated spectral weight being short of expectations based on a purely localized picture [61].…”

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…The availability of large single crystals of iron chalcogenides Fe 1+y Te 1−x Se x means that spin excitations in these materials have been carefully mapped out (Lipscombe et al, 2011;Zaliznyak et al, 2011). In particular, application of the sum rules of neutron scattering indicate that the integrated spin excitation intensity of Fe 1+x Te is inconsistent with an S = 1 Fe 2+ ground state expected in the presence of a strong crystalline electric field (Stock et al, 2014), suggesting the importance of itinerant electrons even for the iron chalcogenides, which exhibit strong electron correlations and localized moments .…”

“…The outcome suggests that pnictogen height is correlated with the strength of electron-electron correlations and consequently the effective bandwidth of magnetic excitations in iron pnictides (Yin et al, 2014;Zhang et al, 2014a). Figure 10 summarizes spin wave measurements for the iron chalcogenide Fe 1+x Te Fruchart et al, 1975;Li et al, 2009d), the parent compound of the 11 family of iron-based superconductors (Lipscombe 28.5 ± 2.5 meV 7.5 ± 2.5 meV Te are sensitive to the excess iron in the interstitial sites Stock et al, 2011;Wen et al, 2011). This is rather different from the iron pnictides, which cannot accommodate any excess iron in the crystal structure.…”