Coexistence of Ferromagnetic and Stripe Antiferromagnetic Spin Fluctuations in SrCo2As2

Abstract

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo2As2, derived from SrFe2−xCoxAs2 iron pnictide superconductors. Our data reveals the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors Q AF =(1,0) and Q FM =(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo2As2 are closely associated with a flat band of the eg orbitals near the Fermi level, different from the t2g orbitals in superconducting SrFe2−xCoxAs2. Therefore, Co-substitution in SrFe2−xCoxAs2 induces a t2g to eg orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.Flat electronic bands can give rise to a plethora of interaction-driven quantum phases, including ferromagnetism [1], Mott insulating phase due to electron correlations [2], and superconductivity [3]. Therefore, an understanding how the flat electronic bands can influence the electronic, magnetic, and superconducting properties of solids is an important topic in condensed matter physics. In iron pnictide superconductors such as AFe 2−x Co x As 2 (A = Ba, Sr) [Figs. 1(a)-1(d)], the dominate interactions are stripe antiferromagnetic (AF) order, and superconductivity, which has singlet electron pairing, arises by doping electron with Co-substitution to suppress static AF order [4][5][6]. While AF spin fluctuations and superconductivity in iron pnictides are believed to arise from nested hole Fermi surfaces at Γ and electron Fermi surfaces at M [ Fig. 1(e)] [7], the density functional theory (DFT) calculations suggest the competing ferromagnetic (FM) and AF spin fluctuations with the balance controlled by doping [8,9]. For Co-overdoped ACo 2 As 2 [10, 11], where the DFT calculations find a tendency for both the FM and AF order, neutron scattering revealed only the AF spin fluctuations [12] while angle resolved photoemission spectroscopy (ARPES) experiments found no evidence of the Fermi surface nesting [13,14]. On the other hand, nuclear magnetic resonance (NMR) measurements on AFe 2−x Co x As 2 provided evidence for FM spin fluctuations at all Co-doping levels in addition to the AF spin fluctuations [15,16]. In particular, strong FM spin fluctuations in AFe 2−x Co x As 2 are believed to compete with AF spin fluctuations and prevent superconductivity for Co-overdoped samples [15,16], contrary to the Fermi surface nesting picture where superconductivity is suppressed via vanishing hole Fermi surfaces with increasing Co-doping [7,17]. Finally, action of physical, chemical pressure, or aliovalent substitution in BCo 2 As 2 (B = Eu, Ca) can drive these AF materials into ferromagnets [18]. In particular, CaCo 1.84 As 2 with a collapsed tetragonal structure [19] forms A-type AF ground state with coexisting FM spin fluctuations within the CoAs layer and A-type AF s...

Ferromagnetic cluster-glass phase in Ca(Co1−xIrx)2−yAs2Pakhira1, Sangeetha2, Smetana3 et al. 2020Phys. Rev. B14062View full textAdd to dashboardCite

“…Inelastic neutron-scattering measurements revealed the presence of stripe-type AFM fluctuations at the same wave vector as observed for the high-T c parent compounds AFe 2 As 2 [26,27]. However, in contrast to the AFe 2 As 2 compounds, no obvious Fermi-surface nesting was observed at that wave vector for SrCo 2 As 2 and, furthermore, FM fluctuations occur that evidently hinder the occurrence of SC [28,29]. FM fluctuations were also observed in FeAs-based superconductors, suggesting that the different T c 's observed in these materials may be at least partially explained by different levels of FM fluctuations in the compounds [30].…”

Helical magnetic ordering in Sr(Co1−xNix)2As2

“…Here, we study the Ni-doped compounds using neutron and high-energy x-ray diffraction to determine the microscopic details of the AF ground state. Rather than the stripe-AF order one may expect from the spin-fluctuation spectrum of SrCo 2 As 2 [24,25,34], we find that the series develops incommensurate AF order consisting of FM-aligned transition-metal layers where the in-plane ordered magnetic moment (μ ⊥ c) forms a helix propagating perpendicular to the layers (i.e., along c). These results support recent evidence that SrCo 2 As 2 possesses frustrated magnetic interactions driven by flat-band instabilities that place the system on the border between itinerant two-dimensional FM (2D-FM) and stripe-AF [24,34].…”

“…Rather than the stripe-AF order one may expect from the spin-fluctuation spectrum of SrCo 2 As 2 [24,25,34], we find that the series develops incommensurate AF order consisting of FM-aligned transition-metal layers where the in-plane ordered magnetic moment (μ ⊥ c) forms a helix propagating perpendicular to the layers (i.e., along c). These results support recent evidence that SrCo 2 As 2 possesses frustrated magnetic interactions driven by flat-band instabilities that place the system on the border between itinerant two-dimensional FM (2D-FM) and stripe-AF [24,34]. Far more subtle, compositiondependent variations in interlayer interactions and magnetic anisotropy, as recently observed in Sr 1−x Ca x Co 2 As 2 , are at play in determining the details of the layer stacking (such as helical, A-type, or more complex collinear magnetic order) [27].…”

“…Many of these phenomena extend in unique ways to the structurally related ACo 2 As 2 , A = Ca, Sr, Ba, Eu, cobalt arsenides. While the cobalt-arsenide materials are not found to be superconducting, they harbor signatures of weakly itinerant ferromagnetism (FM) [23,24], unusual spin fluctuations [25,26], and magnetic frustration [26,27], which are tied to flat-band-driven Stoner instabilities [24,28].…”

First-order phase transition to a nonmagnetic ground state in nonsymmorphic NbCrP

“…Therefore, the temperature dependence of χ and 1/T 1 T can be interpreted in two ways. One is the coexistence of AFM and FM correlations, which was reported in SrCo 2 As 2 [13][14][15]. Another is a contribution from a sharp peak in the density of states near the Fermi energy, which has been discussed in the skutterudite compounds CePt 4 Ge 12 [16] and SrFe 4 As 12 [17].…”