Spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe2(As0.7P0.3)2

Abstract

We use neutron polarization analysis to study spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe 2 (As 0.7 P 0.3 ) 2 (T c = 30 K). Different from optimally hole-and electron-doped BaFe 2 As 2 , where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above T c , we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe 2 (As 0.7 P 0.3 ) 2 . Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center Q AF = (1,0,L = odd), while the magnetic spectrum is isotropic at the zone boundary Q = (1,0,L = even). By comparing the temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe 2 (As 0.7 P 0.3 ) 2 and hole-doped Ba 0.67 K 0.33 Fe 2 As 2 (T c = 38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity. DOI: 10.1103/PhysRevB.96.180503 Spin-orbit coupling (SOC) is an interaction of an electron's spin with its motion. While the importance of SOC to the electronic properties of the 4d and 5d correlated electron materials such as Sr 2 RuO 4 and Sr 2 IrO 4 is long recognized [1,2], its relevance to the physics of the 3d correlated electron materials such as iron pnictide superconductors is much less clear. Since iron pnictide superconductors are derived from metallic parent compounds exhibiting antiferromagnetic (AF) order at T N below a tetragonal-to-orthorhombic structural transition temperature T s associated with orbital ordering and nematic phase [ Fig. 1(a)] [3][4][5][6][7], most microscopic theories for iron-based superconductors are focused on the role of spin [8][9][10], orbital [11], or nematic [7,12] fluctuations to the electron pairing and superconductivity. Although angleresolved photoemission spectroscopy (ARPES) experiments on different families of iron-based superconductors have identified the presence of SOC through observation of electronic band splitting at the Brillouin zone center (ZC) below T s [13][14][15], much is unknown concerning the role of SOC to the AF order, nematic phase, electron pairing mechanism, and superconductivity [16][17][18][19].In addition to its impact on the Fermi surface and electronic band dispersions, SOC also brings lattice anisotropies into anisotropies of magnetic fluctuations [20,21] [46][47][48][49][50][51]. Although polarized INS experiments have conclusively established the presence of SOC induced lowenergy spin excitation anisotropy near the resonance mode in different families of iron-based superconductors [23][24][25][26][27][28][29][30][31][32][33][34], the spin excitation anisotropy persists in the paramagnetic tetragonal state, and becomes isotropic at temperatures well above T N and T s [25,26,[30][31][32]. Therefore, it is still unclear how SOC is coupled to spin fluctuation anisotropy and superconductivity.To resolve this problem, we used polarized INS to study low-energy spin ...

Strong spin resonance mode associated with suppression of soft magnetic ordering in hole-doped Ba1-xNaxFe2As2

“…In addition to a broad SRM that is isotropic and lies at a higher energy corresponding to E res $ 5:3k B T c there is a sharp low-energy SRM-1 that is anisotropic in spin space. Qualitatively the same superposition of a low-energy anisotropic feature SRM-1 and a broader isotropic one at higher energies, SRM-2, has been established in various FeAs-based superconductors: in Ba(Fe 0.94 Co 0.06 ) 2 As 2 (optimally electron doped), 12,13 in NaFe 1−x Co x As, 14,15 in P-doped BaFe 2 As 2 , 16 in Ni-doped BaFe 2 As 2 , 17 and in hole-doped BaFe 2 As 2. 18,19 Various attempts were made to attribute the split and anisotropic SRMs to either persisting antiferromagnetic correlations and their corresponding excitations [20][21][22] or to an orbital-selective pairing.…”

“…The higher SRM-2 lacks c polarisation at an energy transfer of 10 meV but exhibits a In contrast the lower SRM-1 has no b contribution again in agreement with numerous observations for the lower SRM in other materials. [12][13][14]16,19 However, SRM-1 is still essentially polarised along the c direction (79(5)% c polarised and 21% [110] = a polarized) although the static magnetism is rotated from a towards c direction. 29,45 The rotation of the AFM ordered moment is thus not reflected by a rotation of the polarisation of this sharp extra low-energy SRM.…”

“…29,45 The rotation of the AFM ordered moment is thus not reflected by a rotation of the polarisation of this sharp extra low-energy SRM. c polarised extra SRMs are therefore characteristic for the SC phase of FeAs-based materials, [12][13][14]16,19,39 independently on the alignment of their ordered moments.…”

Spin-Orbital-Intertwined Nematic State in FeSe

“…Recently, a nontrivial topological band structure has been successfully verified in iron chalcogenide superconductors [19][20][21] that confirms the importance of SOC on the band structure in FeSCs. A natural follow-up question is how to understand the SOC effect on the various quantum matter states, such as nematic and superconducting states, due to electronic correlation in FeSCs [13][14][15][16][17][18].…”

Preferred magnetic excitations in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$

“…The energy scale of SOC in these materials is non-negligible [6][7][8] compared to the typical phase transition temperatures. Indeed, inclusion of SOC into theoretical models has already allowed researchers to reproduce some of the most intricate electronic groundstate phase behaviors seen in experiments [9][10][11], and the influence of SOC on magnetic excitations has been observed with inelastic neutron scattering (INS) in both undoped [12][13][14] and doped systems [15][16][17][18][19]. In spite of these efforts and the widely accepted notion that magnetic excitations may mediate Cooper pairing [1,20], how SOC might affect the formation of superconductivity has remained largely unexplored.…”