Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1−xCox)2As2

Abstract

We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned Ba(Fe1−xCox)2As2 with x = 0.048 and 0.054. Both samples exhibit a tetragonal-to-orthorhombic structural transition at Ts, a collinear static antiferromagnetic (AF) order at wave vector Q 1 = QAF = (1, 0) below the Néel temperature TN , and superconductivity below Tc (Ts > TN > Tc).In the high temperature paramagnetic tetragonal phase (T Ts), spin excitations centered at Q1 and Q2 = (0, 1) are gapless and have four-fold (C4) rotational symmetry. On cooling to below TN but above Tc, spin excitations become highly anisotropic, developing a gap at Q2 but still are gapless at Q1. Upon entering into the superconducting state, a neutron spin resonance appears at Q1 with no magnetic scattering at Q2. By comparing these results with those from angle resolved photoemission spectroscopy experiments, we conclude that the anisotropic shift of the dyz and dxz bands in detwinned Ba(Fe1−xCox)2As2 below Ts is associated with the spin excitation anisotropy, and the superconductivity-induced resonance arises from the electron-hole Fermi surface nesting of quasiparticles with the dyz orbital characters. arXiv:1910.01871v1 [cond-mat.supr-con]

Orbital selective spin waves in detwinned NaFeAs

“…The observation of low-energy d yz -d yz fluctuations is consistent with the idea that these excitations are itinerant and responsible for superconducting pairing [21][22][23], while the d xy -d xy fluctuations are more localized and do not observably change across the electron-doped superconducting dome [68]. Since the spin fluctuation spectrum in BaFe 2 As 2 also exhibits qualitatively similar results to that in NaFeAs, and with DFT+DMFT calculations showing high-energy scattering centered at Q xy = (1, 1) but extending to higher energy in BaFe 2 As 2 , we conclude that the overall magnetic bandwidth in iron pnictides is controlled by localized magnetic superexchange interactions.…”

“…Since it is not possible to rotate the "weak" magnetic 054430-3 Bragg peak at (0, ±1, 1.5) positions onto the detector bank at fixed E i , one cannot use the above-mentioned method to directly measure the sample assembly detwinning ratio, defined as η = [I (1, 0, 1.5) − I (0, 1, 1.5)]/[I (1, 0, 1.5) + I (0, 1, 1.5)], where I (1, 0, 1.5) and I (0, 1, 1.5) correspond to the strong and weak magnetic Bragg peaks, respectively [22]. Nevertheless, one can still determine the sample detwinning ratio η using the low-energy inelastic part of the spin fluctuation spectrum, which at least provides a firm lower bound, and was in fact observed to have one-to-one correspondence to η in previous experiments on the NaFeAs and BaFe 2 As 2 systems [22,23].…”

“…Within the t-J model, this means that E ex (T ) = 2J[ S i+x • S i N − S i+x • S i S ], where J is the nearest-neighbor magnetic exchange coupling and S i+x • S i is the magnetic scattering in absolute units at temperature T [71]. In this picture, the quasiparticle excitations across the nested hole-electron Fermi surfaces in the nematic phase arise mostly from the d yz -d yz intraorbital excitations, and give rise to the C 2 neutron spin resonance seen in different families of iron-based superconductors [21][22][23]. The increased electronelectron correlations from BaFe 2 As 2 to NaFeAs, reflected in reduced bandwidth of the d xy orbital and high-energy spin excitations [30], correspond to the reduced magnetic exchange coupling J.…”

“…Differentiation of these mechanisms has important consequences for the origins of superconductivity [20]. Inelastic neutron scattering experiments on detwinned FeSe [21], underdoped superconducting NaFe 0.985 Co 0.015 As [22], and Ba(Fe 1−x Co x ) 2 As 2 [23] suggest that the neutron spin resonance, a collective magnetic excita-tion coupled to the superconducting order parameter [24][25][26][27][28][29], likely arises from orbital selective electron-hole Fermi surface nesting of quasiparticles within the single d yz orbital submanifold.…”

Orbital Selectivity in Electron Correlations and Superconducting Pairing of Iron-Based Superconductors

“…This behavior is understood by the strong orbital-selective pairing [72], which also splits into two the neutron resonance peak as a function of energy in the superconducting state [73,148]. The orbital-selective pairing is also being explored in other FeSCs [149].…”