[Paper Review] Nematic spin fluid in the tetragonal phase of BaFe2As2

Leland Harriger, Huiqian Luo+6 more|arXiv (Cornell University)|Nov 16, 2010

Iron-based superconductors research18 citations

TL;DR

This study uses inelastic neutron scattering to demonstrate that the tetragonal phase of BaFe2As2 exhibits a nematic spin fluid characterized by in-plane spin anisotropy above the antiferromagnetic transition temperature (TN). The persistence of anisotropic spin excitations near the Brillouin zone boundary in the paramagnetic phase indicates that spin nematic order—driven by electronic nematicity—precedes and likely mediates the structural and magnetic transitions, offering key insight into the role of electronic nematicity in iron-based superconductors.

ABSTRACT

Magnetic interactions are generally believed to play a key role in mediating electron pairing for superconductivity in iron arsenides; yet their character is only partially understood. Experimentally, the antiferromagnetic (AF) transition is always preceded by or coincident with a tetragonal to orthorhombic structural distortion. Although it has been suggested that this lattice distortion is driven by an electronic nematic phase, where a spontaneously generated electronic liquid crystal state breaks the C4 rotational symmetry of the paramagnetic state, experimental evidence for electronic anisotropy has been either in the low-temperature orthorhombic phase or the tetragonal phase under uniaxial pressure that breaks this symmetry. Here we use inelastic neutron scattering to demonstrate the presence of a large in-plane spin anisotropy above TN in the unstressed tetragonal phase of BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe2As2 is the source of the electronic and orbital anisotropy observed above TN by other probes, and has profound consequences for the physics of these materials.

Motivation & Objective

• To investigate the presence of spin anisotropy in the paramagnetic tetragonal phase of BaFe2As2, where no long-range magnetic order exists.
• To determine whether electronic nematicity—breaking C4 rotational symmetry—manifests in spin degrees of freedom prior to the antiferromagnetic transition.
• To clarify the role of spin nematicity in mediating the structural transition from tetragonal to orthorhombic symmetry.
• To establish a connection between spin anisotropy and the observed electronic and orbital anisotropy in the tetragonal phase.

Proposed method

• Inelastic neutron scattering (INS) was employed to probe spin excitation spectra in single-crystalline BaFe2As2 across the tetragonal and orthorhombic phases.
• Measurements were performed at temperatures both below and above the Néel temperature (TN) to compare spin dynamics in the paramagnetic and antiferromagnetic states.
• The spin wave dispersion and spectral weight were analyzed to detect anisotropy in spin excitation intensity and energy dependence.
• The data were compared with theoretical models of spin nematic order and spin wave theory to assess the nature of the anisotropic excitations.
• A detailed analysis of the momentum dependence of the spin excitations was conducted, particularly near the zone boundary.
• Supplementary data from neutron scattering were used to validate the presence of persistent anisotropy in the paramagnetic phase.

Experimental results

Research questions

• RQ1 Does spin anisotropy persist in the paramagnetic tetragonal phase of BaFe2As2 above TN, despite the absence of long-range magnetic order?
• RQ2 What is the origin of the observed in-plane spin anisotropy in the tetragonal phase, and is it linked to electronic nematicity?
• RQ3 How do spin excitations evolve from the low-temperature orthorhombic phase to the paramagnetic tetragonal phase?
• RQ4 To what extent do the anisotropic spin excitations near the zone boundary survive in the paramagnetic state, indicating a nematic spin fluid?
• RQ5 Is the observed spin anisotropy consistent with a spin nematic state that breaks rotational symmetry?

Key findings

• A large in-plane spin anisotropy is observed in the paramagnetic tetragonal phase of BaFe2As2 above TN, indicating the presence of spin nematic order.
• In the low-temperature orthorhombic phase, spin waves exhibit strong damping along the AF a-axis, signaling anisotropic spin dynamics.
• Upon warming into the tetragonal phase, low-energy spin waves evolve into quasi-elastic excitations, yet the anisotropic spin excitations near the zone boundary persist.
• The persistence of anisotropic spin excitations in the paramagnetic phase strongly suggests a nematic spin fluid state above TN.
• The results indicate that spin nematicity in the tetragonal phase is the source of the electronic and orbital anisotropy observed by other probes, such as resistivity and optical measurements.
• The findings imply that spin nematic order plays a fundamental role in driving the structural transition and may influence superconducting pairing in iron arsenides.