[Paper Review] Antiferromagnetic spin excitations in single crystals of nonsuperconducting Li1-xFeAs

M Wang, XC Wang+10 more|arXiv (Cornell University)|Apr 19, 2011

Advanced Chemical Physics Studies4 citations

TL;DR

This study uses inelastic neutron scattering to investigate spin excitations in nonsuperconducting Li1-xFeAs single crystals, revealing antiferromagnetic spin excitations at Q=(1,0) at low energies and shifting to Q'=(±0.5,±0.5) near the zone boundary. Despite poor Fermi surface nesting, strong spin fluctuations dominate low-energy physics, indicating that both itinerancy and electron correlations are essential for understanding magnetic excitations in iron arsenides.

ABSTRACT

We use neutron scattering to determine spin excitations in single crystals of nonsuperconducting Li1-xFeAs throughout the Brillouin zone. Although angle resolved photoemission experiments and local density approximation calculations suggest poor Fermi surface nesting conditions for antiferromagnetic(AF) order, spin excitations in Li1-xFeAs occur at the AF wave vectors Q = (1, 0) at low energies, but move to wave vectors Q = (\pm 0.5, \pm0.5) near the zone boundary with a total magnetic bandwidth comparable to that of BaFe2As2. These results reveal that AF spin excitations still dominate the low-energy physics of these materials and suggest both itinerancy and strong electron-electron correlations are essential to understand the measured magnetic excitations.

Motivation & Objective

• To determine the nature of spin excitations in nonsuperconducting Li1-xFeAs single crystals, which lack static antiferromagnetic order.
• To investigate whether antiferromagnetic spin fluctuations persist in the absence of Fermi surface nesting, challenging conventional expectations from band structure calculations.
• To assess the role of electron correlations and itinerant magnetism in shaping the magnetic excitation spectrum in iron-based superconductors.
• To compare the magnetic excitation spectrum of Li1-xFeAs with other iron pnictides and determine if it can be described by a simple Heisenberg model.

Proposed method

• Performed inelastic neutron scattering on single crystals of Li0.94FeAs with x=0.06±0.01 to map spin excitations across the entire Brillouin zone.
• Measured energy- and momentum-dependent spin excitation spectra, identifying dispersive modes and wave vector evolution with increasing energy.
• Integrated the imaginary part of the dynamical spin susceptibility χ''(ω) over the full bandwidth to extract the spin fluctuating moment ⟨m²⟩.
• Used a spin-wave Hamiltonian with nearest-neighbor (J1a, J1b), next-nearest-neighbor (J2), and ring-exchange (J3) interactions to model the dispersion.
• Applied a phenomenological damping term Γ=0.15E to account for spin wave broadening in the model calculations.
• Compared experimental constant-energy slices and dispersion relations with theoretical models to constrain exchange couplings and determine the sign of J1b.

Experimental results

Research questions

• RQ1 Do antiferromagnetic spin excitations persist in Li1-xFeAs despite poor Fermi surface nesting predicted by LDA calculations?
• RQ2 How do spin excitation wave vectors evolve with increasing energy, and what do they reveal about the underlying magnetic interactions?
• RQ3 Can the observed high-energy spin excitations near Q'=(±0.5,±0.5) be explained by a minimal Heisenberg model with standard exchange couplings?
• RQ4 What is the role of electron correlations and itinerant magnetism in sustaining strong spin fluctuations in the absence of static long-range order?
• RQ5 What exchange coupling parameters are required to reproduce the experimental spin excitation spectrum, particularly the anisotropy in the dispersion along different crystallographic directions?

Key findings

• Spin excitations in Li0.94FeAs are centered at Q=(1,0) for energies up to ~80 meV, indicating the presence of dynamic antiferromagnetic fluctuations.
• At energies above ~130 meV, spin excitations split and disperse toward Q'=(±0.5,±0.5), suggesting strong magnetic anisotropy and competing exchange interactions.
• The total magnetic bandwidth is comparable to that of BaFe2As2, indicating strong magnetic correlations despite the absence of static order.
• The spin fluctuating moment ⟨m²⟩=2.1±0.6 μB² is comparable to other iron pnictides, confirming strong local moments.
• A simple Heisenberg model with nearest- and next-nearest-neighbor couplings fails to reproduce the high-energy dispersion, requiring additional terms such as ring exchange (J3) and anisotropy.
• Theoretical analysis proves that a negative J1b (next-nearest-neighbor coupling) is both necessary and sufficient to reproduce the observed dispersion maximum along the [1,K] direction, indicating competing ferromagnetic and antiferromagnetic interactions.