One-Dimensional Magnetism Unlocked in Novel Material Could Boost Quantum Technology

Researchers investigating the complex behaviour of iron pnictides have uncovered evidence of quasi-one-dimensional spin excitations in the compound NaFe₀.₅₃Cu₀.₄₇As. Yifan Wang from Zhejiang University, alongside David W. Tam and Weiyi Wang from Rice University, and colleagues including R. A. Ewings, J. Ross Stewart, and Masaaki Matsuda, utilised inelastic neutron scattering to directly observe these excitations, arising from the unique atomic order creating magnetic iron and non-magnetic copper chains within the material. This discovery is significant because it demonstrates how magnetic dilution can reduce the dimensionality of magnetic interactions, offering a potential route to engineer and identify novel low-dimensional quantum materials and providing a compelling link between the magnetic properties of NaFeCuAs and other iron-based superconductors like FeSe.

Identifying truly 1D materials remains a considerable challenge in condensed matter physics, yet these systems hold the key to unlocking more complex behaviours observed in two-dimensional materials like high-temperature superconductors.

This work directly reveals these 1D spin excitations, originating from the unique arrangement of magnetic iron and non-magnetic copper chains within the material’s structure. Inelastic neutron scattering measurements were employed to observe these spin excitations, demonstrating a dominant antiferromagnetic exchange interaction of approximately 90.1(3) meV along the chains.
This value precisely quantifies the strength of the magnetic interactions responsible for the observed 1D spin behaviour and is essential for refining theoretical models. Inter-chain couplings, however, are considerably weaker, measuring approximately −2.4(1) meV and 0.15(5) meV, further confirming the quasi-1D nature of the magnetic interactions.

The observed quasi-1D spin excitations in NaFe₀.₅₃Cu₀.₄₇As arise from both Néel and stripe magnetic orderings, with Néel excitations exhibiting sensitivity to iron impurities present on the copper sites. Remarkably, a striking resemblance exists between the spin excitations in this quasi-1D material and those found in quasi-2D FeSe, suggesting a shared underlying mechanism for their coexisting stripe and Néel excitations.

These findings demonstrate that magnetic dilution within NaFeAs leads to a reduction in the dimensionality of its magnetic properties, establishing a promising strategy for discovering novel low-dimensional quantum materials. This approach could pave the way for advancements in technologies reliant on quantum phenomena and a deeper understanding of fundamental physics, including the elusive mechanisms behind superconductivity.

Neutron scattering characterisation of magnetic excitations in NaFe0.53Cu0.47As single crystals

Inelastic neutron scattering formed the basis of this work, directly revealing quasi-one-dimensional spin excitations in the material NaFe₀.₅₃Cu₀.₄₇As. Single crystals of NaFe₀.₅₃Cu₀.₄₇As were grown using the self-flux method, a technique employed to produce high-quality samples suitable for neutron scattering experiments.

Precisely 0.43 grams of sample material were co-aligned and subjected to detailed analysis using the MAPS chopper spectrometer at the ISIS Neutron and Muon source, and the Polarized Triple-Axis Spectrometer (HB-1) at the High-Flux Isotope Reactor (ORNL). Momentum transfers were referenced in reciprocal lattice units (H, K, L), where a ≈ b ≈ 5.70 Å and c ≈ 6.88 Å, allowing for precise mapping of the material’s magnetic structure.

MAPS measurements were conducted in the [H0L] plane, utilising incident energies of 50 meV, 81 meV, and 245 meV to probe a wide range of energy scales. Sample rotation about the vertical axis, in 2° steps with an incident energy of 50 meV, generated four-dimensional E-Q data, subsequently analysed using Horace software.

Measured intensities were converted into absolute units through calibration against a vanadium reference, ensuring quantitative accuracy. This meticulous approach enabled the determination that the dominant exchange interaction along the chain is approximately 90.1(3) meV, quantifying the strength of the magnetic interactions responsible for the observed quasi-one-dimensional behaviour. The research demonstrated that magnetic dilution in NaFeAs leads to dimension reduction of its magnetic degree of freedom, presenting a strategy for discovering low-dimensional quantum materials.

Quasi-one-dimensional magnetic excitations and anisotropic exchange in NaFe0.53Cu0.47As

Spectroscopic measurements reveal quasi-one-dimensional (1D) spin excitations in NaFe₀.₅₃Cu₀.₄₇As, demonstrating a dominant exchange interaction along the chain of approximately 90.1(3) meV. This value directly quantifies the strength of magnetic interactions responsible for the observed 1D spin behavior and is a crucial parameter for theoretical modelling of the material.

The research utilized inelastic neutron scattering on single crystals to directly observe these quasi-1D spin excitations, arising from the arrangement of magnetic iron and non-magnetic copper chains. Measurements of the local susceptibility, χ”(ω), at 5 K show a striking resemblance between NaFe₀.₅₃Cu₀.₄₇As and FeSe, with both systems exhibiting spin excitations originating from both stripe and Néel vectors of the square lattice.

The observed anisotropy of exchange interactions confirms the quasi-1D character of magnetism within the material, with a strong preference for interactions along the iron chains. Elastic scattering data, collected in the (0.5, 0, 0) and (0, 0, 0) planes, confirm the presence of magnetic peaks expected from the layered atomic and magnetic orderings.

Temperature-dependent measurements of normalized intensity reveal the evolution of magnetic scattering in NaFe₀.₅₃Cu₀.₄₇As, demonstrating the persistence of magnetic order at varying temperatures. The study establishes that magnetic dilution in NaFeAs leads to a reduction in the dimensionality of its magnetic degrees of freedom, offering a strategy for discovering low-dimensional quantum materials. These findings demonstrate a pathway towards creating and studying materials with potentially novel properties, including superconductivity, and furthering our understanding of fundamental physics.

Dimensionality reduction via magnetic dilution defines low-dimensional magnetic behaviour

Spectroscopic measurements on the material NaFe₀.₅₃Cu₀.₄₇As have directly revealed quasi-one-dimensional spin excitations, originating from the arrangement of magnetic iron and non-magnetic copper chains within the material’s structure. These excitations arise due to a strong antiferromagnetic exchange interaction along these chains, quantified as approximately 90.1(3) meV, while interactions between the chains are considerably weaker.

The observed quasi-one-dimensional spin excitations are linked to both Néel and stripe magnetic orders, with the Néel excitations exhibiting sensitivity to impurities of iron on copper sites. This discovery demonstrates that magnetic dilution in NaFeAs can reduce the dimensionality of its magnetic properties, offering a viable strategy for identifying novel low-dimensional quantum materials.

The resemblance between the spin excitations in NaFe₀.₅₃Cu₀.₄₇As and those in quasi-two-dimensional FeSe suggests a shared underlying mechanism for their coexisting stripe and Néel excitations. While the authors acknowledge the influence of iron impurities on the observed Néel excitations, these findings establish a pathway towards the creation and investigation of low-dimensional quantum materials, potentially advancing technologies and deepening our understanding of fundamental physics, including the phenomenon of superconductivity. Future research may focus on exploring similar material compositions to further refine the control over dimensionality and magnetic interactions.