[Paper Review] Static and fluctuating stripe order observed by resonant soft x-ray diffraction in La1.8Sr0.2NiO4

J. Schlappa, C. F. Chang+12 more|ArXiv.org|Mar 5, 2009

Magnetic and transport properties of perovskites and related materials3 citations

TL;DR

This study uses resonant soft x-ray diffraction (RSXD) and neutron diffraction to investigate stripe order in La1.8Sr0.2NiO4, revealing that RSXD detects both static and fluctuating magnetic order due to its energy-integrated nature. Unlike neutron diffraction, which shows distinct temperature dependencies for spin and charge order, RSXD shows identical decay of spin and charge signals, indicating that fluctuating magnetic order significantly contributes to the RSXD signal and offers a new method to probe dynamic order in quantum materials.

ABSTRACT

We studied the stripe phase of La1.8Sr0.2NiO4 using neutron diffraction, resonant soft x-ray diffraction (RSXD) at the Ni L2,3 edges, and resonant x-ray diffraction (RXD) at the Ni K threshold. Differences in the q-space resolution of the different techniques have to be taken into account for a proper evaluation of diffraction intensities associated with the spin and charge order superstructures. We find that in the RSXD experiment the spin and charge order peaks show the same temperature dependence. In the neutron experiment by contrast, the spin and charge signals follow quite different temperature behaviors. We infer that fluctuating magnetic order contributes considerably to the magnetic RSXD signal and we suggest that this result may open an interesting experimental approach to search for fluctuating order in other systems by comparing RSXD and neutron diffraction data.

Motivation & Objective

• To resolve the discrepancy between neutron diffraction and x-ray diffraction results on charge and spin order in La1.8Sr0.2NiO4, particularly the differing low-temperature behaviors.
• To investigate whether the observed differences in temperature dependence stem from differences in q-space resolution or from intrinsic contributions of fluctuating magnetic order.
• To determine whether resonant soft x-ray diffraction (RSXD) is sensitive to both static and fluctuating magnetic order, unlike conventional elastic neutron diffraction.
• To establish a comparative experimental framework using RSXD, neutron diffraction, and resonant x-ray diffraction (RXD) at the Ni K-edge to disentangle contributions from static and dynamic electronic order.

Proposed method

• Performed resonant soft x-ray diffraction (RSXD) at the Ni L2,3 edges to probe spatial modulations of electronic states and magnetic scattering contrast.
• Conducted neutron diffraction at the Orphée reactor to measure elastic scattering from static spin and charge order, with energy analysis to distinguish static and fluctuating components.
• Carried out resonant x-ray diffraction (RXD) at the Ni K-edge to independently verify the charge order signal and compare q-space resolution effects.
• Used high-quality single crystals of La1.8Sr0.2NiO4 with (101) and (103) surface orientations, confirmed by x-ray diffraction with a rocking width of 0.01° (FWHM).
• Analyzed temperature-dependent peak intensities, widths, and integrated intensities along H and L directions in q-space to assess resolution effects.
• Compared the temperature evolution of spin order (SO) and charge order (CO) signals across techniques, accounting for differences in q-space resolution and detection sensitivity.

Experimental results

Research questions

• RQ1 Why do conventional x-ray diffraction experiments show a maximum in charge order intensity at intermediate temperatures, while neutron diffraction shows monotonic growth upon cooling?
• RQ2 To what extent do differences in q-space resolution between neutron and x-ray diffraction experiments explain the observed discrepancies in temperature-dependent intensities?
• RQ3 Does resonant soft x-ray diffraction (RSXD) detect only static magnetic order or also fluctuating magnetic correlations, given its lack of energy resolution?
• RQ4 How do the temperature dependencies of spin and charge order signals compare in RSXD versus elastic neutron diffraction, and what does this imply about the nature of the magnetic response?
• RQ5 Can the comparison of RSXD and neutron diffraction data serve as a general method to detect fluctuating magnetic order in other quantum materials?

Key findings

• RSXD at the Ni L3 edge shows that spin and charge order signals decay on the same temperature scale upon heating, indicating a common origin in the magnetic response.
• Neutron diffraction reveals a distinct temperature dependence: spin order sets in at TN ≈ 105 K and decays slowly, while charge order increases monotonically and saturates at low temperatures.
• The discrepancy between neutron and x-ray results is partially explained by q-space resolution: neutron experiments integrate over broader q-space, leading to slower decay of integrated intensity compared to peak height in high-resolution x-ray experiments.
• Peak broadening in neutron diffraction causes the charge order signal to appear more robust at higher temperatures than in RSXD, where peak height decays faster.
• The identical temperature dependence of SO and CO in RSXD, despite their different behavior in neutron diffraction, indicates that RSXD detects both static and fluctuating magnetic order, as it lacks energy resolution and thus integrates over inelastic contributions.
• The study suggests that comparing RSXD and neutron diffraction data can serve as a novel experimental approach to identify fluctuating magnetic order in other strongly correlated systems.