[Paper Review] Hidden magnetism uncovered in charge ordered bilayer kagome material ScV_6Sn_6

Zurab Guguchia, D. J. Gawryluk+21 more|arXiv (Cornell University)|Apr 13, 2023

Advanced Condensed Matter Physics4 citations

TL;DR

Using muon spin rotation spectroscopy, this study uncovers hidden time-reversal symmetry-breaking magnetism in the charge-ordered bilayer kagome material ScV6Sn6, despite its lack of long-range magnetic order. The key finding is a substantial enhancement in muon spin relaxation rate and internal field width below the charge ordering transition at ~80 K, which increases further under external magnetic fields, indicating bulk, intrinsic electronic magnetism linked to the charge order.

ABSTRACT

Charge ordered kagome lattices have been demonstrated to be intriguing platforms for studying the intertwining of topology, correlation, and magnetism. The recently discovered charge ordered kagome material ScV_6Sn_6 does not feature a magnetic groundstate or excitations, thus it is often regarded as a conventional paramagnet. Here, using advanced muon-spin rotation spectroscopy, we uncover an unexpected hidden magnetism of the charge order. We observe a striking enhancement of the internal field width sensed by the muon ensemble, which takes place within the charge ordered state. More remarkably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. Taken together with the hidden magnetism found in AV_3Sb_5 (A = K, Rb, Cs) and FeGe kagome systems, our results suggest ubiqitous time-reversal symmetry-breaking in charge ordered kagome lattices.

Motivation & Objective

• To investigate the presence of hidden magnetism in ScV6Sn6, a bilayer kagome material with charge order but no long-range magnetic order.
• To determine whether the charge-ordered state in ScV6Sn6 hosts time-reversal symmetry-breaking electronic order, similar to that observed in AV3Sb5 compounds.
• To probe the microscopic origin of electronic response in ScV6Sn6 using high-sensitivity muon spin relaxation (µSR) techniques.
• To compare the magnetic response in ScV6Sn6 with that of AV3Sb5 (A = K, Rb, Cs) to assess similarities and differences in the nature of the hidden magnetism.
• To provide experimental evidence for the interplay between charge order, electronic correlations, and emergent magnetism in kagome lattices.

Proposed method

• Employed zero-field (ZF) and high-field muon spin rotation (µSR) spectroscopy to detect local magnetic fields and spin relaxation dynamics in ScV6Sn6 single crystals.
• Measured temperature-dependent muon spin relaxation rates (Γ) and internal field width (Δ) to identify changes associated with the charge ordering transition at T* ≈ 80 K.
• Analyzed the field dependence of the relaxation rate to distinguish between Gaussian and Lorentzian relaxation components, identifying the dominant contribution from static or slowly fluctuating internal fields.
• Used laboratory X-ray diffraction and single-crystal structural refinements to confirm the kagome lattice structure and space group P6/mmm (191) of ScV6Sn6.
• Compared the µSR response in ScV6Sn6 with that of AV3Sb5 compounds (A = K, Rb, Cs) to assess the universality of time-reversal symmetry-breaking in charge-ordered kagome systems.
• Correlated µSR data with heat capacity and diffraction data to confirm the bulk nature of the charge order transition and its relationship to electronic response.

Experimental results

Research questions

• RQ1 Does the charge-ordered state in ScV6Sn6 host hidden magnetism despite the absence of long-range magnetic order?
• RQ2 Is the observed enhancement in muon spin relaxation rate below T* indicative of time-reversal symmetry-breaking electronic order?
• RQ3 How does the field dependence of the relaxation rate in ScV6Sn6 compare to that in AV3Sb5 compounds, particularly in terms of field-induced enhancement?
• RQ4 What is the origin of the increased relaxation rate in ScV6Sn6—localized moments or dynamic electronic currents?
• RQ5 To what extent is the observed magnetism in ScV6Sn6 similar or different from that in AV3Sb5, and does it suggest a universal mechanism in kagome lattices?

Key findings

• A significant enhancement in the muon spin relaxation rate (Γ) by 0.12 µs⁻¹ was observed below the charge ordering transition temperature T* ≈ 80 K in ScV6Sn6, indicating the presence of internal magnetic fields.
• The Gaussian relaxation component (Δ) increased by 0.12 µs⁻¹ below T*, suggesting a network of dense, weak electronic moments or fluctuating local fields in the charge-ordered state.
• The application of an external magnetic field (up to 8 T) further enhanced the relaxation rate to 0.23 µs⁻¹, a 1.6-fold increase compared to KV3Sb5, indicating strong field-induced response.
• The relaxation enhancement was observed in at least 50% of the sample volume, confirming the bulk nature of the electronic response in ScV6Sn6.
• The field-induced enhancement of the relaxation rate in ScV6Sn6 was more pronounced than in AV3Sb5 compounds, particularly compared to KV3Sb5, where the increase was only 0.15 µs⁻¹ at 8 T.
• The results suggest that the charge-ordered state in ScV6Sn6 hosts time-reversal symmetry-breaking electronic order, analogous to that in AV3Sb5, but with a distinct microscopic origin due to differences in the CDW nature and lack of theoretical model for orbital currents.