Distinct switching of chiral transport in the kagome metals KV₃Sb₅ and CsV₃Sb₅

Kagome metals, with their unique lattice structure, are ideal platforms to explore correlated topological materials. The AV₃Sb₅ (A = K, Rb, Cs) family exemplifies this, exhibiting charge order transitions at around 100 K and showcasing intriguing electronic instabilities. A central question revolves around the broken symmetries within the ordered state, particularly time-reversal and mirror symmetries. Experimental evidence suggests broken symmetries, including electronic C₂ anisotropy, a chiral charge-density-wave state, and three-state nematicity. Electrical magneto-chiral anisotropy (eMChA), a current-direction dependent voltage response in the absence of mirror symmetry, has been observed in CsV₃Sb₅, indicating electronic chirality switchable by a magnetic field. This study investigates whether this phenomenon is general across the AV₃Sb₅ family by comparing CsV₃Sb₅ and KV₃Sb₅, both exhibiting charge order and superconductivity, to elucidate the crucial factors influencing magneto-chiral transport.

Previous research on AV₃Sb₅ compounds has focused on understanding the interplay between charge order and superconductivity. Studies have reported various broken symmetries, including electronic C₂ anisotropy, chiral charge-density-wave states observed via STM, and three-state nematicity detected through optical Kerr effect. The observation of field-switchable eMChA in CsV₃Sb₅ provided strong evidence for a controllable electronic chirality within the charge-ordered state. However, the universality of this phenomenon within the AV₃Sb₅ family remained unclear, with conflicting reports on the field-controllable chirality in KV₃Sb₅. This gap in the literature prompted the current comparative study.

The study used membrane-based microstructures fabricated via focused-ion-beam (FIB) milling to minimize strain, a crucial factor for observing eMChA. These devices, featuring a Hall-bar geometry, allowed for precise measurements of electronic transport properties in both CsV₃Sb₅ and KV₃Sb₅. Linear response regime measurements determined the charge density wave (CDW) transition temperatures (94 K for CsV₃Sb₅ and 76 K for KV₃Sb₅), consistent with previous reports. Magneto-transport measurements up to 35 T were performed to investigate Shubnikov-de Haas oscillations and extract information about the Fermi surfaces. To probe eMChA, second harmonic voltage generation was measured using low-frequency AC currents. Ab initio band structure calculations were employed to complement the experimental findings and provide a theoretical comparison of the electronic structure between CsV₃Sb₅ and KV₃Sb₅. The angular dependence of magnetoresistance was analyzed to reveal the in-plane anisotropy.

The study revealed a striking contrast in magneto-chiral transport between CsV₃Sb₅ and KV₃Sb₅. CsV₃Sb₅ exhibited a non-saturating, field-asymmetric second harmonic voltage (V_2ω) signal, increasing linearly with the magnetic field, indicative of robust magneto-chiral transport and a field-switchable electronic chirality. In contrast, KV₃Sb₅ displayed an almost negligible V_2ω signal, more than two orders of magnitude smaller than CsV₃Sb₅, with no field switchability. Despite similar electronic band structures revealed by both DFT calculations and quantum oscillation measurements, showing comparable Fermi surfaces, the eMChA drastically differed. This stark difference highlights the crucial role of correlated states in determining magneto-chiral transport, rather than just the single-particle electronic structure. Analysis of magnetoresistance showed a pronounced angular anisotropy in CsV₃Sb₅, indicative of coherent interlayer transport, which was significantly suppressed in KV₃Sb₅ suggesting enhanced scattering. The difference in residual resistivity and weaker Shubnikov-de Haas oscillations in KV₃Sb₅ were attributed to a higher vacancy density compared to CsV₃Sb₅.

The substantial difference in eMChA between CsV₃Sb₅ and KV₃Sb₅, despite their similar electronic band structures, points towards the significant influence of electron correlations and the role of disorder. The higher vacancy density in KV₃Sb₅ is hypothesized to either smear out the chiral signal through increased achiral scattering or to pin chiral domains into an achiral pattern, suppressing the eMChA response. The possibility of an achiral bulk state in KV₃Sb₅, even with reports of chiral surface states, is also considered. The observed suppression of eMChA in KV₃Sb₅ underscores the extreme sensitivity of correlated states in these materials to subtle electronic features. Future research directions include targeted doping studies to control vacancy concentrations and investigate the relationship between defect density and eMChA amplitude. This would help discern between competing mechanisms (smearing effect vs. chiral domain pinning) and confirm the exact origin of the distinct chiral transport behavior.

This study demonstrates a striking difference in magneto-chiral transport between the kagome metals CsV₃Sb₅ and KV₃Sb₅, with CsV₃Sb₅ exhibiting a robust, field-switchable eMChA and KV₃Sb₅ showing negligible eMChA. The results emphasize the crucial role of correlated states beyond single-particle physics in determining chiral transport properties. Further investigations involving controlled defect introduction and high-field measurements are suggested to fully unravel the underlying mechanisms.

The study focuses primarily on two specific members of the AV₃Sb₅ family. Generalizing these findings to the entire family requires further investigation of RbV₃Sb₅ and other potential members. The influence of other subtle factors affecting scattering and domain formation, beyond vacancy concentration, needs more comprehensive study. The analysis relies on several assumptions about the mechanisms driving eMChA, which will require more verification.