Contradictions in (Cs,K,Rb)V₃Sb₅ are a feature, not a bug

Spontaneously broken symmetries are at the heart of many phenomena of quantum matter and physics more generally. However, determining the exact symmetries that are broken can be challenging due to imperfections such as strain, in particular when multiple electronic orders are competing. This is exemplified by charge order in some kagome systems, where evidence of nematicity and flux order from orbital currents remains inconclusive due to contradictory measurements. Now, a team of researchers clarifies ¹ this controversy by fabricating highly symmetric samples of a member of this family, Cs V₃Sb₅, and measuring their transport properties.

Materials hosting intertwined electronic ordering phenomena provide both an outstanding challenge and opportunity in current condensed matter physics. Phases such as magnetism, charge order, spin textures or superconductivity may cooperate or compete, or merely coexist without much commonality, thus complicating the interpretation of experimental data. Disentangling the various order parameters into the most elemental building blocks, separating the primary from secondary orders and understanding their interrelation, is the key to solving their rich puzzle.

With the poor performance of ab-initio predictions for such correlated materials, a promising experimental route is to follow structural motifs that either host highly degenerate states or exhibit geometrically frustrated bonds, which are known to commonly host correlated phases at low temperatures. The kagome lattice, a net of triangles connected at their vertices, combines both bond frustration and sublattice symmetries and thus has been a successful platform for the design of non-trivial quantum materials.

AV₃Sb₅, extraordinarily sensitive to even weak perturbations

Recently, a new family of non-magnetic kagome metals AV₃Sb₅ (A = K, Rb, Cs) has emerged as a fertile playground to investigate correlation-driven topological phases. Derived from the kagome structure, this family of materials features a quasi 2D electronic band structure. Superconductivity sets in at low temperature and is intertwined with an exotic charge density wave phase. Although the electronic structure obtained from density functional theory calculations is well established, controversy surrounds the origin and stabilization of the CDW, and the mechanism of superconductivity. The central question is how carefully conducted experiments on a deceptively simple, stoichiometric material of high crystalline purity have yielded contradictory results.

In a recent work, the researchers propose, and experimentally demonstrate, that these discrepancies are intrinsically rooted in the strong coupling of the various orders it hosts. This renders this material class extraordinarily sensitive to even weak perturbations, which could safely be ignored in conventional compounds.

They fabricate highly symmetric samples of a member of this family, CsV₃Sb₅, and measure their transport properties. They find that a measurable anisotropy is absent at any temperature in the unperturbed material. However, a pronounced in-plane transport anisotropy appears when either weak magnetic fields or strains are present. A symmetry analysis indicates that a perpendicular magnetic field can indeed lead to in-plane anisotropy by inducing a flux order coexisting with more conventional bond order.

Although in-plane strain and magnetic fields are demonstrated sensitive perturbations, still the material may be highly sensitive to others as well. In the experimental reality, weakest and inhomogeneous residual strains (for example, from crystal defects and sample mounting) are ubiquitous and hard to avoid, while magnetic fields of several teslas are used in the spirit of non-invasive probes (quantum oscillations, nuclear magnetic resonance, magnetotransport).

Cosequently, the scientists propose that shielding crystalline samples from perturbations as much as possible will consolidate the field and lead towards the identification of the correlated ground state without perturbations. The main point being that the at-first-glance contradictory state of the literature is a feature, not a bug. (Cs,K,Rb)V₃Sb₅ may well realize the long-standing dream of unusual electronic response functions that directly arise from the near-degeneracy of multiple distinct correlated states.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.