Kagome Superconductors Exhibit Incipient Charge Density Wave Order and Hidden Quantum Critical Point

The complex relationship between superconductivity and charge ordering lies at the heart of modern condensed matter physics, and researchers continually seek to understand how these phenomena evolve in different materials. Ilija K. Nikolov from Brown University, Giuseppe Allodi and Anshu Kataria from the University of Parma, along with colleagues, now present compelling evidence for subtle charge correlations within a hole-doped kagome superconductor, revealing a previously hidden critical point in its behaviour. Using a technique sensitive to local charge ordering, the team investigated the material CsV Sb Sn and discovered static charge density wave patterns that persist to surprisingly high temperatures, suggesting they are anchored by imperfections within the material. Their findings demonstrate that these charge patterns would normally disappear at a specific doping level, indicating a critical point between the material’s two superconducting phases, but disorder within the material preserves them, offering new insights into the interplay between superconductivity, charge order, and material imperfections.

This work explores the fundamental mechanisms governing this interaction, focusing on how material structure and imperfections influence the stability of both superconducting and CDW states, employing theoretical modelling and computer simulations to map material behaviour and account for quantum fluctuations and spatial variations.

Kagome Metal Electronic Properties and Phenomena

Current research focuses on Kagome metals, materials with a unique crystal structure resembling a woven basket, exhibiting unusual electronic behaviour due to their specific atomic arrangement. Scientists are investigating interconnected phenomena, including charge density waves and Van Hove singularities, and the strong interactions between electrons that significantly influence the formation of charge density waves. They are also exploring the potential for topological electronic states and how they might interact with the charge density wave order, alongside the possibility of superconductivity and the quantum anomalous Hall effect. To understand these phenomena, scientists employ a range of experimental techniques, including angle-resolved photoemission spectroscopy, transport measurements, neutron scattering, Raman spectroscopy, and scanning tunneling microscopy to probe the electronic structure, electrical conductivity, atomic arrangement, vibrational modes, and charge density wave at the atomic scale.

They also use X-ray diffraction to determine the crystal structure and measure magnetic susceptibility to study magnetic properties. Researchers are exploring various compounds, including CsV₃Sb₅ and related materials, to understand how changing the chemical composition affects the charge density wave and other electronic properties. Current debates centre on the nature of the charge density wave, whether it is driven by conventional mechanisms or more exotic effects, and the role of electron interactions in its formation. Understanding the coexistence of charge density waves and superconductivity, and the relationship between charge density waves and topological states, are also key research questions.

Inhomogeneous CDW Order and Doping Effects

Scientists have uncovered intricate details of charge density wave (CDW) order in compounds of caesium, vanadium, antimony, and tin. Experiments using nuclear quadrupole resonance (NQR) reveal fragmented CDW features, even at temperatures higher than expected, providing insights into the material’s electronic structure. Measurements show that the volume fraction of the inverse Star-of-David (ISD-) CDW pattern decreases with increasing doping, eventually saturating at approximately 50%, indicating that the material stabilizes in an inhomogeneous configuration where the ISD-π order is fragmented, with doping promoting this fragmentation while imperfections preserve the ISD- patches. Analysis of NQR spectra reveals a critical doping level where the ISD- CDW order would vanish in a perfect material. High-temperature charge correlations, measured through NQR linewidth, exhibit a Curie-Weiss (CW) behaviour, and fitting the data to a CW law determined a CW temperature that changes sign near a specific doping level, signifying the growth of CDW fluctuations pinned by imperfections. The parameter related to the correlation length and amplitude of the CDW modulation increases with imperfections, indicating more pinning sites at higher temperatures, demonstrating that short-range CDW order persists even at higher doping levels.

Disorder Sustains Fragmented Charge Density Waves

This research establishes a detailed understanding of charge density wave (CDW) order and its interplay with superconductivity in a kagome compound. Scientists discovered that even small amounts of tin doping introduce imperfections and disorder, leading to fragmented charge density wave patterns that persist to surprisingly high doping levels. These patterns, specifically an inverse Star-of-David distortion, were difficult to detect previously, but this study demonstrates their resilience due to the random distribution of tin atoms within the material, revealing a critical doping level where the volume fraction of this distorted pattern saturates, indicating that disorder plays a crucial role in sustaining the CDW order. This work clarifies the complex relationship between CDW order and superconductivity in these materials, demonstrating that disorder significantly influences this relationship. The authors acknowledge that comparing results with studies using pressure requires caution, as pressure-induced effects are observed in the absence of disorder, while their measurements are made on samples containing dopants. Future research may focus on further exploring the interplay between disorder, charge density waves, and superconductivity, potentially through theoretical studies that consider the specific roles of different orbitals in stabilizing these phases.