Kagome Superconductors: Charge Order Survives Beyond Limits

Kagome metals, materials with a distinctive honeycomb-like structure, present a fascinating puzzle as they balance competing states of order, including charge density waves and superconductivity. Terawit Kongruengkit, Andrea N. Capa Salinas, Ganesh Pokharel, and colleagues at the University of California, Santa Barbara, investigate these competing states in a hole-doped kagome metal, specifically CsV₃Sb₅Sn, using a technique that tracks incredibly fast vibrations within the material. Their results reveal that even when long-range charge order disappears with increasing doping, signatures of these charge fluctuations persist to surprisingly high levels, lasting for several picoseconds. This discovery suggests that these fluctuations are not simply a precursor to full charge order, but a robust phenomenon intrinsic to the material’s behaviour, potentially influencing or even competing with the emergence of superconductivity within this complex system.

Doping Effects on Kagome Superconductor CDW Order

CsV₃Sb₅ exhibits robust charge density wave (CDW) order, central to its electronic properties, and breaks rotational symmetry leading to complex electronic reconstructions revealed by ultrafast spectroscopy. Optical studies indicate multiple CDW-induced energy gaps, and the CDW is strongly coupled to the lattice, exhibiting significant anharmonicity that influences phonon behavior, with anomalous amplitude modes also observed. Doping can induce incommensurate CDW correlations, and titanium doping is a key focus as it introduces disorder and alters the electronic structure, potentially enhancing superconductivity, though it also affects orbital character. Tin doping leads to incommensurate charge-stripe correlations, and other dopants, such as chromium, tantalum, and molybdenum, are explored to tune the CDW and superconductivity, offering insights into the material’s phase diagram, allowing for the design of materials with tailored properties.

Experimental techniques, including ultrafast spectroscopy, optical spectroscopy, angle-resolved photoemission spectroscopy, Raman spectroscopy, X-ray diffraction, and transport measurements, are used to probe the dynamics of the CDW, reveal the formation of CDW gaps, map the electronic band structure, probe lattice vibrations, characterize the crystal structure, and investigate electrical properties and superconductivity. These studies reveal a complex phase diagram with competing CDW and superconducting phases, where quantum fluctuations are believed to play a significant role in suppressing the CDW and potentially enhancing superconductivity. Understanding the orbital character of the electronic states is crucial for understanding the CDW and superconductivity, with a strong coupling between electrons and phonons being a key feature. Some studies show an absence of phonon softening across the CDW transition, suggesting a different mechanism than traditional structural transitions. This research aims to unravel the intricate interplay between CDW order, superconductivity, and doping in CsV₃Sb₅, with the ultimate goal of understanding and potentially enhancing the superconducting properties of this fascinating material.

Persistent Charge Order Survives High Doping

Researchers have uncovered surprising evidence of persistent charge order within a family of kagome metals, even when conventional indicators suggest it should have disappeared. These materials exhibit a complex interplay between charge density waves and superconductivity. Previous studies indicated that the charge density wave order vanishes with doping, specifically by substituting antimony with tin. However, using ultrafast coherent phonon spectroscopy, scientists detected signatures of fluctuating charge order at doping levels far exceeding those previously thought to support it. This research demonstrates that the charge order doesn’t simply disappear, but instead transforms into persistent fluctuations lasting for several picoseconds.

The team observed these fluctuations across a wide range of doping levels, up to compositions where long-range charge order is undetectable by other methods. They found that the strength of these fluctuations peaks near a specific doping level, coinciding with a dip in the material’s superconducting temperature, suggesting a strong connection between the two phenomena. Importantly, the researchers confirmed that these fluctuations aren’t caused by imperfections or disorder within the material, but are an intrinsic property of the doping process itself. By substituting vanadium with titanium or cesium with potassium, they observed the same behavior, reinforcing the robustness of the findings.

The team’s measurements reveal a doping-tuned quantum phase transition, a fundamental shift in the material’s electronic state, occurring at the doping level where fluctuations are most pronounced. These findings challenge the conventional understanding of charge order and superconductivity in kagome metals, suggesting that fluctuating charge order may play a crucial, and previously underestimated, role in mediating or competing with superconductivity. The persistence of these fluctuations, even in the absence of long-range order, opens new avenues for exploring and potentially controlling the superconducting properties of these intriguing materials.

CDW Fluctuations Link to Superconductivity’s Decline

This research demonstrates that charge density wave (CDW) fluctuations persist to surprisingly high doping levels in the kagome metal CsV₃Sb₅, even beyond the point where long-range CDW order is lost. Using ultrafast spectroscopy, the team observed these fluctuations, lasting for several picoseconds, and found they are enhanced near a specific doping level coinciding with a minimum in the superconducting properties of the material. These findings suggest that CDW fluctuations are not simply a precursor to long-range order, but an intrinsic feature of the material’s electronic behaviour, potentially influencing its superconductivity. The study further establishes that this behaviour is linked to hole doping and not merely caused by material imperfections.

Measurements on samples with titanium and potassium substitutions confirmed the presence of robust CDW fluctuations, even with different types of disorder present. The authors estimate the correlation time of these fluctuations, finding it to be on the order of picoseconds, and propose that both thermal and quantum fluctuations contribute to their persistence. While acknowledging that chemical doping introduces some disorder, the consistency of results across different materials suggests the observed fluctuations are a fundamental property of the material’s electronic structure. Future work could focus on further characterizing the interplay between these CDW fluctuations and superconductivity, potentially revealing new mechanisms for controlling and enhancing superconducting properties in this class of materials.