Heterogeneous superconductivity emerging from a charge-stripe state in kagome lattices

In the world of materials science, electrons usually follow a predictable script. They flow through metals like water in a river, or they pair up and move in perfect lockstep to create superconductivity, a state where electricity flows with zero resistance. For decades, physicists believed that superconductivity and another electronic state called a charge density wave were natural enemies. A charge density wave is a pattern where electrons arrange themselves into static ripples, freezing into a rigid structure. The old rule was simple: if the electrons are busy forming these ripples, they cannot pair up to superconduct, and vice versa. They compete for the same electrons, and one order usually wins, pushing the other out. However, recent discoveries have shown that nature is more complicated than this simple rivalry. In some materials, these two states can coexist, and their interaction can create entirely new, unexpected behaviors that challenge our understanding of how matter works at the quantum level.

Researchers have now uncovered a striking example of this complexity in a family of crystals known as kagome superconductors. These materials are named for their internal structure, which resembles a woven basket pattern made of triangles. By carefully adding a small amount of tin atoms to replace some of the antimony atoms in the crystal, a team of scientists led by researchers at the University of Science and Technology of China observed a dramatic shift in how the material behaves. They found that when the crystal enters a specific phase where the electrons form short, stripe-like patterns, the superconductivity does not simply disappear or weaken uniformly. Instead, it becomes a patchwork. In this state, superconductivity emerges in small, isolated islands that grow and eventually connect to form a path through the material, while a significant portion of the electrons remain unpaired and normal, even deep inside the superconducting state.

The journey to this discovery began with growing high-quality crystals of the material, a compound called CsV3Sb5, and systematically replacing a small fraction of its atoms with tin. The scientists mapped out how the material changed as they increased the amount of tin. They found that at a very specific point, when about seven percent of the antimony atoms were swapped, the nature of the electron patterns changed abruptly. Before this point, the electrons formed a large, repeating pattern across the entire crystal. Just after this point, the pattern broke down into short, localized stripes. Coinciding with this change, the temperature at which the material became a superconductor dropped sharply. But the most surprising finding was not just the drop in temperature, but the strange way the superconductivity behaved in this new stripe phase.

When the scientists measured the electrical resistance and magnetic properties of these stripe-phase samples, they saw a clear mismatch. The material began to show signs of superconductivity, such as repelling magnetic fields, at a much higher temperature than it actually started conducting electricity with zero resistance. This gap suggested that superconductivity was not happening everywhere at once. Instead, small pockets of superconducting material were forming early on, floating in a sea of normal, non-superconducting metal. As the temperature dropped further, these pockets grew and eventually linked up, like islands merging into a continent, allowing electricity to flow freely across the entire sample. This process, known as percolation, explained why the transition was so broad and why the material behaved differently depending on how the current was pushed through it.

To see this patchwork directly, the researchers used a powerful microscope called a scanning tunneling microscope, which can image individual atoms and measure the energy of electrons at specific spots. They found that the size of the superconducting gap—the energy range where electrons are locked into pairs—varied significantly from one spot to another across the crystal surface. In some areas, the gap was large and well-defined; in others, it was smaller and smeared out. Crucially, this variation did not match the distribution of the tin atoms. The tin atoms were spread out evenly, so the unevenness of the superconductivity was not caused by the impurities themselves. Instead, the variation was tied to the short-range electron stripes. The microscope revealed that the electrons were being scattered by these stripes in a way that broke the superconducting pairs, but only in certain regions, creating a landscape of strong and weak superconductivity.

Perhaps the most puzzling aspect of this state was the presence of a large number of unpaired electrons, or "normal fluid," even when the material was superconducting. In a typical superconductor, almost all electrons pair up below a certain temperature. Here, however, the measurements showed that a substantial fraction of electrons, roughly one-third to two-thirds depending on the sample, remained unpaired and active even at the lowest temperatures. These unpaired electrons were not just random leftovers; they were concentrated on specific paths within the material's electronic structure that were directly linked to the stripe patterns. The researchers concluded that the short-range stripes were acting as a source of strong scattering, breaking the electron pairs in a way that is forbidden in conventional superconductors but is a hallmark of a more exotic type of pairing where the sign of the electron wave changes.

The study rules out several simpler explanations for this behavior. The uneven superconductivity was not caused by the tin atoms clumping together or by strain in the crystal lattice, as the distribution of the tin was uniform and the effect vanished when the stripe pattern disappeared. It was also not a case of the material simply separating into two distinct phases of superconductor and metal. Instead, the evidence points to a single, unified state where the superconductivity is inherently fragile and spatially varied because of its interaction with the charge stripes. This finding suggests that the electrons in these kagome materials are forming pairs in a complex, unconventional way, likely involving a change in the sign of their quantum wave function, which makes them highly sensitive to the presence of the stripes.

This work provides a rare glimpse into a state of matter where superconductivity and charge order do not just compete, but cooperate to create a heterogeneous, patchwork reality. It challenges the long-held view that superconductivity must be a uniform, all-or-nothing state and shows that in the right conditions, it can emerge as a collection of connected islands. By identifying this unique behavior in a material that is relatively simple and well-understood, the researchers have opened a new window into the physics of unconventional superconductivity. Their results suggest that the interplay between different electronic orders can lead to rich and unexpected phenomena, offering a new platform for exploring the fundamental mechanisms that govern how electrons pair up and flow without resistance.