Direct Observation of Channelised Supercurrents in a Kagome Superconductor

Using a SQUID microscope, researchers discovered that CsV3Sb5-xSnx kagome superconductors host a network of narrow supercurrent channels behaving as Josephson junctions, providing a spatial explanation for their anomalous transport properties and linking these phenomena to charge density waves and doping-induced disorder.

The Gist

Imagine a superconductor as a magical highway where electricity flows without any friction or heat loss. In most materials, we expect this "super-highway" to be a wide, open road where traffic (the electric current) spreads out evenly across the entire surface, like water flowing smoothly over a flat field.

However, this paper reveals that in a specific type of exotic material called CsV3Sb5 (a "kagome" superconductor), the traffic behaves very differently. Instead of a wide-open road, the electricity is forced into a narrow, winding network of tiny tunnels.

Here is a breakdown of what the researchers found, using simple analogies:

1. The Mystery of the "Invisible Roads"

For a long time, scientists noticed strange things happening when they sent electricity through flakes of this material. The current seemed to jump and oscillate in weird patterns, suggesting the material wasn't behaving like a normal superconductor. But they couldn't see why this was happening because they were looking at the whole material at once, like trying to understand traffic in a city by only looking at the city limits.

The researchers used a special, high-tech microscope (a SQUID-on-tip) that acts like a super-sensitive magnetic camera. This allowed them to take a "satellite photo" of the electricity flowing inside the material.

2. The Discovery: A Network of Narrow Tunnels

When they looked at the doped (chemically altered) samples, they saw something surprising:

• Above the freezing point: The electricity flowed evenly, like water in a calm lake.
• Below the freezing point (when it becomes a superconductor): Suddenly, the water didn't stay in the lake. Instead, it rushed into a specific network of narrow, invisible canals.

These "canals" are incredibly thin—much thinner than anyone expected. They appear exactly when the material turns superconducting and stay in the exact same spots, even if you heat the material up to room temperature and cool it back down again. It's as if the material has a permanent, invisible map of roads that only opens up when it gets cold.

3. The "Traffic Jams" and the Josephson Junctions

The researchers noticed that as they pushed more electricity through these narrow tunnels, the flow didn't just get faster; it got complicated. The current would hit a "speed bump" (a critical current) and then suddenly redistribute itself to other tunnels.

They explain this using an analogy of Josephson Junctions. Imagine these narrow tunnels aren't just solid pipes, but are connected by tiny, magical bridges.

• When the traffic is light, it flows smoothly.
• When the traffic gets heavy, it hits a bridge that acts like a gate. The gate forces the traffic to split and find new paths.
• This explains the strange "jumps" in the data: the electricity is constantly navigating a complex maze of gates and bridges, not just flowing down a single straight road.

4. The "Doping" Factor: Why Only Some Materials Do This

The researchers tested two types of samples:

• The "Doped" Sample (with a little bit of Tin added): This one had the crazy network of narrow tunnels.
• The "Undoped" Sample (pure material): This one mostly flowed like a normal, wide river, though it did have some slight crowding near the edges.

This suggests that the "tunnel network" is triggered by the specific chemical changes (doping) and the way the material's internal structure (specifically the "charge density wave," which is like a frozen pattern of electrons) gets disrupted. The doping seems to break up the smooth landscape, forcing the electricity to find these specific, narrow paths.

5. The "Ghost" Currents

Even when the researchers applied a magnetic field (which usually pushes electricity around), the magnetic effects (like the "Meissner effect" where magnets float above superconductors) only happened inside those same narrow tunnels.

• The Analogy: Imagine a city where only certain streets have streetlights. If you look for light, you only see it on those specific streets. The rest of the city is dark. Similarly, the "superconducting state" (the ability to carry current without resistance) only exists inside those narrow tunnels. The rest of the material is essentially "dark" or inactive regarding superconductivity.

The Big Takeaway

This paper changes how we see these materials. We used to think the electricity was flowing through the whole chunk of material evenly. Now we know it's actually flowing through a hidden, microscopic highway system of narrow channels.

This discovery explains why previous experiments showed such weird, oscillating results: the scientists were seeing the interference patterns created by this complex network of tiny tunnels, not a simple flow of electricity. It turns out that in these exotic materials, the "road" to superconductivity is much more like a maze of narrow alleys than a broad highway.

Technical Summary

Technical Summary: Direct Observation of Channelised Supercurrents in a Kagome Superconductor

Problem Statement
The AV3Sb5 (A = Cs, K, Rb) family of kagome superconductors exhibits a complex interplay between superconductivity and unconventional charge density wave (CDW) order. Recent transport studies on CsV3Sb5 have reported anomalous phenomena, including intrinsic Josephson junctions, higher-order Cooper pairing, and a zero-field diode effect. However, a critical roadblock in understanding these materials is the lack of spatially resolved information regarding electrical transport. Conventional bulk transport measurements cannot distinguish between intrinsic bulk physics and mesoscopic geometric effects, nor can they resolve how competing orders influence local current flow. Without local imaging, the mechanisms driving these transport anomalies remain speculative.

Methodology
To address this, the authors employed a recently developed tapping-mode SQUID-on-tip (TM-SOT) microscope. This technique combines a tilted SQUID sensor (optimized for in-plane magnetic fields) with atomic force microscopy (AFM) in tapping mode. This setup allows for the correlation of nanoscale transport with device topography at high spatial resolution.

The study focused on mechanically exfoliated flakes of Sn-doped CsV3Sb5-xSnx (specifically x = 0.03), alongside control samples of undoped CsV3Sb5 and 2H-NbSe2. The researchers performed:

1. Local Magnetic Field Imaging: Mapping the magnetic field generated by AC transport currents and Meissner screening currents across the superconducting transition.
2. Current Profile Extraction: Using an iterative Biot-Savart model to reconstruct current density profiles from the magnetic field data.
3. Variable Parameter Testing: Investigating the stability of current distributions across temperature (from above Tc down to 50 mK), injected current magnitude (ramping through multiple critical currents), and magnetic field exposure.
4. Comparative Analysis: Contrasting the doped kagome flakes with undoped kagome flakes and the prototypical CDW-superconductor NbSe2.

Key Results

1. Emergence of Channelised Supercurrents: In Sn-doped CsV3Sb5-xSnx flakes, supercurrents are not homogeneously distributed. Instead, they are exclusively confined to a network of narrow channels. These channels emerge precisely at the onset of the superconducting transition (where resistivity drops) and persist down to the lowest temperatures.
2. Stability and Robustness: The spatial locations of these channels are fixed; they do not shift or change width with temperature variations (even after thermal cycling to room temperature) or changes in injected current. This stability contrasts with the temperature-dependent length scales (such as the Pearl length) expected in conventional thin-film superconductors.
3. Non-Linear Transport and Josephson Junctions: The flakes exhibit a cascade of critical currents. Crucially, crossing these critical currents does not destroy the supercurrent channels or revert the material to a normal metallic state. Instead, the current redistributes non-monotonically among the channels and the bulk. This behavior is consistent with a model of intrinsic Josephson junctions connected by narrow supercurrent filaments, where quasiparticle currents flow in parallel to the supercurrents once a critical current is exceeded.
4. Correlation with Interference Patterns: The complex interference patterns observed in transport measurements (oscillating critical currents with varying periods) are directly explained by the multi-channel current distribution. The different channel paths form loops of varying sizes, acting as intrinsic SQUIDs.
5. Doping and Disorder Dependence: The channelised network is a feature of light hole-doping (x = 0.03). Undoped samples show significantly weaker channelisation, with currents flowing more homogeneously in the interior, though edge accumulation remains. Control samples of NbSe2 exhibit uniform current distributions consistent with London theory, confirming that the channelisation is specific to the doped kagome system.
6. Phase Coherence: Magnetic response imaging reveals that Meissner screening currents and vortices are also confined to the same narrow channels. This indicates that the coherent superconducting condensate exists only within these filaments, while the regions between them lack phase coherence.

Significance and Claims
The paper establishes a firm connection between the previously observed anomalous transport properties in CsV3Sb5-xSnx flakes and a spatially inhomogeneous current distribution. The authors claim that the "intrinsic Josephson junctions" and "higher-order pairing" signatures reported in bulk transport are likely manifestations of this channelised network of Josephson junctions linked by nanowires, rather than bulk phenomena.

The findings suggest that the channelisation is driven by an underlying structural or electronic order that is sensitive to doping and disorder (potentially linked to the fragmentation of the CDW order by Sn substitution). The results highlight that mesoscopic geometric factors and local competing orders can dominate transport in quantum materials, potentially leading to misinterpretations of bulk physics if local imaging is absent. This work opens new frontiers for the local investigation of charge transport and competing orders in strongly correlated electron systems, providing a necessary spatial context for interpreting the exotic physics of kagome superconductors.