Researchers at Okayama University and collaborating institutions have boosted superconductivity in the kagome metal CsV₃Sb₅ to approximately 2.5 Kelvin through the application of tensile strain. This work demonstrates a path toward tuning superconductivity independently of charge density wave order within the material, a long-standing challenge in the field. The team synthesized high-quality single crystals of CsV₃Sb₅, essential for accurate measurements of the material’s response to strain.

At a tensile strain of ε = +0.90%, the ¹²¹Sb-NQR data revealed a double transition: an upper transition at 3.6 Kelvin to a nodal gap state, and a lower one at 3.0 Kelvin characterized by a nodeless gap. These results suggest degenerate superconducting states with different gap symmetry at ambient pressure that split under strain.

The team’s findings offer a resolution, indicating that there are degenerate states at ambient pressure that split under strain, and highlight the tunability of superconductivity via uniaxial pressure. This research builds on studies of unconventional superconductivity appearing near quantum critical points, as seen in cuprates and pnictides.

Tensile Strain Enhances Superconductivity to 3.6 Kelvin in CsV₃Sb₅

CsV₃Sb₅ achieved a superconducting transition temperature of 3.6 Kelvin when subjected to tensile strain, a notable increase from its approximately 2.5 K baseline and a temperature that brings this kagome metal closer to potentially practical superconducting applications. Researchers at Okayama University, led by Yusuke Takeuchi, investigated this enhancement using in-situ uniaxial pressure and ¹²¹Sb nuclear quadrupole resonance (NQR) to observe the material’s behavior under stress.

Their work demonstrates a way to manipulate superconductivity independently from the charge density wave (CDW) order already present within the material’s structure, a crucial step in understanding and optimizing these complex quantum states. The investigation focused on applying tensile strain to a single crystal of CsV₃Sb₅ along its crystallographic a-axis, carefully monitoring the resulting changes in its superconducting properties. ¹²¹Sb-NQR measurements confirmed that increasing tensile strain significantly elevates the superconducting transition temperature, while leaving the CDW state unaffected.

This decoupling of superconductivity and charge order is a key result, suggesting that these two phenomena, often intertwined in unconventional superconductors, can be tuned separately. The researchers precisely measured strain using a homemade piezoelectric-driven cell, defining strain (ε) as 100 times the change in length divided by the original length. Below 3.6 Kelvin, the nuclear spin-lattice relaxation rate (1/T₁T) exhibited a behavior characteristic of a nodal gap state, where the superconducting gap has nodes, points where it vanishes.

However, a further decrease in temperature to 3.0 Kelvin triggered a second transition, marked by the appearance of a coherence peak indicative of a nodeless gap. The researchers state in their published work that these results provide direct evidence for degenerate states with different gap symmetry that split under strain.

This splitting suggests that the superconducting state at ambient pressure features degenerate possibilities which split under strain. The ability to tune superconductivity through mechanical strain opens new avenues for exploring and controlling these exotic materials. This work demonstrates a high tunability of superconductivity by uniaxial pressure, offering a new degree of freedom for materials scientists seeking to engineer advanced superconducting materials.

Charge-Order and Superconductivity Relationship in Kagome Metal CsV₃Sb₅

The kagome metal CsV₃Sb₅ exhibits a surprising response to mechanical stress; applying tensile strain elevates its superconducting transition temperature to 3.6 Kelvin, a potentially significant step toward realizing more practical superconducting applications. This enhancement, detailed in recent work from Okayama University and collaborating institutions, demonstrates a previously unrealized degree of control over superconductivity in this complex material. Unlike many other unconventional superconductors, the team discovered that manipulating strain boosts superconductivity without impacting the material’s charge density wave (CDW) order, a competing electronic state.

Researchers utilized ¹²¹Sb nuclear quadrupole resonance (NQR) to probe the evolution of superconductivity under precisely controlled uniaxial pressure. Their experiments revealed a double transition at a tensile strain of ε = +0.90%, initially observing a shift to a nodal gap state at 3.6 Kelvin, followed by a transition to a nodeless gap state at 3.0 Kelvin.

The observation of these distinct transitions is particularly noteworthy given the ongoing debate surrounding the superconducting gap symmetry in CsV₃Sb₅. Prior studies have yielded conflicting results, with some suggesting conventional s-wave pairing and others pointing towards more exotic, unconventional mechanisms. The ability to disentangle these states through strain provides a powerful tool for understanding the underlying pairing mechanism. The researchers found that the material’s charge density wave order remained stable even as superconductivity was significantly enhanced, a result that challenges conventional understanding regarding the interplay between these two phenomena.

This tunability is not merely academic; it suggests a pathway toward engineering materials with tailored superconducting properties. The crystals were synthesized by the self-flux method, a crucial step in ensuring the clarity of their NQR measurements.

The temperature dependence of the ¹²¹Sb-NQR 1/T₁T measurements under varying strains revealed the double transition, with the 1/T₁T exhibiting a power-law-like behavior characteristic of a nodal state at ε = +0.90%. This detailed analysis provides a clear picture of how strain modifies the electronic structure and pairing symmetry within CsV₃Sb₅.

CsV₃Sb₅ Crystal Structure and Synthesis via Self-Flux Method

This technique proved essential for enabling precise strain manipulation, a key element in their investigation of the material’s superconducting properties and its relationship to charge density wave order. The ability to grow these crystals allowed for in-situ uniaxial pressure application, facilitating the observation of subtle changes in the material’s behavior under stress. This controlled environment was vital for accurately measuring the impact of strain on both the superconducting transition temperature and the charge density wave state.

This finding demonstrates a level of tunability previously unseen in CsV₃Sb₅, suggesting that superconductivity can be independently engineered without disrupting the underlying charge order. At a tensile strain of ε = +0.90%, the superconducting transition temperature reached 3.6 Kelvin, a relatively high temperature for this material and a potential step toward more practical applications, even if significant cooling is still required.

NQR Reveals Double Transitions in Strained Superconducting State

At a tensile strain of ε = +0.90%, the ¹²¹Sb-NQR data revealed a particularly striking phenomenon: a double transition in the superconducting state. This ability to disentangle and manipulate these states is particularly noteworthy given the challenges in controlling superconductivity in strongly correlated electron systems. Unconventional superconductivity frequently emerges near quantum critical points, often linked to the ordering of electronic or spin states. Materials like CsV₃Sb₅, with their unique kagome lattice structure, offer a platform for exploring these phenomena.

Competing Gap Symmetries in CsV₃Sb₅ at Ambient Pressure

CsV₃Sb₅, a material distinguished by its kagome lattice structure, defies expectations for superconductivity by exhibiting tunable properties without altering its charge density wave order. This ability to decouple superconductivity from the charge density wave state is a significant step toward designing materials with tailored electronic properties. The team’s investigation, utilizing ¹²¹Sb nuclear quadrupole resonance, revealed that tensile strain markedly elevates the superconducting transition temperature.

Crucially, this enhancement occurs without impacting the charge density wave order, indicating the two phenomena are not inextricably linked in this material. This finding challenges the conventional understanding that superconductivity and charge order must compete or coexist in a fixed relationship.

The nuclear spin-lattice relaxation rate measurements revealed an upper transition at 3.6 Kelvin to a nodal gap state, and a lower transition at 3.0 Kelvin characterized by a nodeless gap. The emergence of these competing gap symmetries is not merely a curiosity; it points to a complex interplay of electronic interactions within the kagome lattice. CsV₃Sb₅, with its unique structure and the observed decoupling of superconductivity from the charge density wave, offers a valuable platform for exploring these phenomena.

Uniaxial Pressure Tuning of Superconducting Gap Symmetry

Researchers utilized ¹²¹Sb nuclear quadrupole resonance to observe this behavior, providing insights into the complex interplay between these quantum states of matter. The ability to independently tune superconductivity from the charge density wave order is a key finding, challenging conventional understanding of these competing phenomena. At a tensile strain of ε = +0.90%, the ¹²¹Sb-NQR data revealed a remarkable double transition: an upper transition at 3.6 Kelvin to a nodal gap state, and a lower one at 3.0 Kelvin characterized by a nodeless gap.

These results suggest degenerate superconducting states with different gap symmetry in the kagome metal at ambient pressure which split under strain. The researchers meticulously measured ¹²¹Sb-NQR to track the evolution of the superconducting gap under varying strain conditions.

They found that while the CDW state remained largely unaffected by the applied strain, the superconducting transition temperature increased significantly, reaching 3.6 Kelvin at the maximum tensile strain. The precise control over gap symmetry through strain offers a potential route towards designing novel superconducting devices with tailored properties, and further research will focus on understanding the underlying mechanisms driving this tunability.

👉 More information
🗞 Strain-Tuned Nodal Superconductivity in the Charge-Ordered Kagome Metal CsV₃Sb₅
✍️ Yusuke Takeuchi et al.
🧠 DOI: http://link.aps.org/doi/10.1103/mzgp-2lzb

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