dual superconducting states kagome lattice CsV₃Sb₅

Physicists Discover Dual Superconducting States in Kagome Lattice Material CsV₃Sb₅

Introduction to Superconductivity and Its Mystery

Superconductivity, characterized by the complete absence of electrical resistance at extremely low temperatures, is a quantum phenomenon of great interest. While the phenomenon is traditionally associated with the formation of Cooper pairs—electron pairs bound together—the precise factors that lead to superconductivity in quantum materials remain elusive.

Study on Kagome Lattice Superconductor CsV₃Sb₅

Researchers from Princeton University, the National High Magnetic Field Laboratory, Beijing Institute to Technology, and the University of Zurich recently undertook a study to explore the superconductivity of CsV₃Sb₅ a material with a Kagome lattice, which consists of atoms arranged in a hexagonal configuration resembling the traditional Kagome basket pattern.

The study, published in Nature Physics, establishes the presence of two superconducting regimes within this material, each linked to distinct transport and thermodynamic responses.

Discovery of Chiral Charge Density Wave in Kagome Superconductors

Excitement in the Quantum Materials Community

"In 2021, our identification of a chiral charge density wave in the Kagome superconductor AV₃Sb₅ (A = K, Rb, Cs) generated significant excitement within the quantum materials community," stated Shafayat Hossain, the study's first author, in an interview with the publishing website.

Interplay of Symmetry Breaking and Superconductivity

"Kagome superconductors exhibit multiple symmetry-breaking phenomena in the charge-ordered state before undergoing a transition to a superconducting ground state. Given the interplay between symmetry breaking, the multiband characteristics of AV₃Sb₅, and its topological band structure, the emergence of an unconventional superconducting state appeared highly probable."

Investigation into the Superconducting Nature of CsV₃Sb₅

Research Motivation and Methodology

When Hossain and his collaborators began exploring the origins of superconductivity in Kagome superconductors, existing literature provided no indication that the superconductivity in AV₃Sb₅ was unconventional. However, the intricate interplay of competing orders in the material's normal state suggested a possible impact on its superconducting behavior.

"Motivated by this, we employed transport and thermodynamic techniques to systematically explore the superconducting state CsV₃Sb₅," Hossain explained. "Unexpectedly, our initial transport measurements immediately revealed the presence of two distinct superconducting regimes, a discovery we had not foreseen."

Experimental Findings: Two Superconducting Regimes

Upper Critical Fields Across Temperature Variations

As part of their investigation, the researchers examined the upper critical fields of CsV₃Sb₅ across varying temperatures and for two distinct field orientations, specifically along the conducting planes and perpendicular to them.

Notably, the measurements revealed the existence of two distinct superconducting regimes in CsV₃Sb₅, delineated by a step-like enhancement in the upper critical fields.

Heat Capacity and Thermal Conductivity Observations

"Our observations revealed two distinct anomalies in the heat capacity as a function of temperature, signifying the emergence of two superconducting gaps," stated Luis Balicas, senior author of the study. "Furthermore, thermal conductivity exhibited a finite, constant contribution with temperature before the second gap formed, suggesting that certain regins of the Fermi surface remained ungapped in the superconducting state until further cooling induced gap formation in these electronic states."

Anisotropic Behavior and Unconventional Superconductivity 

Magnetic Field Rotation Effects on Thermal Conductivity

The researchers observed that when magnetic fields were rotated within the conducting planes, the thermal conductivity of the Kagome superconductor exhibited anisotropic behavior upon transitioning to a superconducting state.

This finding implies that the superconducting phase in CsV₃Sb₅ possesses a complex gap structure, suggesting a potential unconventional nature.

Gap Anisotropies and Pairing Symmetry

"Thermal conductivity is expected to be mediated by carriers excited across the superconducting gap, indicating a mildly anisotropic gap function," Balicas explained. "Interestingly, this anisotropy undergoes rotation upon the emergence of the second superconducting gap, suggesting distinct gap anisotropies. However, the precise pairing symmetry remains undetermined."

Implications of the Findings: Band-Selective Superconductivity

Presence of Multiple Superconducting Gaps

The results obtained by this research team suggest that the Kagome-lattice material CsV₃Sb₅ may exhibit band-selective superconductivity, a phenomenon where distinct electron bands develop independent superconducting gaps.

"While the precise symmetry of the gap function remains elusive, our study confirms the presence of multiple superconducting gaps in  CsV₃Sb₅ and suggests the potential existence of an unconventional pairing symmetry yet to be fully understood," said Balicas.

Charge Density Wave and Anomalous Hall Response

"The charge density wave (CDW) state, from which superconductivity emerges, exhibits unconventional characteristics. Notably, despite the absence of magnetism, it reportedly demonstrates an anomalous Hall response. Consequently, the coexistence of superconductivity with such a chiral CDW state suggests the likelihood of an unconventional pairing mechanism."

Future Research and Broader Impact on Superconductors

Significance of Kagome Superconductors in Quantum Research

The research conducted by Hossain, Balicas, and their team provides valuable insights into the superconducting behavior of CsV₃Sb₅, with potential implications for other Kagome-lattice superconductors. Their future studies will focus on further investigating multiband superconductors with intrinsic symmetry-breaking in their normal state.

"Kagome superconductors such as CsV₃Sb₅ are part of a broader class of materials, extending decades of research on cuprates and iron pnictides," Hossain stated. "The discovery of novel superconductors within this category remains an exciting frontier, as each new, material holds the potential to unveil unprecedented quantum states.

Exploring Topological Properties in Future Studies

"Our future research on Kgome superconductors will delve deeper into their unconventional gap structures and in-gap states, exploring the potential for nontrivial topological propterties."

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stable superconductivity ambient pressure

Physicists Achieve Stable Superconductivity at Ambient Pressure

Breakthrough in Ambient-Pressure Superconductivity

Researchers at the University of Houston's Texas Center for Superconductivity have reached another groundbreaking milestone in their pursuit of ambient-pressure high-temperature superconductivity, advancing the quest for superconductors that function in real-world conditions and paving the way for next-generation energy-efficient technologies.

Investigating Superconductivity in Bi₀.₅Sb₁.₅Te₃ (BST)

Research by Liangzi Deng and Paul Ching-Wu Chu

Professors Liangzi Deng and Paul Ching-Chu of the UH Department of Physics investigated the induction of superconductivity in Bi₀.₅Sb₁.₅Te₃ (BST) under pressure while preserving its chemical and structural properties, as detailed in their study, "Creation, stabilization, and investigation at ambient pressure of pressure-induced superconductivity in Bi₀.₅Sb₁.₅Te₃" published in the Proceeding of the National Academy of Sciences.

Link Between Pressure, Topology, and Superconductivity

"The idea that high-pressure treatment of BST might reconfigure its Fermi surface topology and enhance thermoelectric performance emerged in 2001," Deng stated. "That intricate relationship between pressure, topology and superconductivity drew our interest."

Challenges in High-Pressure Superconductors

Metastable States and Practical Limitations

"As materials scientist Pol Duwez once observed, most industrially significant solids exist in a metastable state," Chu explained. "The challenge lies in the fat that many of the most intriguing superconductors require high pressure to function, making them difficult to analyze and even more challenging to implement in real-world applications."

Deng and Chu's innovation offers a solution to this pressing issue.

The Pressure-Quench Protocol (PQP) - A Key Innovation

Deng and Chu pioneered the pressure-quench protocol (PQP), a method introduced in an October UH news release, to stabilize BST's superconducting states at ambient pressure—removing the necessity for high-pressure environments.

Significance of This Discovery

A Novel Approach to Material Phases

Why is this significant? It introduces a novel approach to preserving valuable material phases that typically require high-pressure conditions, enabling both fundamental research and practical applications.

Evidence of High-Pressure Phase Stability

"This experiment provides clear evidence that high-pressure-induced phases can be stabilized at ambient pressure through a delicate electronic transition, without altering symmetry," Chu stated. "This breakthrough opens new possibilities for preserving valuable material phases typically confined to high-pressure conditions and could aid in the quest for superconductors with higher transition temperatures."

Exploring New States of Matter

"Remarkably, this experiment unveiled a groundbreaking method for identigying new states of matter that neither naturally exist at ambient pressure nor emerge under high-pressure conditions," Deng noted. "It underscores PQP's potential as a powerful tool for mapping and expanding material phase diagrams."

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Physicists at the University of Houston have unlocked a new path to stable superconductivity at ambient pressure, paving the way for next-generation energy-efficient technologies. This revolutionary advancement could transform materials science, energy storage, and beyond.

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