A kagome lattice superconductor reveals a 'cascade' of quantum electron states

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A complex network of electronic states can co-exist on the kagome structure, according to researchers from Boston College. This is similar to high-temperature superconductors.

A bulk single crystal of a topological Kagome Metal, CsV3 Sb 5, was the focus of the study. This metal is superconducting at temperatures below 2.5 degrees Kelvin or minus 455 Fahrenheit. The exotic material is made from Vanadium atoms in atomic planes. They are arranged on a so called kagome lattice, which is a pattern of interlaced hexagons and triangles placed on top of each other. There are Cesium and Antimony spacers layers between the kagome plans.

This material provides a window into the relationship between the underlying geometry and the physical properties of quantum solids, such as light transmission, electrical conduction or response to magnetic fields. Kagome lattice material is prized because it causes destructive interference and frustrates the kinetic motions of traversing electrons.

Most of the experimental work has been focused on kagome magnetics. The team found that the material was not magnetic. This opens up the possibility of investigating how electrons in kagome system behave without magnetism. These crystals are classified as "topological" in their electronic structure, while their high electrical conductivity makes them a "metal".

"This topological metallic becomes superconducting at very low temperatures, which is a rare occurrence of superconductivity within a Kagome material," stated Ilija Zeljkovic (Boston College Associate Professor of Physics), who was a co-author of the report titled "Cascades of correlated electron state in a Kagome superconductor, CsV3 Sb 5".

The liquid state of a metal is formed by electrons within the crystal. When the charged liquid flows under a voltage bias, it is called electrical conduction. Zeljkovic was part of the team that used scanning tunneling to examine the quantum interference effects of electron liquids. He is a Professor of Physics at Boston College, along with graduate student Hong Li and He Zhao. He also earned his doctorate of Physics at BC in 2020.

The team reported that the experiments revealed a "cascade of symmetry-broken phase of the electron liquid driven primarily by the correlation of electrons in the material."

As the temperature of the material drops, ripples or standing waves appear in the electron liquid. These waves are also known as charge density waves. They have a periodicity that is different from the underlying nuclear lattice. A new standing wave component that nucleates at a lower temperature does not nucleate in the crystal axes. This is because electrical conduction in this direction is different from any other.

Wang stated that these phases can form in either the non-superconducting metal state or the normal state. They also persist below the superconducting transition. These experiments show that superconductivity in CsV3 Sb 5 arises from and coexistswith a correlated quantum electron state that breaks the spatial symmetries within the crystal.

These findings could have important implications on how electrons form "Cooper", or superconductors that can conduct electricity without resistance. Zeljkovic said that other research suggests the possibility of unusual electron pairings in this family of superconductors made from kagome.

Researchers have discovered a phenomenon known as time-reversalsymmetry breaking in CsV3 Sb 5. This rule, which states that actions will be performed in reverse if the time runs backwards, is typically broken in magnetic materials. However, the kagome metallic does not have any significant magnetic moments. Zeljkovic stated that the next steps in this research include understanding this apparent contradiction, and finding out how the electronic states discovered in this work relate to time-reversalsymmetry breaking.

An associated Nature article in the same electronic advance edition reflects the importance and research into these newly-discovered kagome superconductors. The paper co-authored with BC's Ziqiang Wang and titled "Roton couple density wave in strong-coupling Kagome Superconductor" reports on the observation of new standing waves created by Cooper pairs with another periodicity within the same kagome supraconductor, called CsV3 Sb 5.

Wang stated that the publication of the two reports side by side not only provides new insights into kagome superconductors but also signals the high level and excitement surrounding these materials, their unique properties, and phenomena which researchers at Boston College, and other institutions around the globe are discovering more frequently.

More information: He Zhao and colleagues, Cascades of correlated electron state in a CsV3 Sb 5 Kagome superconductor, Nature (2021). Information from Nature He Zhao et. al., Cascades of correlated electron state in a Kagome superconductor (2021). DOI: 10.1038/s41586-021-03946-w