Kagome lattice superconductor reveals ‘cascade’ of quantum electron states

Researchers have discovered a complex landscape of electronic states that can coexist on a kagome lattice, resembling those of high-temperature superconductors, a team of Boston College physicists report in an advanced electronic publication from the journal. Nature.

The objective of the study was a loose single crystal of a topological kagome metal, known as CsV₃Sb₅– a metal which becomes superconducting below 2.5 degrees Kelvin, or minus 455 degrees Fahrenheit. The exotic material is constructed from atomic planes composed of vanadium atoms arranged on a so-called kagome lattice – described as a pattern of interlocking triangles and hexagons – stacked on top of each other, with spacing layers of cesium and antimony between the kagome planes.

The material provides a window into how the physical properties of quantum solids, such as light transmission, electrical conduction, or the response to a magnetic field, relate to the underlying geometry of the atomic lattice structure. Because its geometry causes destructive interference and “frustrates” the kinetic motion of traversing electrons, kagome lattice materials are valued for providing a unique and fertile ground for the study of quantum electronic states described as frustrated, correlated, and topological.

So far, the majority of experimental efforts have focused on kagome magnets. The material the team examined is not magnetic, which opens the door to studying the behavior of electrons in kagome systems in the absence of magnetism. The electronic structure of these crystals can be classified as “topological”, whereas high electrical conductivity makes them a “metal”.

“This topological metal becomes superconducting at low temperatures, which is a very rare occurrence of superconductivity in a kagome material,” said Ilija Zeljkovic, associate professor of physics at Boston College, co-lead author of the report titled “Cascade of States. correlated electronics in a CsV kagome superconductor₃Sb₅. “

In a metal, the electrons in the crystal form a liquid state. Electrical conduction occurs when charged liquid flows under a bias voltage. The team used tunneling spectroscopy to probe the effects of quantum interference from electronic liquid, said Zeljkovic, who led the research with colleagues at Boston College, physics professor Ziqiang Wang, graduate student Hong Li and He Zhao, who received his doctorate in physics from BC in 2020, as well as colleagues at the University of California, Santa Barbara.

The experiments revealed a “cascade” of phases broken by the symmetry of the electronic liquid driven by the correlation between electrons in the material, the team reported.

Occurring consecutively as the temperature of the material is lowered, ripples or standing waves first emerge in the electronic liquid, called charge density waves, with a periodicity different from the underlying atomic lattice. At a lower temperature, a new standing wave component nucleats only along one direction of the crystal axes, so the electrical conduction along that direction is different from any other direction.

These phases develop in the normal state – or in the non-superconducting metallic state – and persist below the superconducting transition, Wang said. Experiments show that superconductivity in CsV₃Sb₅ emerges and coexists with a correlated quantum electronic state that breaks the spatial symmetries of the crystal.

The findings could have strong implications for how electrons form “Cooper” pairs and transform into a charged superfluid at an even lower temperature, or a superconductor capable of electrical conduction without resistance. In this family of kagome superconductors, other research has already suggested the possibility of unconventional electronic pairing, Zeljkovic said.

Researchers in the field have noted a phenomenon called time-inversion symmetry breaking in CsV₃Sb₅. This rule of symmetry – which states that actions would be performed in reverse if time were to move back – is usually broken in magnetic materials, but kagome metal shows no substantial magnetic moment. Zeljkovic said the next steps in this research are to understand this apparent contradiction and how the electronic states revealed in this recent work are related to time inversion symmetry breaking.

The level of importance and research on these recently discovered kagome lattice superconductors is reflected in a Nature article published in the same advance electronic edition. Also co-authored by Ziqiang Wang of BC, the article, titled “Roton pair density wave in a strong-coupling kagome superconductor”, reports the observation of new standing waves formed by Cooper pairs with another periodicity in the same. kagome superconductor, CsV₃Sb₅.

“The publication of these two reports side by side not only reveals new and extensive information about the superconductors of the kagome lattice, but also signals the high level of interest and enthusiasm surrounding these materials and their unique properties and phenomena, that the Boston College researchers and institutions around the world are finding out more and more frequently, ”Wang said.