Complex landscape of electronic states discovered in kagome lattice

“This topological metal becomes superconducting at low temperature, which is a very rare occurrence of superconductivity in a kagome material.”Ilija Zeljkovic, Boston College

Researchers have discovered a complex landscape of electronic states that can co-exist on a kagome lattice, resembling those in high-temperature superconductors. Led by a team of physicists from Boston College, the researchers report their findings in a paper in Nature.

The focus of the study was a bulk single crystal of a topological kagome metal known as CsV₃Sb₅ – a metal that becomes superconducting below 2.5K (-455°F). This exotic material is built from atomic planes composed of vanadium atoms arranged on a so-called kagome lattice – described as a pattern of interlaced triangles and hexagons – stacked on top of one another, with cesium and antimony spacer layers between the kagome planes.

This material offers a window into how the physical properties of quantum solids – such as light transmission, electrical conduction and response to a magnetic field – relate to the underlying geometry of the atomic lattice structure. Because their geometry causes destructive interference and 'frustrates' the kinetic motion of traversing electrons, kagome lattice materials are prized for offering unique and fertile ground for the study of quantum electronic states described as frustrated, correlated and topological.

The majority of experimental efforts thus far have focused on kagome magnets. The material the team examined is not magnetic, which opens the door to investigating how electrons in kagome systems behave in the absence of magnetism. The electronic structure of this material can be classified as 'topological', while high electrical conductivity makes it a 'metal'.

“This topological metal becomes superconducting at low temperature, which is a very rare occurrence of superconductivity in a kagome material,” said Ilija Zeljkovic, an associate professor of physics at Boston College and a lead co-author of the paper.

In a metal, electrons in the crystal form a liquid state; electrical conduction happens when the charged liquid flows under a voltage. The researchers used scanning tunneling spectroscopy to probe the quantum interference effects of the electron liquid in the kagome material, said Zeljkovic, who conducted the research with colleagues from Boston College and the University of California, Santa Barbara.

These experiments revealed a 'cascade' of symmetry-broken phases of the electron liquid, driven by the correlation between the electrons in the material. Occurring consecutively as the temperature of the kagome material is lowered, standing waves known as charge density waves emerge first in the electron liquid, with a periodicity different from the underlying atomic lattice. At lower temperatures, a new standing wave component nucleates along just one direction of the crystal axes, such that electrical conduction along this direction is different than in any other direction.

These phases develop in the normal state – or the non-superconducting metallic state – and persist below the superconducting transition, said Ziqiang Wang, professor of physics at Boston College and co-author of the paper. The experiments demonstrate that superconductivity in CsV₃Sb₅ emerges from, and coexists with, a correlated quantum electronic state that breaks spatial symmetries of the crystal.

These findings could have strong implications for how the electrons form so-called Cooper pairs and turn 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 electron pairing, said Zeljkovic.

Researchers in the field have also noted a phenomenon called time-reversal symmetry breaking in CsV₃Sb₅. This symmetry rule – which holds that actions would be performed in reverse if time were to run backwards – is typically broken in magnetic materials, but the kagome metal shows no substantial magnetic moments. According to Zeljkovic, 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-reversal symmetry breaking.

The amount of research being conducted into these recently discovered kagome lattice superconductors is reflected in an associated paper in Nature. Also co-authored by Wang, the paper reports the observation of novel standing waves formed by Cooper pairs with yet another periodicity in the same kagome superconductor, CsV₃Sb₅.

“The publishing of these two reports side-by-side not only reveals new and broad insights into kagome lattice superconductors, but also signals the high level of interest and excitement surrounding these materials and their unique properties and phenomena, which researchers at Boston College and institutions around the world are discovering with increasing frequency,” Wang said.

This story is adapted from material from Boston College, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.