&Bullet; physics 14, p58
A new experimental method based on adsorption can indicate whether a material is a Mott insulator or a common insulator.
Mott isolators are no ordinary isolators. These materials have unfilled orbitals that would normally make them conductors, but electron-electron interactions prevent current from flowing. The distinction between a Mott isolator and an ordinary (trivial) isolator was typically based on theoretical calculations of the band structure. Now, Han Woong Yeom of Pohang University of Science and Technology in South Korea and colleagues have developed an experimental method that can reveal the true identity of an isolator  .
Mott insulators can be charged easily because they are bound to high temperature superconductors and quantum spin fluids. However, identifying Mott isolators is not straightforward. An example is trigonal tantalum sulfide (1T-TaS)2), which consists of layers in a lattice structure “Star of David”. 1T TaS for thirty years2 was classified as a Mott isolator based on the presumed presence of unfilled orbitals in each layer. However, recent experiments showed that the layers are stacked as bilayers, which could mean that electron sharing fills the orbitals and degrades 1T TaS2 to a trivial isolator.
To determine which category of insulators applies, Yeom and colleagues split a 1T TaS2 Crystal that exposes two types of surfaces: one with a full double layer and one with only a half double layer. They adsorbed potassium atoms on both types of surfaces and observed the adsorption pattern with a scanning tunneling microscope. The “complete” surfaces showed a less ordered pattern than the “half” surfaces – proof that a complete bilayer does not offer any unfilled orbital sites to which potassium atoms can bind. The team confirmed this interpretation with site-by-site conductivity measurements. The results show that bulk 1T TaS2 is a trivial insulator, but a single layer film – if it could be made – would be a Mott insulator.
Michael Schirber is the corresponding editor for physics based in Lyon, France.
- J. Lee et al., “Differentiating a Mott isolator from a trivial isolator with atomic adsorbates” Phys. Rev. Lett.126196405 (2021).