Theoretically tantalum disulfide should be a conductor but experimentally it is an insulator. Why this is so has now been explained thanks to scanning tunneling microscopy results from the RIKEN Center for Emergent Matter Science in Japan.
Crystalline solids with an odd number of electrons in the unit cell should be good conductors and those with an even number should be insulating. However, rules of thumb are there to be broken and if there is strong repulsion between the electrons in the structure some of them become so localized in the structure that they cannot carry a current. Additionally, some layered materials see interactions in different layers to form paired bilayer structures that contains an even number of electrons making them insulators too.
Tantalum disulfide has 13 electrons in each repeating structure so ought to be a conductor. The RIKEN team experimented with crystals of tantalum disulfide cleaved under vacuum conditions to generate a pristine surface they could study at near absolute zero using STM. Their scans revealed stacking of layers although sometimes the crystal cleaves so that bilayers are divided other times not. However, additional spectroscopic studies of the paired and unpaired layers showed that even the unpaired situation is insulating. This suggests that the repulsion theory may be correct, a characteristic known as "Mottness" named for its designer Sir Thomas Mott.
"The exact nature of the insulating state and of the phase transitions in tantalum disulfide have been long-standing mysteries and it was very exciting to find that Mottness is a key player, aside from the pairing of the layers. This is because theorists suspect that a Mott state could set the stage for an interesting phase of matter known as a quantum spin liquid," explains team member Christopher Butler.
Team leader Tetsuo Hanaguri adds that "I am very satisfied we have been able to put a new piece into the puzzle. Future work may help us to find new interesting and useful phenomena emerging from Mottness, such as high-temperature superconductivity." [Butler C J, et al. Nature Commun. (2020) DOI: 10.1038/s41467-020-16132-9]