New approach to calculate electron traffic jam in transition metal dichalcogenide
Many branches of condensed matter theory have focused on the phases of materials, or on phase variations of a given material. In so-called ‘phase changing materials’ the different phases can be accessed by a change in temperature or pressure. The transition metal dichalcogenide TaS2 is one of these fascinating examples. It can adopt various different material states, including the superconducting state, the normal conducting state, and insulating properties. The debateover its low temperature (T < 200K) state continues, however, with many experimentalists reporting that TaS2 is an insulator in the low temperature phase, but many theorists claiming that it must be in a metallic state.
Figure (a): The geometry of 1T-TaS2, with Ta atoms in red, S atoms in orange and the charge density wave (blue isosurface) highlighted by the hexagonal star shape. Figures (b) and (c) show the computed band structure of 1T-TaS2 using (b) the previous calculation method and (c) the newly suggested calculation method. © Dongbin Shin, MPSD
In Mott insulators, electron transport is stalled, just like in a traffic jam. This phenomenon is well understood within the framework of density functional theory (DFT) - a mathematical procedure that deals with the quantum mechanics of many-electron system. Scientists combine DFT and another parameter called the Hubbard-type U potential to calculate how the electron transport traffic jam is caused.
However, now a research team from the MPSD and UNIST has found that combining DFT and the Hubbard-type U potential can easily lead to erroneous results, particularly for the charge density wave (CDW) phases of ‘phase changing materials’, like 1T-TaS2. The team closely examined this mathematical procedure underlying this method.Lead author Dongbin Shin says: "We know that, in the low temperature phase, the CDW state produces the reconstruction in the hexagonal star pattern. In this CDW state, the Hubbard-type U potential must be accommodated on this star pattern. We have found, though, that the application of the U potential onto the atomic site and not on the star pattern leads to a substantial error in the calculation of Mott insulating state. This is significant for the correct description of the Coulomb interaction between CDW states.”
The team’s study may not only solve the long-standing problem relating to the Mott insulating state of 1T-TaS2, but also suggests the generalized Hubbard U-type potential approach to describe the Mott insulator in the CDW phase and to correct the on-site Coulomb interaction in molecular solids.