Structural Transition of Li₂RuO₃Induced by Molecular-Orbit Formation

Abstract

A pseudo honeycomb system Li 2 RuO 3 exhibits a second-order-like transition at temperature T=T c~5 40 K to a low-T nonmagnetic phase with a significant lattice distortion forming Ru-Ru pairs. For this system, we have calculated the band structure, using the generalized gradient approximation (GGA) in both the high-and low-T phases, and found that the results of the calculation can naturally explain the insulating behavior observed in the low-T phase. The detailed characters of the Ru 4d t 2g bands obtained by the tight-binding fit to the calculated dispersion curves show clear evidence that the structural transition is driven by the formation of the Ru-Ru molecular-orbit, as proposed in our previous experimental studies.

Valence bond liquid phase in the honeycomb lattice materialLi2RuO3

“…Our calculations find rather small energy differences between ferro-and antiferromagnetic arrangement of the u ± spins, so when they are unbound and their covalent bond is broken at high temperature, they behave magnetically as nearly free spin 1/2 electrons. Indeed the difference between the experimental susceptibility at low temperatures (spin gap) and at high temperatures [7] is consistent with this scenario. Finally, (6) all investigated long range orders of the structural dimers are energetically strongly favorable when compared with the undimerized structure.…”

“…Finally, by analyzing the total density of states (DOS), we conclude that not only the orbital identified in Ref. 7 contributes to the covalency of the dimerized bond via direct overlap, but also another orbital provides an additional contribution via an O-assisted hopping. These findings play a very important role in understanding the microscopic physics of other 4d and 5d honeycomb oxides.…”

“…On the other hand, an itinerant scenario was put forward in Ref. 7. These authors suggested that the driving force of dimerization is covalent bonding.…”

First-order magnetostructural transition in single crystals of the honeycomb lattice ruthenate Li2RuO3

“…Nearly perfect hexagons of the high temperature C2/m phase undergo strong distortion, leading to a low temperature structure with significant shortening of one of the three inequivalent Ru-Ru bonds on each honeycomb 20 . Based on DFT calculations on the low and high temperature structures it was proposed that Li 2 RuO 3 undergoes a transition from a highly correlated metal to a molecular orbital insulator involving Ru-Ru dimerization and spin-singlet formation 20,21 . An alternative mechanism of spin-singlet formation driven by magnetoelastic coupling has also been proposed 22 .…”

Effect of disorder on the dimer transition of the honeycomb-lattice compoundLi2RuO3

“…Interestingly, this dimer transition is accompanied by a strong decrease of the magnetization. In order to explain the origin of this transition, scenarios such as the transition from a highly correlated metal to a molecularorbital insulator accompanied by bond-dimer formation [20,22] and the formation of spinless dimers by magnetoelastic mechanism [23] have been proposed. More recently, a pair distribution function (PDF) analysis based on high-energy X-ray diffraction (XRD) revealed that the dimers exist even above T d but the positions of the dimers change dynamically [21].…”