Interorbital charge transfers as the driving force for the metal-insulator transition of BaVS_3

(see: Frank Lechermann, Silke Biermann, and Antoine Georges, cond-mat/0409463 (2004))

The intriguing physics of the vanadium sulphide BaVS_3 is keeping condensed matter physicists busy for already more than 30 years. By exhibiting three distinct continuous phase transition on cooling, this compound is an outstanding example of the interplay of various degrees of freedom. A structural transition at T_S~240K from hexagonal (P6_3/mmc) to orthorhombic (Cmc2_1) is followed by a highlighted metal-insulator transition (MIT) into a paramagnetic phase at T_MIT~70K. Finally, at T_X~30K a transition to an incommensurate antiferromagnetic ordered state seems to occur.
For all temperature ranges, the microscopic structure of BaVS_3 shows chains of tilted VS_6 octahedra along the c axis, "glued" together by Ba atoms in between. In the orthorhombic (Cmc2_1) structure (70K < T < 240K) there is a zigzag distortion of these chains in the bc-plane.


  
(a) hexagonal (P6_3/mmc) structure along c axis.
(b-e) orthorhombic (Cmc2_1) structure: (b) along c axis, (c) along a axis, (d) 3d view, (e) closeup on VS_6 chain.
(f) schematic term scheme for BaVS_3.

From experimental investigations it became quickly clear that the multi-orbital 3d^1 system BaVS_3 is subject to a strong competition between localized and itinerant electronic states. The resistivity above the MIT is rather high for a metal and the existence of local moments was verified. However, an understanding for the occurence of the MIT was lacking until recently a doubling of the crystal unit cell below 70 K was observed, indicating the appearance of a charge density wave (CDW) state. This experimental finding is however in contradiction to the conventional electronic structure resulting from LDA to DFT, which does not provide an appropriate nesting condition on the obtained Fermi surface. The reason therefore is due to the fact that the LDA overrates the occupation of the itinerant states originating from 3d orbitals with dominant A_1g symmetry. The strongly localized 3d states stemming from the E_g orbitals, responsible for the local moment behavior, are hardly occupied within LDA. The A_1g orbital is mainly directed along the c axis of the system, i.e., in chain direction, whereas the E_g orbitals point in between the surrounding sulphur ions.
We show that by incorporating explicit many-body effects in the LDA+DMFT framework, the imbalance in the {A_1g, E_g} occupations is reduced. Notably the Hund's coupling J favors an interorbital charge transfer from the A_1g to the E_g states for moderate electronic correlations. The accompanied Fermi surface reconstruction opens now the possibility for a CDW state, as observed in experiments. Additionally, the Curie-Weiss behavior for the E_g and the Pauli behavior for the A_1g electrons can theoretically be extracted.



(g) quasiparticle bands crossing the Fermi level along the c* direction, corresponding to a propagation along the c axis of the systen. Clearly seen is the shifting of the Fermi wave vector of the A_1g band towards lower values within LDA+DMFT.
(i,h) integrated spectral functions for T=1160K (i) and T=332K. The low coherence scale of the narrower E_g bands seems to be obvious.