Theoretical model for the structural phase transition at the metal-insulator transition in polymerizedKC60

Abstract

The recently discovered structural transition in polymerized KC 60 at about 50 K results in a doubling of the unit cell volume and accompanies the metal-insulator transition. Here we show that the (a ជ ϩc ជ ,b ជ ,a ជ Ϫc ជ ) superstructure results from small orientational charge density waves along the polymer chains and concomitant displacements of the surrounding K ϩ ions. The effect is specific for the space group Pmnn of KC 60 and is absent in RbC 60 and CsC 60 ͑space group I2/m). The mechanism is relevant for the metal-insulator transition. 66 165425-1 THEORETICAL MODEL FOR THE STRUCTURAL PHASE . . . PHYSICAL REVIEW B 66, 165425 ͑2002͒ 165425-3

Orientational charge density waves and the metal-insulator transition in polymerizedKC60

“…12 The similarity between a displacement wave with wavelength =2 /2͉k F ͉ =2a in a 1D atomic chain and an OCDW in a ͑C 60 − ͒ n polymeric chain -also essentially a 1D objectis obvious: both double the periodicity. Therefore, it is tempting to conclude that the formation of OCDWs, originally invoked to explain the structural transition in polymerized KC 60 , 9 is responsible for the metal-insulator transition as well, just as lattice distortions doubling the primitive unit cell yield an insulating state for a 1D chain of atoms. In this paper, we present a rigorous analysis and show that the analogy is not as evident as one might intuitively expect; it turns out that a 1D picture does not yield the desired band gap.…”

“…9, for the insulating phase ͑T Ͻ T c Ϸ 50 K͒, orientational deviations of the C 60 − valence charge distributions of +⌬ 0 and −⌬ 0 along every polymer chain are added to the fixed chain orientations ± 0 . The resulting superstructure has a basecentered orthorhombic ͑bco͒ Bravais lattice; its primitive unit cell contains four inequivalent C 60 − monomers ͑Fig.…”

“…The positions r ជ t A are tabulated in Table IV; they reflect the periodicity doubling scheme of Ref. 9. ͓We recall that in FIG.…”

“…9, a theoretical model explaining the experimentally observed 8 ͑a ជ + c ជ , b ជ , a ជ − c ជ͒ superstructure of polymerized KC 60 below T Ϸ 60 K has been presented. The model involves ͑small͒ rotations of the valence charge distributions of the C 60 − monomers inducing average center-of-mass displacements of the K + ions.…”

“…␦ a a Ref 9,. the OCDWs are found to induce displacements of the alkali ions along the a ជ and c ជ directions, compatible with the ͑a ជ + c ជ , b ជ , a ជ − c ជ͒ superstructure.͔ Since the positively charged alkali ions act as attractive centers, the interaction strength U A is negative, as U 0 , U 1 and U 2 .…”