Quantum phases in endofullerene zigzag chains

Abstract

We employ large-scale density matrix renormalization group calculations to study the quantum phases of dipolar molecules confined in bent (zigzag) endofullerene chains, as a function of the chain angle ${\hat{I}}^3$. For LiF, ferroelectric order persists across the full range $60^{\circ }<{\hat{I}}^{3}180^{\circ }$, with the critical effective dipole moment increasing as the chain bends and parallel alignment becomes less favorable. Near the equilateral configuration (${\hat{I}}^3=60^{\circ }$), geometric frustration drives a transition to an antiferroelectric Néel-ordered phase in which neighboring dipoles anti-align along the chain axis. We show that capturing this reorientation requires including dipolar couplings beyond the nearest-neighbor approximation, since next-nearest-neighbor interactions become equally strong at ${\hat{I}}^3=60^{\circ }$. For confined water, o-D${}_2$O reproduces both ordered phases, whereas p-H${}_2$O -- owing to its large rotational constants -- develops no order at any chain angle despite the enhanced coordination of the bent geometry. Because a zigzag chain is the narrowest stripe of a two-dimensional lattice, these results suggest that engineered endofullerene layers could host a rich variety of dipole-ordered quantum phases beyond the ferroelectric ordering observed in previous work.

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