Abstract
Atomic chains are perfect systems for getting fundamental insights into the electron dynamics and coupling between the electronic and ionic degrees of freedom in one-dimensional metals. Depending on the band filling, they can exhibit Peierls instabilities (or charge density waves), where equally spaced chain of atoms with partially filled band is inherently unstable, exhibiting spontaneous distortion of the lattice that further leads to metal-insulator transition in the system. Here, using high-resolution scanning transmission electron microscopy, we directly image the atomic structures of a chain of iodine atoms confined inside carbon nanotubes. In addition to long equidistant chains, the ones consisting of iodine dimers and trimers were also observed, as well as transitions between them. First-principles calculations reproduce the experimentally observed bond lengths and lattice constants, showing that the ionic movement is largely unconstrained in the longitudinal direction, while naturally confined by the nanotube in the lateral directions. Moreover, the trimerized chain bears the hallmarks of a charge density wave. The transition is driven by changes in the charge transfer between the chain and the nanotube and is enabled by the charge compensation and additional screening provided by the nanotube.
Highlights
Depending on the band filling, they can exhibit Peierls instabilities, where spaced chain of atoms with partially filled band is inherently unstable, exhibiting spontaneous distortion of the lattice that further leads to metal−insulator transition in the system
Using high-resolution scanning transmission electron microscopy, we directly image the atomic structures of a chain of iodine atoms confined inside carbon nanotubes
I t was asserted in the early work of Peierls that at low temperature a regular chain structure of a one-dimensional metal with a partly filled band will never be stable because distortion of the lattice with periodicity corresponding to the
Summary
With the nanotube walls and that charge transfer of electrons from the nanotube to the chain leads to electrostatic attraction, the cylindical symmetry of the interaction leads to rather flat radial potential profile at the center of the tube. The binding energy is barely negative in the (5,5) tube due to the inner diameter being smaller than the ionic radius of iodine, it interestingly forces the chain to be straight and shows minor trimerization for any strain This is in qualitative agreement with the finding that small bond length trimers are observed in small diameter chains, the calculated energy minimum is at larger Lz,av. If the linear chain is charged by additional one-third electron per atom, the σ-band obtains two-thirds filling and the system could undergo Peierls transition, as seen in the trimer band structure and as evident from simple tight binding calculations.[26] The calculated phonon spectrum shown in Figure 4c exhibits a pronounced imaginary frequency mode at k = 2 · π , exactly matching with the Fermi nesting vector.
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