A carbon chain 48 atoms long. Pure sp-hybridization — alternating single and triple bonds, the simplest possible one-dimensional carbon structure. Approaching the theoretical limit of carbyne, a hypothetical infinite carbon chain.
Sheridan, Andréasson, and Anderson (arXiv:2602.20401) find that as the chain lengthens, its ground state and excited state evolve in opposite directions. The ground state delocalizes — electrons spread across the entire chain, Peierls distortion weakens, the chain approaches a uniform conductor. The excited state localizes — after excitation, the electronic energy rapidly self-traps, confining itself to a small section of the chain, then decaying through topology-dependent pathways.
The longer the chain, the more pronounced the split. In short chains, ground and excited states are both somewhat localized. In long chains, the ground state covers the whole chain while the excited state covers a fraction of it. Length amplifies the difference between the two responses.
The mechanism is the Peierls distortion — the alternation between single and triple bonds that opens a gap in the electronic structure. In the ground state, lengthening the chain dilutes this alternation. The distortion weakens, electrons delocalize, the chain approaches a metal. In the excited state, the extra energy creates a local structural distortion — a defect in the bond alternation pattern — that self-traps. The chain is too flexible to sustain a delocalized excitation. The ground state is rigid enough to stay spread out; the excited state is not.
The general observation: the same system can be simultaneously becoming more delocalized in one state and more localized in another. Whether a structure promotes spreading or trapping depends on which state you're asking about. The property is not “the chain delocalizes” or “the chain localizes.” It is “the chain delocalizes in its ground state and localizes in its excited state.” The two behaviors are not competing — they are coexisting in the same system at the same length.