2021

We investigate the statistics of photons emitted by tunneling electrons in a single electronic level plasmonic nanojunction. We compute the waiting-time distribution of successive emitted photons w(τ). When the cavity damping rate κ is larger than the electronic tunneling rate Γ, we show that in the photon-antibunching regime, w(τ) indicates that the average delay time between two successive photon-emission events is given by 1/Γ. This is in contrast with the usually considered second-order correlation function of emitted photons, g(2)(τ), which displays the single time scale 1/κ. Our analysis shows a relevant example for which w(τ) gives independent information on the photon-emission statistics with respect to g(2)(τ), leading to physical insight into the problem. We discuss how this information can be extracted from experiments even in the presence of a nonperfect photon-detection yield.

Triangulene nanographenes are open-shell molecules with predicted high spin state due to the frustration of their conjugated network. Their long-sought synthesis became recently possible over a metal surface. Here, we present a macrocycle formed by six [3]triangulenes, which was obtained by combining the solution synthesis of a dimethylphenyl-anthracene cyclic hexamer and the on-surface cyclodehydrogenation of this precursor over a gold substrate. The resulting triangulene nanostar exhibits a collective spin state generated by the interaction of its 12 unpaired π-electrons along the conjugated lattice, corresponding to the antiferromagnetic ordering of six S = 1 sites (one per triangulene unit). Inelastic electron tunneling spectroscopy resolved three spin excitations connecting the singlet ground state with triplet states. The nanostar behaves close to predictions from the Heisenberg model of a S = 1 spin ring, representing a unique system to test collective spin modes in cyclic systems.

Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional π-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergo a topological phase transition. We fabricated atomically precise chGNRs of different chirality and size by on surface synthesis using predesigned molecular precursors. Combining scanning tunneling microscopy (STM) measurements and theory simulations, we follow the evolution of topological properties and bulk band gap depending on the width, length, and chirality of chGNRs. Our findings represent a new platform for producing topologically protected spin states and demonstrate the potential of connecting chiral edge and defect structure with band engineering.

Deposition of a one-atom-thick layer of Ag or Cu on Au electrodes proves to be an effective strategy to tune the band alignment and conductivity of molecular junctions.