2025
S. Edalatmanesh and T. Frederiksen
Non-Hermitian effects in the Su–Schrieffer–Heeger model: Exploring substrate coupling and decoupling dynamics
submitted [arXiv:2501.08299]
The substrate-adsorbate interaction can significantly influence the adsorbate's electronic structure, stability, reactivity, and topological properties. In this study, we investigate the emergence of non-Hermitian physics in the Su-Schrieffer-Heeger (SSH) model when coupled to a substrate, focusing on the impact of substrate interaction on the electronic states of the adsorbate. We demonstrate how the coupling between the SSH chain and the underlying substrate induces non-Hermitian effects, which manifest as amplification or attenuation of zero-energy electronic states. Furthermore, inspired by novel experimental techniques such as using a scanning tunneling microscope tip to lift part of the nanomaterial, we present simulations of scenarios where a segment of the SSH chain is decoupled from the substrate. By examining various configurations, including cases with odd or even numbers of sites coupled to the substrate, we demonstrate that tuning the coupling strength induces novel phenomena, such as the emergence of a zero-energy monomode or additional zero-energy states localized at the boundary between on-surface and suspended chain segments. Our results reveal the role of substrate coupling in shaping the topological properties of non-Hermitian SSH chains, offering new insights into tunable non-Hermitian effects and their potential applications in quantum technologies and nanodevices.
M. Frankerl, L. L. Patera, T. Frederiksen, J. Repp, and A. Donarini
Substrate stabilization of Jahn–Teller distortion in a single molecule
submitted
[arXiv:2408.00478]
Charge-state transitions of a single Cu-phthalocyanine molecule adsorbed on an insulating layer of NaCl on Cu(111) are probed by means of alternate charging scanning tunneling microscopy. Real-space imaging of the electronic transitions reveals the Jahn–Teller distortion occurring upon formation of the first and second anionic charge states. The experimental findings are rationalized by a theoretical many-body model which highlights the crucial role played by the substrate. The latter enhances and stabilizes the intrinsic Jahn–Teller distortion of the negatively charged molecule hosting a degenerate pair of single-particle frontier orbitals. Consequently, two excess electrons are found to occupy, in the ground state, the same localized orbital, despite a larger Coulomb repulsion than the one for the competing delocalized electronic configuration. Control over the charging sequence by varying the applied bias voltage is also predicted.
A. Domínguez-Celorrio, L. Edens, S. Sanz, M. Vilas-Varela, J. Martinez-Castro, D. Peña, V. Langlais, T. Frederiksen, J. I. Pascual, and D. Serrate
Engineering open-shell extended edge states in chiral graphene nanoribbons on MgO
submitted
[arXiv:2406.03927]
Graphene nanostructures are a promising platform for engineering electronic states with tailored magnetic and quantum properties. Synthesis strategies on metallic substrates have made it possible to manufacture atomically precise nanographenes with controlled size, shape and edge geometry. In these nanographenes, finite spin magnetic moment can arise as a result of many-body interactions in molecular orbitals with π-conjugated character and subject to strong spatial confinement, for example at the zig-zag edges. However, owing to the mixing of the molecular orbitals and metallic states from the catalysing substrate, most of their expected quantum phenomenology is severely hindered. The use of in-situ ultra-thin decoupling layers can impede nanographene-metal hybridization and facilitate the expression of predicted properties. Here we show that the edges of narrow chiral graphene nanoribbons (GNRs) over MgO monolayers on Ag(001) can host integer charge and spin-1/2 frontier states. The electron occupation varies with the GNR length, which alternates even or odd number of electrons, thus resulting correspondingly in a non-magnetic closed-shell state or an open-shell paramagnetic system. For the latter, we found the spectral fingerprint of a narrow Coulomb correlation gap. Charged states, up to 19 additional electrons, were identified by comparing mean-field Hubbard (MFH) simulations of the density of states with experimental maps of the discretized molecular orbitals acquired with a scanning tunnelling microscope (STM). In consideration of the length-dependent magnetic moment and the discrete nature of the electronic structure, we envisage that GNRs supported by thin insulating films can be used as tailor-made active elements in quantum sensing and quantum information processing.
S. Jiang, F. Scheurer, Q. Sun, P. Ruffieux, X. Yao, A. Narita, K. Müllen, R. Fasel, T. Frederiksen, and G. Schull
Length-independent quantum transport through topological band states of graphene nanoribbons
submitted
[arXiv:2208.03145]
[HTML5]
Atomically precise graphene nanoribbons (GNRs) have emerged as promising candidates for nanoelectronic applications due to their widely tunable energy band gaps resulting from lateral quantum confinement and edge effects. Here we report on the electronic transport characterization of an edge-modified GNR suspended between the tip of a scanning tunneling microscope (STM) and a Au(111) substrate. Differential conductance measurements on this metal-GNR-metal junction reveal loss-less transport properties (inverse decay length β<0.001Å) with high conductance (0.1 G0) at low voltages (50 meV) over long distances (z>10 nm). The transport behavior is sensitive to the coupling between ribbon and electrodes, an effect that is rationalized using tight-binding and density functional theory simulations. From extensive modelling we infer that the length-independent transport is a manifestation of band transport through topological valence states, which originate from the zigzag segments on the GNR edges.
D.-Y. Li, Y. Zheng, R. Ortiz, B.-X. Wang, Y. Jiang, B. Yuan, X.-Y. Zhang, C. Li, L. Liu, X. Liu, D. Guan, Y. Li, H. Zheng, C. Liu, J. Jia, T. Frederiksen, P.-N. Liu, and S. Wang
Magnetic exchange interaction between unpaired π- and d-electrons in nanographene-metal coordination complexes
National Science Review, in press (2025)
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[DOI]
The combination of open-shell nanographenes (NGs) and magnetic transition metals holds great promise for generating various new quantum phases applicable in spintronics and quantum information technologies. However, a crucial aspect in accomplishing this is to comprehend the magnetic exchange interactions between unpaired π- and d-electrons, a topic that has been seldom addressed. In this study, we focus on magnetic π-d exchange interactions between open-shell NGs and a magnetic coordination center of Fe or Co by employing scanning tunneling microscopy and spectroscopy. We synthesize two sets of NG-metal coordination complexes on a Au(111) substrate, secured by coordination bonds of carboxyl acid-Fe (Co). Through analysis of the excitation spectra, we observe a characteristic exchange coupling of 9 meV (5 meV) between the unpaired π-electron and the Fe (Co) d-shell electrons. Our experimental findings are qualitatively in agreement with multiconfigurational quantum chemistry calculations. This work evidences that a substantial magnetic exchange coupling can be achieved and engineered in metal-organic coordination systems, paving the way for designing and customizing extended radical metal-organic frameworks with precisely tailored magnetic properties.