Engineer­ing Covalent Quantum Model Systems

The project builds covalently linked porphyrin spin chains on ultra­thin insulat­ing films to create designer quantum‑model systems. By coupling low‑energy electrospray/ion‑beam deposi­tion (LEIBD) with ESR‑STM, mass‑selected metallotetraphenyl‑porphyrin fragments are placed on MgO/Ag(100) or NaCl/Au(111) and assem­bled into dimers and short 1‑D arrays (2–6 units); site‑resolved spectroscopy yields g‑factors, exchange and dipolar couplings, while pulsed ESR (Rabi, Ramsey, echo) demon­strates coher­ent control of the result­ing spin Hamil­ton­ian, provid­ing a gener­ally applic­a­ble platform for molec­u­lar quantum simulators.

The project aims to fabri­cate covalently linked porphyrin‑spin chains on ultra­thin insulat­ing films and to employ them as designer quantum‑model systems. Conven­tional on‑surface synthe­sis on Au(111) can produce atomi­cally precise spin lattices, but the strong hybridi­s­a­tion with the metal substrate dramat­i­cally short­ens spin lifetimes. In contrast, thin insulat­ing layers such as MgO/Ag(100) or NaCl/Au(111) decou­ple the spins from the conduc­tive substrate, enabling electron‑spin‑resonance scanning tunnelling microscopy (ESR‑STM) with MHz‑wide linewidths; however, these insula­tors do not support the metal‑catalysed coupling reactions required to build extended struc­tures. The central challenge there­fore is to create chemi­cally defined, covalent spin chains that reside on a decou­pling surface while preserv­ing long coher­ence times.

In the first work step we will use mass‑selective soft‑landing (LEIBD). Metallotetraphenyl‑porphyrins such as FeTPP are ionised either by electron‑impact or by electro­spray, mass‑selected, and softly deposited on the insulat­ing films with low impact energies. By system­at­i­cally varying the landing energy, substrate temper­a­ture and post‑annealing condi­tions we will gener­ate a yield map that identi­fies the optimal parame­ters for forming dimers and short chains. In paral­lel, a refer­ence set of intact FeTPP molecules will be thermally evapo­rated to deter­mine adsorp­tion geome­try, electronic struc­ture and single‑spin proper­ties; these data will serve as bench­marks for the covalently linked systems.

Spin‑sensitive charac­ter­i­sa­tion will be performed with ESR‑STM. Site‑resolved spectroscopy will extract the g‑factor, zero‑field split­ting and both exchange and dipolar couplings between neigh­bour­ing porphyrins. Pulsed microwave sequences (Rabi, Ramsey and Hahn‑echo) will allow us to measure T₁ and T₂ times and to imple­ment quantum‑logic opera­tions, thereby demon­strat­ing coher­ent control of the emergent spin Hamil­ton­ian. The exper­i­men­tal results will be comple­mented by exact diago­nal­i­sa­tion of Heisen­berg models, enabling us to test theoret­i­cal predic­tions of edge states and magnon/spinon bands.

The work is organ­ised into three packages. The first, led by Willke, will build a compre­hen­sive refer­ence database for intact FeTPP on metal and insulat­ing surfaces, includ­ing STS, IETS and continuous‑wave ESR measure­ments. The second, led by Kappes, will optimise the LEIBD parame­ters, produce quanti­ta­tive yield maps for open‑end porphyrin fragments and analyse the struc­tural proper­ties of the result­ing dimers and chains on HOPG, Au(111) and finally on MgO/Ag and NaCl/Au films. The third, a joint effort, will handle UHV trans­fer between the two labora­to­ries, perform site‑resolved ESR‑STM on the covalently linked dimers and chains, and recon­struct the under­ly­ing spin Hamil­to­ni­ans to identify emergent quantum phenomena.

Prelim­i­nary work demon­strates feasi­bil­ity: Willke has already shown ESR‑STM on single transition‑metal ions and on molec­u­lar spin centres such as Fe‑phthalocyanine, includ­ing Rabi oscil­la­tions and Hahn‑echo exper­i­ments. Kappes has success­fully employed LEIBD to deposit mass‑selected fragments of fullerenes and metal­lo­por­phyrins on various substrates, confirm­ing chemi­cal purity with MS‑TDS and XPS analyses.

The integra­tion of mass‑selected ion soft‑landing with ESR‑STM repre­sents a method­olog­i­cal break­through that is not avail­able at any single site today. It provides a modular platform on which arbitrary molec­u­lar build­ing blocks can be arranged with controlled spin coupling, and the approach is readily exten­si­ble to larger biomol­e­cules such as metal­lo­pro­teins. In the long term the project will deliver an open toolbox for the commu­nity, linking funda­men­tal surface physics with quantum‑information and sensing appli­ca­tions and laying the founda­tion for the next gener­a­tion of molec­u­lar quantum simulators.