arXiv subjects

Fabian Paschke

Publications and source records attributed to Fabian Paschke.

5 recordsLinked to original sources

arXiv2026

On-surface synthesis and aromaticity of large cyclocarbons

Molecular rings of N carbon atoms, that is, cyclo[N]carbons, or C$_N$, can be formed by tip-induced chemistry [1-7]. Because of their monocyclic geometry, cyclocarbons are fundamentally important for testing theoretical models of aromaticity [8-11]. Here, we synthesized large cyclo[N]carbons, with N up to 88, by tip-induced chemistry on a NaCl surface and studied their aromaticity by measuring their transport gaps by scanning tunnelling spectroscopy. We first generated C$_{20}$ and C$_{22}$, and then fused multiple cyclocarbons [5-7] by means of atom manipulation, obtaining C$_{42}$, C$_{44}$, C$_{46}$, C$_{66}$ and C$_{88}$. In agreement with predictions obtained using a finely tuned density functional [12-15] and large active space approximate configuration interaction calculations executed on quantum hardware [16, 17], we observe a substantially smaller transport gap for C$_{20}$ (N = 4n) compared to C$_{22}$ (4n+2), and for C$_{44}$ (4n) relative to C$_{42}$ (4n+2). In larger cyclocarbons, the oscillation of the transport gap between anti-aromatic N = 4n and aromatic N = 4n+2 cyclocarbons becomes smaller, and is expected to eventually vanish with increasing N, indicating non-aromaticity. Our experimental results show that aromaticity persists at N = 42, and theory predicts ring currents comparable in magnitude to that of benzene in cyclocarbons of this size. In the future, such large cyclocarbons could be used to study conductance, quantum interference, and the effects of aromaticity in single atomic carbon wires and circuits.

cond-mat.mes-hall↗

arXiv2026

A molecule with half-Möbius topology

Stereoisomers of C$_{13}$Cl$_2$ exhibiting helical orbitals around a ring of carbon atoms were synthesized by atom manipulation on NaCl surfaces. We resolved the enantiomeric geometries of the singlet states by atomic force microscopy and mapped their helical orbital densities by scanning tunnelling microscopy. A $π$-orbital basis of the helical, non-planar singlets that twists by 90° in one circulation is consistent with a half-Möbius topology. In such a topology, the $π$-orbital basis changes sign with respect to two circumnavigations and is periodic with respect to four circumnavigations. A quasiparticle on a ring with this boundary condition could be interpreted as carrying a Berry phase of $π$/2. We demonstrate reversible switching of the topology, between the two singlets of oppositely threaded half-Möbius topology, and the planar, topologically trivial, triplet state. Multireference calculations, including large-scale sample-based ab initio calculations executed on quantum hardware, reveal that the switching is associated with a helical pseudo Jahn-Teller effect.

cond-mat.mes-hall↗

arXiv2025

Synthesis and characterization of a $π$-extended Clar's goblet

Concealed non-Kekulé polybenzenoid hydrocarbons have no sublattice imbalance yet cannot be assigned a classical Kekulé structure, leading to an open-shell ground state with potential application in organic spintronics. They constitute an exceedingly small fraction of the total number of polybenzenoid hydrocarbons that can be constructed for a given number of benzenoid rings, and their synthesis remains challenging. The archetype of such a system is Clar's goblet (C$_{38}$H$_{18}$), a diradical proposed by Erich Clar in 1972 and recently synthesized on a Au(111) surface. Here, we report the synthesis of a pi-extended Clar's goblet (C$_{76}$H$_{26}$), a tetraradical concealed non-Kekulé polybenzenoid hydrocarbon, by a combined in-solution and on-surface synthetic approach. By means of low-temperature scanning tunneling microscopy and atomic force microscopy, we characterized individual molecules adsorbed on a Cu(111) surface. We provide insights into the electronic properties of this elusive molecule, including the many-body nature of its ground and excited states, by mean-field and multiconfigurational quantum chemistry calculations.

cond-mat.mes-hall↗

arXiv2024

A Route Toward the On-Surface Synthesis of Organic Ferromagnetic Quantum Spin Chains

Engineering sublattice imbalance is an intuitive way to induce high-spin ground states in bipartite polycyclic conjugated hydrocarbons (PCHs). Such high-spin molecules can be employed as building blocks of quantum spin chains, which are outstanding platforms to study many-body physics and fundamental models in quantum magnetism. Recent reports on the bottom-up synthesis of antiferromagnetic molecular spin chains provided insights into paradigmatic quantum phenomena such as fractionalization. In contrast to antiferromagnetism, demonstration of ferromagnetic coupling between PCHs has been scarce. Previous attempts in this direction were limited by the formation of non-benzenoid rings leading to spin quenching, or the use of spacer motifs that considerably weaken the magnitude of ferromagnetic exchange. Here, we demonstrate the on-surface synthesis of short ferromagnetic spin chains based on dibenzotriangulene (DBT), a PCH with a triplet ground state. Our synthetic strategy centers on achieving a direct (that is, without a spacer motif) majority-minority sublattice coupling between adjacent units. This leads to a global sublattice imbalance in spin chains scaling with the chain length, and therefore a ferromagnetic ground state with a strong intermolecular ferromagnetic exchange. By means of scanning probe measurements and multiconfigurational quantum chemistry calculations, we analyze the electronic and magnetic properties of ferromagnetic dimers and trimers of DBT, and confirm their quintet and septet ground states, respectively, with an intermolecular ferromagnetic exchange of 7 meV. Furthermore, we elucidate the role of sublattice coupling on magnetism through complementary experiments on antiferromagnetic DBT dimers with majority-majority and minority-minority couplings. We expect our proof-of-principle study to provide impetus for the design of purely organic ferromagnetic materials.

cond-mat.mes-hall↗

arXiv2018

The graphene/n-Ge(110) interface: structure, doping, and electronic properties

The implementation of graphene in semiconducting technology requires the precise knowledge about the graphene-semiconductor interface. In our work the structure and electronic properties of the graphene/$n$-Ge(110) interface are investigated on the local (nm) and macro (from $μ\mathrm{m}$ to mm) scales via a combination of different microscopic and spectroscopic surface science techniques accompanied by density functional theory calculations. The electronic structure of freestanding graphene remains almost completely intact in this system, with only a moderate $n$-doping indicating weak interaction between graphene and the Ge substrate. With regard to the optimization of graphene growth it is found that the substrate temperature is a crucial factor, which determines the graphene layer alignment on the Ge(110) substrate during its growth from the atomic carbon source. Moreover, our results demonstrate that the preparation routine for graphene on the doped semiconducting material ($n$-Ge) leads to the effective segregation of dopants at the interface between graphene and Ge(110). Furthermore, it is shown that these dopant atoms might form regular structures at the graphene/Ge interface and induce the doping of graphene. Our findings help to understand the interface properties of the graphene-semiconductor interfaces and the effect of dopants on the electronic structure of graphene in such systems.

cond-mat.mtrl-sci↗