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Manuel Vilas-Varela

Publications and source records attributed to Manuel Vilas-Varela.

At least 19 recordsLinked to original sources
arXiv2026

Topological Engineering of a Frustrated Antiferromagnetic Triradical in Aza-Triangulene Architectures

Open-shell nanographenes provide a versatile platform to host unconventional magnetic states within their π-conjugated networks. Particularly appealing are graphene architectures that incorporate spatially separated radicals and tunable interactions, offering a scalable route toward spin-based quantum architectures. Triangulenes are ideal for this purpose, as their radical count scales with size, although strong hybridization prevents individual spin control. Here, we realize a radical reconfiguration strategy that transforms a single-radical aza-triangulene into three topologically protected spin states by covalently extending it with armchair anthene moieties of increasing length. Scanning tunnelling spectroscopy reveals emergent correlated spins forming a frustrated spin trimer whose interaction weakens with anthene length. This trend is captured by multi-reference electronic-structure calculations, which trace a progressive rise in polyradical character driven by the progressive reorganization of the correlated frontier orbitals into three edge-localized natural orbitals. Consequently, the initial single-radical doublet reorganizes into non-interacting edge spins, a molecular analog of a three-qubit quantum register.

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arXiv2026

Selectivity in tip-induced skeletal editing via heteroatom substitution

Skeletal editing enables precise structural modifications of molecules at late stages of a synthetic sequence, with applications in drug discovery and materials science. We recently demonstrated skeletal editing on the single-molecule scale. Voltage pulses applied by the tip of a scanning probe microscope to an oxygen-containing seven-membered heterocycle led to both oxygen deletion and ring-contraction rearrangement reactions. An open question is whether selective skeletal editing of a heterocyclic core can be achieved by an appropriate choice of the heteroatom. Here, we show that tip-induced reactions of an analogous sulfur-containing seven-membered ring results in sulfur deletion in virtually all cases. Our results demonstrate that the combination of tip-induced chemistry and heteroatom selection in the molecular design is a powerful strategy for single-molecule skeletal editing, with the potential to enable diverse structural transformations of heterocyclic frameworks.

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arXiv2025

A Single-Molecule Quantum Heat Engine

Particle-exchange heat engines operate without moving parts or time-dependent driving, relying solely on static energy-selective transport. Here, we realize a particle-exchange quantum heat engine based on a single diradical molecule, only a few nanometers in size. We experimentally investigate its operation at low temperatures and demonstrate that both the power output and efficiency are significantly enhanced by Kondo correlations, reaching up to 53 % of the Curzon-Ahlborn limit. These results establish molecular-scale particle-exchange engines as promising candidates for low-temperature applications where extreme miniaturization and energy efficiency are paramount.

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arXiv2025

Spin and Charge Control of Topological End States in Chiral Graphene Nanoribbons on a 2D Ferromagnet

Tailor-made graphene nanostructures can exhibit symmetry-protected topological boundary states that host localized spin-$1/2$ moments. However, one frequently observes charge transfer on coinage metal substrates, which results in spinless closed-shell configurations. Using low temperature scanning tunneling spectroscopy, we demonstrate here that pristine topologically nontrivial chiral graphene nanoribbons synthesized directly on the ferromagnet $\textrm{GdAu}_2$ can either maintain a charge-neutral diradical singlet or triplet configuration, or exist in a singly anionic doublet state. As an underlying mechanism, we identify a moiré-modulated work function and exchange field, as corroborated by Kelvin-probe force microscopy and spin-flip spectroscopy. The joint electrostatic and magnetic interactions allow reversibly switching between the three spin multiplicities by atomic manipulation. We introduce an effective Hubbard dimer model that unifies the effects of local electrostatic gating, electron-electron-correlation, hybridization and exchange field to outline the phase diagram of accessible spin states. Our results establish a platform for the local control of $π$-radicals adsorbed on metallic substrates.

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arXiv2025

On-surface Synthesis of a Ferromagnetic Molecular Spin Trimer

Triangulenes are prototypical examples of open-shell nanographenes. Their magnetic properties, arising from the presence of unpaired $π$ electrons, can be extensively tuned by modifying their size and shape or by introducing heteroatoms. Different triangulene derivatives have been designed and synthesized in recent years, thanks to the development of on-surface synthesis strategies. Triangulene-based nanostructures with polyradical character, hosting several interacting spin units, can be challenging to fabricate but are particularly interesting for potential applications in carbon-based spintronics. Here, we combine pristine and N-doped triangulenes into a more complex nanographene, \textbf{TTAT}, predicted to possess three unpaired $π$ electrons delocalized along the zigzag periphery. We generate the molecule on an Au(111) surface and detect direct fingerprints of multi-radical coupling and high-spin state using scanning tunneling microscopy and spectroscopy. With the support of theoretical calculations, we show that its three radical units are localized at distinct parts of the molecule and couple via symmetric ferromagnetic interactions, which result in a $S=3/2$ ground state, thus demonstrating the realization of a molecular ferromagnetic Heisenberg-like spin trimer

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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.

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arXiv2025

Systematic Modulation of Charge and Spin in Graphene Nanoribbons on MgO

Graphene nanostructures can be engineered with atomic precision to display customized electronic states with application in spintronics or quantum technologies. In order to take advantage of their full potential, their charge and spin state must be precisely controlled. Graphene systems exchange charge to reach thermodynamic equilibrium with their environment, requiring external gating potentials to tune their ground state. Alternative strategies like intrinsic doping or substrate modifications provided small variations of their equilibrium charge and poor control over their spin. Here, we show systematic manipulation of the electron occupation in graphene nanoribbons (GNRs) laying on MgO layers grown on Ag(001). Owing to the extraordinary decoupling properties of MgO, and the electropositive character of the substrate, GNRs are found to host an integer number of electron charges that depend on their length and shape. This results in the alternation between a non-magnetic closed-shell state and an open-shell paramagnetic system for even and odd electron occupations respectively. For the odd case, we found the spectral fingerprint of a narrow Coulomb correlation gap, which is the smoking gun of its spin 1/2 paramagnetic state. Comparisons of scanning tunnelling microscopy (STM) data with mean-field Hubbard (MFH) simulations confirm the practical discretization of the GNR electronic states and point to charge excess of up to 19 electrons in a single ribbon. We anticipate that GNRs supported by MgO ultra-thin insulating films can open the door to customized devices for quantum sensing and quantum processing applications..

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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.

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arXiv2024

Observation of SOMO-HOMO inversion in a polycyclic conjugated hydrocarbon

We report the generation of a non-benzenoid polycyclic conjugated hydrocarbon, which consists of a biphenyl moiety substituted by indenyl units at the 4,4' positions, on ultra-thin sodium chloride films by tip-induced chemistry. Single-molecule characterization by scanning tunneling and atomic force microscopy reveals an open-shell biradical ground state, with a peculiar electronic configuration wherein the singly-occupied molecular orbitals (SOMOs) are lower in energy than the highest occupied molecular orbital (HOMO).

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arXiv2024

Tuning the Spin Interaction in Non-planar Organic Diradicals Through Mechanical Manipulation

Open-shell polycyclic aromatic hydrocarbons (PAHs) represent promising building blocks for carbon-based functional magnetic materials. Their magnetic properties stem from the presence of unpaired electrons localized in radical states of $π$ character. Consequently, these materials are inclined to exhibit spin delocalization, form extended collective states, and respond to the flexibility of the molecular backbones. However, they are also highly reactive, requiring structural strategies to protect the radical states from reacting with the environment. Here, we demonstrate that the open-shell ground state of the diradical 2-OS survives on a Au(111) substrate as a global singlet formed by two unpaired electrons with anti-parallel spins coupled through a conformational dependent interaction. The 2-OS molecule is a protected derivative of the Chichibabin's diradical, featuring a non-planar geometry that destabilizes the closed-shell quinoidal structure. Using scanning tunneling microscopy (STM), we localized the two interacting spins at the molecular edges, and detected an excited triplet state a few millielectronvolts above the singlet ground state. Mean-field Hubbard simulations reveal that the exchange coupling between the two spins strongly depends on the torsional angles between the different molecular moieties, suggesting the possibility of influencing the molecule's magnetic state through structural changes. This was demonstrated here using the STM tip to manipulate the molecular conformation, while simultaneously detecting changes in the spin excitation spectrum. Our work suggests the potential of these PAHs for a new class of all-carbon spin-crossover materials.

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arXiv2023

Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects

On-surface synthesis has paved the way towards the fabrication and characterization of conjugated carbon-based molecular materials that exhibit π-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative, was recently shown to display rich on-surface chemistry, offering an ideal platform to investigate structure-property relations regarding spin-selective charge transfer and magnetic fingerprints. Herein, we study electronic changes upon fusing single molecules into larger dimeric derivatives. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell dimers covalently coupled through sterically accessible carbon atoms. Meanwhile, its open-shell structure on Au(111) leads to coupling via atoms displaying high spin density, resulting in symmetric or asymmetric dimer products. Interestingly, whereas all fused products on Au(111) exhibit similar charge transfer properties, only asymmetric dimers show magnetic fingerprints. We rationalize this in terms of molecular bonding structure and π-conjugation: in contrast to the symmetric and highly conjugated dimer, asymmetric dimers display more localized orbitals, which result in a larger Coulomb repulsion and thus promote single electron occupancies with associated spin density and π-magnetism. These results expose a clear relationship between molecular symmetry, charge transfer, and spin states in π-conjugated carbon-based nanostructures.

cond-mat.mtrl-sci↗
arXiv2023

Bistability between $π$-diradical open-shell and closed-shell states in indeno[1,2-a]fluorene

Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in non-linear optics and photovoltaics. Here, we report the generation of unsubstituted indeno[1,2-a]fluorene, the final and yet unreported parent indenofluorene isomer, on various surfaces by cleavage of two C-H bonds in 7,12-dihydroindeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunneling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, while it exhibits charge bistability between neutral and anionic states on the lower work function surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits either of two ground states: an open-shell pi-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. Switching between open- and closed-shell states of a single molecule is observed by changing its adsorption site on NaCl.

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arXiv2023

On-Surface Synthesis and Characterization of a High-Spin Aza-[5]-Triangulene

Triangulenes are open-shell triangular graphene flakes with total spin increasing with their size. In the last years, on-surface-synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) and the detection of its high spin ground state on a Au(111) surface. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S = 2), instead of the expected quartet state (S = 3/2), confirming the positively charged state of the molecule on the surface. We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in a S = 2 aza-[5]-triangulene on Au(111).

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arXiv2023

Detecting the spin-polarization of edge states in graphene nanoribbons

Low dimensional carbon-based materials are interesting because they can show intrinsic $π$-magnetism associated to p-electrons residing in specific open-shell configurations. Consequently, during the last years there have been impressive advances in the field combining indirect experimental fingerprints of localized magnetic moments with theoretical models. In spite of that, a characterization of their spatial- and energy-resolved spin-moment has so far remained elusive. To obtain this information, we present an approach based on the stabilization of the magnetization of $π$-orbitals by virtue of a supporting substrate with ferromagnetic ground state. Remarkably, we go beyond localized magnetic moments in radical or faulty carbon sites: In our study, energy-dependent spin-moment distributions have been extracted from spatially extended one-dimensional edge states of chiral graphene nanoribbons. This method can be generalized to other nanographene structures, representing an essential validation of these materials for their use in spintronics and quantum technologies.

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arXiv2021

Aza-Triangulene: On-Surface Synthesis, Electronic and Magnetic Properties

Nitrogen heteroatom doping into a triangulene molecule allows tuning its magnetic state. However, the synthesis of the nitrogen-doped triangulene (aza-triangulene) has been challenging. Herein, we report the successful synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with their characterizations by scanning tunneling microscopy and spectroscopy in combination with density functional theory (DFT) calculations. Aza-triangulenes were obtained by reducing ketone-substituted precursors. Exposure to atomic hydrogen followed by thermal annealing and, when necessary, manipulations with the scanning probe, afforded the target product. We demonstrate that on Au(111) aza-triangulene donates an electron to the substrate and exhibits an open-shell triplet ground state. This is derived from the different Kondo resonances of the final aza-triangulene product and a series of intermediates on Au(111). Experimentally mapped molecular orbitals match perfectly with DFT calculated counterparts for a positively charged aza-triangulene. In contrast, aza-triangulene on Ag(111) receives an extra electron from the substrate and displays a closed-shell character. Our study reveals the electronic properties of aza-triangulene on different metal surfaces and offers an efficient approach for the fabrication of new hydrocarbon structures, including reactive open-shell molecules.

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arXiv2021

Magnetic Interactions Between Radical Pairs in Chiral Graphene Nanoribbons

Magnetic graphene nanoribbons (GNRs) have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone group effectively adds one p_z electron to the π-electron network, thus producing an unpaired π radical. A closely related scenario occurs for regular ketone-functionalized chGNRs in which one oxygen atom is missing. Two such radical states can interact via exchange coupling and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding GNR, i.e., with or without ketone functionalization. In addition, we determine the parameters whereby this type of systems with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide new insights to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties.

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arXiv2021

Circumventing the Stability Problems of Graphene Nanoribbon Zigzag Edges

Carbon nanostructures with zigzag edges exhibit unique properties with exciting potential applications. Such nanostructures are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable exploitation. Here, we prove the viability of chemical protection/deprotection strategies for this aim, demonstrated on labile chiral graphene nanoribbons (chGNRs). Upon hydrogenation, the chGNRs survive an exposure to air, after which they are easily converted back to their original structure via annealing. We also approach the problem from another angle by synthesizing a chemically stable oxidized form of the chGNRs that can be converted to the pristine hydrocarbon form via hydrogenation and annealing. These findings may represent an important step toward the integration of zigzag-edged nanostructures in devices.

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