Ferromagnetic Order in 2D Layers of Transition Metal Dichlorides.

Magnetism in two dimensions is traditionally considered an exotic phase mediated by spin fluctuations, but far from collinearly ordered in the ground state. Recently, 2D magnetic states have been discovered in layered van der Waals compounds. Their robust and tunable magnetic state by material composition, combined with reduced dimensionality, foresee a strong potential as a key element in magnetic devices. Here, a class of 2D magnets based on metallic chlorides is presented. The magnetic order survives on top of a metallic substrate, even down to the monolayer limit, and can be switched from perpendicular to in-plane by substituting the metal ion from iron to nickel. Using functionalized STM tips as magnetic sensors, local exchange fields are identified, even in the absence of an external magnetic field. Since the compounds are processable by molecular beam epitaxy techniques, they provide a platform with large potential for incorporation into current device technologies.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies.

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

On-Surface Synthesis and Determination of the Open-Shell Singlet Ground State of Tridecacene.

The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many-body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet-triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

Interplay between π-Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers.

The synthesis of novel polymeric materials with porphyrinoid compounds as key components of the repeating units attracts widespread interest from several scientific fields in view of their extraordinary variety of functional properties with potential applications in a wide range of highly significant technologies. The vast majority of such polymers present a closed-shell ground state, and, only recently, as the result of improved synthetic strategies, the engineering of open-shell porphyrinoid polymers with spin delocalization along the conjugation length has been achieved. Here, we present a combined strategy toward the fabrication of one-dimensional porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents on Au(111). Scanning tunneling microscopy and noncontact atomic force microscopy describe the thermal-activated intra- and intermolecular oxidative ring closure reactions as well as the controlled tip-induced hydrogen dissociation from the porphyrinoid units. In addition, scanning tunneling spectroscopy measurements, complemented by computational investigations, reveal the open-shell character, that is, the antiferromagnetic singlet ground state (S = 0) of the formed polymers, characterized by singlet-triplet inelastic excitations observed between spins of adjacent porphyrinoid units. Our approach sheds light on the crucial relevance of the π-conjugation in the correlations between spins, while expanding the on-surface synthesis toolbox and opening avenues toward the synthesis of innovative functional nanomaterials with prospects in carbon-based spintronics.

A closed local-orbital unified description of DFT and many-body effects.

Density functional theory (DFT) is usually formulated in terms of the electron density as a function of positionn(r). Here we discuss an alternative formulation of DFT in terms of the orbital occupation numbers {nα} associated with a local-orbital orthonormal basis set {ϕα}. First, we discuss how the building blocks of DFT, namely the Hohenberg-Kohn theorems, the Levy-Lieb approach and the Kohn-Sham method, can be adapted for a description in terms of {nα}. In particular, the total energy is now a function of {nα},E[{nα}], and a Kohn-Sham-like Hamiltonian is derived introducing the effects of the electron-electron interactions via effective potentials,{Vαeff=∂Eee[{nß}]/∂nα}. In a second step we consider the Hartree and exchange energies and discuss how to describe them, in the spirit of a DFT approach, in terms of the orbital occupation numbers. In this contribution special attention is paid to the description of the (intra-atomic) correlation energy and corresponding correlation potentials {Vcorr,α}. For this purpose, a model system is analyzed in detail, whereby an atomic Hamiltonian interacts with the environment via a simplified model; the use of this model allows us to obtain the correlation energy and potentials (in terms of {nα}) for different cases corresponding to low, intermediate and high electron correlations.

A local-orbital density functional formalism for a many-body atomic Hamiltonian: Hubbard-Hund's coupling and DFT + U functional.

In the conventional DFT + U approach, the mean field solution of the Hubbard Hamiltonian associated with thedorf(iσ) electrons of a transition metal atom is used to define the DFT + U potential acting on theiσ-electrons. In this work, we go beyond that mean field solution by analyzing the correlation energy and potential for a multi-level atom described by a Kanamori Hamiltonian connected to different channels representing the environment. As a first step, we analyze the many-body solution of our model, using a local-orbital density functional formalism that takes as independent variables the orbital occupancies,niσ, of the atomic orbitals; accordingly, we present the corresponding density functional solution describing the correlation energy and potential as a function ofniσ. Then, we use this analysis to introduce a DFT + U potential extending previous proposals to materials with arbitrarily high correlation. In particular, we find that this potential mainly screens the conventional mean field potential contribution, and also yields new terms associated with the number of atomic electrons. Our results show that the atomic correlation effects enhance the role played by the intra-atomic exchange interaction and favor the formation of magnetic solutions.

Significance Of Nuclear Quantum Effects In Hydrogen Bonded Molecular Chains.

In hydrogen-bonded systems, nuclear quantum effects such as zero-point motion and tunneling can significantly affect their material properties through underlying physical and chemical processes. Presently, direct observation of the influence of nuclear quantum effects on the strength of hydrogen bonds with resulting structural and electronic implications remains elusive, leaving opportunities for deeper understanding to harness their fascinating properties. We studied hydrogen-bonded one-dimensional quinonediimine molecular networks which may adopt two isomeric electronic configurations via proton transfer. Herein, we demonstrate that concerted proton transfer promotes a delocalization of π-electrons along the molecular chain, which enhances the cohesive energy between molecular units, increasing the mechanical stability of the chain and giving rise to distinctive electronic in-gap states localized at the ends. These findings demonstrate the identification of a class of isomeric hydrogen-bonded molecular systems where nuclear quantum effects play a dominant role in establishing their chemical and physical properties. This identification is a step toward the control of mechanical and electronic properties of low-dimensional molecular materials via concerted proton tunneling.

Ultrahigh-yield on-surface synthesis and assembly of circumcoronene into a chiral electronic Kagome-honeycomb lattice.

On-surface synthesis has revealed remarkable potential in the fabrication of atomically precise nanographenes. However, surface-assisted synthesis often involves multiple-step cascade reactions with competing pathways, leading to a limited yield of target nanographene products. Here, we devise a strategy for the ultrahigh-yield synthesis of circumcoronene molecules on Cu(111) via surface-assisted intramolecular dehydrogenation of the rationally designed precursor, followed by methyl radical-radical coupling and aromatization. An elegant electrostatic interaction between circumcoronenes and metallic surface drives their self-organization into an extended superlattice, as revealed by bond-resolved scanning probe microscopy measurements. Density functional theory and tight-binding calculations reveal that unique hexagonal zigzag topology of circumcoronenes, along with their periodic electrostatic landscape, confines two-dimensional electron gas in Cu(111) into a chiral electronic Kagome-honeycomb lattice with two emergent electronic flat bands. Our findings open up a new route for the high-yield fabrication of elusive nanographenes with zigzag topologies and their superlattices with possible nontrivial electronic properties.

Proton Transfer in Guanine-Cytosine Base Pairs in B-DNA.

A double proton transfer reaction in a guanine-cytosine (GC) base pair has been proposed as a possible mechanism for rare tautomer (G*C*) formation and thus a source of spontaneous mutations. We analyze this system with free energy calculations based on extensive Quantum Mechanics/Molecular Mechanics simulations to properly consider the influence of the DNA biomolecular environment. We find that, although the G*C* rare tautomer is metastable in the gas phase, it is completely unstable in the conditions found in cells. Thus, our calculations show that a double proton reaction cannot be the source of spontaneous point mutations. We have also analyzed the intrabase H transfer reactions in guanine. Our results show that the DNA environment gives rise to a large free energy difference between the rare and canonical tautomers. These results show the key role of the DNA biological environment for the stability of the genetic code.

Two-step ATP-driven opening of cohesin head.

The cohesin ring is a protein complex composed of four core subunits: Smc1A, Smc3, Rad21 and Stag1/2. It is involved in chromosome segregation, DNA repair, chromatin organization and transcription regulation. Opening of the ring occurs at the "head" structure, formed of the ATPase domains of Smc1A and Smc3 and Rad21. We investigate the mechanisms of the cohesin ring opening using techniques of free molecular dynamics (MD), steered MD and quantum mechanics/molecular mechanics MD (QM/MM MD). The study allows the thorough analysis of the opening events at the atomic scale: i) ATP hydrolysis at the Smc1A site, evaluating the role of the carboxy-terminal domain of Rad21 in the process; ii) the activation of the Smc3 site potentially mediated by the movement of specific amino acids; and iii) opening of the head domains after the two ATP hydrolysis events. Our study suggests that the cohesin ring opening is triggered by a sequential activation of the ATP sites in which ATP hydrolysis at the Smc1A site induces ATPase activity at the Smc3 site. Our analysis also provides an explanation for the effect of pathogenic variants related to cohesinopathies and cancer.