arXiv subjects

Nils Krane

Publications and source records attributed to Nils Krane.

At least 19 recordsLinked to original sources

arXiv2026

One-dimensional carbon nanostructures with periodic graphitic nitrogen substitution

Heteroatom substitution is a powerful route to tune the chemical and electronic properties of carbon nanomaterials. In particular, replacement of an sp2 hybridized carbon atom in the graphene lattice with a nitrogen atom (denoted as graphitic nitrogen) induces substantial changes in the electronic properties. These include changes in the band structure that can influence electronic transport, and magnetism. A key requirement for applications is both the periodic and precise incorporation of the heteroatoms in extended carbon lattices. Here, we report the on-surface synthesis and characterization of two one dimensional carbon nanostructures, a polymer and a graphene nanoribbon, consisting of periodically incorporated graphitic nitrogen atoms. The on-surface reactions toward formation of the nanostructures were monitored by scanning tunneling microscopy. The bond-resolved chemical structures of the reaction intermediates and products were investigated by atomic force microscopy, which enabled atomic-scale visualization of the graphitic nitrogen sites. The electronic properties of the nanostructures were studied by scanning tunneling spectroscopy and density functional theory calculations. Our analyses revealed the presence of localized nitrogen-centered electronic states. In the gas phase where the nanostructures are in a neutral charge state, these states undergo spin polarization leading to an open-shell ground state. Upon adsorption on Au(111), the nanostructures exhibit electron transfer to the surface, which resulted in a closed-shell ground state. Our results demonstrate a straightforward and generally applicable route to synthesize graphitic nitrogen-substituted carbon nanomaterials with potential applications in spintronics, catalysis and energy storage.

cond-mat.mes-hall↗

arXiv2026

Engineering Ferrimagnetic Interactions in Molecular Quantum Systems

Achieving long-range ferrimagnetic order in purely organic systems remains a major challenge in molecular magnetism. Here we report the synthesis and characterization of heterospin-coupling motifs, formed by covalently linking spin-1/2 and spin-1 triangular nanographenes. A combined solution-phase and on-surface synthetic strategy yields three distinct compounds, whose structures are elucidated by bond-resolved scanning probe microscopy. Starting from a spin-1/2--spin-1 dimer as the elemental ferrimagnetic unit, we employ inelastic electron tunneling spectroscopy to resolve low-energy magnetic excitations and extract the parameters of the Heisenberg Hamiltonian. Extension to trimeric architectures results in two distinct spin configurations, with compensated ($S=0$) and uncompensated ($S=3/2$) ferrimagnetic ground states. The Heisenberg model accurately describes all magnetic transitions, offering direct insight into increasingly complex spin Hamiltonians. These findings establish a molecular platform for designing tunable heterospin systems with robust exchange interactions, opening routes toward multi-level spin encoding in qudit-based quantum technologies.

cond-mat.mes-hall↗

arXiv2026

Observation of the Ferromagnetic Kondo Effect

The quest for quantum ground states beyond the conventional Fermi-liquid paradigm remains a central challenge in many-body physics. The ferromagnetic Kondo effect represents a particularly intriguing case: an exotic variant of the Kondo effect in which an asymptotically free spin gives rise to singular Fermi-liquid behavior. Despite its theoretical importance, this regime has long eluded experimental observation owing to its subtle spectroscopic signatures, vanishingly small energy scales, and strict symmetry constraints in conventional nanostructures. Here, we demonstrate the coexistence of the ferromagnetic and overscreened Kondo effects within a single molecular spin system$\unicode{x2014}$a triangulene dimer comprising spin-1 and spin-1/2 units adsorbed on a metal surface. Low-temperature scanning tunneling spectroscopy reveals characteristic signatures of singular Fermi-liquid behavior, which are fully supported by many-body calculations. The unique molecular design provides intrinsic control over spin configuration and coupling asymmetry, allowing distinct many-body regimes to be accessed within the same platform. Our results establish a robust strategy for realizing non-Fermi-liquid physics at the atomic scale and demonstrate that ferromagnetic Kondo behavior can not only be observed but also deliberately engineered in molecular systems.

cond-mat.mes-hall↗

arXiv2025

Tunable Luminescence From a Single Free-Base Porphyrin Molecule By Controlled Access to Optically Active States

Scanning tunneling microscopy-induced luminescence (STML) provides access to the optical properties of individual molecules through a cascade of relaxation processes between many-body states. Insufficient charge attachment energies prevent relaxation via optically active states, causing even intrinsically bright molecules to remain dark in STML. Here, we utilize substrate work function control and tip-induced gating of the double-barrier tunnel junction to induce an energy shift of the ionic transition state of a single free-base tetrabenzoporphyrin (H2TBP) to control access to optically active states and bright exciton emission. The experimental observations are validated by a rate equation and a polaron model considering the relaxation energy of the NaCl decoupling layer upon charging of the molecule.

cond-mat.mes-hall↗

arXiv2025

The multiconfigurational ground state of a diradicaloid characterized at the atomic scale

We report the tip-induced generation and scanning probe characterization of a singlet diradicaloid, consisting of two phenalenyl units connected by an sp-hybridized C$_{4}$ chain, on an ultrathin insulating NaCl surface. The bond-order contrast along the C$_{4}$ chain measured by atomic force microscopy and mapping of charge-state transitions by scanning tunneling microscopy, in conjunction with multiconfigurational calculations, reveal that the molecule exhibits a many-body ground state. Our study experimentally demonstrates the manifestation of strong electronic correlations in the geometric and electronic structures of a single molecule.

cond-mat.mes-hall↗

arXiv2025

Sensing a magnetic rare-earth surface alloy by proximity effect with an open-shell nanographene

Open-shell nanographenes have attracted significant attention due to their structurally tunable spin ground state. While most characterization has been conducted on weakly-interacting substrates such as noble metals, the influence of magnetic surfaces remains largely unexplored. In this study, we investigate how TbAu2, a rare-earth-element-based surface alloy, affects the magnetic properties of phenalenyl (or [2]triangulene (2T)), the smallest spin-1/2 nanographene. Scanning tunneling spectroscopy (STS) measurements reveal a striking contrast: while 2T on Au(111) exhibits a zero-bias Kondo resonance - a hallmark of a spin-1/2 impurity screened by the conduction electrons of the underlying metal - deposition on TbAu2 induces a symmetric splitting of this feature by approximately 20 mV. We attribute this splitting to a strong proximity-induced interaction with the ferromagnetic out-of-plane magnetization of TbAu2. Moreover, our combined experimental and first-principles analysis demonstrates that this interaction is spatially modulated, following the periodicity of the TbAu2 surface superstructure. These findings highlight that TbAu2 serves as a viable platform for stabilizing and probing the magnetic properties of spin-1/2 nanographenes, opening new avenues for the integration of {\pi}-magnetic materials with magnetic substrates.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast Coulomb blockade in an atomic-scale quantum dot

Controlling electron dynamics at optical clock rates is a fundamental challenge in lightwave-driven nanoelectronics. Here, we demonstrate ultrafast charge-state manipulation of individual selenium vacancies in monolayer and bilayer tungsten diselenide (WSe$_2$) using picosecond terahertz (THz) source pulses, focused onto the picocavity of a scanning tunneling microscope (STM). Using THz pump--THz probe time-domain sampling of the defect charge population, we capture atomic-scale snapshots of the transient Coulomb blockade, a signature of charge transport via quantized defect states. We identify back tunneling of localized charges to the tip electrode as a key challenge for lightwave-driven STM when probing electronic states with charge-state lifetimes exceeding the pulse duration. However, we show that back tunneling can be mitigated by the Franck-Condon blockade, which limits accessible vibronic transitions and promotes unidirectional charge transport. Our rate equation model accurately reproduces the time-dependent tunneling process across the different coupling regimes. This work builds on recent progress in imaging coherent lattice and quasiparticle dynamics with lightwave-driven STM and opens new avenues for exploring ultrafast charge dynamics in low-dimensional materials, advancing the development of lightwave-driven nanoscale electronics.

cond-mat.mes-hall↗

arXiv2024

Vibrational excitations in magnetic triangular nanographenes

Inelastic electron tunneling spectroscopy (IETS) is a powerful measurement technique often used in scanning tunneling spectroscopy to probe excited states of various nanostructures, e.g., the magnetic properties of complex spin systems. The observed excited states can be of magnetic and vibrational origin and it is therefore necessary to differentiate between these two excitation mechanisms. Here, we investigate the spin S = 1/2 phenalenyl radical on Au(111). IETS measurements feature inelastic excitations, whereas the spatial distribution of their intensity excludes any spin excitations. Comparison to theoretical simulations proves the vibrational origin of those excitations and allows us to assign the observed features to distinct vibrational modes.

cond-mat.mes-hall↗

arXiv2024

Conformational tuning of magnetic interactions in coupled nanographenes

Phenalenyl (C$_{13}$H$_9$) is an open-shell spin-$1/2$ nanographene. Using scanning tunneling microscopy (STM) inelastic electron tunneling spectroscopy (IETS), covalently-bonded phenalenyl dimers have been shown to feature conductance steps associated with singlet-triplet excitations of a spin-$1/2$ dimer with antiferromagnetic exchange. Here, we address the possibility of tuning the magnitude of the exchange interactions by varying the dihedral angle between the two molecules within a dimer. Theoretical methods, ranging from density functional theory calculations to many-body model Hamiltonians solved within different levels of approximation, are used to explain STM-IETS measurements of twisted phenalenyl dimers on a h-BN/Rh(111) surface. By means of first-principles calculations, we also propose strategies to induce sizable twist angles in surface-adsorbed phenalenyl dimers via functional groups, including a photoswitchable scheme. This work paves the way toward tuning magnetic couplings in carbon-based spin chains and two-dimensional lattices.

cond-mat.mes-hall↗

arXiv2024

Layer-Dependent Charge State Lifetime of Single Se Vacancies in WSe$_2$

Defect engineering in two-dimensional semiconductors has been exploited to tune the optoelectronic properties and introduce new quantum states in the band gap. Chalcogen vacancies in transition metal dichalcogenides in particular have been found to strongly impact charge carrier concentration and mobility in 2D transistors as well as feature sub-gap emission and single-photon response. In this letter, we investigate the layer-dependent charge state lifetime of Se vacancies in WSe$_2$. In one monolayer WSe$_2$, we observe ultrafast charge transfer from the lowest unoccupied orbital of the top Se vacancy to the graphene substrate within (1.0 $\pm$ 0.2) ps measured via the current saturation in scanning tunneling approach curves. For Se vacancies decoupled by TMD multilayers, we find a sub-exponential increase of the charge lifetime from (62 $\pm$ 14) ps in bilayer to few nanoseconds in four-layer WSe$_2$, alongside a reduction of the defect state binding energy. Additionally, we attribute the continuous suppression and energy shift of the dI/dV in-gap defect state resonances at very close tip--sample distances to a current saturation effect. Our results provide a key measure of the layer-dependent charge transfer rate of chalcogen vacancies in TMDs.

cond-mat.mes-hall↗

arXiv2024

Magnetic Excitations in Ferromagnetically Coupled Spin-1 Nanographenes

In the quest for high-spin building blocks to form covalently bonded 1D or 2D materials with controlled magnetic interactions, $π$-electron magnetism provides an ideal framework to engineer large ferromagnetic interactions between nanographenes. As a first step in this direction, we investigate the spin properties of ferromagnetically coupled triangulenes, triangular nanographenes with spin $S = 1$. Combining in-solution synthesis of rationally designed molecular precursors and on-surface synthesis, we achieve covalently bonded $S = 2$ triangulene dimers and $S = 3$ trimers on Au(111). Starting from the triangulene dimer, we thoroughly characterize its low-energy magnetic excitations using inelastic electron tunneling spectroscopy (IETS). IETS reveals conductance steps identified as a quintet to triplet excitation, and a zero-bias peak stemming from higher-order spin-spin scattering of the 5-fold degenerate ferromagnetic ground state. The Heisenberg picture captures the relevant parameters of inter-triangulene ferromagnetic exchange, and its successful extension to the larger $S = 3$ system confirms the model's accuracy. We expect that the addition of ferromagnetically coupled building blocks to the toolbox of magnetic nanographenes opens new opportunities to design carbon materials with complex magnetic ground states.

cond-mat.mes-hall↗

arXiv2024

Demonstrating Kondo behavior by temperature-dependent scanning tunneling spectroscopy

The Kondo effect describes the scattering of conduction electrons by magnetic impurities, manifesting as an electronic resonance at the Fermi energy with a distinctive temperature evolution. In this letter, we present a critical evaluation of the current methodology employed to demonstrate Kondo behavior in transport measurements, underscoring the limitations of established theoretical frameworks and the influence of extrinsic broadening. We introduce a novel approach for analyzing spectroscopic indicators of the Kondo effect, employing the Hurwitz-Fano lineshape as a model for the Kondo resonance in the presence of extrinsic broadening. Through precise scanning tunneling spectroscopy measurements on an exemplary spin-1/2 Kondo system, phenalenyl on Au(111), we demonstrate the efficacy of our proposed protocol in extracting accurate intrinsic Kondo linewidths from finite-temperature measurements. The extracted linewidths exhibit a robust fit with a recently derived expression for the temperature-dependent intrinsic Kondo linewidth, providing compelling evidence for the validity of the underlying theory.

cond-mat.str-el↗

arXiv2023

Electronic and magnetic properties of single chalcogen vacancies in MoS$_2$/Au(111)

Two-dimensional (2D) transition-metal dichalcogenides (TMDC) are considered highly promising platforms for next-generation optoelectronic devices. Owing to its atomically thin structure, device performance is strongly impacted by a minute amount of defects. Although defects are usually considered to be disturbing, defect engineering has become an important strategy to control and design new properties of 2D materials. Here, we produce single S vacancies in a monolayer of MoS$_2$ on Au(111). Using a combination of scanning tunneling and atomic force microscopy, we show that these defects are negatively charged and give rise to a Kondo resonance, revealing the presence of an unpaired electron spin exchange coupled to the metal substrate. The strength of the exchange coupling depends on the density of states at the Fermi level, which is modulated by the moiré structure of the MoS$_2$ lattice and the Au(111) substrate. In the absence of direct hybridization of MoS$_2$ with the metal substrate, the S vacancy remains charge-neutral. Our results suggest that defect engineering may be used to induce and tune magnetic properties of otherwise non-magnetic materials.

cond-mat.mes-hall↗

arXiv2023

Variations of vibronic states in densely-packed structures of molecules with intramolecular dipoles

Electrostatic potentials strongly affect molecular energy levels and charge states, providing the fascinating opportunity of molecular gating. Their influence on molecular vibrations remains less explored. Here, we investigate Ethyl-Diaminodicyanoquinone molecules on a monolayer of MoS$_2$ on Au(111) using scanning tunneling and atomic force microscopy and spectroscopy. These molecules exhibit a large dipole moment in gas phase, which we find to (partially) persist on the MoS$_2$ monolayer. The self-assembled structures consist of chains, where the dipoles of neighboring molecules are aligned anti-parallel. Thanks to the decoupling efficiency of the molecular states from the metal by the MoS$_2$ interlayer, we resolve vibronic states of the molecules, which vary in intensity depending on the molecular surrounding. We suggest that the vibrations are strongly damped by electrostatic interactions with the environment.

cond-mat.mes-hall↗

arXiv2022

Direct observation of the magnetic ground state of the two smallest triangular nanographenes

Fusion of three benzene rings in a triangular fashion gives rise to the smallest open-shell graphene fragment, the phenalenyl radical, whose $π$-extension leads to an entire family of non-Kekulé triangular nanographenes with high-spin ground states. Here, we report the first synthesis of unsubstituted phenalenyl on a Au(111) surface, which is achieved by combining in-solution synthesis of the hydro-precursor and on-surface activation by atomic manipulation, using the tip of a scanning tunneling microscope (STM). Single-molecule structural and electronic characterization confirm its open-shell S = 1/2 ground state that gives rise to Kondo screening on the Au(111) surface. In addition, we compare the phenalenyl's electronic properties with those of triangulene, the second homolog in the series, whose S = 1 ground state induces an underscreened Kondo effect. Our results set a new lower size-limit in the on-surface synthesis of magnetic nanographenes that can serve as building blocks for the realization of new exotic quantum phases of matter.

cond-mat.mes-hall↗

arXiv2020

Monolayers of MoS$_{2}$ on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral lineshapes. Insertion of thin decoupling layers reduces the linewidths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS$_{2}$ on Ag(111) provides a semiconducting band gap that may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetra-cyano-quino-dimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck-Condon model, we reveal their vibronic origin and identify the modes with strong electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2020

Vibrational excitation mechanism in tunneling spectroscopy beyond the Franck-Condon model

Vibronic spectra of molecules are typically described within the Franck-Condon model. Here, we show that highly resolved vibronic spectra of large organic molecules on a single layer of MoS$_{2}$ on Au(111) show spatial variations in their intensities, which cannot be captured within this picture. We explain that vibrationally mediated perturbations of the molecular wave functions need to be included into the Franck-Condon model. Our simple model calculations reproduce the experimental spectra at arbitrary position of the STM tip over the molecule in great detail.

cond-mat.mes-hall↗

arXiv2019

Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS$_2$

Scanning tunneling spectroscopy requires the application of a potential difference between the sample and a tip. In metal-vacuum-metal junctions, one can safely assume that the potential is constant along the metallic substrate. Here, we show that the inhomogeneous shape of the electric potential has to be taken into account when probing spatially extended molecules on a decoupling layer. To this end, oligothiophene-based molecules were deposited on a monolayer of molybdenum disulfide (MoS$_2$) on a Au(111) surface. By probing the delocalized molecular orbital along the thiophene-backbone, we found an apparent intramolecular shift of the positive ion resonance, which can be ascribed to a perturbation potential caused by the tip. Using a simple model for the electrostatic landscape, we show that such a perturbation is caused by the inhomogeneity of the applied bias potential in the junction and may be further modified by an electric dipole of a functionalized tip. The two effects can be disentangled in tunneling spectra by probing the apparent energy shift of vibronic resonances along the molecular backbone. We suggest that extended molecules on MoS$_2$ can be used as a sensor for the shape of the electrostatic potential of arbitrary tips.

cond-mat.mes-hall↗