arXiv subjects

Carmen Herrmann

Publications and source records attributed to Carmen Herrmann.

10 recordsLinked to original sources

arXiv2025

Molecular Cross-linking of MXenes: Tunable Interfaces and Chemiresistive Sensing

MXenes, a family of 2D transition metal compounds, have emerged as promising materials due to their unique electronic properties and tunable surface chemistry. However, the translation of these nanoscale properties into macroscopic devices is constrained by suitable cross-linking strategies that enable both processability and controlled inter-flake charge transport. Herein, we demonstrate the tunability of interfaces and the inter-layer spacing between Ti$_3$C$_2$T$_x$ MXene flakes through molecular cross-linking with homologous diamines. Oleylamine was first used to stabilize MXenes in chloroform, followed by diamine-mediated cross-linking to tune precisely the interlayer spacing. Grazing incidence X-ray scattering (GIXRD/GIWAXS) confirmed the correlation between ligand chain length and inter-layer spacing, which was further supported by Density Functional Theory (DFT) calculations. Furthermore, we investigated the charge transport properties of thin films consisting of these diamine-crosslinked Ti$_3$C$_2$T$_x$ MXenes and observed a strong dependence of the conductivity on the interlayer spacing. The dominating charge transport mechanism is variable range hopping (VRH) in accordance with the structure analysis of the films. Finally, we probed chemiresistive vapor sensing in MXene composites, observing pronounced water sensitivity and selectivity, highlighting their potential for use in humidity sensors. Insights into molecular cross-linking and its impact on charge transport open avenues for next-generation MXene-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Singlet-Triplet Kondo Effect in Blatter Radical Molecular Junctions: Zero-bias Anomalies and Magnetoresistance

The Blatter radical has been suggested as a building block in future molecular spintronic devices due to its radical character and expected long-spin lifetime. However, whether and how the radical character manifests itself in the charge transport and magnetotransport properties seems to depend on the environment or has not yet been studied. Here, we investigate single-molecule junctions of the Blatter radical molecule in a mechanically controlled break junction device at low temperature. Differential conductance spectroscopy on individual junctions shows two types of zero-bias anomalies attributed to Kondo resonances revealing the ability to retain the open-shell nature of the radical molecule in a two-terminal device. Additionally, a high negative magnetoresistance is also observed in junctions without showing a zero-bias peak. We posit that the high negative magnetoresistance is due to the effect of a singlet-triplet Kondo effect under magnetic field originating from a double-quantum-dot system consisting of a Blatter radical molecule with a strong correlation to a second side-coupled molecule. Our findings not only provide the possibility of using the Blatter radical in a two-terminal system under cryogenic conditions but also reveal the magnetotransport properties emerging from different configurations of the molecule inside a junction.

cond-mat.mes-hall↗

arXiv2020

Kondo screening in Co adatoms with full Coulomb interaction

Using a numerically exact first-principles many-body approach, we revisit the "prototypical" Kondo case of a cobalt impurity on copper. Even though this is considered a well understood example of the Kondo effect, we reveal an unexpectedly strong dependence of the screening properties on the parametrization of the local Coulomb tensor. As a consequence, the Kondo temperature can vary by orders of magnitude depending on the complexity of the parametrization of the electron-electron interaction. Further, we challenge the established picture of a spin-$1$ moment involving two cobalt $d$-orbitals only, as orbital-mixing interaction terms boost the contribution of the remainder of the $d$-shell.

cond-mat.str-el↗

arXiv2020

A Chirality-Based Quantum Leap

Chiral degrees of freedom occur in matter and in electromagnetic fields and constitute an area of research that is experiencing renewed interest driven by recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials. The CISS effect underpins the fact that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision. Any technology that relies on optimal charge transport, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which is presently lacking. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking perspective provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects, and presents a vision for their future roles in enabling room-temperature quantum technologies.

cond-mat.mes-hall↗

arXiv2020

Current vortices in aromatic carbon molecules

The local current flow through three small aromatic carbon molecules, namely benzene, naphthalene and anthracene, is studied. Applying density functional theory and the non-equilibrium Green's function method for transport, we demonstrate that pronounced current vortices exist at certain electron energies for these molecules. The intensity of these circular currents, which appear not only at the anti-resonances of the transmission but also in vicinity of its maxima, can exceed the total current flowing through the molecular junction and generate considerable magnetic fields. The $π$ electron system of the molecular junctions is emulated experimentally by a network of macroscopic microwave resonators. The local current flows in these experiments confirm the existence of current vortices as a robust property of ring structures. The circular currents can be understood in terms of a simple nearest-neighbor tight-binding Hückel model. Current vortices are caused by the interplay of the complex eigenstates of the open system which have energies close-by the considered electron energy. Degeneracies, as observed in benzene and anthracene, can thus generate strong circular currents, but also non-degenerate systems like naphthalene exhibit current vortices. Small imperfections and perturbations can couple otherwise uncoupled states and induce circular currents.

cond-mat.mes-hall↗

arXiv2018

Co(CO)n/Cu(001): Towards understanding chemical control of the Kondo effect

The Kondo effect is a many-body phenomenon allowing insight into the electronic and atomistic structure of spin-polarized adsorbates on metal surfaces. Its chemical control is intriguing because it deepens such insight, but the underlying mechanisms are only partly understood. We study the effect of increasing the number of CO ligands attached to a cobalt adatom on copper(001), which correlates with an increase in the Kondo temperature $T_K$ experimentally (P. Wahl et al, Phy. Rev. Lett. 95, 166601 (2005)), by solving an Anderson impurity model parametrized by density functional theory (DFT++). Our results suggest that the orbital responsible for the Kondo effect is $d_{x^2-y^2}$ for the tetracarbonyl, and its combination with $d_{z^2}$ for the dicarbonyl. The molecular structures depend considerably on the approximate exchange--correlation functional, which may be related to the known difficulty of describing CO binding to metal surfaces. These structural variations strongly affect the Kondo properties, which is not only a concern for predictive studies, but also of interest for detecting mechanical deformations and for understanding the effect of tip--adsorbate interactions in the scanning tunneling microscope. Still, by constraining the tetracarbonyl to $C_{4v}$ symmetry, as suggested by experimental data, we find structures compatible with the experimental trend for $T_K$ (employing BLYP-D3+U). This is not possible for the tricarbonyl despite the range of computational parameters scanned. For the tetra- and dicarbonyl, the increased $T_K$ correlates with a larger hybridization function at the Fermi level, which we trace back to an increased interaction of the Co $3d$ orbitals with the ligands.

cond-mat.str-el↗

arXiv2018

Structural diradical character

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. Since diradical and closed-shell electronic structures differ in their distribution of single, double, triple and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones. Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation.

physics.comp-ph↗

arXiv2017

The interplay between strong correlation and adsorption distances

Adsorbed transition metal atoms can have partially filled $d$- or $f$-shells due to strong on-site Coulomb interaction. Capturing all effects originating from electron correlation in such strongly correlated systems is a challenge for electronic structure methods. It requires a sufficiently accurate description of the atomistic structure (in particular bond distances and angles), which is usually obtained from Kohn-Sham density functional theory (DFT), which due to the approximate nature of the exchange-correlation functional may provide an unreliable description of strongly correlated systems. To elucidate the consequences of this popular procedure, we apply a combination of DFT with the Anderson impurity model (AIM), as well as DFT+U for a calculation of the potential energy surface along the Co/Cu(001) adsorption coordinate, and compare the results with those obtained from DFT. The adsorption minimum is shifted towards larger distances by applying DFT+AIM, or the much cheaper DFT+U method, compared to the corresponding spin-polarized DFT results, by a magnitude comparable to variations between different approximate exchange-correlation functionals (0.08 to 0.12 Angstrom). This shift originates from an increasing correlation energy at larger adsorption distances, which can be traced back to the Co 3$d_{xy}$ and 3$d_{z2}$ orbitals being more correlated as the adsorption distance is increased.

cond-mat.str-el↗

arXiv2017

Towards colloidal spintronics through Rashba spin-orbit interaction in lead sulphide nanosheets

Employing the spin degree of freedom of charge carriers offers the possibility to extend the functionality of conventional electronic devices, while colloidal chemistry can be used to synthesize inexpensive and tuneable nanomaterials. In order to benefit from both concepts, Rashba spin-orbit interaction has been investigated in colloidal lead sulphide nanosheets by electrical measurements on the circular photo-galvanic effect. Lead sulphide nanosheets possess rock salt crystal structure, which is centrosymmetric. The symmetry can be broken by quantum confinement, asymmetric vertical interfaces and a gate electric field leading to Rashba-type band splitting in momentum space at the M points, which results in an unconventional selection mechanism for the excitation of the carriers. The effect, which is supported by simulations of the band structure using density functional theory, can be tuned by the gate electric field and by the thickness of the sheets. Spin-related electrical transport phenomena in colloidal materials open a promising pathway towards future inexpensive spintronic devices.

cond-mat.mtrl-sci↗

arXiv2014

Mode-Tracking Based Stationary-Point Optimization

In this work, we present a transition-state optimization protocol based on the Mode-Tracking algorithm [J. Chem. Phys. 118 (2003) 1634]. By calculating only the eigenvector of interest instead of diagonalizing the full Hessian matrix and performing an eigenvector following search based on the selectively calculated vector, we can efficiently optimize transition-state structures. The initial guess structures and eigenvectors are either chosen from a linear interpolation between the reactant and product structures, from a nudged-elastic band search, from a constrained-optimization scan, or from the minimum-energy structures. Alternatively, initial guess vectors based on chemical intuition may be defined. We then iteratively refine the selected vectors by the Davidson subspace iteration technique. This procedure accelerates finding transition states for large molecules of a few hundred atoms. It is also beneficial in cases where the starting structure is very different from the transition-state structure or where the desired vector to follow is not the one with lowest eigenvalue. Explorative studies of reaction pathways are feasible by following manually constructed molecular distortions.

physics.comp-ph↗