arXiv subjects

Per Hedegård

Publications and source records attributed to Per Hedegård.

At least 19 recordsLinked to original sources

arXiv2025

Atomic-scale observation of $d$-$π$-$d$ spin coupling in coordination structures

Spin coupling between magnetic metal atoms and organic radicals plays a pivotal role in high-performance magnetic materials. The complex interaction involving multi-spin centers in bulk materials makes it challenging to study spin coupling at the atomic scale. Here, we investigate the $d$-$π$-$d$ spin interaction in well-defined metal-organic coordinated structures composed of two iron (Fe) atoms and four all-trans retinoic acid (ReA) molecules, using low-temperature scanning tunneling microscopy and atomic force microscopy. The ReA molecule is turned into a spin-$1/2$ radical state by dehydrogenation, facilitating strong magnetic coupling with the coordinated Fe atoms. Comprehensive theoretical analysis, based on density functional theory and valence bond theory, further elucidates the intrinsic mechanism of ferrimagnetic spin coupling in the coordination structure. Specifically, simultaneous antiferromagnetic coupling of Fe dimer to ReA radicals parallelizes the dimer spin orientation. This work contributes to the fundamental understanding of spin interaction in metal-organic coordination structures and provides microscopic insights for designing advanced magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

On the Ferrimagnetic State of CrCl$_2$(pyz)$_2$

Van der Waals layered ferromagnetic compounds with high two-dimensional electronic conductivity holds strong potential for quantum computing, future unconventional superconductors, catalysts, batteries, and fuel cells. We suggest a minimal theoretical model to understand the magnetic properties of the metal-organic framework CrCl$_2$(pyz)$_2$ (pyz=pyrazine). Using a Hubbard model we show that the groundstate is dominated by a specific configuration of delocalized electrons on the pyz sites with a ferrimagnetic coupling to the localized spins on the Cr sites. This model suggests a magnetic moment of $2μ_B$ which is remarkably close to the experimental value of $1.8 μ_B$ [K. S. Pedersen et al., Nat. Chem. 10, 1056-1061 (2018)]. From Weiss mean-field theory we predict a weak ferromagnetic Cr-Cr coupling of $\approx 0.9$ meV. This is consolidated by second order perturbation theory of the RKKY interaction yielding a Cr-Cr coupling of $\approx 5$ meV. Understanding the interactions in these types of compounds can facilitate designs of metal-organic compounds with tailored magnetic properties.

cond-mat.str-el↗

arXiv2022

Observation of biradical spin coupling through hydrogen bonds

Investigation of intermolecular electron spin interaction is of fundamental importance in both science and technology.Here, radical pairs of all-trans retinoic acid molecules on Au(111) are created using an ultra-low temperature scanning tunneling microscope. Antiferromagnetic coupling between two radicals is identified by magnetic-field dependent spectroscopy.The measured exchange energies are from 0.1 to 1.0 meV. The biradical spin coupling is mediated through O-H$\cdots$O hydrogen bonds, as elucidated from analysis combining density functional theory calculation and a modern version of valence bond theory.

cond-mat.mes-hall↗

arXiv2021

Theory of Chirality Induced Spin Selectivity: Progress and Challenges

We provide a critical overview of the theory of the chirality-induced spin selectivity (CISS) effect, i.e., phenomena in which the chirality of molecular species imparts significant spin selectivity to various electron processes. Based on discussions in a recently held workshop, and further work published since, we review the status of CISS effects - in electron transmission, electron transport, and chemical reactions. For each, we provide a detailed discussion of the state-of-the-art in theoretical understanding and identify remaining challenges and research opportunities.

cond-mat.mtrl-sci↗

arXiv2020

Multinomial, Poisson and Gaussian statistics in count data analysis

It is generally known that counting statistics is not correctly described by a Gaussian approximation. Nevertheless, in neutron scattering, it is common practice to apply this approximation to the counting statistics; also at low counting numbers. We show that the application of this approximation leads to skewed results not only for low-count features, such as background level estimation, but also for its estimation at double-digit count numbers. In effect, this approximation is shown to be imprecise on all levels of count. Instead, a Multinomial approach is introduced as well as a more standard Poisson method, which we compare with the Gaussian case. These two methods originate from a proper analysis of a multi-detector setup and a standard triple axis instrument.We devise a simple mathematical procedure to produce unbiased fits using the Multinomial distribution and demonstrate this method on synthetic and actual inelastic scattering data. We find that the Multinomial method provide almost unbiased results, and in some cases outperforms the Poisson statistics. Although significantly biased, the Gaussian approach is in general more robust in cases where the fitted model is not a true representation of reality. For this reason, a proper data analysis toolbox for low-count neutron scattering should therefore contain more than one model for counting statistics.

physics.data-an↗

arXiv2018

Ground-state spin blockade in a single-molecule junction

It is known that the quantum-mechanical ground state of a nano-scale junction has a significant impact on its electrical transport properties. This becomes particularly important in transistors consisting of a single molecule. Due to strong electron-electron interactions and the possibility to access ground states with high spins, these systems are eligible hosts of a current-blockade phenomenon called ground-state spin blockade. This effect arises from the inability of a charge carrier to account for the spin difference required to enter the junction, as that process would violate the spin selection rules. Here, we present a direct experimental demonstration of ground-state spin blockade in a high-spin single-molecule transistor. The measured transport characteristics of this device exhibit a complete suppression of resonant transport due to a ground-state spin difference of 3/2 between subsequent charge states. Strikingly, the blockade can be reversibly lifted by driving the system through a magnetic ground-state transition in one charge state, using the tunability offered by both magnetic and electric fields.

cond-mat.mes-hall↗

arXiv2018

Semi-classical generalized Langevin equation for equilibrium and nonequilibrium molecular dynamics simulation

Molecular dynamics (MD) simulation based on Langevin equation has been widely used in the study of structural, thermal properties of matters in difference phases. Normally, the atomic dynamics are described by classical equations of motion and the effect of the environment is taken into account through the fluctuating and frictional forces. Generally, the nuclear quantum effects and their coupling to other degrees of freedom are difficult to include in an efficient way. This could be a serious limitation on its application to the study of dynamical properties of materials made from light elements, in the presence of external driving electrical or thermal fields. One example of such system is single molecular dynamics on metal surface, an important system that has received intense study in surface science. In this review, we summarize recent effort in extending the Langevin MD to include nuclear quantum effect and their coupling to flowing electrical current. We discuss its applications in the study of adsorbate dynamics on metal surface, current-induced dynamics in molecular junctions, and quantum thermal transport between different reservoirs.

cond-mat.mes-hall↗

arXiv2016

Electron and phonon drag in thermoelectric transport through coherent molecular conductors

We study thermoelectric transport through a coherent molecular conductor connected to two electron and two phonon baths using the nonequilibrium Green's function method. We focus on the mutual drag between electron and phonon transport as a result of `momentum' transfer, which happens only when there are at least two phonon degrees of freedom. After deriving expressions for the linear drag coefficients, obeying the Onsager relation, we further investigate their effect on nonequilibrium transport. We show that the drag effect is closely related to two other phenomena: (1) adiabatic charge pumping through a coherent conductor; (2) the current-induced nonconservative and effective magnetic forces on phonons.

cond-mat.mes-hall↗

arXiv2015

Current-induced runaway vibrations in dehydrogenated graphene nanoribbons

We employ a semi-classical Langevin approach to study current-induced atomic dynamics in a partially dehydrogenated armchair graphene nanoribbon. All parameters are obtained from density functional theory. The dehydrogenated carbon dimers behave as effective impurities, whose motion decouples from the rest of carbon atoms. The electrical current can couple the dimer motion in a coherent fashion. The coupling, which is mediated by nonconservative and pseudo-magnetic current-induced forces, change the atomic dynamics, and thereby show their signature in this simple system. We study the atomic dynamics and current-induced vibrational instabilities using a simplified eigen-mode analysis.Our study shows that the armchair nanoribbon serves as a possible testbed for probing the current-induced forces.

cond-mat.mes-hall↗

arXiv2015

On the illusory connection between cross-conjugation and quantum interference

Quantum interference, be it destructive or constructive, has a substantial influence on the magnitude of molecular conductance and consequently there is significant interest in predicting these effects. It is commonly thought that cross-conjugated paths result in suppressed conductance due to destructive quantum interference. Using Hückel theory and DFT calculations we investigate systems that break this cross-conjugation rule of thumb. We predict and rationalize how a class of conjugated molecules containing closed loops can exhibit destructive interference despite being linearly conjugated and exhibit constructive interference despite being cross-conjugated. The arguments build on the graphical rules derived by Markussen et al. [1] and the hitherto neglected effects of closed loops in the molecular structure. Finally, we identify the 1,3 connected Azulene molecule as belonging to the closed-loop class and argue that this explains recent measurements of its electrical conductance.

cond-mat.mes-hall↗

arXiv2015

Current-induced forces and hot-spots in biased nano-junctions

We investigate theoretically the interplay of current-induced forces (CIF), Joule heating, and heat transport inside a current-carrying nano-conductor. We find that the CIF, due to the electron-phonon coherence, can control the spatial heat dissipation in the conductor. This yields a significant asymmetric concentration of excess heating (hot-spot) even for a symmetric conductor. When coupled to the electrode phonons, CIF drive different phonon heat flux into the two electrodes. First-principles calculations on realistic biased nano-junctions illustrate the importance of the effect.

cond-mat.mes-hall↗

arXiv2014

Current-induced forces: a simple derivation

We revisit the problem of forces on atoms under current in nanoscale conductors. We derive and discuss the five principal kinds of force under steady-state conditions from a simple standpoint that - with the help of background literature - should be accessible to physics undergraduates. The discussion aims at combining methodology with an emphasis on the underlying physics through examples. We discuss and compare two forces present only under current - the non-conservative electron wind force and a Lorentz-like velocity-dependent force. It is shown that in metallic nanowires both display significant features at the wire surface, making it a candidate for the nucleation of current-driven structural transformations and failure. Finally we discuss the problem of force noise and the limitations of Ehrenfest dynamics.

cond-mat.mes-hall↗

arXiv2014

Triplet excitations in graphene-based systems

In this article we investigate the excitations in a single graphene layer and in a single-walled carbon nanotube, i.e. the spectrum of magnetic excitations is calculated. In the absence of interactions in these systems there is a unique gap in the electron-hole continuum. We show that in the presence of Coulomb correlations new states, magnons, appear in this forbidden region. Coulomb interaction is examined in the context of Pariser-Parr-Pople (PPP) model which takes into account long range nature of interaction. The energy of new bound states depends on the strength of Coulomb forces. The calculations are performed for arbitrary electron-hole ($e-h$) momentum $\textbf{q}$ what allows to find the magnons dispersion law $\varepsilon(\textbf{q})$, effective mass $m^*$ and velocity $v_{gr}$. Finally, we determine the critical values of system parameters when this type of excitations can exist.

cond-mat.str-el↗

arXiv2014

Quantum Interference in Off-Resonant Transport through Single Molecules

We provide a simple set of rules for predicting interference effects in off-resonant transport through single-molecule junctions. These effects fall in two classes, showing respectively an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same, or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diag- onalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions, interference causes no transmission nodes, but argue that it may lead to a non-standard gate-dependence of the zero-bias Kondo resonance.

cond-mat.mes-hall↗

arXiv2013

Phonon excitation and instabilities in biased graphene nanoconstrictions

We calculate the phonons in a graphene nanoconstriction(GNC) in the presence of a high current density. The Joule-heating, current-induced forces, and coupling to electrode phonons is evaluated using first principles nonequilibrium DFT-NEGF calculations. Close to a resonance in the electronic structure we observe a strongly nonlinear heating with bias and breakdown of the harmonic approximation. This behavior results from negatively damped phonons driven by the current. The effect may limit the stability and capacity of graphene nanoconstrictions to carry high currents.

cond-mat.mes-hall↗

arXiv2012

Current-induced atomic dynamics, instabilities, and Raman signals: Quasi-classical Langevin equation approach

We derive and employ a semi-classical Langevin equation obtained from path-integrals to describe the ionic dynamics of a molecular junction in the presence of electrical current. The electronic environment serves as an effective non-equilibrium bath. The bath results in random forces describing Joule heating,current-induced forces including the non-conservative wind force,dissipative frictional forces, and an effective Lorentz-like force due to the Berry phase of the non-equilibrium electrons. Using a generic two-level molecular model, we highlight the importance of both current-induced forces and Joule heating for the stability of the system. We compare the impact of the different forces, and the wide-band approximation for the electronic structure on our result. We examine the current-induced instabilities (excitation of runaway "waterwheel" modes) and investigate the signature of these in the Raman signals.

cond-mat.mes-hall↗

arXiv2011

Current-induced dynamics in carbon atomic contacts

The effect of electronic current on the atomic motion still poses many open questions, and several mechanisms are at play. Recently there has been focus on the importance of the current-induced non-conservative forces (NC) and Berry-phase derived forces (BP) regarding the stability of molecular-scale contacts. Systems based on molecules bridging electrically gated graphene electrodes may offer an interesting test-bed for these effects. We employ a semi-classical Langevin approach in combination with DFT calculations to study the current-induced vibrational dynamics of an atomic carbon chain connecting electrically gated graphene electrodes. This illustrates how the device stability can be predicted solely from the modes obtained from the Langevin equation including the current induced forces. We point out that the gate offers control of the current independent of bias voltage which can be used to explore current-induced vibrational instabilities due the NC/BP forces. Furthermore, using tight-binding and the Brenner potential we illustrate how Langevin-type molecular dynamics can be performed including the Joule heating effect for the carbon chain systems. Molecular dynamics including current-induced forces enables an energy redistribution mechanism among the modes, mediated by anharmonic interactions, which is found to be vital in the description of the electronic heating. We have developed a semi-classical Langevin equation approach which can be used to explore current-induced dynamics and instabilities. We find instabilities at experimentally relevant bias and gate voltages for the carbon chain system.

cond-mat.mes-hall↗

arXiv2011

Laser-like vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor(D-A) type molecular rectifiers. At finite voltage additional non-equilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser-action. We investigate the effect in realistic molecular rectifier structures using first principles calculations.

cond-mat.mes-hall↗