arXiv subjects

J. Fransson

Publications and source records attributed to J. Fransson.

At least 19 recordsLinked to original sources

arXiv2026

Phonon mediated spin-spin interactions

Indirect long range interactions between localized magnetic moments are in metals mediated by itinerant electrons. In insulators and semi-conductor, such interactions need to be small, if not negligible, due to the absence of mediating carriers. The existence of magnetically ordered insulators, for instance, metal-oxides, is therefore an everlasting source for proposals of various mechanisms that may support the order. Here, phonon mediated interactions between localized magnetic moments is considered as a mechanism that can provide quantifiable symmetric and anti-symmetric anisotropic spin-spin interactions. It is demonstrated that while a symmetric anisotropic interaction exists for all types of phonons, the existence of anti-symmetric anisotropic interactions requires broken inversion symmetry. The latter mechanism may explain the weak ferromagnetic order observed in chiral, e.g., CuO and CoO compounds. Furthermore, the interaction is nearly independent of the temperature at low temperature while approaches a linear growth at high. Spatially, the interactions have an oscillatory power law decay with the inter-nuclei distance.

cond-mat.other↗

arXiv2026

An argument why the Spinterface model cannot explain the chirality induced spin selectivity effect

In the context of chirality induced spin selectivity effect, it has been argued that a chiral molecule when adsorbed on a metal facilitates the formation of a local spin moment at the interface between the metal and molecule, given a strong spin-orbit coupling in the metal. The possibility for such spin moment formation is analyzed in terms of general arguments and effective modeling of a pertinent set-up. The conclusion from this analysis is that a strong spin-orbit coupling in the metal does not provide a sufficient mechanism to sustain a stabilized spin moment at the interface. It is, moreover, shown that an electron flux in to or out from the molecule does not provide conditions for a spin moment formation, regardless of whether the flux is spin-polarized or not.

cond-mat.mes-hall↗

arXiv2025

Macroscopically distinguishable superposition in infinitely many degrees of freedom

We investigate the concept of macroscopically distinguishable superpositions within an infinite array of boson sites. Our approach is rigorous within the frame of Hilbert space theory. In this context, it is natural to differentiate between states -- and corresponding dynamics -- that involve only finitely many degrees of freedom, referred to as local, and those that are inherently nonlocal. Previous studies have shown that such systems can support nonlocal coherent states (NCS). In this work, we demonstrate that NCS can dynamically evolve into nonlocal cat states under the influence of a nonlocal Hamiltonian -- specifically, the square of the total number operator. Crucially, the resulting dynamics cannot be decomposed into local factors. Furthermore, we explore broader mathematical implications of these phenomena within the framework of generalized bosons. Our findings highlight that the concepts of coherent states and nonlocal cat states are not inherently bound together; rather, their fusion is a distinctive feature of standard bosons. Finally, we propose that if the generalized boson framework can be physically realized in engineered quantum systems, the phenomena described here may hold significant relevance for both physics and materials science.

quant-ph↗

arXiv2025

Should it really be that hard to model the chirality induced spin selectivity effect?

The chirality induced spin selectivity effect remains a challenge to capture with theoretical modeling. While at least a decade was spent on independent electron models, which completely fail to reproduce the experimental results, the lesson to be drawn out of these efforts is that a correct modeling of the effect has to include interactions among the electrons. In the discussion of the phenomenon ones inevitably encounters the Onsager reciprocity and time-reversal symmetry, and questions whether the observations violate these fundamental concepts, or whether we have not been able to identify what it is that make those concepts redundant in this context. The experimental fact is that electrons spin-polarize by one or another reason, when traversing chiral molecules. The set-ups are simple enough to enable effective modeling, however, overcoming the grand failures of the theoretical efforts, thus far, and formulating a theory which is founded on microscopic modeling appears to be a challenge. A discussion of the importance of electron correlations is outlined, pointing to possible spontaneous breaking of time-reversal symmetry and Onsager reciprocity.

cond-mat.mes-hall↗

arXiv2025

Breaking of Time-Reversal Symmetry and Onsager Reciprocity in Chiral Molecule Interfacd with an Environment

Molecular closed shell structures are known to form spin-singlet configurations, resulting from the spin-exchange associated with electron-electron interactions. While the vanishing total spin-moment is an immanent property of the spin-singlet, the vanishing local moments is a result of fluctuations between degenerate spin-configurations. Here, it is demonstrated that the spin-configuration of a chiral molecule is enantiospecifically locked when coupled to an electron reservoir. The coupling opens up the molecular closed shell structure, which broadens the energy levels. Together with the molecular spin-orbit coupling, this dissipative coupling generates an effective spin-splitting of the molecular energy levels and facilitates stabilization of a chirality determined spin-configuration. Simultaneously, the charge molecular charge distribution is shown to depend linearly on the magnetization of the reservoir such that no linear response regime is established with respect to the external magnetization. Accordingly, the Onsager reciprocity theorem does not apply to a chiral molecule attached to a reservoir, hence, providing a theoretical foundation for the chirality induced spin selectivity effect.

cond-mat.mes-hall↗

arXiv2025

Chiral induced Spin Polarized Electron Current: Origin of the Chiral Induced Spin Selectivity Effect

The discovery of the chiral induced spin selectivity effect has provided a novel tool to study how active physical and chemical mechanism may differ in chiral enantiomers, however, the origin of the effect itself is yet an open question. In this article, it is theoretically shown that two aspects have to be fulfilled for the chiral induced spin selectivity effect to arise. First, chirality is a necessary condition for breaking spin-degeneracy in molecular structures that do not comprise heavy elements. Second, dissipation is indispensable for the molecule to develop a non-vanishing spin-polarization. These theoretical conclusions are illustrated in terms of a few examples, showing the necessity of the two aspects to be coordinated for the emergence of the chiral induced spin selectivity effect.

cond-mat.mes-hall↗

arXiv2024

Nonlocal coherent states in an infinite array of boson sites

A regular coherent state (CS) is a special type of quantum state for boson particles placed in a single site. The defining feature of the CS is that it is an eigenmode of the annihilation operator. The construction easily generalizes to the case of a finite number of sites. However, the challenge is altogether different when one considers an infinite array of sites. In this work we demonstrate a mathematically rigorous construction that resolves the latter case. The resulting nonlocal coherent states (NCS) are simultaneous eigenmodes for all of the infinitely many annihilation operators acting in the infinite array's Fock space. Our construction fundamentally relies on Dirichlet series-based analysis and number theoretic arguments.

quant-ph↗

arXiv2023

Vibrationally Induced Magnetism in Supramolecular Aggregates

Magnetic phenomena are in chemistry and condensed matter physics considered to be associated with low temperatures. That a magnetic state, or order, is stable below a critical temperature as well as becoming stronger the lower the temperature is a nearly unquestioned paradigm. It is, therefore, surprising that recent experimental observation made on supramolecular aggregates suggest that, for instance, the magnetic coercivity may increase with increasing temperature, as well as the chiral induced spin selectivity effect may be enhanced. Here, a mechanism for vibrationally stabilized magnetism is proposed and a theoretical model is introduced with which the qualitative aspects of the recent experimental findings can be explained. It is argued that anharmonic vibrations, which become increasingly occupied with increasing temperature, enables nuclear vibrations to both stabilize and sustain magnetic states. The theoretical proposal, hence, pertains to structures without inversion and/or reflection symmetries, for instance, chiral molecules and crystals.

cond-mat.mes-hall↗

arXiv2023

Emergent magnetism as a cooperative effect of interactions and reservoir

Closed shell molecular structures are under normal conditions time-reversal invariant. Experimental evidences point, however, towards that this invariance may be locally violated when the structure is in contact with a particle reservoir. The mechanisms behind such local symmetry breaking are not clear by any means. By considering a minimal model for a closed shell structure, here we propose that the symmetry breaking may result from a combination of internal and/or external interactions. It is shown that a magnetic moment of a localized electron level can be generated and maintained under the influence of such combination. The theoretical results should cast new light on the mechanisms that may form magnetic properties in molecular compounds.

cond-mat.mes-hall↗

arXiv2022

Chiral Phonon Induced Spin-Polarization

The current understanding of chirality suggests the existence of a connection between structure and angular momentum, including spin. This is particularly emphasised in the chiral induced spin selectivity effect, where chiral structures act as spin filters. However, the recent discovery of chiral phonons have demonstrated that phonons too may carry angular momentum which also can be regarded as magnetic moments which add to the total moment. Here, it is shown that chiral phonons may induce a non-trivial spin-texture in an otherwise non-magnetic electronic structure. By considering a set-up in which electrons and phonons are interfaced with each other, it is shown that chiral phonons may transfer its angular momentum into the electron reservoir which, thereby, becomes spin-polarized. It is, moreover, shown that an equivalent mechanism does not exist whenever the electrons are interfaced with achiral phonons.

cond-mat.mes-hall↗

arXiv2022

The Chiral Induced Spin Selectivity Effect -- What It Is, What It Is Not, And Why It Matters

The chiral induced spin selectivity effect is an excited states phenomenon, which can be probed using photo-spectroscopy as well as transport measurements. On the one hand such measurements represent averaged quantities, on the other hand nearly all theoretical descriptions, with only a few exceptions, have been concerned with energy dependent properties of the pertinent structures. While those properties may or may not be relevant for the chiral induced spin selectivity effect, many of those properties have been attributed as being the, or part of the, origins of the effect. Here, it is demonstrated that, for instance, the spin-resolved transmission provides little, if any, information about the chiral induced spin selectivity effect. Moreover, although effective single-electron theory can be used in this context, reasons are given for why such descriptions are not viable.

cond-mat.mes-hall↗

arXiv2021

Charge and Spin Dynamics and Enantioselectivity in Chiral Molecules

Charge and spin dynamics is addressed in chiral molecules immediately after the instantaneous coupling to an external metallic reservoir. It is shown how a spin-polarization is induced in the chiral structure as a response to the charge dynamics. The dynamics indicate that chiral induced spin selectivity is an excited states phenomenon which, in the transient regime partly can be captured using a simplistic single particle description, however, in the stationary limit definitively shows that electron correlations, e.g., electron-vibration interactions, crucially contribute to sustain an intrinsic spin anisotropy that can lead to a non-vanishing spin selectivity. The dynamics, moreover, provide insight to enantiomer separation, due to different acquired spin-polarizations.

cond-mat.mes-hall↗

arXiv2021

Solving the Bose-Hubbard model in new ways

We introduce a new method for analysing the Bose-Hubbard model for an array of bosons with nearest neighbor interactions. It is based on a number-theoretic implementation of the creation and annihilation operators that constitute the model. One of the advantages of this approach is that it facilitates computation with arbitrary accuracy, enabling nearly perfect numerical experimentation. In particular, we provide a rigorous computer assisted proof of quantum phase transitions in finite systems of this type. Furthermore, we investigate properties of the infinite array via harmonic analysis on the multiplicative group of positive rationals. This furnishes an isomorphism that recasts the underlying Fock space as an infinite tensor product of Hecke spaces, i.e., spaces of square-integrable periodic functions that are a superposition of non-negative frequency harmonics. Under this isomorphism, the number-theoretic creation and annihilation operators are mapped into the Kastrup model of the harmonic oscillator on the circle. It also enables us to highlight a kinship of the model at hand with an array of spin moments with a local anisotropy field. This identifies an interesting physical system that can be mapped into the model at hand.

quant-ph↗

arXiv2021

Charge Redistribution and Spin Polarization Driven by Correlation Induced Electron Exchange in Chiral Molecules

Chiral induced spin selectivity is a phenomenon that has been attributed to chirality, spin-orbit interactions, and non-equilibrium conditions, while the role of electron exchange and correlations have been investigated only marginally until very recently. However, as recent experiments show that chiral molecules acquire a finite spin-polarization merely by being in contact with a metallic surface, these results suggest that electron correlations play a more crucial role for the emergence of the phenomenon than previously thought. Here, it is demonstrated that molecular vibrations give rise to molecular charge redistribution and accompanied spin-polarization when coupling a chiral molecule to a non-magnetic metal. It is, moreover, shown that enantiomer separation, due to spin-polarization intimately related to the chirality, can be understood in terms of the proposed model.

cond-mat.mes-hall↗

arXiv2020

First-principles Dzyaloshinskii-Moriya interaction in a non-collinear framework

We have derived an expression of the Dzyaloshinskii-Moriya (DM) interaction, where all the three components of the DM vector can be calculated independently, for a general, non-collinear magnetic configuration. The formalism is implemented in a real space - linear muffin-tin orbital - atomic sphere approximation (RS-LMTO-ASA) method. We have chosen the Cr triangular trimer on Au(111) and Mn triangular trimers on Ag(111) and Au(111) surfaces as numerical examples. The results show that the DM interaction (module and direction) is drastically different for collinear and non-collinear states. Based on the relation between the spin and charge currents flowing in the system and their coupling to the non-collinear magnetic configuration of the triangular trimer, we demonstrate that the DM interaction can be significant, even in the absence of spin-orbit coupling. This is shown to emanate from the non-collinear magnetic structure, that can induce significant spin and charge currents even with spin-orbit coupling is ignored.

cond-mat.mtrl-sci↗

arXiv2020

Remote Mesoscopic Signatures of Induced Magnetic Texture in Graphene

Mesoscopic conductance fluctuations are a ubiquitous signature of phase-coherent transport in small conductors, exhibiting universal character independent of system details. In this work, however, we demonstrate a pronounced breakdown of this universality, due to the interplay of local and remote phenomena in transport. Our experiments are performed in a graphene-based interaction-detection geometry, in which an artificial magnetic texture is induced in the graphene layer by covering a portion of it with a micromagnet. When probing conduction at some distance from this region, the strong influence of remote factors is manifested through the appearance of giant conductance fluctuations, with amplitude much larger than $e^2/h$. This violation of one of the fundamental tenets of mesoscopic physics dramatically demonstrates how local considerations can be overwhelmed by remote signatures in phase-coherent conductors

cond-mat.mes-hall↗

arXiv2020

Vibrational Origin of Exchange Splitting and Chirality Induced Spin Selectivity

Electron exchange and correlations emerging from the coupling between ionic vibrations and electrons are addressed. Spin-dependent electron-phonon coupling originates from the spin-orbit interaction, and it is shown that such electron-phonon coupling introduces exchange splitting between the spin channels in the structure. By application of these results to a model for a chiral molecular structure mounted between metallic leads, the chirality induced spin selectivity is found to become several tens of percents using experimentally feasible parameters.

cond-mat.mes-hall↗

arXiv2019

Investigation of the Yu-Shiba-Rusinov states of a multi-impurity Kondo system

Recent studies of mutually interacting magnetic atoms coupled to a superconductor have gained enormous interest due to the potential realization of topological superconductivity. The Kondo exchange coupling J_K of such atoms with the electrons in the superconductor has a pair-breaking effect which produces so-called Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap, whose energetic positions are intimately connected with the requirements for topological superconductivity. Here, using the tip of a scanning tunneling microscope, we artificially craft a multi-impurity Kondo system coupled to a superconducting host consisting of an Fe adatom interacting with an assembly of interstitial Fe atoms on an oxygen-reconstructed Ta(100) surface and we experimentally investigate the signatures of Kondo screening and the YSR states. With the help of numerical renormalization group (NRG) calculations, we show that the observed behavior can be qualitatively reproduced by a two-impurity Kondo system whose inter-impurity antiferromagnetic interaction J is adjusted by the number of interstitial Fe atoms in the assembly. When driving the system from the regime of two decoupled Kondo singlets (small J) to that of an antiferromagnetic dimer (large J), the YSR state shows a characteristic cross-over in its energetic position and particle-hole asymmetry.

cond-mat.supr-con↗