epl draft Nonequilibrium Kondo-vs-RKKY Scenarios in Nanoclusters

Ultrafast manipulations of magnetic phases are eliciting increasing attention from the scientific community, because potentially relevant to the understanding of nonequilibrium phase transitions and to novel technologies. Here, we focus on manipulations applied to magnetic impurities in metallic hosts. By considering small nanoring geometries, we show how currents can induce a dynamical switching between different types of exchange interactions in these systems. Our work thus opens a study window on nonequilibrium Doniach’s magnetic phase diagrams, and time-dependent Kondo-vs-RKKY scenarios. Introduction. Magnetic phenomena are known to man from antiquity [1], with documented references to lodestone as early as nearly three millennia ago. A proper notion of the basic nature of magnetism, though, is a much more recent outcome, made possible in the last century by the advent of quantum mechanics. Then it became clear that it is the exchange interaction [2, 3], a produce of Coulomb force and symmetry properties of the manyparticle wavefunction, to be responsible for the ordering of magnetic moments (e.g. electron spins) in a system. Nowadays, it is possible to create and employ ultrashort electromagnetic pulses to probe and control quantum mechanical systems and alter nature and strength of their many-body interactions, making feasible the study of dynamical phase transitions [4]. For magnetism, this means a direct manipulation of the exchange interaction, to attain different magnetic orders and be able to dynamically switch between them [5–7] for technological aims. A type of systems which is attractive in this context is localized magnetic impurities in metallic hosts: due to strong electronic correlations, they exhibit a fascinating behavior, depending on temperature, impurity density, and disorder [8–11]. For their magnetic properties, a key feature is the interplay of Kondo screening (due to the exchange coupling between conduction electrons and magnetic moments at the impurities) and Ruderman-KittelKasuya-Yoshida (RKKY) exchange (the conduction electrons induce an indirect magnetic coupling among the moments at the impurities) [11, 12]. This competition provides the rationale behind the so-called Doniach-Phase diagram [8], which shows how different magnetic phases relate to temperature and exchange interaction. This subject has received great attention for bulk samples, in connection to heavy-fermions and mixed-valence compounds [11, 12]. However, finite samples are also of high importance, in relation to nanotechnology [13–19]. Finite-size effects were initially studied for isolated impurities in the so-called Kondo-box model [20], with the average conduction level spacing introducing a new energy scale. For dense impurities, such spacing was found to be a regulating factor to the interplay of Kondo and RKKY interactions [21], leading to a Doniach-like magnetic phase diagram for nanoclusters [22–25]. These studies on dense impurities focussed on systems in equilibrium; it could then be of interest to consider the dynamical regime. This possibility was recently pointed out for bulk samples [26]; however, the case of finite systems, where a finite conduction level spacing plays a role, has remained unexplored so far. This is the issue we wish to address here: Namely, can we dynamically manipulate the interplay of Kondo and RKKY exchange interactions in finite systems with dense magnetic impurities? Our answer is in the affirmative: By studying nanorings with dense magnetic impurities, and using magnetic fluxes to induce ring currents in the conduction levels, we show that i) it is possible to go across different magnetic configurations in time, and in fact ii) realise a non-equilibrium Doniach-like phase diagram. We also iii) show how such p-1 ar X iv :1 80 2. 00 83 8v 2 [ co nd -m at . s tr -e l] 1 6 A ug 2 01 8