Mechanism of structural phase transitions in KCrF3

We study the origin of the cubic to tetragonal and tetragonal to monoclinic structural transitions in KCrF3, and the associated change in orbital order, paying particular attention to the relevance of super-exchange in both phases. We show that super-exchange is not the main mechanism driving these transitions. Specifically, it is not strong enough to be responsible for the high-temperature cubic to tetragonal transition and does not yield the type of orbital order observed in the monoclinic phase. The energy difference between the tetragonal and the monoclinic structure is tiny, and most likely results from the interplay between volume, covalency, and localization effects. The transition is rather driven by Slater exchange than super-exchange. Nevertheless, once the monoclinic distortions are present, super-exchange helps in stabilizing the low symmetry structure. The orbital order we obtain for this monoclinic phase is consistent with the magnetic transition at 80 K.Comment: 8 pages, 6 figure

Dominant Reaction Pathways in High Dimensional Systems

This paper is devoted to the development of a theoretical and computational framework to efficiently sample the statistically significant thermally activated reaction pathways, in multi-dimensional systems obeying Langevin dynamics. We show how to obtain the set of most probable reaction pathways and compute the corrections due to quadratic thermal fluctuations around such trajectories. We discuss how to obtain predictions for the evolution of arbitrary observables and how to generate conformations which are representative of the transition state ensemble. We present an illustrative implementation of our method by studying the diffusion of a point particle in a 2-dimensional funneled external potential.Comment: 18 pages, 7 figures. Improvement in the text and in the figures. Version submitted for publicatio

Altermagnetic surface states: towards the observation and utilization of altermagnetism in thin films, interfaces and topological materials

The altermagnetism influences the electronic states allowing the presence of non-relativistic spinsplittings. Since altermagnetic spin-splitting is present along specific k-paths of the 3D Brillouin zone, we expect that the altermagnetic surface states will be present on specific surface orientations. We unveil the properties of the altermagnetic surface states considering three representative space groups: tetragonal, orthorhombic and hexagonal. We calculate the 2D projected Brillouin zone from the 3D Brillouin zone. We study the surfaces with their respective 2D Brillouin zones establishing where the spin-splittings with opposite sign merge annihilating the altermagnetic properties and on which surfaces the altermagnetism is preserved. Looking at the three principal surface orientations, we find that for several cases two surfaces are blind to the altermagnetism, while the altermagnetism survives for one surface orientation. Which surface preserves the altermagnetism depends also on the magnetic order. We show that an electric field orthogonal to the blind surface can activate the altermagnetism. Our results predict which surfaces to cleave in order to preserve altermagnetism in surfaces or interfaces and this paves the way to observe non-relativistic altermagnetic spin-splitting in thin films via spin-resolved ARPES and to interface the altermagnetism with other collective modes. We open future perspectives for the study of altermagnetic effects on the trivial and topological surface states.Comment: 8 pages, 10 figure

Puckering Free Energy of Pyranoses: an NMR and Metadynamics--Umbrella Sampling Investigation

We present the results of a combined metadynamics--umbrella sampling investigation of the puckered conformers of pyranoses described using the gromos 45a4 force field. The free energy landscape of Cremer--Pople puckering coordinates has been calculated for the whole series of alpha and beta aldohexoses, showing that the current force field parameters fail in reproducing proper puckering free energy differences between chair conformers. We suggest a modification to the gromos 45a4 parameter set which improves considerably the agreement of simulation results with theoretical and experimental estimates of puckering free energies. We also report on the experimental measurement of altrose conformers populations by means of NMR spectroscopy, which show good agreement with the predictions of current theoretical models