Dr. Óscar Nájera

This article is in German.

In checkmk 1.6 I implemented the possibility to calculate forecasts over monitored IT-resources in order to predict future utilization. This feature was developed to allow on-premise service providers to independently evaluate their resource utilization and plan their resource allocation.

Vanadium dioxide (VO\(_2\)) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO\(_2\) thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO\(_2\), which account for a continuous temperature dependence of the optical response of the VO\(_2\) in the insulating state.

We consider the dimer Hubbard model within dynamical mean-field theory to study the interplay and competition between Mott and Peierls physics. We describe the various metal-insulator transition lines of the phase diagram and the breakdown of the different solutions that occur along them. We focus on the specific issue of the debated Mott-Peierls insulator crossover and describe the systematic evolution of the electronic structure across the phase diagram. We found that at low intradimer hopping, the emerging local magnetic moments can unbind above a characteristic singlet temperature T\(^*\). Upon increasing the interdimer hopping, subtle changes occur in the electronic structure. Notably, we find Hubbard bands of a mix character with coherent and incoherent excitations. We argue that this state might be relevant for materials such as VO\(_2\) and its signatures may be observed in spectroscopic studies, and possibly through pump-probe experiments.

We study in detail the solution of a basic strongly correlated model,namely, the dimer Hubbard model. This model is the simplest realization of a cluster DMFT problem.

We provide a detailed description of the solutions in the “coexistent region” where two (meta)stable states of the DMFT equations are found, one a metal and the other an insulator. Moreover, we describe in detail how these states break down at their respective critical lines. We clarify the key role played by the intra-dimer correlation, which here acts in addition to the onsite Coulomb correlations.

We consider a minimal model to investigate the metal-insulator transition in VO\(_2\). We adopt a Hubbard model with two orbitals per unit cell, which captures the competition between Mott and singlet-dimer localization. We solve the model within dynamical mean-field theory, characterizing in detail the metal-insulator transition and finding new features in the electronic states. We compare our results with available experimental data, obtaining good agreement in the relevant model parameter range. Crucially, we can account for puzzling optical conductivity data obtained within the hysteresis region, which we associate with a metallic state characterized by a split heavy quasiparticle band. Our results show that the thermal-driven insulator-to-metal transition in VO\(_2\) is compatible with a Mott electronic mechanism, providing fresh insight to a long-standing “chicken-and-egg” debate and calling for further research of “Mottronics” applications of this system.