Theoretical Solid State Physics

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The recently discovered iron pnictide superconductors (as well as chalcogenides, ruthenates, and other 4d transition metal oxides) show puzzling anomalous properties, like a coherence- incoherence crossover, also in the normal state. While there is consensus about strong correlation effects playing a key role in these materials, their precise origin (Coulomb repulsion or Hund's rule coupling between electrons of different orbitals) has been under debate as one of the major open questions in the field many years. In order to better understand the differences between Mott insulators and Hund metals, in particular in the context of the coherence-incoherence crossover, we used - for the first time- the numerical renormalization group to obtain a numerically exact dynamical mean-field theory solution to the Hund metal problem for a three-band model on a Bethe lattice at 1/3 filling. Our main result is the explicit demonstration of "spin-orbital separation'': spin screening occurs at much lower energies than orbital creening. The ground state is a Fermi liquid. With increasing temperature we observe a coherence-incoherence crossover which is clearly driven by Hund's rule coupling and not by Coulomb interaction effects as in Mott-Hubbard systems or by thermal broadening (in agreement with recent ARPES measurements).

Spin exchange between a single-electron charged quantum dot and itinerant electrons leads to an emergence of Kondo correlations. When the quantum dot is driven resonantly by weak laser light, the resulting emission spectrum allows for a direct probe of these correlations. In the opposite limit of vanishing exchange interaction and strong laser drive, the quantum dot exhibits coherent oscillations between the single-spin and optically excited states. Here, we show that the interplay between strong exchange and nonperturbative laser coupling leads to the formation of a new nonequilibrium, quantum-correlated state, characterized by the emergence of a laser-induced secondary spin screening cloud, and examine the implications for the emission spectrum.

The conductance through quantum point contacts (QPCs) is quantized in units of the conductance quantum. In addition to this well understood quantization, measured curves exhibit a shoulder at around 0.7 times the conductance quantum. In this regime, the electrical and thermal conductance show anomalous behavior in their dependence of parameters such as temperature, magnetic field or applied bias. These effects are collectively known as the 0.7-anomaly in QPCs. Their origin has been controversially discussed ever since they were first mentioned in 1996. We have now shown that the 0.7-anomaly can be explained by a model with a parabolic potential barrier and a purely local interaction. We demonstrated via detailed calculations, using either the functional Renormalization Group approach or second order perturbation theory, that this model can reproduce all generic features of the 0.7 anomaly. Our results show excellent agreement with experimental data measured by the group of Stefan Ludwig.

We study electronic transport in a Luttinger liquid with an embedded impurity, which is either a weak scatterer or a weak link, when interacting electrons are coupled to one-dimensional massless bosons (e.g., acoustic phonons). We find that the duality relation, $\Delta_{ws}\Delta_{wl}=1 $, between scaling dimensions of the electron backscattering in the WS and WL limits, established for the standard LL, holds in the presence of the additional coupling for an arbitrary fixed strength of boson scattering from the impurity. This means that at low temperatures such a system remains either an ideal insulator or an ideal metal, regardless of the scattering strength. On the other hand, when fermion and boson scattering from the impurity are correlated, the system has a rich phase diagram that includes a metal-insulator transition at some intermediate values of the scattering.