Theoretical Solid State Physics

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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. Based on previous work in our group (Nature 501, 73–78, 2013), we show a possible path to unify different points of view on the origin of the 0.7-anomaly: Using a real-frequency Keldysh-fRG calculation, we find that throughout the subopen 0.7-anomaly region the barrier-induced peak of the local density of states is pinned to the chemical potential. Throughout the pinning region electrons traversing the QPC experience a spatially extended, slowly fluctuating spin background. This view bridges the gap between two previous, seemingly contradictory phenomenological descriptions, one based on localized but dynamic spins, the other on spatially extended but static spin structures.

We study the Kondo chain in the regime of high spin concentration where the low energy physics is dominated by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. As has been recently shown (A. M. Tsvelik and O. M. Yevtushenko, Phys. Rev. Lett {\bf 115}, 216402 (2015)), this model has two phases with drastically different transport properties depending on the anisotropy of the exchange interaction. In particular, the helical symmetry of the fermions is spontaneously broken when the anisotropy is of the easy plane type (EP). This leads to a parametrical suppression of the localization effects. In the present paper we substantially extend the previous theory, in particular, by analyzing a competition of forward- and backward-scattering, including into the theory short range electron interactions and calculating spin correlation functions. We discuss applicability of our theory and possible experiments which could support the theoretical findings.

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.