Theoretical Solid State Physics
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Welcome!
Our group focuses on the study of strongly correlated many particle systems, where interactions play an important role. To do so we use and develop advanced theoretical tools, in particular tensor network and functional renormalization group methods.

Recent research highlights:

We demonstrate that low dimensional Kondo-Heisenberg systems, consisting of itinerant electrons and localized magnetic moments (Kondo impurities), can be used as a principally new platform to realize scalar chiral spin order. The underlying physics is governed by a competition of the Ruderman-Kittel-Kosuya-Yosida (RKKY) indirect exchange interaction between the local moments with the direct Heisenberg one. When the direct exchange is weak and RKKY dominates, the isotropic system is in the disordered phase. A moderately large direct exchange leads to an Ising-type phase transition to the phase with chiral spin order. Our finding paves the way towards pioneering experimental realizations of the chiral spin liquid in systems with spontaneously broken time-reversal symmetry.

We show that the paradigmatic Ruderman-Kittel-Kasuya-Yosida (RKKY) description of two local magnetic moments coupled to propagating electrons breaks down in helical Luttinger liquids when the electron interaction is stronger than some critical value. In this novel regime, the Kondo effect overwhelms the RKKY interaction over all macroscopic interimpurity distances. This phenomenon is a direct consequence of the helicity (realized, for instance, at edges of a time-reversal invariant topological insulator) and does not take place in usual (nonhelical) Luttinger liquids.

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.