# Theoretical Solid State Physics

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.

## Current research highlights:

## Projective symmetry group classification of chiral Z2 spin liquids on the pyrochlore lattice: Application to the spin-1/2 XXZ Heisenberg model

Simplified models for quantum spin-ice materials, such as the XXZ model on the pyrochlore lattice, may host interesting new spin-liquid ground states. In this work, we provide a complete classification of nearest-neighbor chiral Z2 spin-liquid states on the pyrochlore lattice using a projective symmetry group analysis. Moreover, the newly

classified states are applied to the XXZ model within a self-consistent mean-field scheme. We find a new class of spin-liquid states, the pi/3-states, that inherit their properties from fractionalization of the C3 symmetry of the pyrochlore lattice. Further, we discuss observable signatures that can be measured in neutron scattering experiments. Remarkably, the spin liquids preserve the SU(2) symmetry even in the presence of SU(2) symmetry breaking interactions.

## Multipoint Correlation Functions: Spectral Representation and Numerical Evaluation &

Computing Local Multipoint Correlators Using the Numerical Renormalization Group

We show how to compute multipoint correlation functions of quantum many-body systems with unprecedented accuracy. Multipoint functions describe correlations between multiple quantum particles at different space-time points. Such functions pervade many branches of physics and are relevant for interpreting numerous types of experimental measurements, like electrical conductivities and scattering spectra of photons or neutrons. However, multipoint functions are notoriously difficult to compute, especially for strong interactions and low temperatures of interest for understanding quantum systems such as high-temperature superconductors, heavy fermions, or Hund metals.

In [F. B. Kugler, S.-S. B. Lee, and J. von Delft, Phys. Rev. X 11, 041006 (2021)], we develop a theoretical framework ("spectral representations") expressing general multipoint functions through the system's quantum states, revealing their structure with great clarity. In [S.-S. B. Lee, F. B. Kugler, and J. von Delft, Phys. Rev. X 11, 041007 (2021)], we devise a powerful method for numerically evaluating the spectral representations of local multipoint functions. With this, we compute effective two-particle interactions ("vertex functions") and, as an example, spectra for resonant inelastic x-ray scattering (RIXS), a promising experimental tool in solid-state physics. Our novel approach allows us to accurately treat energies and temperatures ranging over many orders of magnitude, beyond the reach of other numerical methods. This makes it a valuable addition to the toolbox of theoretical techniques for studying quantum matters.

## Uncovering Non-Fermi-Liquid Behavior in Hund Metals: Conformal Field Theory Analysis of an SU(2)×SU(3) Spin-Orbital Kondo Model

Hund metals have attracted attention in recent years due to their unconventional superconductivity, which supposedly originates from non-Fermi-liquid (NFL) properties of the normal state. When studying Hund metals using dynamical mean-field theory (DMFT), one arrives at a self-consistent "Hund impurity problem" involving a multiorbital quantum impurity with nonzero Hund coupling interacting with a metallic bath. If its spin and orbital degrees of freedom are screened at different energy scales, the intermediate energy window is governed by a novel NFL fixed point, whose nature had not yet been clarified. We resolve this problem by providing an analytical solution of a paradigmatic example of a Hund impurity problem, involving two spin and three orbital degrees of freedom. To this end, we combine a state-of-the-art implementation of the numerical renormalization group, capable of exploiting non-Abelian symmetries, with a generalization of Affleck and Ludwig’s conformal field theory (CFT) approach for multichannel Kondo models.

Welcome our new member:

Dr. Markus Scheb: "My main area of research is the finite Projected Entangled Pair State algorithm, one of the most promising numerical methods for describing strongly correlated, two-dimensional quantum systems. More specifically, I focus on the implementation and technical intricacies of the algorithm to make it as fast and memory-efficient as possible."

Many correlated metallic materials are described by Landau Fermi-liquid theory at low energies, but for Hund metals the Fermi-liquid coherence scale T_FL is found to be surprisingly small. In this Letter, we study the simplest impurity model relevant for Hund metals, the three-channel spin-orbital Kondo model, using the numerical renormalization group (NRG) method and compute its global phase diagram. In this framework, T_FL becomes arbitrarily small close to two new quantum critical points that we identify by tuning the spin or spin-orbital Kondo couplings into the ferromagnetic regimes. We find quantum phase transitions to a singular Fermi-liquid or a novel non-Fermi-liquid phase. We compute dynamical susceptibilities in these phases, which reveal universal power laws. These can be fully understood using conformal field theory arguments, which also clarify the nature of the non-Fermi-liquid phase.

The crossover from fluctuating atomic constituents to a collective state as one lowers temperature or energy is at the heart of the dynamical mean-field theory description of the solid state. We demonstrate that the numerical renormalization group is a viable tool to monitor this crossover in a real-materials setting. The renormalization group flow from high to arbitrarily small energy scales clearly reveals the emergence of the Fermi-liquid state of Sr2RuO4. We find a two-stage screening process, where orbital fluctuations are screened at much higher energies than spin fluctuations, and Fermi-liquid behavior, concomitant with spin coherence, below a temperature of 25 K. Our work demonstrates the potential of DFT+DMFT+NRG as a new computational paradigm for real-material systems to (i) directly access real-frequency properties at arbitrarily low temperatures and (ii) reveal and elucidate the intricate renormalization process that occurs during the dressing of atomic excitations by their solid-state environment.

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.