Ultra-clean epitaxy methods were developed for the growth of high quality transition metal dichalcogenide (TMDC) atomically thin layers and for complex oxide heterostructures. A01 will exploit van-der-Waals (vdW) epitaxy and pulsed-laser deposition to produce new electronic and magnetic phases. The project addresses three major topical areas: i) atomically thin materials, ii) interfaces between high-k oxides and atomically thin materials and iii) ferromagnetic/large spin-orbit coupling oxide interfaces.

The aim of this project is the preparation and characterization of bulk materials which constitute the prerequisite for many of the experimental studies in the planned CRC. Our experimental techniques range from different methods of polycrystal preparation to sophisticated techniques of single-crystal growth, such as the floating-zone method in image furnaces or the chemical transport reaction growth. We will focus on multiferroic or magnetoelectric materials, on compounds with a strong impact of spin-orbit coupling and on systems in which frustration or topology result in new phenomena.

Materials possessing topologically nontrivial electronic wavefunctions are called topological materials and comprise insulators, semimetals, and superconductors. In this project, to form a materials basis for their investigations, we grow high-quality single crystals of topological insulators (TIs), three-dimensional Dirac or Weyl semimetals, and superconductors derived from TIs. We also explore new materials in these classes. In addition, we grow TI thin films using MBE techniques and fabricate devices to tune the Fermi level through electrostatic gating.

In this project, the ground states and low-lying excitations of magnetic and topological materials are investigated by means of a broad range of thermodynamic and transport measurements. The main topics to be addressed are: (I) the dynamics of magnetic monopole excitations in spin ice, (II) novel phases and excitations arising from coupled order parameters, (III) peculiar magnetotransport properties of topological insulators and 3D Dirac/Weyl semimetals, and (IV) the nature of superconductors derived from topological insulators.

The interplay of spin-orbit coupling, topology, and electronic correlations manifests itself on very different energy scales. Covering 18 decades in frequency, we aim at the electrodynamic properties and the dynamic behavior of quantum matter, and at the control of the order-parameter dynamics via external fields. The topics range from the search for the universal Faraday effect in topological insulators (TIs) via spin-orbit-driven excitations such as spin-plasmons in TIs or electro-magnons in multiferroics to the control of the excitation dynamics via external fields in systems with topological excitations such as skyrmions in chiral magnets or monopoles in spin ice.

In spin-orbit-dominated correlated 4d and 5d transition-metal oxides, we will use Raman scattering and resonant inelastic x-ray scattering (RIXS) to explore novel exotic phenomena, to clarify the hierarchy of energy scales, and to achieve a profound understanding of the underlying interactions. Key issues are the physics of j=1/2 moments coupled via bond-directional Kitaev interactions in d5 compounds, the search for Majorana-like spin excitations, and the role of spin-orbit coupling in the ruthenates.

This neutron-scattering project will explore the magnetic and nuclear structure and excitations of electronically correlated quantum matter that can be controlled by external parameters such as field, temperature, pressure, or by chemical doping. We will study magnetoelectric or multiferroic systems, materials in which spin-orbit coupling induces particular phenomena or functionalities like 4d and 5d compounds, as well as systems characterized by topological effects.

This project focuses on the study of dynamical non-equilibrium properties of spin-orbit coupled matter. First, we plan to investigate the near-equilibrium response of these materials after optical excitation to elucidate microscopic coupling mechanisms between the spin, orbital, lattice, and charge degrees of freedom. Second, using intense excitation often addressing one specific degree of freedom, we also intend to optically drive systems far from equilibrium into non-thermal, typically metastable, states.

Strong spin-orbit interactions give rise to exciting phenomena at the surfaces of metals and topological insulators, in 2D materials, and even in bulk crystals. In project B06, scanning tunneling microscopy and spectroscopy, as well as their spin-polarized refinements, will be used to unravel the local electronic and magnetic structure of surfaces with an emphasis on phenomena stemming from spin-orbit coupling, including topological superconductivity or novel spin textures.

Thanks to advanced transport methods (current correlations, high-frequency transport), we aim at unravelling unexplored aspects of quantum anomalous Hall systems and further assess the role of charge puddles, of magnetic domain walls and magnons, and of Coulomb interaction. We want to develop tools to explore topological physics at the most fundamental level of the single excitation, thus unveiling its most quantum aspects.

The spin-orbit interaction of electrons in the presence of a structure inversion-asymmetric environment such as surfaces, interfaces, or heterostructures gives rise to a plethora of novel electronic and transport phenomena, as well as non-trivial spin textures. We investigate these by means of density functional theory and relate to photoemission, scanning tunneling spectroscopy, and transport experiments. Having a material-specific theory, we explore possibilities of fine-manipulation of properties by means of chemical functionalization, intercalation, alloying, or strain.

Strong spin-orbit interactions can stabilize novel topological forms of matter. Our focus will be on Dirac matter — a powerful unifying concept which can be used to model diverse physical systems, including surface states of topological insulators, Weyl metals, and frustrated magnets. The projects investigate magnetotransport in disordered and interacting Dirac matter, charged impurities in topological insulators, disorder and competing interactions in spin liquids described by Majorana excitations, and novel phase-space Berry phase effects.

One of the most intriguing phenomena of interacting quantum many-body systems is the fractionalization of quantum numbers, that is the low-temperature emergence of novel quantum numbers which are distinct from the ones of the original constituents of the quantum many-body system. Project C03 studies the fractionalization of magnetic moments for spin-orbit entangled j=1/2 Mott insulators, relevant to a number of 5d transition-metal oxides. Our focus will be on the thermodynamic signatures of this fractionalization and its underlying Z2 gauge theory description studied in the context of three-dimensional Kitaev-Heisenberg models, which will be explored by means of quantum Monte Carlo simulations, the numerical renormalization group, and a novel functional renormalization group approach.

The project addresses fundamental aspects governing the dynamics of correlated quantum matter and their manifestation in concrete experiments. Three key questions will be at the focus. First, how does a quantum system react to a sudden perturbation (e.g., a laser pulse), and how does it ultimately reach thermal equilibrium? Second, which dynamical properties characterize phase transitions? Third, how can topological excitations (e.g. magnetic skyrmions) dynamically be created and manipulated? We will approach these questions using Keldysh renormalization group techniques, quantum kinetic equations, and other field theoretical methods combined with numerics.

In project C05, we theoretically investigate how quantum materials can be controlled by

driving them externally, for example, by laser light. We focus especially on the interplay of an external driving and the simultaneous coupling to environments such as phonon modes. We plan to explore whether transitions can be induced efficiently in the presence of coupled order. Additionally, we will investigate materials with spin-orbit coupling and its influence on the dynamics of these systems by the coupling of the different excitation and relaxation channels.