Periodic Reporting for period 3 - ExQuiSid (Extreme Quantum Matter in Solids) Reporting period: 2021-06-01 to 2022-11-30 Summary of the context and overall objectives of the project Quantum stochastic processes in solids, representing many-body systems par excellence, are believed to lead to extreme forms of quantum entanglement and non-local correlations (extreme quantum matter), that offer a well-defined starting point for an understanding of a wide range of anomalous materials properties, as well as emergent electronic phases such as magnetically mediated superconductivity or partial spin and charge order. While overwhelming experimental evidence seem to suggest a breakdown of traditional concepts such as well-defined quasi-particle excitations, present-day disagreement between experiment and theory may be traced to the lack of experimental information on the spectrum of low-lying excitations close to and far from equilibrium under controlled conditions. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The ERC project ExQuiSid focusses on the experimental investigation of quantum correlations and quantum entanglement in materials with strong electronic correlations under extreme conditions (ultra-low temperatures, high magnetic fields and high pressures). To resolve the nature and relevance of quantum correlations and quantum entanglement in real materials, the methodology comprises materials preparation with measurements of bulk and transport properties and neutron and x-ray spectroscopy. Major highlights include:• High-precision studies of magnetic anisotropies by means of torque magnetometry and elastic neutron scattering under carefully conceived conditions. This defines the parameter space for studies of transverse-field quantum phase transitions.• Identification of topologically non-trivial order in the zero-temperature limit and identification of a novel stabilization mechanism. This paves the way to a new class of quantum phase transitions.• Identification of a new class of coupled elementary excitations (here magneto-elastic). This identifies a new avenue in the pursuit of novel phases near quantum phase transitions.• Discovery of emergent mesoscale antiferromagnetism at the transverse-field quantum phase transition of an Ising ferromagnet. This identifies a show-case for the formation of textures in a parameter regime dominated by by quantum entanglement.• Identification of symmetry-enforced topological nodal planes in chiral magnets. This identifies an approach how to readily control topological protectorates of the Fermi surface.• Identification topological magnons and emergent Landau levels born out of the non-trivial real space topology of a magnetic structure. • Implementation and commissioning of several new experimental methods: transverse field susceptibility, transverse field calorimetry, neutron-scattering under microwave pumping, and various slow pump-probe techniques, and the commissioning of novel crystal growth apparatus (annealing furnaces for post-growth treatment). Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) ExQuiSid advances the understanding of the nature of extreme quantum matter in the most extensively studied model systems, notably simple magnetic materials (insulators and metals) tuned through a quantum phase transition. For the proposed studies a new generation of experimental methods has been implemented comprising neutron spectroscopy with an unprecedented nano-eV resolution even under large magnetic fields, transverse-field vector magnetometry, calorimetry and transport down to milli-Kelvin temperatures, and, ultra-high purity single-crystal growth combined with advanced materials characterisation. ExQuiSid resolves long-standing mysteries in model-systems of extreme quantum phase transitions, experimentally enables pioneering studies on the creation, nature and classification of non-equilibrium quantum matter in solids at ultra-low energies and temperatures, and experimentally enables pioneering studies of quantum matter driven periodically out of equilibrium in the search for dynamical quantum instabilities and dynamical quantum phases. Fluctuating skyrmion textures in a chiral magnet Magneto-elastic hybrid excitation
