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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ExQuiSid (Extreme Quantum Matter in Solids)

Teaser

Summary

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 clearly suggests a breakdown of traditional concepts such as well-defined quasi-particle excitations, the striking present-day disagreement between experiment and theory may be traced to the lack of experimental information on the spectrum of quantum stochastic many-body processes in solids in the low-energy and low-temperature limit close to and far from equilibrium.

Work performed

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. The implementation of the project builds on a large body of existing expertise. It proceeded extremely well in the first funding period, exceeding our expectations.

Major highlights included
â€¢ High-precision studies of magnetic anisotropies by means of torque magnetometry and elastic neutron scattering under carefull designed conditions. This defines the parameter space for studies of transverse-field quantum phase transitions.
â€¢ 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.
â€¢ Identification of a new class of coupled elementary excitations (here magneto-elastic). This identifies a change of paradigm in the search for novel phases near quantum phase transitions.
â€¢ Implementation and commissioning of several new experimental methods: transverse field susceptibility, transverse field calorimetry, x-ray scattering under microwave pumping.

Part of these results have been published. In addition, considerable progress has been made with the development of neutron scattering under microwave pumping and the commissioning of novel crystal growth apparatus (annealing furnaces for post-growth treatment).

Final results

ExQuiSid will advance 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 my group has implemented a new generation of methods covering for the first time 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 will (i) solve long-standing mysteries in model-systems of extreme quantum phase transitions, (ii) experimentally enable and permit pioneering studies on the creation, nature and classification of non-equilibrium quantum matter in solids at ultra-low energies and temperatures, and (iii) experimentally enable and permit pioneering studies of quantum matter driven periodically out of equilibrium to identify dynamical quantum instabilities and dynamical quantum phases such as many-body localisation.