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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - HyperQC (Hyper Quantum Criticality)

Teaser

Summary

Hyper Quantum Criticality (HyperQC) is a major initiative with the aim of generating and controlling novel phases of correlated magnetic quantum matter, and of exploring them in high-precision experiments. A combination of new capabilities enabled by the development of instrumentation, pioneering ultra-fast studies and experiments on magnetic model materials will allow both the exploration of fundamental Hamiltonians and fully quantitative tests of detailed predictions for quantum criticality in hyper-parameter space: temperature, magnetic field, pressure, energy, momentum and time.

Direct control of the dimensionality, symmetry, chemical potential and interactions in magnetic materials is achieved by a new experimental set-up combining high magnetic fields and pressures with ultra-low temperatures, which will be installed on neutron scattering instruments at the Swiss Spallation Neutron Source SINQ. Experiments on a number of magnetic model materials allow the realization and high-precision measurements of the multi-dimensional quantum critical properties of systems including magnon Bose-Einstein Condensates, spin Luttinger-liquids and renormalized classical ordered phases, as well as of other many-body phenomena in quantum spin systems.

Experiments on the time-dependent, non-equilibrium properties of quantum magnets and quantum critical points are new. Ultra-short laser and X-ray pulses, e.g. from the new Swiss free electron laser SwissFEL, are able to alter and measure the lattice, spin, orbital and electronic properties of solids, which has been demonstrated in recent experiments on multiferroic materials and superconductors. The effects of such pulses on a number of well-characterized model quantum magnets are investigated with the aim of studying the time-dependent dynamics of quantum critical systems for the first time.

The results of HyperQC are relevant for our understanding of processes like sensing and switching in devices, for the exploitation of many-body quantum states in future applications as well as for our fundamental understanding and control of correlated quantum systems.

Work performed

Experiments on static quantum criticality focused in the first project period on systems with separate magnetic-field or pressure-controlled quantum critical points. Combination of both parameters at ultra-low temperatures will be possible in the second project period.

Arrays of quantum spins on a square lattice were studied in a series of metal-organic materials, in which the square lattices are formed by Cu2+ ions and organic ligands and chlorine or bromine atoms. We demonstrated that pressure is able to modify the magnetic superexchange between magnetic ions beyond expectations enabling unprecedented continuous and discontinuous control of the dimensionality of the spin systems from being two- to one- or three-dimensional. On the other hand, some model materials e.g. for the quantum spin ladder showed a continuous increase of the relevant exchange parameters and no quantum criticality.

Two model materials with two-dimensional lattices of quantum spin dimers were studied. The materials cover two important limits of potential inter-layer and intra-layer frustration and quantum critical points that are reached by the application of very strong magnetic fields beyond 20 T. We were able to clarify the nature of dimensional reduction at quantum critical points and of field-induced order of the Bose-Einstein Condensate universality class in this case.

Quantum criticality was explored in a material, which for the first time shows a spin-orbital singlet ground-state and an excitation gap to spin-orbital triplets. This scenario is of special interest because the control of spin-orbit coupling would allow simultaneous criticality of spin and orbital degrees of freedom.

Combined studies in high magnetic fields and at high pressures at ultra-low temperatures will become possible in the second project period, when a new superconducting-magnet dilution system will finally become available. New high-pressure cells and a controlled helium compressor system were designed and procured from companies for future experiments in combination with the superconducting magnet.

First studies of out-of-equilibrium quantum criticality in quantum magnets addressed fundamental questions concerning dynamic control of the superexchange parameters via strong pumping of phonon modes in the THz and IR range. Spin chains, two- and three-dimensional quantum dimer systems were investigated theoretically by ab-initio calculations and experimentally by pump-probe laser spectroscopy in preparation for future experiments using a free electron laser to test dynamic quantum criticality.

Final results

Concerning the control of dimensionality of quantum critical systems we have by now achieved a detailed understanding of possible mechanisms like ligand selection and pressure-control of the superchange pathways (inhomogeneous compression by homogeneous pressure), pseudo Jahn-Teller distortions, and the frustration. The superconducting-magnet dilution system will allow us to do experiments as a function of multiple control parameters until the end of the project.

Dynamic control of superexchange parameters by pumping of phonon modes has revealed new mechanisms for phonon-magnon conversion in systems with flat magnon bands. In the second project period experiments will focus on systems with intrinsic phonon-magnon instabilities like electro-magnons and spin-Peierls transitions.