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Hyper Quantum Criticality

Periodic Reporting for period 3 - HyperQC (Hyper Quantum Criticality)

Reporting period: 2019-12-01 to 2021-05-31

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 laser 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 magnetically ordered phases, as well as of other many-body phenomena in quantum spin systems.

Experiments on the time-dependent, out-of-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.
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, 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- , one- and finally 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 for the pure case and that with modulated inter-layer stacking. For the former large-single crystals of a historic material, the ancient pigment Han purple, was synthesized and grown as large high-quality single crystals in the lab.

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 became possible at the very end of the project, when a pioneering superconducting-magnet dilution system was commissioned. New high-pressure cells and a controlled helium compressor system are used to simultaneously apply hydrostatic pressure to quantum materials. The instrument offers a world-wide unique range of magnetic field and pressure at ultra-low temperatures.

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. For the latter a special laser setup generating ultra-short, ultra-intense pulses with tunable energies in the THz range was developed and exploited to study materials with Shastry-Sutherland and alternating chain dimer lattices under out-of-equilibrium conditions and on ultra-short timescale. The first out-of-equilibrium population of magnon modes in a gapped quantum magnet was observed via a new non-linear magnetophononic mechanism. The latter can be explored for many other materials and offers an new approach for coherent control of quantum matter on an ultra-short timescale.

The ground-breaking results of the project were presented at major international conferences and workshops and at seminars and colloquia at a large number of universities in Europe, in the US and in Asia by the PI and all project collaborators. The results were published in leading peer-reviewed journals e.g. in Physical Review Letters, Applied Physics Letters and Nature Physics. One of the stories was on the cover of one of these journals and another was covered by a press release and on social media.

Important achievements of the project in instrumentation and methods development (e.g. ultra-short and -intense THz laser pulses) will have an immediate impact beyond the scope of HyperQC and serve the wider physics, chemistry and material science community.
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 frustration. The superconducting-magnet dilution system enables us to do experiments as a function of multiple control parameters at ultra-low temperatures for the first time.

Dynamic control of superexchange parameters and selection rules by pumping of phonon modes has revealed new mechanisms for phonon-magnon conversion in systems with flat magnon bands. The non-linear magnetophononic mechanism or “magnetophononics” is completely new and allows – beyond investigation of ultra-fast phenomena in quantum magnetism – the dynamic coherent population of a critical density of magnetic quasi-particles, which for the equilibrium case has led to the first observation of Bose-Einstein Condensation in a condensed matter system. This dynamic coherent control can be applied to a large number of very interesting scenarios and materials, including systems with electro-magnons and spin-Peierls transitions.