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Quantum materials under extreme conditions

Periodic Reporting for period 2 - ExtremeQuantum (Quantum materials under extreme conditions)

Reporting period: 2018-03-01 to 2019-08-31

The exploration of new and exotic states of matter is as fundamental to our understanding of the universe as is the detection of elementary particles or the discovery of celestial objects. What is more, many of these states exhibit properties that could have significant impact upon future technologies. States of particular current interest include unconventional superconductors, low-dimensional ordered magnets, spin liquids and ices, topological insulators, bosonic superfluids, shape memory phases and multiferroics. All of these examples emerge from a complex soup of many-body quantum interactions, making them difficult to understand. Nevertheless, finding out how the states arise is the first, but essential step towards fully harnessing their capacity for application.

Two key factors governing the systems that support these states are crystal symmetry and cooperative electronic or magnetic interactions. It is, however, now becoming apparent that other ingredients are likely playing an equally vital role: quantum mechanical fluctuations, the underlying topology, and random configurational disorder are all suspected of deep involvement in these so-called quantum materials.

This project seeks to advance our knowledge of these issues by using extreme conditions of magnetic field and pressure to enable a continuous, clean and reversible tuning of quantum interactions, thereby shedding light on the building blocks of exotic magnetism and unconventional superconductivity. By developing the materials and methodology to achieve this, we intend to push our understanding of quantum systems beyond current limitations and open a route for exploiting the untapped potential of these materials to underpin future technology.

The project takes as its starting point recent theoretical and experimental discoveries in the area of quantum materials. By utilizing both atomic and molecular substitution, the project will focus on a series of materials that are on the verge of a phase instability. Ultra-high fields and applied pressure will push these systems through the critical region where the state of matter changes and inherently quantum effects dominate. Electronic, magnetic and structural properties will be measured as the tipping point is breached and the resulting data compared with predictions of theoretical models. The overall objective is to provide answers to questions of deep concern to modern physics, specifically how quantum fluctuations, topology and disorder give rise to states of matter with novel and functional properties.
"1.1 Spin-1/2 chains and other morphologies

An ongoing research effort is to create extreme one-dimensional (1D) and two dimensional (2D) spin-1/2 antiferromagnets (AFMs) to accentuate quantum fluctuations. In our paper ""Magnetic order and enhanced exchange in the quasi-one-dimensional molecule-based antiferromagnet Cu(NO3)2(pyrazine)3"" (Phys. Chem. Chem. Phys., 2019) we report a new highly 1D spin-1/2 AFM and discuss why the chains are more isolated than in similar materials.

The paper ""Unconventional Field-Induced Spin Gap in an S = 1/2 Chiral Staggered Chain"" (PRL, 2019) describes the remarkable properties of the quantum spin chain [Cu(pyrimidine)(H2O)4]SiF6.H2O. Not only does this appear to be one of the most 1D spin-1/2 chains yet found it also exhibits an unusual energy gap that evolves linearly in applied field and does not fit with the existing theories. The study also raises the possibility of combining different chiral symmetries with anisotropic interactions to create new ground states and exotic excitations.

1.2 Spin-1 chains and other morphologies

Phase diagrams of 1D and 2D spin-1 AFMs are yet to be fully explored experimentally. There are barriers: (i) many of the newest materials are only available in powdered form, leading to difficulties in determining magnetic parameters, and (ii) methods to chemically control the parameters of new materials remain incomplete.

We have addressed item (i) in two publications: ""Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet"" in (PRB, 2017) and ""Determining the anisotropy and exchange parameters of polycrystalline spin-1 magnets"" (NJP, 2019).

Item (ii) is ongoing, but the results in the papers above enabled us to make good headway. Our study ""Strong easy-plane anisotropy in bespoke Ni(II) quantum magnets"" has been submitted for publication, and “Towards control of the magnetic properties of Ni(II) chains using halide substitution” is nearly complete.

Progress in these two areas allow us to explore specific, novel S = 1 systems. ""Implications of bond disorder in a S=1 kagome lattice"" (Sci. Rep., 2018) reports interplay between order, disorder and geometric frustration, an area of considerable interest. In ""A Near-Ideal Molecule-Based Haldane Spin-Chain"" (arXiv:1909.08427 2019) we report a new material which promises to be an important testbed of issues of topology and low-dimensionality. Measurements of this system at high pressure and with induced disorder are underway.

1.3 Spin-1/2 dimers

""Adiabatic physics of a magnetic quantum fluid: magnetocaloric effect, zero-point fluctuations, and two-dimensional universal behavior"" (Physical Review B, 2017) describes the effect of ultra-high magnetic fields on quantum magnets and presents evidence that quantum fluctuations play a significant role. Pressure measurements are in progress.

2.1 Non-centrosymmetric superconductors

We have two papers in this area: ""Multigap superconductivity in chiral non-centrosymmetric TaRh2B2"" (Physical Review B, 2018) and ""Superconductivity and the upper critical field in the chiral non-centrosymmetric superconductor NbRh2B2"" report high critical fields and multi-gap superconductivity.

2.2 Strongly correlated electron systems

""Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs4Sb12"" (arXiv:1907.09181) reports ultra-high magnetic field work suggests a strong influence on the properties of this Kondo semimetal from quantum fluctuations and the proximity of a topological phase of matter. Pulsed-field x-ray, neutron and high pressure studies are ongoing.

We have also spent time studying frustrated pyrochlore iridates and obtained intriguing data that are being prepared for publication.
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These results are significant steps forward in our understanding of quantum materials. The creation of new, tuneable molecular magnets pushes back the boundaries of our understanding of interacting spin systems. In this context the project has produced and characterized a new spin-chain close to an experimentally unexplored quantum tricritical point, a new near-ideal Haldane chain and a molecular kagome system. Work is ongoing to push these materials further by tuning their structure or introducing random disorder.

In addition, this project has made, and will continue to make, significant headway in developing methods for characterising polycrystalline materials without use of a large user facility. The new approaches allow speedier feedback from synthesis to experiment, permitting identification of the most interesting materials on which to spend time perfecting single-crystal growth.

Further new results and improved understanding of the effects of quantum fluctuations, disorder and topology on the newest materials will be produced throughout the project.
Kagome structure of the molecule-based S = 1 system [H2F]2[Ni3F6(3-fluoropyridine)12][SbF6]2
Crystal structure of the chiral spin-1/2 chain [Cu(pyrazine)(H2O)4]SiF6.H2O
Zero-field magnetic structure of the spin-1 chain [Ni(pyridine)2(HF2)]SbF6
Phase diagram of molecular dimer system. The extended dome is measured in pulsed fields.