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ExtremeQuantum Report Summary

Project ID: 681260
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - ExtremeQuantum (Quantum materials under extreme conditions)

Reporting period: 2016-09-01 to 2018-02-28

Summary of the context and overall objectives of the project

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 in areas as diverse as the generation, transmission and storage of electricity; fast and secure communications; quantum information processing; medical imaging and treatment; and advanced sensors and actuators. 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 systems to understand. Nevertheless, finding out how the states arise is the first, but essential step towards fully harnessing their capacity for application.

It has long been appreciated that two key factors governing the systems that display such states of matter are the crystalline 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 of the system, 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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

1) Spin-half dimer systems

Towards the beginning of the project we published a paper titled "Adiabatic physics of a magnetic quantum fluid: magnetocaloric effect, zero-point fluctuations, and two-dimensional universal behavior" in which we hoped to understand the effect of ultra-high magnetic fields on quantum magnets. Here I give an overview of this work:

A key goal of physicists, materials scientists and chemists is to develop materials to test the theoretical predictions of quantum mechanics in real systems. High impact research in this field includes a focus on magnetic quantum fluids that exhibit the Bose Einstein condensation (BEC) of magnons. The wide interest in this topic can be seen from the number of publications in PRL and Nature journals to which we refer in our paper. Often the experimental investigations of these systems are reliant on the results of measurements in pulsed magnetic fields. One of the results of our work is that, without careful consideration of the sample temperature in these measurements, a magnetocaloric effect can arise undetected and a naïve interpretation of the resulting data can lead to erroneous physics. This conclusion has possible implications for several existing and ongoing studies in this area, where we suggest similar distortions of the phase diagram occur.

The discovery of a strong magnetocaloric effect in our BEC system is a significant extension of the PRL that first characterised this material (Lancaster et al., PRL 2014). Our results derive from the application of several state-of-the-art experimental techniques, including measurements of the susceptibility (at radio frequencies) and the magnetocaloric effect in pulsed magnets. Given these experiments revealed new physics in our material, we believe our work will encourage others to consider the consequences of studying quantum materials with ultra-high magnetic fields.
Our experimental results are explained with a simple and general theoretical model. Thus our work is relevant to the wider investigation of spin-gapped and correlated materials. Furthermore, we present evidence to show that quantum fluctuations play a role in determining the magnetic properties of our low-dimensional system and that the critical exponents on either side of the BEC phase may be different, the possibility of which is an ongoing debate in this field.

We believe this material is an attractive low-dimensional system in which to pursue the experimental study of quantum fluids. This point is further exemplified in that we are able to access the complete phase diagram of our organic system using readily achievable fields and temperatures, whereas investigations of related inorganic materials are often limited to the study of smaller portions of the phase diagram.

2) Spin-one chains

In Spring 2017 we published a paper entitled "Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet". Here I summarize the main points:

Many of the newest and most exciting materials are frequently only available in powdered form. This can lead to difficulties in determining the energy scales and anisotropy of competing interactions, particularly when these energies are similar in magnitude. For this paper, we synthesize a quasi-1D molecular antiferromagnet, namely [Ni(HF2)(pyrazine)2]SbF6, composed of Ni(II) moments and H···F hydrogen bridges. Further connectivity through pyrazine ligands afford a robust 3D structural network. Such S = 1 chains are of interest especially if the single-ion anisotropy D and magnetic exchange energy J can be controlled and tuned. We show that a combination of high magnetic fields and neutron scattering could reveal and untangle these parameters, an unprecedented feat in a powder considering D/J is approximately 1 for this material. While D/J is close to the experimentally elusive quantum tricritical point where Haldane, XY and quantum paramagnet.

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)

The new materials, published results and progressing sub-projects described above represent significant steps forward in our understanding of strongly correlated electron systems and molecule-based materials. The creation of new, tuneable molecular materials that can be used to explore uncharted regions of the phase diagrams of quantum magnets pushes back the boundaries of our understanding of interacting spin systems. In this context this project has produced and characterized a new spin-chain close to a hitherto experimentally unexplored quantum tricritical point, a new near-ideal (isotropic) Haldane chain and a molecular kagome system. Work is ongoing to push these materials further either by tuning their structure or introducing random disorder.

In addition, this project has made, and will continue to make, significant headway in developing methodologies for performing a quantitative analysis of polycrystalline S = 1 magnets and extracting information without recourse to a large user facility. These new approaches will allow a much speedier feedback time from synthesis to experiment and back again, permit chemists to identify the most interesting materials on which to spend time perfecting the single-crystal growth techniques.

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.

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