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 systems to understand. Nevertheless, finding out how the states arise is the first, but essential step towards fully harnessing their capacity for application.
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. The overall objective is to better understand how quantum interactions and fluctuations, topology and disorder can give rise to states of matter with novel and functional properties.
Below are details of some of the results that have emerged from this project.
Unconventional Field-Induced Spin Gap in an S = 1/2 Chiral Staggered Chain
We have measured the quantum spin chain [Cu(pyrimidine)(H2O)4]SiF6.H2O, which has a chiral structure and intriguing properties.
Typically the theory of low-dimensional magnetism precedes experimental testing, largely because of the difficulties in creating real materials described by theoretical models. However, in some cases experimental work uncovers physics not previously predicted. An example is the spin chain copper-benzoate where the local environment around the spins alternates along the chain. In this case, experiments revealed that a magnetic energy gap forms when a magnetic field is applied. This was entirely perplexing until Oshikawa and Affleck found that the behaviour could be described by the sine-Gordon model of quantum-field theory. Their theoretical work inspired a great deal more activity.
In this paper, we highlight a new material, which at first glance could be a sine-Gordon chain, but with an added twist: a four-fold chiral structure. Using heat capacity, magnetometry and electron-spin resonance measurements, we show that the size and field-dependence of the energy gap, as well as the complex excitation spectrum, do not fit with the existing theories. We offer a qualitative explanation for the observations, however considerable theoretical effort is called for to quantitatively account for the results. The study also raises the possibility of combining different chiral symmetries with anisotropic interactions to create new ground states and exotic excitations.
Magnetic order and enhanced exchange in the quasi-one-dimensional molecule-based antiferromagnet Cu(NO3)2(pyrazine)3
The quasi-one-dimensional molecule-based Heisenberg antiferromagnet Cu(NO3)2(pyz)3 has an intrachain coupling J=13.7(1) K and exhibits a state of long-range magnetic order below TN = 0.105(1) K. The ratio of interchain to intrachain coupling is estimated to be |J′/J| = 3.3 × 10−3, demonstrating a high degree of isolation for the Cu chains.
Implications of bond disorder in a S=1 kagome lattice
A kagome lattice consists of triangular arrangements of magnetic moments. If the moments interact antiferromagnetically then this arrangement can lead to frustration—an inability of all interactions to be simultaneously satisfied—and give rise to unusual magnetic properties. Creating real-world examples of frustrated kagome lattices is a particular goal of scientists investigating magnetic materials based on molecular building blocks.
Strong hydrogen bonds such as F···H···F offer new strategies to fabricate molecular architectures exhibiting novel structures and properties.
Along these lines and, to potentially realize hydrogen-bond mediated superexchange interactions in a frustrated material, we synthesized [H2F]2[Ni3F6(Fpy)12][SbF6]2 (Fpy = 3-fluoropyridine).
It was found that positionally-disordered H2F+ ions link neutral NiF2(Fpy)4 moieties into a kagome lattice with perfect 3-fold rotational symmetry.
Detailed magnetic investigations combined with density-functional theory revealed weak antiferromagnetic interactions (J ~ 0.4 K) and a large positive single-ion anisotropy of 8.3 K with ms = 0 lying below ms = ±1. The observed weak magnetic coupling is attributed to bond-disorder of the H2F+ ions which leads to disrupted Ni-F···H-F-H···F-Ni exchange pathways. Despite this result, we argue that networks such as this may be a way forward in designing tunable materials with varying degrees of frustration.
Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S = 1 quantum antiferromagnet
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 synthesized a quasi-1D molecular antiferromagnet, [Ni(HF2)(pyrazine)2]SbF6, composed of Ni(II) moments and H···F hydrogen bridges. Further connectivity through pyrazine ligands affords 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 paramagnetic phases coexist, the small interchain exchange energy is still sufficient to prompt long-range XY antiferromagnetic order near 12 K. This work can enable characterisation of polycrystalline samples of other low-dimensional magnets with competing energy scales.
Future work involves tuning of the parameters of this material through metal-ion doping and high pressure in an effort to tip the delicate balance of interactions, pushing towards the nearby phase boundaries and into Haldane physics territory.
Adiabatic physics of a magnetic quantum fluid: magnetocaloric effect, zero-point fluctuations, and two-dimensional universal behavior
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.
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.
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 hope the 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 and is relevant to the wider investigation of spin-gapped and correlated materials. Furthermore, we 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.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 681260).