Magnetic order and disorder in a quasi-two-dimensional quantum Heisenberg antiferromagnet with randomized exchange
Here we investigate the effect of randomizing exchange coupling in a S = 1/2 square-lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QHAF). Pulsed-field magnetization measurements allow us to estimate an effective in-plane exchange strength J, while the temperature TN at which long-range order occurs is found using muon-spin relaxation, allowing us to construct a phase diagram for the series. We find that disorder suppresses the effective size of the exchange, the long-range order and the ordered moment size, and we see an extended disordered phase in the region 0.4 ≲ x ≲ 0.8. We also find evidence for the formation of small clusters of spins whose fluctuations act to destabilize the magnetic order more swiftly than would be expected from the reduction in the exchange strength alone. This is a feature that has been predicted theoretically and has implications for other disordered quasi-two-dimensional QHAFs.
My research interest lies in the development and understanding of materials in which strong electronic correlations give rise to theoretically or technologically significant properties. These functional materials are not merely of concern to condensed matter physicists, they are also relevant for society as a whole - the useful materials of the future being those that are just beyond our understanding today.
The properties displayed by functional materials, which could be superconductivity, magnetism, or merely a marked response to external stimuli, are key ingredients for transformative new technologies. Shedding light on how the interesting properties emerge from the interplay between the quasiparticles, the magnetic moments and the vibrational excitations present in these systems thus remains one of the major challenges facing physics today.
Typically I use low temperatures and high magnetic fields, often combined with molecular chemistry and applied hydrostatic pressure, to understand how the structure of these materials relates to their electronic and magnetic properties.
High Magnetic Fields
The application of high magnetic fields is a powerful method for revealing the often complex behaviour found in functional materials. Not only do magnetic fields couple to conduction electrons and magnetic moments, the key ingredients in a strongly-correlated system, they are also directional, allowing the topology of electronic interactions to be investigated.
High magnetic fields can be used to access the normal state of high-temperature superconductors and map out the topology of their Fermi surface via either quantum oscillations (as in the example data shown on the left) or angle-dependent magnetoresistance oscillations. Fields can also change the energy level structure of a material and allow excited states to be explored, identify quantum phase transitions, align spins and lift the frustration in low-dimensional magnets, alter the character of quasiparticles in heavy fermion compounds, and reveal hidden order or induce new phases in magnetic or charge-ordered materials.
To obtain fields in excess of about 20 tesla, one needs to go to specialised facilities. I am a regular user of the Nicholas Kurti Magnetic Field Laboratory in the Clarendon Laboratory at the University of Oxford. Here it is possible to perform measurements of magnetization and resistivity on a variety of novel materials in fields up to 60 tesla and temperatures down to 400 mK. I also make frequent visits to the National High Magnetic Field Laboratory in the United States, and the High Field Magnet Laboratory in the Netherlands.
Here is a short article I wrote for the UK Magnetics Society on measuring electronic properties in high fields.
Low-dimensional and Molecular Magnets
Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics could make possible the design and growth of bespoke magnetic devices. Progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal.
Coordination polymers, like the one shown on the left, are self-organising materials consisting of arrays of metal ions linked via molecular ligands. Here, the choice of initial components dictates the form of the final product, enabling many different polymeric architectures to be obtained. These materials are thus a possible route to successful crystal engineering and, as well as magnetism, a number of functionalities are being actively studied around the world, including gas storage, optoelectronics, and ferroelectricity.
I use high- and low-field magnetometry to understand the balance of competing interactions in low-dimensional magnet systems constructed from organic and molecular building blocks. By working closely with sample growth experts, materials can be studied that are near-ideal realizations of model quantum systems, allowing the predictions of quantum theory to be tested in the laboratory.
Department of Physics
University of Warwick
Coventry CV4 7AL
A full list of my publications can be accessed here.
I currently teach the 2nd Year Undergraduate Core Module on statistical mechanics:
and a portion of the 4th Year module on CMP:
I am the PI on an ERC Consolidator Grant, that began in September 2016.
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).
I am extremely grateful to the ERC for this funding.