Strain tuning and precision instrumentation
We perform technically challenging measurements on correlated electron materials. Our most important tool is controlled lattice distortion, using home-built piezoelectric-based uniaxial pressure cells, versions of which have been commercialised. Through careful sample preparation and mounting, we have been able to compress hard oxide materials including Sr2RuO4 and YBa2Cu3O6+x by over 1%. In the process, we induce strong changes in electronic properties. For example, at a uniaxial compression of ~0.6% one of the Fermi surfaces of Sr2RuO4 passes through a topological transition, and in connection with this change the superconducting transition temperature more than doubles.
Uniaxial pressure was until recently an underused experimental technique, due to the technical difficulties. The largest difficulty was to achieve both large and highly homogeneous uniaxial stress in samples; we have solved this by preparing samples as relatively narrow bars, mounting them carefully, and applying force along the long axis. The quantitative effects of laboratory-achievable uniaxial pressures often far exceeds those of achievable magnetic field: Depending on the material, uniaxial compression by 1% can be equivalent to an applied field of around a thousand tesla. By lifting lattice symmetries, for example by applying an orthorhombic lattice distortion to a tetragonal crystal, the effects of uniaxial pressure are also often qualitatively different from hydrostatic pressure.
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Piezoelectric-based strain tuning is a recently-developed technique, and we are expanding its use to other materials. We are also pursuing a number of technical developments, including:
- Heat capacity, thermal conductivity and elastocaloric measurements of samples under uniaxial pressure.
- stress-strain and feed-back controlled apparatus
- Reducing the sample size: using a focused ion beam to prepare small samples into precisely-defined shapes with smooth surfaces, we expect to achieve higher strains.
- Bespoke apparatus that allows much larger samples to be pressurised, in setups compatible with muon spin relaxation and neutron scattering experiments.Below is an SEM image of a sample that has been shaped with a plasma focused ion beam. The sample is placed across a ~100 µm-wide gap, and we apply stress by varying the width of this gap.
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