Magnetic susceptibility is a vital thermodynamic probe of materials, and measurements are typically performed using commercial apparatus that averages over the entire sample. However even the highest-quality samples frequently have a surprising level of inhomogeneity, and much higher-quality information can be gained from local measurements, i.e. measurements over a small portion of the sample across which its properties are highly uniform. For example, the colour map below shows the local Tc of a sample of the superconductor LaFePO, measured with a scanning SQUID susceptometer. Tc varies by ~0.3 K across the sample, so without local measurement it would be impossible to measure with precision properties close to Tc. One such measurement, for example, is the rate at which the superfluid density increases below Tc, which provides information on the pairing mechanism.
We have also developed methods to study materials under high uniaxial pressure, which can qualitatively alter their properties. For example, Tc of the superconductor Sr2RuO4 doubles under uniaxial pressure, while that of YBa2Cu3O6+x is suppressed. The size of the pressure cell however means that we cannot put the sample and cell into commercial measurement apparatus. Also, to reach the highest possible pressures it is better to use small samples. A scanning probe microscope becomes essential to have optimal placement of the sensor relative to the portions of the sample under high stress, and simply to find the high-stress regions while excluding the low-stress regions of the sample from measurement.
A SQUID is an exceptionally sensitive magnetic probe: it can be used to measure the magnetic penetration depth of superconducting samples to sub-nanometre precision, and also subtle features in the normal-state susceptibility of a material. In this project, you will develop and build a general-purpose long-range scanning probe microscope, and also a high-resolution SQUID susceptometer to be used with it.
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