Investigation of correlated electron systems under uni-axial strain
Abstract
A central paradigm for classifying the phases of correlated electron systems is their
symmetry. Having the ability to controllably tune symmetry-related properties of
the system is therefore a powerful probe.
In this thesis experiments on quasi-two-dimensional metals Sr₃Ru₂O₇ and Sr₂RuO₄
are reported, where uni-axial strain was used as a means of lifting the native tetragonal
symmetry. Uni-axial strain was applied to the samples using a piezo-electric
based device which can apply both positive and negative strains to the sample, to
study the symmetry of the response about zero strain.
Sr₃Ru₂O₇ exhibits a magnetic-field-tuned quantum critical point, in the vicinity
of which a novel phase is stabilized. The transport properties of the phase were
previously shown to be highly susceptible to in-plane magnetic fields. We show
that resistivity inside the phase responds strongly to strain applied along one of the
in-plane crystal axes, with the responses parallel and perpendicular to that of the
applied strain mirroring each other about zero strain. Our results suggest that the
underlying symmetry of the phase is C₄ rather than C₂ symmetric.
Sr₂RuO₄ is an unconventional superconductor which was predicted to have an
order parameter of the form pₓ ± ip[sub]y. This should result in a splitting of the transitions
of the two components as a function of strain, with a cusp in T[sub]c versus strain
at zero strain, where T[sub]c is the upper of the two transitions. We find that the response
of T[sub]c to strain along [100] is large and symmetric about zero strain, whilst
the response to [110] strain is weak and mostly anti-symmetric. No cusp is observed
for either strain direction. We argue that although our results are in contradiction
with the simplest pₓ ± ip[sub]y models, they may still be consistent with certain scenarios
where the cusp would have been too small to be observed.
Type
Thesis, PhD Doctor of Philosophy
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