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Abstract:
Scanning Hall probe microscopy has been used to study vortex structures
in very-high-quality single crystals of the unconventional
superconductor Sr2RuO4 (T-c congruent to 1.5 K). In none of our samples
do we find credible evidence for the existence of the spontaneous fields
or chiral domains predicted for the expected time-reversal
symmetry-breaking order parameter. Even in our highest-quality samples
we observe very strong vortex pinning and anomalous broadening of vortex
profiles. The best samples also exhibit a clear field-driven triangular
to square vortex lattice transition at low fields, as predicted by
extended London theory calculations. In stark contrast, slightly less
well-ordered samples exhibit pronounced vortex chaining/banding that we
tentatively attribute to an extrinsic source of disorder.
