Аннотация
We have directly imaged the anisotropic nonlinear Meissner effect in an
unconventional superconductor through the nonlinear electrodynamic response of
both (bulk) gap nodes and (surface) Andreev bound states. A superconducting
thin film is patterned into a compact self-resonant spiral structure, excited
near resonance in the radio-frequency range, and scanned with a focused laser
beam perturbation. At low temperatures, direction-dependent nonlinearities in
the reactive and resistive properties of the resonator create photoresponse
that maps out the directions of nodes, or of bound states associated with these
nodes, on the Fermi surface of the superconductor. The method is demonstrated
on the nodal superconductor YBa$_2$Cu$_3$O$_7-\delta$ and the results are
consistent with theoretical predictions for the bulk and surface contributions.
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