  In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux, the
diamagnetic Meissner effect, from the interior of superconducting lead. This
discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS) theory
of superconductivity. In exotic superconducting systems BCS theory does not
strictly apply. A classical example is a superconductor-magnet hybrid system
where magnetic ordering breaks time-reversal symmetry of the superconducting
condensate and results in the stabilisation of an odd-frequency superconducting
state. It has been predicted that under appropriate conditions, odd-frequency
superconductivity should manifest in the Meissner state as fluctuations in the
sign of the magnetic susceptibility meaning that the superconductivity can
either repel (diamagnetic) or attract (paramagnetic) external magnetic flux.
Here we report local probe measurements of faint magnetic fields in a Au/Ho/Nb
trilayer system using low energy muons, where antiferromagnetic Ho (4.5 nm)
breaks time-reversal symmetry of the proximity induced pair correlations in Au.
From depth-resolved measurements below the superconducting transition of Nb we
observe a local enhancement of the magnetic field in Au that exceeds the
externally applied field, thus proving the existence of an intrinsic
paramagnetic Meissner effect arising from an odd-frequency superconducting
state.
