  It is shown that in a structure consisting of a superconducting ring-shaped
electrode overlapped by a normal metal contact through a thin oxide barrier,
measurements of the tunnel current in magnetic field can probe persistent
currents in the ring. The effect manifests itself as periodic oscillations of
the tunnel current through the junction at a fixed bias voltage as function of
perpendicular magnetic field. The magnitude of oscillations depends on bias
point. It reaches maximum at energy eV which is close to the superconducting
gap and decreases with increase of temperature. The period of oscillations dF
in units of magnetic flux is equal neither to h/e nor to h/2e, but
significantly exceeds these values for larger loop circumferences. The
phenomenon is explained by formation of metastable states with large vorticity.
The pairing potential and the superconducting density of states are
periodically modulated by the persistent currents at sub-critical values
resulting in corresponding variations of the measured tunnel current.
