Deviation From Fraunhofer-Type Modulation of Josephson Current Through Niobium Tunnel Junctions by Applying Vertical Magnetic Field

Anomalous modulation characteristics of Josephson current I
_c
through a niobium tunnel junction have been measured by applying the external magnetic field in perpendicular direction. Josephson current I
_c
through a superconducting niobium/aluminum-oxide/niobium tunnel junction is modulated by applying the parallel magnetic field (H
_x
, H
_y
) and the vertical field H
_z
to the junction plane using three pairs of Helmholtz coils. Modulation characteristics of the I
_c
value upon the vertical field H
_z
have hysteresis. Before applying the vertical field H
_z
, the I
_c
- (H
_x
, H
_y
) dependences are the product of the two Fraunhofer diffraction patterns in the H
_x
and H
_y
directions parallel to each edge of the square-shaped junction and show a square-shaped main peak area. After applying the vertical field H
_z
more than 5000 A/m, during the removing process of this vertical field H
_z
, the I
_c
- (H
_x
, H
_y
) modulation pattern shows a triangle-shaped main peak area due to the remained trapping and shielding magnetic flux at niobium electrodes.