Advanced HTS DC SQUIDs with Step-Edge Josephson Junctions for Geophysical Applications

For ground-based electromagnetic methods in geophysics dc superconducting quantum interference devices based on high-temperature superconductors are well suited as magnetic field sensors. Therefore, we introduce in this paper an advanced fabrication technology for high-temperature superconducting dc SQUIDs based on step-edge Josephson junctions. The dc SQUIDs are prepared on structured magnesium-oxide substrates. A trilayer of YBa
₂
Cu
₃
O
_7-x
/SrTiO
₃
/YBa
₂
Cu
₃
O
_7-x
is deposited by pulsed laser deposition and structured with ion beam etching. The galvanometer-type design consists of four different dc SQUIDs directly coupled to a pickup loop. They exhibit large I
_c
R
_n
products and a voltage swing of more than 30 μV. The magnetic field noise amounts to about 20 fT/Hz
^1/2
and 35 fT/Hz
^1/2
for the white noise region as well as 25 fT/Hz
^1/2
and 100 fT/Hz
^1/2
at 100 Hz for ac-bias and dc-bias SQUID electronics, respectively. The advanced fabrication process thus enables to produce HTS dc SQUIDs intended for the use in geophysical applications with high throughput.