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 YBa₂Cu₃O_7-x/SrTiO₃/YBa₂Cu₃O_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.

I. Introduction

Superconducting quantum interference devices (SQUIDs) have the potential to be used in various applications in geosciences [1]–[3]. Especially with the method called transient electromagnetics or TEM, SQUID-based instruments have created significant impact for mineral exploration of conductive subsurface ore bodies. In this method a primary magnetic field produced by a current through a large surface transmitter loop is abruptly switched off and a secondary magnetic field appears due to eddy currents in subsurface conductive geological structures [4]. The system of eddy currents, often called smoke rings, dissipates and diffuses into depth and to the sides with a diffusion velocity depending on the conductance. Thus, the temporal decay of the secondary magnetic field amplitude represents a depth-conductivity profile of the subsurface geology. In this context, magnetic field sensors such as fluxgates or SQUIDs have a significant advantage compared to induction coils (herein, abbreviated as coils): they can detect conductors in larger depth and underneath conductive overburden due to the longer decay times of the secondary magnetic field compared with the time derivative measured by the coil [3] . Moreover, SQUIDs show superior noise performance compared to other commercial devices for magnetically unshielded environment operation.