Step edge YBa2Cu3O7−δ dc SQUIDs on sapphire substrates

Abstract

We have made YBa2Cu3O7−δ (YBCO) thin film dc superconducting quantum interference devices (SQUIDs) on sapphire substrates that operate above 77 K. The YBCO thin films are free of weak links and have critical current densities (Jc) of 106 A/cm2 at 77 K. Across the step edge, typical Jc ranges from 105 to 103 A/cm2 at 77 K and increases linearly with decreasing temperature. Typically, we obtain IcRn products near 30 μV at 77 K. We have operated the dc SQUIDs between 4.2 and 77 K and observed voltage modulation in rough agreement with the expected values. The flux noise is typically 1×10−10 Φ20/Hz, with a corresponding noise energy, SE, of 1.3×10−29 J/Hz at 77 K and 1 kHz.

New techniques for fabricating step-edge junctions for high-T/sub c/ SQUIDs on MgO substrates

“…The use of steps milled into substrates to engineer grain boundaries which behave as Josephson junctions in highquality epitaxial films of Y B a 2 C~3 0 7 -~ has been widely used [1,2] in the fabrication of DC and RF SQUIDs. This technique relies on the formation of grain boundaries with a tilt-angle greater than 14" [3].…”

A simple route for synthesis of a bi-epitaxial junction in thin films of YBa2Cu3O7 ? x

“…The typical values of critical currents at 4.2 K and 70 K are 36mA and 2.5 mA, which correspond to a critical current density of 2.4 x lo3 Acmp2 and 2.0 x lo2 A cme2, respectively. Table I shows that these values of the critical current density are higher than those obtained in S-N-S junctions of YBCO-Au-Nb thin films [3], lower than those obtained in step edge junctions [14,15] and comparable to those obtained in bi-epitaxial junctions on MgO substrate [6]. The critical currents also show a linear variation as a function of temperature, similar to the behaviour obtained in step edge junctions [14].…”

“…Table I shows that these values of the critical current density are higher than those obtained in S-N-S junctions of YBCO-Au-Nb thin films [3], lower than those obtained in step edge junctions [14,15] and comparable to those obtained in bi-epitaxial junctions on MgO substrate [6]. The critical currents also show a linear variation as a function of temperature, similar to the behaviour obtained in step edge junctions [14]. This has been attributed to the microbridge type S-N-S Josephson junction with short weak links operating in the junction device [16].…”

Superconducting properties of YBa2Cu3O7step-edge Josephson junctions prepared on CeO2-buffered sapphire substrates

“…Step-edge weak links in c-axis oriented YBa 2 Cu 3 O 7 (YBCO) films can be used for high-temperature Josephson junction devices operating at 77 K. High-T c Josephson junction technology and superconducting quantum interference devices (SQUID) based on step-edge junctions have been investigated by a large number of groups [1][2][3][4][5][6][7][8][9][10][11][12].…”

“…Stepedge junctions have been fabricated on various substrates of SrTiO 3 [1,7,8,12], LaAlO 3 [4,10,12], MgO [2,11], and sapphire [3,9]. The sapphire substrate is of considerable interest in view of its modest dielectric constant and the commercial availability of large diameter substrates at low cost, compared with the other oxide single-crystal substrates commonly used for the high-T c superconducting thin-film deposition.…”