Grooved Dayem Nanobridges as Building Blocks of High-Performance YBa₂Cu₃O_7−δ SQUID Magnetometers

Abstract

We present noise measurements performed on a YBa 2 Cu 3 O 7−δ nanoscale weak-linkbased magnetometer consisting of a Superconducting QUantum Interference Device (SQUID) galvanically coupled to a 3.5 × 3.5 mm 2 pick-up loop, reaching white flux noise levels and magnetic noise levels as low as 6 µΦ 0 / √ Hz and 100 fT/ √ Hz at T = 77 K, respectively. The low noise is achieved by introducing Grooved Dayem Bridges (GDBs), a new concept of weak-link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

“…The 1/ f knee is below 3 Hz and the white magnetic field noise is around 63 fT/ √ Hz. This result is almost a factor 2 lower than that which was previously obtained with GDB weak links 18 In conclusion, we have studied the temperature dependence of GDB based SQUIDs. At low temperatures, the GDBs behave like short and thin Dayem bridges, whereas at temperatures above…”

“…By coupling a GDB-based SQUID galvanically to a 9 mm × 8.7 mm pick-up loop on a 10 mm × 10 mm substrate we achieve a magnetic field sensitivity as low as 63 fT/ √ Hz at 77 K. that is significantly larger as compared to our previous work. 18 The deposition conditions have been tuned so that the YBCO film is close to optimally doped (critical temperature T c 89.0 K) and c-axis oriented. 19,20 An amorphous carbon film, with thickness t C = 100 nm, is deposited by PLD on top of the YBCO film, which is subsequently patterned by electron beam lithography (EBL) and oxygen plasma reactive ion etching.…”

“…The etching rate during Ar + ion milling of the material inside the nano-gap is decreased compared to the rest of the sample, when t C /L G > ∼ 2. 18,24 In this regime, the increased re-deposition rate inside the nano-gap leads to a reduced effective etching rate. The final result of the Ar + ion milling is a nano-bridge with width W = 200 nm and length L = 150 nm and with a groove etched in the center, which acts as a weak link (see Fig.1(b)).…”

SQUID magnetometer based on Grooved Dayem nanobridges and a flux transformer

“…This fabrication technique has been used to obtain YBCO nanostructures with pristine quality and lateral dimension down to 65 nm [7] [8]. To obtain a GDB, a narrow gap is opened in the nanowire mask, as described in detail in [6]. When the gap in the mask is in the order of tmask/2 or smaller, the etching rate of the material inside the gap is reduced compared to other areas of the sample.…”

“…The final devices show lower critical current Ic and higher differential resistance δV/ δI at T=77 K compared to standard nanowires. A more detailed description of the etching dynamic of GDB is reported in [6]. The result presented here have been obtained on a GDB-based SQUID implementing 200 nm wide GDBs with a gap length of 50 nm.…”