Improved noise performance of ultrathin YBCO Dayem bridge nanoSQUIDs

Abstract

We have fabricated YBa2Cu3O (YBCO) nano superconducting quantum interference devices (nanoSQUIDs), realized in Dayem bridge configuration, on films with thickness down to 10 nm. The devices, which have not been protected by a Au capping layer during the nanopatterning, show modulations of the critical current as a function of the externally applied magnetic field from 300 mK up to the critical temperature of the nanobridges. The absence of the Au shunting layer and the enhancement of the sheet resistance in ultrathin films lead to very large voltage modulations and transfer functions, which make these nanoSQUIDs highly sensitive devices. Indeed, by using bare YBCO nanostructures, we have revealed an upper limit for the intrinsic white flux noise level .

NanoSQUIDs from YBa$_2$Cu$_3$O$_7$/SrTiO$_3$ superlattices with bicrystal grain boundary Josephson junctions

“…We further note that the flux noise spectrum of YBCO nanoSQUIDs often shows a superposition of a few Lorentzians rather than a pure 1/f spectrum, which can be explained by the relatively small number of dominating defects in JJs with small size 23,32 . In addition we find excess 1/f noise in our GBJJ YBCO nanoSQUIDs, which has been attributed to the fluctuations of spins of unknown origin, most likely due to defects at the substrate/YBCO interface or at the edges of the nanopatterned structures 32 .…”

“…Another open issue is the unclear origin of strong 1/f noise in YBCO nanoSQUIDs based on cJJs. For those devices, this excess noise has been also attributed to I 0 fluctuations, although, there is no barrier in such nanowires 23 .…”

“…Au) for resistive shunting and protection during nanopatterning 7 . Josephson junctions in such single layer devices are based on constrictions (cJJs) with widths down to ∼ 50 nm 22 , that yield a root-mean-square (rms) spectral density of flux noise S 1/2 Φ in the thermal white noise limit down to < 450 nΦ 0 /Hz 1/2 at T = 18 K (Φ 0 is the magnetic flux quantum) 23 . Another recently developed JJ type is based on creating Josephson barriers in YBCO thin films via focused ion beam (FIB) irradiation with a He ion beam 24 .…”

“…The dominant source of 1/f noise comes from critical current I 0 fluctations due to localized defects in the JJ barrier 34 , which can be several orders of magnitude larger than for conventional Nb tunnel junctions [35][36][37] . For YBCO nanoSQUIDs with ultralow thermal white noise, this excess noise can dominate the spectral density of flux noise up to high frequencies in the MHz range 23,32 . An established procedure to suppress this 1/f noise contribution from I 0 fluctuations in dc SQUIDs is the application of bias reversal schemes (including proper modulation of flux bias), which suppresses 1/f noise below the applied bias reversal frequency f br 38,39 .…”

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

“…The Ar + ion milling parameters have been tuned to minimize damage to the superconducting nanostructures. [21][22][23] A Grooved Dayem Bridge (GDB) is obtained by designing a gap in the carbon mask, along the whole width of a short nanowire (Dayem bridge), with a typical gap length L G = 50 nm, see Fig.1(a). 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.…”