Junction characteristics and magnetic field dependencies of low noise step edge junction Rf-SQUIDs for unshielded applications

Abstract

Step edge grain boundary (GB) junctions and rf-SQUIDs have been made using pulsed laser deposited Y-Ba-Cu-O films on crystalline LaAlO3 substrates. The steps were developed using various ion-beam etching processes resulting in sharp and ramp type step structures. Sharp step based GB junctions showed behavior of serial junctions with resistively shunted junction (RSJ) -like I-V characteristics. The ramped type step structures resulted in relatively high critical current, Ic, junctions and noisy SQUIDs. The sharp steps resulted in low noise rf-SQUIDs with a noise level below 140 fT/Hz1/2 down to few Hz at 77 K while measured with conventional tank circuits. The Ic of the junctions and hence the operating temperature range of the SQUIDs made using sharp steps was controlled by both the step height and the junction widths. The junction properties of the SQUIDs were also characterized showing RSJ-like characteristics and magnetic field sensitivities correlated to that of the SQUIDs. Two major low and high background magnetic field sensitivities have been observed for our step edge junctions and the SQUIDs made on sharp steps. High quality step edge junctions with low magnetic field sensitivities made on clean sharp steps resulted in low I/f noise rf-SQUIDs proper for applications in unshielded environment

Analysis of electrical characteristics and magnetic field dependences of YBCO step edge and bicrystal grain boundary junctions for rf-SQUID applications

“…The I -V curves of our junctions exhibit a non-linear behaviour at V ≈ I c R N with deviation from the simple RSJ model for the larger W j at low temperatures [8,16]. As shown in figure 1, there is a strong non-linearity in the I -V curve of the 5 and 8 µm wide junctions as also reported for our SEJs [16]. This, which can enhance the noise of the devices, is associated with the Josephson flux motion effect occurring in junctions with widths larger than the Josephson penetration depth, λ J = (h/4πe J c µ 0 (2λ + d)) 0.5 [17,21] 7.8 for the 3, 5, and 8 µm wide junctions respectively, which are within the range of the reported values [15,21].…”

“…The R N of the 3, 5, and 8 µm wide BGBJs in figure 1 are about 14, 6, and 2.5 , respectively, giving sheet resistances (ρ N ) in the range of about 48-95 n cm2 and I c R N values in the range of 1.8-2.1 mV at 7 K. The obtained I c R N values are in the range of and slightly higher than the reported values for the BGBJs[11,[13][14][15].The measured R N of most of our BGBJs showed a slight temperature dependence, decreasing by about 5-10% as the temperature increased from 7 K to their T c . Relatively high ρ N values were obtained for our SEJs[16]. The R N of our I -V curve and the corresponding dV /dI versus I at ∼10 K of the 2 µm wide SEJ of an rf-SQUID magnetometer made on LaAlO 3 substrate with 255 nm deep steps.…”

“…The multiple modulation types in figure 6 are interpreted to be due to the field dependent interferences in the V s−pp of the inevitable serial junctions in the SEJ rf-SQUIDs [1][2][3]. The study of the high B a sensitivity of our SEJ rf-SQUIDs and its correlation with their junction parameters has been previously reported [6,16]. The V s−pp of the low B a -sensitive devices dropped by less than about 10% under B a ∼ 50 µT, as also observed from figure 7.…”

“…The I -V and dV /dI curves of a 2 µm wide junction on a 270 nm deep ditch are shown in figure 3. While the R N values of the RSJ-like behaviour SEJs mostly decreased with increase of temperature further than that of the BGBJs, the typical higher ρ N of the SEJs is interpreted to be a major advantage of the SEJ technology in obtaining lower white noise devices [16]. [15,18].…”

Optimization of step-edge substrates for high-TC superconducting devices

“…The results show the high uniformity and quality of the fabricated step-edge Josephson junctions. The step-edge SQUID exhibit excellently in good result in flux-to-voltage transfer function [16][17][18][19][20][21][22].…”