As you YBCO is a layered materials with anisotropic critical field. Out of plane coherence length is extremely small so in-plane tunnel junction may be easy to prepared. But for layered materials it is hard to obtain a realistic tunnel barrier. People have already tried with natural barrier because of lower transparency.

I think, in case oxides best barrier can be achieved with epitaxial uniform growth, which is of course challenging :).

Dear Miaofang,

Muhammad correctly noticed why it is difficult to fabricate SIS YBCO Josephson

junctions (JJ). However, YBCO JJs are still fabricated and investigated; a common YBCO JJ is the so called "step edge" junction, for example. There are also other ways of fabricating YBCO JJs, e.g., by Focused Helium-Ion-Beam Irradiation of YBCO.

See for example a recent article written by experts in this field:

PHYSICAL REVIEW APPLIED 11, 044082 (2019)

Josephson Junctions and SQUIDs Created by Focused Helium-Ion-Beam

Irradiation of YBa2 Cu3 O7

B. Müller, M. Karrer, F. Limberger, M. Becker, B. Schröppel, C.J. Burkhardt, R. Kleiner, E. Goldobin, and D. Koelle.

YBCO JJs have also found their own applications, since they have the obvious advantage to operate in liquid nitrogen temperatures.

See for example the article

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8990942

concerning quantum voltage devices comprising YBCO JJs.

Also, YBCO grain boundary JJs have been fabricated and employed in several

applications, see e.g.:

https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8326529

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 28, NO. 7, 2018

HTS DC SQUIDs with Step-EdgeJosephson Junctions for Geophysical Applications,

Leonie Lina Kaczmarek, Rob IJsselsteijn, Viatcheslav Zakosarenko, Andreas Chwala, Hans-Georg Meyer,Matthias Meyer, and Ronny Stolz.

More on the applications of YBCO step-edge JJs can be found in a very recent

article in the Journal of Superconductivity and Novel Magnetism (volume 34, issue 6, 2021, Special Issue Dedicated to Brian Josephson on the Occasion of His 80th Birthday):

Journal of Superconductivity and Novel Magnetism (2021) 34:1611–1619

https://doi.org/10.1007/s10948-020-05698-6

"A Grain Boundary Josephson Junction that Supported Many Careers

and Led to Applications with Impact",

Catherine Patricia Foley & Simon K. H. Lam & Jia Du & E. E. Mitchell & Jeina Lazar & Wendy Purches & Shane Keenan & Marcel Bick & David Clark & Keith Leslie.

Best regards,

Nick Lazarides