  Since the discovery of the unconventional copper-oxide
high-transition-temperature superconductors (HTS), researchers have explored
many methods to fabricate superconducting tunnel junctions from these materials
for both superconducting electronics operating at the practical temperature of
liquid nitrogen (77 K) and for fundamental measurements essential for testing
and guiding theories of these remarkable superconductors. The difficulty is
that the traditional estimate of the superconducting coherence length is very
short and anisotropic in these materials, typically ~2 nm in the a-b plane and
~0.2 nm along the c-axis. The coherence volume encloses very few
superconducting pairs, so even the presence of small scale inhomogeneities can
locally disrupt superconductivity unlike in conventional superconductors.
Therefore the electrical properties of Josephson junctions are sensitive to
chemical variations and structural defects on atomic length scales, thus to
make multiple uniform HTS junctions, control at the atomic level is required.
In this letter, we demonstrate very high-quality all-HTS Josephson
superconducting tunnel junctions (both Josephson and quasiparticle tunneling)
created by using a 500 pm diameter focused beam of helium ions to direct-write
tunnel barriers into YBa2Cu3O7(YBCO) thin films. With this method we
demonstrate the ability to control the barrier properties continuously from
conducting to insulating by varying the irradiation dose. This technique
provides a reliable and reproducible pathway for the scaling up of quantum
mechanical circuits operating at practical temperatures (~77 K) as well as an
avenue to conduct superconducting tunneling studies in HTS for basic science.
