Quantum size effects in ultra-thin YBa₂Cu₃O_7 - x films.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Technologies

Background:

• High-temperature superconductors like Yttrium Barium Copper Oxide (YBCO) possess d-wave symmetry, creating a zero energy gap in nodal directions, hindering their use in quantum applications.
• The inherent properties of bulk YBCO limit its potential for advanced quantum technologies requiring a fully energy-gapped state.

Purpose of the Study:

• To investigate the order parameter symmetry in ultra-thin YBCO films.
• To explore the impact of film thickness on the energy gap and superconducting properties.
• To determine the suitability of these films for quantum applications.

Main Methods:

• Fabrication of ultra-thin YBCO films and nanowires.
• Measurement of electrical transport properties, including critical current anisotropy.
• Andreev reflection spectroscopy on nanoconstrictions.

Main Results:

• Anisotropy of critical current in nodal and antinodal directions decreases with reduced film thickness.
• A thickness-dependent energy gap, absent in bulk YBCO, was observed.
• Quantum size effects in ultra-thin YBCO films were found to open an energy gap across the entire Fermi surface.

Conclusions:

• Ultra-thin YBCO films exhibit a fully gapped superconducting state due to quantum size effects.
• This fully gapped state makes ultra-thin YBCO films a promising platform for quantum applications.
• The findings pave the way for advancements in quantum computing and quantum communications.