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Title: Pulsed laser deposition of superconducting Sr2RuO4 thin-films and the role of crystallographic defects
Author: Palomares Garcia, Carla
ISNI:       0000 0004 9353 860X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Sr2RuO4 (SRO214) is a prototypical unconventional superconductor. However, since the discovery of superconductivity a quarter of a century ago, the symmetry of the bulk and surface superconducting states in single-crystal SRO214 remains highly controversial. Solving this problem is impeded by the fact that superconducting SRO214 is extremely challenging to achieve in thin-films as structural defects and impurities sensitively annihilate the superconducting state. Despite a handful of successful results on the growth of superconducting SRO214 thin-films by molecular beam epitaxy (MBE), reproducible growth by pulsed laser deposition (PLD) remains extremely challenging. In this thesis, we developed a protocol for the reliable growth of superconducting SRO214 thin-films by pulsed laser deposition and identify the type of structural defects that are responsible for destroying superconductivity. We have systematically investigated the structure-electrical-properties relationship of epitaxial SRO214 thin-films grown by PLD on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT). Growth parameters including temperature, substrate and target composition have been varied to understand their influence on the microstructure and electrical properties of SRO214. We also investigated the presence of impurities and crystallographic defects using X-ray diffraction techniques in conjunction with electron microscopy to identify the underlying microstructural features that suppress superconductivity in SRO214 thin-films. We demonstrate that, careful control of the starting material is essential to achieve superconductivity. By replacing the conventional SRO214 polycrystalline target with a single crystal of Sr3Ru2O7 (SRO327), we succeed in growing reproducible, high quality, superconducting SRO214 thin-films. By varying the SRO214 thickness, we observe the absence of superconductivity in films thinner than 50 nm (grown using a pulse frequency of 2 Hz). The suppression of superconductivity in thinner films is found to correlate to in-plane misorientation and mosaic twist, caused by in-plane screw dislocations that form close to the SRO214/LSAT interface. In conclusion, this work provides the first reliable pulsed laser deposition protocol for superconducting SRO214 thin-films. The results are of immediate interest to the superconducting and magnetism communities and form the foundation for a whole new spectrum of proximity experiments involving unconventional superconducting symmetries.
Supervisor: Robinson, Jason Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Superconductivity ; Thin films ; Sr2RuO4 ; Pulsed Laser Deposition (PLD) ; Crystallographic defects ; X-ray diffraction