Unconventional superconductivity continues to be one of the most striking chapters in condensed matter physics, by posing challenges to our theoretical understanding
of its origin. During the last three decades a large number of unconventional superconductors with exotic properties have been found arising great interest, such
as the heavy fermion systems, high Tc cuprates as well as the Iron based superconductors etc. Sr2RuO4, the material I have studied, can be considered as an
exemplary case in this regard. In spite of more than two decades of comprehensive research, Sr2RuO4 remains one of the most compelling superconductors till date.
Various experimental results give evidence that the superconductivity of Sr2RuO4 is chiral: including measurements of the Kerr effect, sound velocities, critical currents
across junctions, and muon spin relaxation(μSR), the experimental technique at the heart of this dissertation. Recent NMR Knight shift measurements suggests that
the pairing is most likely spin-singlet, and in the tetragonal lattice of Sr2RuO4, the combination of singlet pairing and chirality compels consideration of an seemingly
unlikely order parameter: dxz ± idyz. It is unlikely because it comes along with a horizontal line node at kz = 0, whereas Sr2RuO4 has a very low c-axis conductivity.
And that makes the question whether or not the superconductivity of Sr2RuO4 is chiral, of great importance. This calls for an unique scenario in regard to our
understanding of unconventional superconductivity, as the presence of chirality in Sr2RuO4 might imply a new form of pairing, which is yet to be firmly determined.
Chiral superconductors break time reversal symmetry by definition, and in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two component order parameters. Probing Sr2RuO4 under uniaxial pressure offers the possibility to lift the degeneracy between such components. However, despite strenuous efforts, a splitting of the superconducting and TRSB transitions under uniaxial pressure has not been observed so far. In this thesis, I report muon spin relaxation measurements on Sr2RuO4 samples, placed under uniaxial stress. The relatively large sample size suitable for μSR demanded for a customized uniaxial pressure cell in order to perform our experiments. It has been a technically challenging task to have a fully fledged uniaxial pressure cell with stringent requirements, that is suitable for time restricted facility experiments like μSR. The technical advancement has been documented thoroughly in this thesis. Using the dedicated uniaxial pressure cell, we observed the much awaited stress induced splitting between the onset temperatures of superconductivity and time reversal symmetry breaking, consistent with the qualitative expectations for a chiral order parameter in Sr2RuO4. In addition to that, we report the appearance of a bulk magnetic order in Sr2RuO4 under higher uniaxial stress for the first time, above the critical pressure at which a Lifshitz transition is known to occur. The signal in the state appearing at high stress qualitatively differs from that in the TRSB
state in unstressed Sr2RuO4, which provides evidence that the enhanced muon spin relaxation at lower stresses is not a consequence of conventional magnetism. As
a whole, our results strongly support the idea of two-component superconducting order parameter in Sr2RuO4, that breaks time-reversal symmetry.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:76124 |
| Date | 30 September 2021 |
| Creators | Ghosh, Shreenanda |
| Contributors | Klauss, Hans-Henning, Curro, Nicholas, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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