Superconductivity and Magnetism in Strontium Titanate Thin Films and Heterostructures
2020-06
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Superconductivity and Magnetism in Strontium Titanate Thin Films and Heterostructures
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2020-06
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In this thesis we present our investigation of magnetism and superconductivity in SrTiO3 thin films and heterostructures in the polar/non-polar interface of NdTiO3/SrTiO3, using milli-Kelvin transport measurements. In Chapter 2 we describe our experimental method, including our low temperature transport measurement setup. In Chapter 3 we present our investigation of the magnetic properties of NdTiO3/SrTiO3 using milli-Kelvin temperature magnetotransport measurements. We observe large negative magnetoresistance (in excess of -90%) and robust hysteresis. We interpret our data in the context of spin-dependent transport and argue for the presence of ferromagnetic ordering in this anti-ferromagnetic/ paramagnetic interface. In addition, using time dependent magnetoresistance measurement we observe butterfly-shaped transit hysteretic features near zero-magnetic field. Such butterfly-shaped hysteretic features are previously interpreted as evidence for interfacial magnetism. We argue that these transient hysteretic signals are consistent with magnetocaloric effects arising from magnetic materials extrinsic to the sample. In Chapter 4 we discuss our investigation of effects of paramagnetic pair breaking and spin-orbital coupling on multi-band superconductivity. Using low temperature critical field measurements, we observe robust evidence for multi-band superconductivity from the unconventional dependence of critical field on temperature. We observe a pronounced positive curvature of the critical field curve as function temperature. Furthermore, we observe that the out-of-plane critical field exceeds the Pauli limit, while typically the out-of-plane critical field saturates about an order of magnitude smaller than the Pauli limit. We interpret this as due to the enhancement of the critical field due to multi-band superconductivity, as has been theoretically predicted. Furthermore, we propose an original model for critical field for multi-band superconductors, which includes orbital pair breaking, paramagnetic pair breaking and spin-orbital coupling. We find excellent agreement of this model with our data and that this model has wide reaching applications to any multi-band superconductors in the dirty limit, in which all these effects are present. Chapter 5 focuses on the comparison of effects of magnetic impurity scattering and non-magnetic impurity scattering in the context of multi-band superconductors. We did extensive measurements of magnetically doped (Nd-doped STO) and non-magnetically doped (La-doped STO) samples. We measured samples with carrier densities spanning about an order magnitude, for both sets of samples. We find that for magnetically doped samples the superconducting dome (critical temperature as a function of carrier density) is shifted to lower densities. As a result, at lower densities we observe that the critical temperatures of magnetically doped samples are higher than those of non-magnetically doped samples, while at higher densities it is the opposite. Analysis of out-of-plane critical field measurements reveals the multi-band nature of superconductivity in both sets of samples. In–plane critical field data reveal the importance of spin-orbital coupling in Nd doped samples. We argue that the reason why, at lower densities, Nd doped samples have higher critical temperature despite being magnetically doped is the suppression of magnetic scattering by spin-orbital coupling. Furthermore, we studied several very high mobility samples at extremely low densities (<1018 cm-3) and we did not observe superconductivity down to less than 30mK, contrary to observations reported in literature on oxygen vacancy doped samples.
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University of Minnesota Ph.D. dissertation. June 2020. Major: Physics. Advisor: Vlad Pribiag. 1 computer file (PDF); ix, 113 pages.
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Ayino, Yilikal. (2020). Superconductivity and Magnetism in Strontium Titanate Thin Films and Heterostructures. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/224611.
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