Possible signatures of mixed-parity superconductivity in doped polar SrTiO3 films

Superconductors that possess both broken spatial inversion symmetry and spin-orbit interactions exhibit a mix of spin singlet and triplet pairing. Here, we report on measurements of the superconducting properties of electron-doped, strained SrTiO3 films. These films have an enhanced superconducting transition temperature and were previously shown to undergo a transition to a polar phase prior to becoming superconducting. We show that some films show signatures of an unusual superconducting state, such as an in-plane critical field that is higher than both the paramagnetic and orbital pair breaking limits. Moreover, nonreciprocal transport, which reflects the ratio of odd versus even pairing interactions, is observed. Together, these characteristics indicate that these films provide a tunable platform for investigations of unconventional superconductivity

Point group symmetry of cadmium arsenide thin films determined by convergent beam electron diffraction

Cadmium arsenide (Cd3As2) is one of the first materials to be discovered to belong to the class of three-dimensional topological semimetals. Reported room temperature crystal structures of Cd3As2 reported differ subtly in the way the Cd vacancies are arranged within its antifluorite-derived structure, which determines if an inversion center is present and if Cd3As2 is a Dirac or Weyl semimetal. Here, we apply convergent beam electron diffraction (CBED) to determine the point group of Cd3As2 thin films grown by molecular beam epitaxy. Using CBED patterns from multiple zone axes, high-angle annular dark-field images acquired in scanning transmission electron microscopy, and Bloch wave simulations, we show that Cd3As2 belongs to the tetragonal 4/mmm point group, which is centrosymmetric. The results show that CBED can distinguish very subtle differences in the crystal structure of a topological semimetal, a capability that will be useful for designing materials and thin film heterostructures with topological states that depend on the presence of certain crystal symmetries.Comment: Accepted for publication in Physical Review Material

Enhanced Critical Field of Superconductivity at an Oxide Interface

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remains a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching 25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing the condensate significant immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against magnetic field

Atomic Scale Understanding of Ferroelectricity and Superconductivity in SrTiO3

The central material of interest for this thesis is strontium titanate (SrTiO3). Doped SrTiO3 exhibits a wide range of remarkable properties. The focus of this work is to investigate its ferroelectric and superconducting behavior. We utilize scanning transmission electron microscopy (STEM) to understand the role of the nanoscale SrTiO3 structure in these properties. SrTiO3 is an incipient ferroelectric in unstrained, pure form, but easily becomes ferroelectric when subjected to small perturbations. Understanding the nature of the ferroelectric transition in SrTiO3 is essential as it can play an important role in other properties. A hallmark of order-disorder transitions is the formation of polar domains above the Curie temperature. These polar regions percolate below the Curie temperature to form a long-range ordered ferroelectric state. Using high-angle annular dark-field STEM and analyzing off-centering of Ti columns, we show local polar regions at room temperature in compressively strained SrTiO3 films, highlighting the order-disorder nature of the ferroelectric transition in this material. Next, we focus on understanding the competition between mobile carriers and polar crystal distortions. Elucidating the nature of this competition is of great interest for polar superconductors, which have attracted significant interest for their potential to host unconventional superconducting states. We observe a systematic suppression of the nanodomains and ferroelectricity with increasing amount of Sm dopant atoms. The itinerant electrons screen polar distortions and disrupt the nanodomains above the Curie temperature. The results provide direct evidence that long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. This disorder destroys the nanodomains at high dopant concentrations. The focus of the second part of this thesis concerns the superconductivity of SrTiO3. Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. Superconductivity in SrTiO3 occurs in the vicinity of a ferroelectric instability, and recent theories suggest a link between two orders. Here, by using atomic structure imaging, we find that the superconducting critical temperature correlates with the length scale of polar order. The superconducting transition temperature is enhanced when polar nanodomains are sufficiently large. In these cases, the Cooper pairs reside in a non-centrosymmetric environment with strong spin-orbit coupling. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of SrTiO3. We investigate the robustness of superconductivity to magnetic and nonmagnetic disorder and discuss the importance of findings in explaining the unconventional nature of superconductivity in this material