Session G41: Superconducting Electron Gas in KTaO3

Nature of the electron-phonon coupling in KTaO3

The discovery of superconductivity in KTaO3 heterostructures, with critical temperatures an order of magnitude higher than that of SrTiO3 heterostructures, has spurred significant interest in the last couple of years. The relevance of the polar modes for pairing, as well as the role of spin-orbit coupling are still under debate. Here, we present our findings on the Rashba-like coupling between conduction electrons and soft transverse polar modes. Combining ab initio computations and a microscopic model involving spin-orbit coupled t2g electrons, we find a significant electron-phonon coupling to the ferroelectric mode in KTaO3 and SrTiO3. We compare both systems and discuss the implications for the origin of superconductivity in bulk and heterostructures.   

Superfluid stiffness and superconducting gap of KTaO3-based 2D gas.

The achievement of high-quality epitaxial interfaces involving transition metal oxides offers a unique opportunity to design artificial materials that host novel electronic phases.  After fifteen years of dedicated research on SrTiO₃ -based interfaces [1], the recent discovery of a superconducting 2-DEG) in (111)-oriented KTaO₃-based heterostructures injected new momentum into the realm of oxide interfaces [2,3]. In this system, the superconducting T_c can exceed 2K, nearly an order of magnitude higher than that observed in SrTiO₃-based interfaces. Additionally, the increased mass of Ta compared to Ti leads to significantly enhanced spin-orbit effects, as recently demonstrated [4,5]. Consequently, KTaO₃-based 2-DEGs have the potential to enable the realization of topological superconducting phases—a concept originally proposed for SrTiO₃-based 2-DEGs but hitherto unattainable due to the limitations of the relevant energy scales. In this talk, I will present dc and microwave transport experiments on gate tunable superconducting 2-DEGs formed at the (111)-oriented AlOx/KTaO₃ interface. The temperature dependence of the superfluid stiffness, extracted from the microwave response of the 2-DEG, is found to be consistent with a node-less superconducting order parameter having a gap value larger than expected within a simple BCS weak-coupling limit model (Δ₀/k_BT_c= 2.3) [6]. Moreover, the superconducting transition follows the Berezinskii-Kosterlitz-Thouless scenario, a phenomenon not previously reported in SrTiO₃-based interfaces. In addition, I will also present recent measurements of the superconducting gap obtained via tunneling spectroscopy in Au/AlOx/(111)-KTaO₃  planar junctions and discuss its temperature dependence and evolution under a perpendicular magnetic field. Finally, I will provide perspectives on the realization of superconducting devices

References

[1] A. Caviglia et al., Nature 456, 624–627 (2008).[2] Liu, C. et al. Science 371, 716–721 (2021).

[3] Chen, Z. et al. Science 372, 721–724 (2021).

[4] Vicente-Arche, L. M. et al. Adv. Mater. 2102102 (2021).

[5] S. Varotto, et al. Nature Commun. 13, 6165 (2022).

[6] S. Mallik et al. Nature Commun. 13, 4625 (2022)   

Role of optical phonons for normal and superconducting state properties of KTO

Emergent superconductivity at the LaAlO$_3$/KTaO$_3$ interfaces exhibits a mysterious dependence on the KTaO$_3$ crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO$_3$/KTaO$_3$ interfacial superconductors and the non-superconducting counterpart. We find that the mobile electrons that contribute to the interfacial superconductivity show strong $k_perp$ dispersion. Comparing the superconducting and non-superconducting interfaces, the quasi-three-dimensional electron gas with over 5.5~nm spatial distribution ubiquitously exists, while the main difference lies on the signature of electron-phonon coupling, which shows intriguingly different strengths depending on the interfacial orientations. Remarkably, the stronger electron-phonon coupling correlates with the higher superconducting transition temperature. Our observations offer a straightforward explanation for the orientation-dependent superconductivity and suggest that interfacial orientations can affect electron-phonon coupling strength over several nanometers, which may have profound implications for the applications of oxide interfaces in general.   

Strong violation of the Pauli limit for the upper critical field of the KTaO3(111) interfaces

The nature of superconductivity and its interplay with strong spin–orbit coupling at the KTaO₃(111) interfaces remain a subject of debate. I will talk about our recent experiments on epitaxial growth and superconductivity of KTaO₃(111) heterostructures. Our results 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 the carrier density and orientation of the magnetic field. The strong violation of Pauli limit for the upper critical field of the KTaO₃(111) interfaces is due to spin–orbit coupling which motivates the formation of anomalous quasiparticles with vanishing magnetic moment. These results offer design opportunities for superconductors with extreme resilience against the applied magnetic fields.