Two-dimensional superconductivity at the surfaces of KTaO3 gated with ionic liquid

Abstract: The recent observation of superconductivity at the interfaces between KTaO3 and EuO (or LaAlO3) offers a new example of emergent phenomena at oxide interfaces. This superconductivity exhibits an unusual strong dependence on the crystalline orientation of KTaO3 and its superconducting transition temperature Tc is nearly one order of magnitude higher than that of the seminal LaAlO3/SrTiO3 interface. To understand its mechanism, it is crucial to address if the formation of oxide interfaces is indispensable for the presence of superconductivity. Here, by exploiting ionic liquid (IL) gating, we obtain superconductivity at KTaO3 (111) and (110) surfaces with Tc up to 2.0 K and 1.0 K, respectively. This oxide-interface-free superconductivity gives a clear experimental evidence that the essential physics of KTaO3 interface superconductivity lies in the KTaO3 surfaces doped with electrons. Moreover, the ability to control superconductivity at surfaces with IL provides a simple way to study the intrinsic superconductivity in KTaO3.

Overview of Two-Dimensional Superconductivity at the Surfaces of KTaO$_3$ Gated with Ionic Liquid

This paper explores the emergence of superconductivity at the surfaces of potassium tantalate (KTaO$_3$, KTO) when gated with ionic liquids (ILs), focusing on its two-dimensional (2D) properties. Conducted by researchers from several prestigious Chinese institutions, this study advances our understanding of oxide-based superconductors, particularly in the absence of traditional oxide interfaces. The research highlights the significant role of crystalline orientation in dictating superconducting properties, contrasting previously studied LaAlO$_3$/SrTiO$_3$ (LAO/STO) interfaces.

Systematic Analysis of Orientation-Dependent Superconductivity

The study employs IL gating to explore superconductivity at KTO surfaces of three principal crystalline orientations: (111), (110), and (001). Enhanced superconducting transition temperatures $T_c$ of up to 2.0 K and 1.0 K were observed for (111) and (110) orientations, respectively, while (001) orientation showed no superconductivity down to 0.4 K. These marked differences underscore the orientation-dependent nature of KTO's superconductivity, akin to the effects observed in EuO (or LAO)/KTO interfaces. Furthermore, it leverages ionic liquids to modulate carrier densities on the surface, enabling a precise probing of intrinsic superconducting properties without forming oxide interfaces.

Methodology and Device Fabrication

The fabrication of devices followed rigorous procedures involving optical lithography and lift-off techniques. Key steps included confining KTO surfaces with amorphous aluminum oxide layers, constructing metallic gate electrodes, and depositing thin amorphous LaAlO$_3$ layers for enhanced electrical contact. A critical stage involved coating the sample with IL, namely N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), and applying a gate voltage to induce superconductivity.

Results: Robust and Tunable 2D Superconductivity

The experimental data reveal superconducting states below specific temperatures in IL-gated KTO surfaces for (111) and (110) orientations. Both Hall effect measurements and $V-I$ characteristics provide evidence for electron-driven superconductivity with clearly defined critical currents and zero-resistance states. A prominent feature is the Berezinskii-Kosterlitz-Thouless (BKT) transition, indicating 2D superconductivity, with transition temperatures aligned closely with mid-point $T_c$ values. Additionally, magnetoresistance studies indicate significant anisotropy, further validating the 2D nature concomitant with the superconductivity observed.

Implications and Future Directions

This study has significant implications both theoretically and practically. It corroborates the notion that intrinsic 2D superconductivity extends beyond oxide interfaces and into bare, electron-doped surfaces. This provides a simplified model for investigating 2D superconducting physics, promoting practical applications in future electronic devices where minimizing additional interfacing layers is paramount.

Future developments could explore the roles of other gating agents and different crystalline orientations to gain deeper insights into charge dynamics and mobility, potentially illuminating further aspects of 2D superconductivity mechanisms. Additionally, understanding gating effects and their interplay with structural and electronic properties could lead to the development of more sophisticated superconducting materials.

In conclusion, this comprehensive and systematic investigation into KTaO$_3$ opens new doors to exploring emergent 2D superconductivity in oxides, emphasizing the importance of surface orientation and IL gating. This work not only enriches the current understanding of oxide superconductors but also sets a foundational framework for transitioning theoretical exploration into viable quantum technologies.