C-axis transport in YBCO nanostructures

Abstract

Despite nearly four decades since the discovery of high-temperature superconductivity (HTS) in cuprates, the underlying microscopic mechanisms remain one of the most profound open questions, in Condensed Matter Physics. The presence of multiple, intertwined electronic orders, driven by strong electron-electron correlations, adds further complexity to their behavior. This is reflected by a highly intricate phase diagram. Specifically we focus on YBa2Cu3O7−δ (YBCO), as our choice of HTS. While in-plane transport (ab-plane) has been extensively studied, little is known about the out-of-plane (c-axis) transport, especially in thin film systems. This is due to the challenges associated with growing the film in other, suitable orientations (most convenient and highest quality YBCO films are c-axis oriented) or fabricating suitable nanostructures to probe it. In addition, the out-of-plane conduction pathway is significantly more resistive than the in-plane one. Despite these challenges, thin film platforms offer the ability to investigate properties that are inaccessible in bulk, single crystals. Modifications of the YBCO ground state by an engineered superpotential can only manifest in thin film systems, where the substrate morphology has been patterned into a superstructure (for example, nanofacets). This superpotential has been shown to induce changes such as enhancement of superconductivity, unidirectional Charge-Density Wave (CDW) and even nematicity. Therefore, developing a platform to gain access to c-axis transport properties in thin films (where the effect of such superpotential modifications on c-axis can also be observed) offers a complementary perspective to the ab-plane studies. Together, they may hold the key to unlocking the mystery of HTS. Previous studies on c-axis transport in YBCO are mostly performed on bulk single crystals. They have revealed that charge transport along the c-axis differs fundamentally from that in the ab-plane. In underdoped samples, a crossover from metallic to semiconducting behavior is observed with decreasing temperature. This behavior could be related with the opening of the pseudogap. Moreover, the anisotropy between the in-plane and out-of-plane responses becomes more pronounced at lower dopings, suggesting a quasi-two-dimensional character. This thesis focuses on creating a reliable platform for probing the out-of-plane transport by mesa devices. These devices are made on thin film YBCO, which is grown by pulsed-laser-deposition (PLD). Furthermore, we describe two fabrication routes for the mesa devices using single-lithography (but with larger areas), or by utilizing a more intricate three-lithography method, which grants much smaller areas. Then we use them to probe the c-axis transport on 50nm thick, YBCO films.