PhD defense – Dongxin Zhang

On December 19 at 10 am (TRT Auditorium), Dongxin Zhang will defend his PhD thesis entitled “Innovative synthesis routes to superconducting infinite-layer nickelate thin films”, prepared under the supervision of Lucía Iglesias & Manuel Bibes.

Abstract:

In 1986, the discovery of high-temperature Tc superconductivity in cuprate marked a major breakthrough in condensed matter physics. Despite decades of intensive research, the unconventional nature of superconductivity in these materials remains unresolved. In 2019, superconductivity was discovered in hole-doped infinite-layer (IL) nickelates (R(1-x)BxNiO2; R = La, Pr or Nd, B = Sr, Ca or Eu, x = dopant concentration), compounds structurally analogous to cuprates. This discovery opened new avenues for understanding the mechanisms behind high-Tc superconductivity. The so-called IL structure consists of two-dimensional NiO2 planes separated by rare-earth layers. Achieving this specific phase requires selective removal of all apical oxygens from the perovskite precursor phase (R(1-x)BxNiO3) through a complex topotactic reduction process, traditionally performed using CaH2 as a reducing agent.

Since the initial discovery, progress in this field has been limited by significant synthesis challenges. Producing high-quality superconducting IL nickelate films remains technically demanding, with low success rate, and has been achieved by only a handful of laboratories worldwide. Additionally, the resulting films often lack surface crystallinity, hindering the use of surface-sensitive techniques such as scanning tunneling microscopy (STM) or angle-resolved photoelectron spectroscopy (ARPES). Two alternative reduction strategies have been proposed to overcome these difficulties:in situ deposition of a thin aluminum (Al) overlayer by molecular beam epitaxy, and in situ atomic hydrogen. While promising, these approaches are either technically challenging or provide limited spatial control over the topotactic reduction, restricting their widespread adoption and limiting the potential for device fabrication. This highlights the need for simpler reduction methods offering tunable spatial resolution.

In this thesis, we developed and investigated two alternative approaches to overcome these challenges. The first method employs ex situ (after air exposure) and in situ Al overlayer deposition via magnetron sputtering, enabling the fabrication of high-quality superconducting Pr0.8Sr0.2NiO2 films with a maximum Tc(onset) ~ 17.2 K, comparable to the state-of-the-art for this compound. We performed a systematic comparison between ex situ and in situ reduced samples, studying in depth their transport properties and superconducting properties, including the magnetic-field dependence, critical current and penetration depth. The development of this Al-assisted reduction method allowed us to begin exploring the physics underlying superconductivity in nickelates, by conducting preliminary ARPES measurements of the electronic structure, and initiating the fabrication and characterization of planar junctions.
In parallel, we explored a second method based on the electric field generated by a conductive atomic force microscopy (AFM) tip to induce localized topotactic reduction of the perovskite films. Preliminary experiments demonstrate the feasibility of this method, enabling precise spatial control over the formation of superconducting regions and opening new possibilities for studying device-relevant phenomena at the nanoscale.
Together, these two alternative approaches, which are significantly more accessible than existing methods, are aimed at enhancing reproducibility and providing spatial control over the reduction process, thereby lowering barriers for the broader exploration of superconducting nickelates.