Advances in Tl-based high-temperature superconducting <?A3B2 pi6?>thin films

Thallium-based (Tl-based) high-temperature superconducting (HTS) thin films have garnered significant attention due to their exceptional properties, including a high critical temperature (Tc), high critical current density (Jc), extremely low microwave surface resistance (Rs), and excellent stability. These attributes make them highly promising for applications in microwave devices and in quantum devices. However, their development has been consistently hindered by a major fabrication challenge: the high volatility of thallium oxides during high-temperature processing. This results in thallium loss, leading to poor reproducibility and degraded performance.This review details the progress made in overcoming these challenges, emphasizing the groundbreaking methodologies developed at Nankai University. The foundational advancement was the establishment of the “two-step method”. This process begins with the deposition of an amorphous Tl-Ba-Ca-Cu-O precursor film onto substrates such as LaAlO3. The crucial second step involves a high-temperature post-annealing treatment in conjunction with a Tl-rich “companion target”. This target serves as a vapor source, releasing Tl2O to replenish the thallium lost from the film, thereby enabling the crystallization of high-performance phases such as Tl-2212 and Tl-2223. Despite its achievements, the traditional “two-step method” had limitations, including process complexity and suboptimal reproducibility. To overcome these, the team introduced a revolutionary advancement: the “companion-target-free two-step method”. This innovative technique eliminates the external Tl source entirely. It maintains the deposition of a precursor film but revolutionizes the annealing by conducting it within a sealed sapphire crucible. It confines the Tl2O vapor volatilized from the precursor film, establishing a stable, high partial pressure atmosphere of Tl2O and O2. This enhances crystallinity and uniformity, and significantly improves reproducibility.Rigorous structural characterizations confirm the superior quality of films produced by this novel technique. X-ray diffraction (XRD) analyses of Tl-2212 films with varying thicknesses reveal only (001) peaks, indicating perfect c-axis orientation and epitaxial growth on LaAlO3. Rocking curve measurements for the (0012) peak yield full width at half maxima (FWHM) values consistently below 1.0°, demonstrating exceptional out-of-plane alignment. Phi-scans of the (1019) plane exhibit a distinct fourfold symmetry, confirming high-quality in-plane epitaxy. The method’s versatility is proven by its successful application in synthesizing pure-phase Tl-2223 films. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images of these Tl-2223 films confirm a defect-free periodic lattice, while selected-area electron diffraction (SAED) patterns verify the tetragonal crystal structure.The enhanced properties have directly enabled high-performance devices. In passive microwave components, superconducting bandpass filters fabricated from Tl-2212 and Tl-2223 films exhibit exceptionally low insertion losses. A 10-pole Tl-2212 filter showed losses of only 0.1 dB at 77 K and 0.58 dB at 95 K. A Tl-2223 filter maintained low losses of 0.27 dB at 102 K, proving stable operation in the 90–106 K range. In active devices, Tl-2212 bicrystal Josephson junctions exhibited clear multilevel Shapiro steps under microwave irradiation at temperatures up to 100 K, confirming strong Josephson coupling and opening avenues for high-temperature SQUIDs. Furthermore, a Tl-2212-based terahertz metamaterial demonstrated a low-loss resonance at 0.44 THz, highlighting its potential for tunable, low-loss terahertz components.In conclusion, significant progress in fabrication methodologies, particularly the “companion-target-free two-step method”, has enabled the reproducible synthesis of high-quality, epitaxial Tl-based HTS thin films. These achievements represent critical steps toward practical utilization. Future endeavors should focus on scaling up the process for large-area film production and on the miniaturization and integration into complex circuits for industrial adoption.