Resilient Growth of a Highly Crystalline Topological Insulator-Superconductor Heterostructure Enabled by an Ex Situ Nitride Film.

Highly crystalline and easily feasible topological insulator-superconductor (TI-SC) heterostructures are crucial for the development of practical topological qubit devices. The optimal superconducting layer for TI-SC heterostructures should be highly resilient against external contamination and structurally compatible with TIs. In this study, we provide a solution to this challenge by showcasing the growth of a highly crystalline TI-SC heterostructure using refractory TiN (111) as the superconducting layer. This approach can eliminate the need for in situ cleavage or growth. More importantly, the TiN surface shows high resilience against contaminations during air exposure, as demonstrated by the successful recyclable growth of Bi2Se3. Our findings indicate that TI-SC heterostructures based on nitride films are compatible with device fabrication techniques, paving the way to the realization of practical topological qubit devices in the future.

Momentum-Resolved Electronic Structures and Strong Electronic Correlations in Graphene-like Nitride Superconductors.

Although transition-metal nitrides have been widely applied for several decades, experimental investigations of their high-resolution electronic band structures are rare due to the lack of high-quality single-crystalline samples. Here, we report on the first momentum-resolved electronic band structures of titanium nitride (TiN) films, which are remarkable nitride superconductors. The measurements of the crystal structures and electrical transport properties confirmed the high quality of these films. More importantly, from a combination of high-resolution angle-resolved photoelectron spectroscopy and first-principles calculations, the extracted Coulomb interaction strength of TiN films can be as large as 8.5 eV, whereas resonant photoemission spectroscopy yields a value of 6.26 eV. These large values of Coulomb interaction strength indicate that superconducting TiN is a strongly correlated system. Our results uncover the unexpected electronic correlations in transition-metal nitrides, potentially providing a perspective not only to understand their emergent quantum states but also to develop their applications in quantum devices.

Electronic structure of superconducting VN(111) films.

Vanadium nitride (VN) is a transition-metal nitride with remarkable properties that have prompted extensive experimental and theoretical investigations in recent years. However, there is a current paucity of experimental research investigating the temperature-dependent electronic structure of single-crystalline VN. In this study, high-quality VN(111) films were successfully synthesized on α -Al 2 O 3 (0001) substrates using magnetron sputtering. The crystal and electronic structures of the VN films were characterized by a combination of high-resolution X-ray diffraction, low-energy electron diffraction, resonant soft X-ray absorption spectroscopy, and ultraviolet photoelectron spectroscopy. The electrical transport measurements indicate that the superconducting critical temperature of the VN films is around 8.1 K. Intriguingly, the temperature-dependent photoelectron spectroscopy measurements demonstrate a weak temperature dependence in the electronic structure of the VN films, which is significant for understanding the ground state of VN compounds.

Flexible but Refractory Single-Crystalline Hyperbolic Metamaterials.

The fabrication of flexible single-crystalline plasmonic or photonic components in a scalable way is fundamentally important to flexible electronic and photonic devices with high speed, high energy efficiency, and high reliability. However, it remains a challenge. Here, we have successfully synthesized flexible single-crystalline optical hyperbolic metamaterials by directly depositing refractory nitride superlattices on flexible fluorophlogopite-mica substrates with magnetron sputtering. Interestingly, these flexible hyperbolic metamaterials show dual-band hyperbolic dispersion of dielectric constants with small dielectric losses and high figures of merit in the visible to near-infrared ranges. More importantly, the optical properties of these nitride-based flexible hyperbolic metamaterials show remarkable stability during 1000 °C heating or after being bent 1000 times. Therefore, the strategy developed in this work offers an easy and scalable route for fabricating flexible, high-performance, and refractory plasmonic or photonic components, which can significantly expand the applications of current electronic and photonic devices.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.