Yuhang Zhang

FeSe-based superconductors have become a hot topic with regard to high-temperature superconductor mechanisms and applications due to their broadly adjustable critical temperatures and the underlying rich physics. This has led to the emergence of numerous experimental approaches for regulating important critical parameters, particularly superconducting transition temperature, T_c. Owing to its powerful and effective control, electrochemical intercalation has become a widely adopted technique for tailoring the chemical and physical properties of layered materials in recent years. This short review concisely introduces FeSe-based superconductors and an electrochemical intercalation method and summarizes the research progress that has been made in utilizing this method to modulate the structure and superconductivity of FeSe-based materials.

We report a comprehensive investigation of the superconducting properties of the mineral superconductor Covellite (CuS) using high-quality single crystals. First, we establish that CuS is an intrinsic Type-II superconductor, correcting its long-standing classification as Type-I. Second, a complete set of anisotropic superconducting parameters was determined, including the critical fields, penetration depth and coherence length, which yield a Ginzburg-Landau parameter κ ~ 1.5 and a moderate anisotropy of γ ~ 2. Our results indicate that this Type-II superconductivity can be well-described by a conventional, weak-coupling, single-band s-wave pairing mechanism. This work fills a long-standing gap in the understanding of this archetypal superconductor.

The cascade of correlated topological quantum states in the newly discovered vanadium-based kagome superconductors, AV₃Sb₅ (A = K, Rb, and Cs), with a Z₂ topological band structure has sparked immense interest. Here, we report the discovery of superconductivity and electronic nematic order in high-quality single-crystals of a new titanium-based kagome metal, CsTi₃Bi₅, that preserves the translation symmetry, in stark contrast to the charge density wave superconductor AV₃Sb₅. Transport and magnetic susceptibility measurements show superconductivity with an onset superconducting transition temperature T_c of approximately 4.8 K. Using the scanning tunneling microscopy/spectroscopy and Josephson scanning tunneling spectroscopy, we demonstrate that the single crystals of CsTi₃Bi₅ exhibit two distinct superconducting gaps. Furthermore, the superconducting gaps break the six-fold crystal rotational symmetry down to two-fold. At low energies, we find that the quasiparticle interference patterns exhibit rotational-symmetry-breaking C₂ patterns, revealing a nematic ordered normal state with the same nematic direction as in the superconducting state. Our findings uncover a novel superconducting state in CsTi₃Bi₅ and provide new insights for the intrinsic electron liquid crystal phases in kagome superconductors.

Vortex pinning is a crucial factor that determines the critical current of practical superconductors and enables their diverse applications. However, the underlying mechanism of vortex pinning has long been elusive without a clear microscopic description^1,2. Here using high-resolution scanning tunneling microscopy, we studied single vortex pinning induced by point defect in layered FeSe-based superconductors. We found the defect-vortex interaction drives low-energy vortex bound states away from E _F, resulting a “mini” gap which effectively lowered the system energy and caused the pinning. By measuring the local density-of-states, we directly obtained the elementary pinning energy and estimated the pinning force through the spatial gradient of pinning energy. The results align with the bulk critical current measurement. We further show that a general microscopic quantum model with considering defect-vortex interaction can naturally capture our observation. It indicates the local pairing near pinned vortex core is actually enhanced, which is beyond the traditional understanding that non- superconducting regions pin vortices^1-3. Our study thus reveals a general microscopic mechanism of vortex pinning in superconductors, and provides guidance for enhancing the critical current of real superconductors.