The recently emerging superconductivity in infinite-layer nickelates with the isostructural and isoelectronic characteristics of cuprates provides a new platform to explore the pairing mechanism of high-temperature superconductors. In this work, we studied the upper critical field $\left(H_{\mathrm{c}2}\right)$ of a high-quality ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ thin film with superconducting transition temperature, $T_{\mathrm{c}}^{\mathrm{onset}}=18.8$ K, using high magnetic field up to 56 T. A very large $H_{\mathrm{c}2}, \sim 40$ T for $H\parallel c$ and $\sim 52$ T for $H\perp c$, was confirmed, which suggests that infinite-layer nickelates also have great application potential. The anisotropy of $H_{\mathrm{c}2}$ monotonically decreases from $\sim 10(\gamma =H_{\mathrm{c}2}^{ab}/H_{\mathrm{c}2}^c)$ near $T_{\mathrm{c}}$ to $\sim 1.5$ at 2 K. Angle dependence of $H_{\mathrm{c}2}$ confirms the crossover of superconductivity from two-dimensional to three-dimensional as the temperature decreases. We discussed that the interstitial orbital effect causes the weakening of anisotropy. The observed abnormal upturning of $H_{\mathrm{c}2}$ at low temperatures is found to be a universal behavior independent of film quality and rare-earth elements. Therefore, it should not be the Fulde-Ferrell-Larkin-Ovchinnikov state due to the fact that it is in the dirty limit and insensitive to disorder.

Recent investigations of FeSe-based superconductors have revealed the presence of two superconducting domes and suggest possible distinct pairing mechanisms. Two superconducting domes are commonly found in unconventional superconductors and exhibit unique normal states and electronic structures. In this study, we conducted electromagnetic transport measurements to establish a complete phase diagram, successfully observing the two superconducting domes in FeSe _{1− x} S _x (0 ≤ x ≤ 0.25) and FeSe _{1− x} Te _x (0 ≤ x ≤ 1) superconductors. The normal state resistivity on SC1 shows the strange metal state, with a power exponent approximately equal to 1 ( ρ ( T ) ∝ T ⁿ with n ~ 1), whereas the exponent on SC2 is less than 1. A bulk Dirac state observed on SC1, completely synchronized with the strange metal behavior, indicating a close relationship between them. While a topological surface Dirac state is witnessed on SC2 and undergoes a sign change near the pure nematic quantum critical point. The evolution of the Dirac states indicates that the appearance of the two superconducting domes may originate from the Fermi surface reconstruction. Our findings highlight distinct Dirac states and normal state resistivity across the two superconducting domes, providing convincing evidence for the existence of the two different pairing mechanisms in FeSe-based superconductors.

Since the discovery of high-temperature superconductivity in cuprates, understanding the unconventional pairing mechanism has remained one of the most significant challenges. The upper critical field ( H c 2 ) is an essential parameter for obtaining information on the pair-breaking mechanism, coherence length ξ, and pairing symmetry, all of which are crucial for understanding unconventional superconducting mechanisms. Here, we provide a brief review of studies on H c 2 in several representative series of cuprate, iron-based, and nickelate superconductors. By comparing the behavior of H c 2 as a function of temperature, doping concentration, and anisotropy across these three major classes of superconductors, we hope to contribute to a better understanding of the complex pairing interactions in high-temperature superconductors.

The iron-based superconductor FeSe has garnered considerable attention, in no small part due to its rich physics as well as the unique coexistence of superconductivity and nematicity. The recent discovery of the superconducting diode effect (SDE)—a non-reciprocal critical current with respect to the bias direction—requires simultaneous breaking of time-reversal symmetry (TRS) and inversion symmetry (IS), making it a powerful transport signature of broken symmetries in superconductors. Notably, most reported SDEs rely on the application of an external magnetic field to break TRS, which significantly limits their practical applications in integrated superconducting electronics. Here, we report a field-free SDE in $45°$-twisted FeSe Josephson junctions below $3 \mathrm{K}$, evidenced directly by the even symmetric dependence of the asymmetric critical current on the magnetic field. Under temperature modulation, the SDE is progressively suppressed and ultimately exhibits a polarity reversal at $2.2 \mathrm{K}$. Our findings provide compelling transport evidence for the field-free SDE in iron-based superconductor FeSe, offering a promising platform for exploring symmetry-breaking physics and developing low-dissipation superconducting electronic devices.

The report of the first room-temperature, ambient-pressure superconductivity in copper-doped lead apatite Pb_10−xCu_x(PO₄)₆O has attracted lots of attention. However, subsequent studies revealed the presence of numerous impurity phases in the polycrystalline sample, and the sharp superconducting-like transition is not due to a superconducting transition but most likely due to a reduction in resistivity caused by the first-order structural phase transition of Cu₂S at around 385 K from the β phase at high temperature to the γ phase at low temperature. Before now, only bulk measurements have been performed on a Pb_10−xCu_x(PO₄)₆O powder sample, which could be affected by the impurity phases, masking the intrinsic properties of Pb_10−xCu_x(PO₄)₆O. In this study, ³¹P and ^63/65Cu nuclear magnetic resonance (NMR) measurements have been performed on a Pb_10−xCu_x(PO₄)₆O powder sample to investigate its physical properties from a microscopic point of view. Our NMR data evidence the non-magnetic insulating nature of Pb_10−xCu_x(PO₄)₆O without any trace of electron correlation effects. Furthermore, the ^63/65Cu NMR results suggest that no copper or very little copper is substituted for Pb in Pb₁₀(PO₄)₆O prepared by sintering Pb₂SO₅ and Cu₃P.