Guangdi Zhou

ABSTRACT Ruddlesden-Popper bilayer nickelate thin film superconductors discovered under ambient pressure enable great possibilities for investigating electronic structures of the superconducting state. Here, we report angle-resolved photoemission spectroscopy (ARPES) measurements of 1, 2, and 3 unit-cell epitaxial La2.85Pr0.15Ni2O7 films grown on SrLaAlO4 substrates, through pure-oxygen in situ sample transportation. Evidence obtained using photons with distinct probing depths shows that conduction is localized primarily at the first unit cell near the interface. Scanning transmission electron microscopy (STEM), together with energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), indicates that interfacial Sr diffusion and pronounced p-d hybridization gradient may collectively account for the interfacial confinement of conduction. Fermi surface maps reveal hole doping compared to non-superconducting ambient-pressure bulk crystals. Measurements of dispersive band structures suggest contributions from both Ni dx2-y2 and dz2 orbitals at the Fermi level. Density functional theory (DFT) + U calculations capture qualitative features of the ARPES results, consistent with a hole-doped scenario. These findings constrain theoretical models of the superconducting mechanism and suggest potential for enhancing superconductivity in nickelates under ambient pressure.

ABSTRACT The recent discovery of ambient-pressure superconductivity in thin-film bilayer nickelates opens new possibilities for investigating electronic structures in this new class of high-transition-temperature ($T_\mathrm{c}$) superconductors. Here, we construct a realistic multi-orbital Hubbard model for the thin-film system based on structural parameters integrating scanning transmission electron microscopy measurements and ab initio calculations. The interaction parameters are calculated with the constrained random phase approximation (cRPA). Density functional theory (DFT) plus cluster dynamical mean-field theory (CDMFT) calculations, with cRPA-calculated on-site Coulomb repulsive $U$ and experimentally measured electron filling $n$, quantitatively reproduce Fermi surfaces from angle-resolved photoemission spectroscopy experiments. The distinct Fermi surface topology from simple DFT+$U$ results features the indispensable role of correlation effects. Based upon the correlated electronic structures, a modified random-phase-approximation (RPA) approach yields a pronounced $s^{\pm }$-wave pairing instability, due to the strong spin fluctuations originating from the Fermi surface nesting between bands with predominantly $d_{z^{2}}$ characters. Our findings highlight the quantitative effectiveness of the DFT+cRPA+CDMFT approach that precisely determines correlated electronic structure parameters without fine-tuning. The revealed intermediate correlation effect may explain the same order-of-magnitude onset $T_\mathrm{c}$ observed both in pressured bulk and strained thin-film bilayer nickelates.

Abstract In this work, the uniformity is significantly improved of the photoresist film spinning-coated on the small-size diamond wafer by inlaying the diamond wafer into a 1-inch polytetrafluoroethylene substrate; consequently, the utilizable surface area of the diamond wafer is remarkably increased. As a result, the interdigital electrodes of 2.5 mm × 2.5 mm in dimension are prepared on the single crystal diamond (5 mm × 5 mm × 0.5 mm) and a metal–semiconductor–metal structured diamond deep-ultraviolet photodetector with a large active area of 3.093 mm 2 has been fabricated. Compared to the maximum values of the interdigital-typed intrinsic diamond deep-ultraviolet photodetectors, the active area is increased by more than six times, and the photocurrent reaches the order of milliampere, which is about two orders of magnitude larger. Meanwhile, the responsivity and external quantum efficiency are 56.3 A W −1 and 328, respectively, at 50 V bias under 3.125 μ W mm −2 213 nm illumination, and the corresponding mobility-lifetime product of the diamond wafer is 1.11 × 10 −5 cm 2 V −1 . As the voltage continued to increase, which still maintained an upward trend and did not appear saturated; the corresponding responsivity is up to 275.9 A W −1 at 120 V. In addition, the ultraviolet-visible light discrimination ratio is 1.4 × 10 4 at 10 V, and the carrier transit time between interdigital electrodes is measured to be only about 1 ns (excited by a 213 nm pulse laser), which shows that the photodetector has an ultrafast response speed.