Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in ${\mathrm{FeSe}}_{1-x}$ thin films with different preferred growth orientations. The films with preferred growth along the $c$ axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

In the archetypal lithium-rich cathode compound Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, a major part of the capacity is contributed from the anionic (O^2−/−) reversible redox couple and is accompanied by the transition metal ions migration with a detrimental voltage fade. A better understanding of these mutual interactions demands for a new model that helps to unfold the occurrences of voltage fade in lithium-rich system. Here we present an alternative approach, a cationic reaction dominated lithium-rich material Li_1.083Ni_0.333Co_0.083Mn_0.5O₂, with reduced lithium content to modify the initial band structure, hence ~80% and ~20% of capacity are contributed by cationic and anionic redox couples, individually. A 400 cycle test with 85% capacity retention depicts the capacity loss mainly arises from the metal ions dissolution. The voltage fade usually from Mn⁴⁺/Mn³⁺ and/or Oⁿ⁻/O²⁻ reduction at around 2.5/3.0 V seen in the typical lithium-rich materials is completely eliminated in the cationic dominated cathode material.

We propose a waveguide directional coupler design with weak coupling using silicon-based suspended coupling microstrip probes and the split-block type of construction common in integrated millimeter wave components. WR4.0 coupler prototypes with standard UG-387 flanges were fabricated and measured. The simulation and measurement results show that this design achieves a coupling of around -18 dB, with a typical ± 2 dB variation across the band from 210 GHz to 270 GHz, while the isolation is better than 30 dB. Tolerance studies confirm that this design could be scaled up to terahertz frequency with modern CNC machining and silicon chip fabrication technology.

Superconducting δ -NbN ultrathin film has become a key element in extremely sensitive detector applications in recent decades because of its excellent electronic properties. We have realized the epitaxial growth of ultrathin δ -NbN films on (100)-oriented 3C-SiC/Si substrates by dc reactive magnetron sputtering at 760 °C with a deposition rate of 0.054 nm s ⁻¹ . High-resolution transmission electron microscope images confirm the excellent epitaxy of these films. Even with a thickness of 1.3 nm (∼3 unit cells), the δ -NbN film shows a superconducting transition above 8 K. Furthermore, our ultrathin δ -NbN films demonstrate a long Ginzburg–Landau superconducting coherent length ( ξ G L ( 0 ) > 5 nm ) with a critical current density of about 2.2 MA cm ⁻² , and good stability in an ambient environment.