We have investigated the low temperature isothermal current-voltage characteristics of FeSe epitaxial films with thickness of ∼ 360 nm at different magnetic fields parallel to the c axis. At zero magnetic field, with decreasing temperature a Berezinskii–Kosterlitz–Thouless transition, which often appears in two-dimensional (2D) superconductors, is observed. The current dependence of the voltage obeys the scaling relation V ∼ I 3 at the transition temperature T B K T ≈ 4.7 K, and the temperature dependence of sheet resistance follows the prediction of the Halperin-Nelson formula in the vicinity of T B K T . When a magnetic field with magnitude 0.1 T ≲ μ 0 H ≲ 6 T is applied, the low temperature isothermal current-voltage data in each field can be scaled onto two different branches by using the scaling relation predicted by the vortex-glass theory. When comparing with the scaling relation, the dimensionality D can only be taken as 2, indicating a vortex-liquid phase to quasi-2D vortex-glass phase transition occurs in the FeSe film. The presence of Berezinskii–Kosterlitz–Thouless transition and the quasi-2D vortex-glass phase are related to the weak coupling between Fe-Se layers in the FeSe film.
In this study, boron powder with different particle sizes was purified by both chemical and heat treatment methods. The reduction in the particle size can improve the chemical purification with no effort on the heat treatment. The superconducting properties of the powder drastically changed even with only a partial elimination of oxygen. On the one hand, less oxygen content resulted in high Tc and Jc values under the low magnetic field, and most importantly, a significant improvement in the superconducting connectivity (Af value). On the other hand, the degradation of Jc under a high field and a change in the pinning mechanism were also found, along with decreasing oxygen. This result indicated that oxygen, probably MgO, might act as the pinning center and as an obstacle for the supercurrent in MgB2 at the same time. This work paves the way for obtaining pure oxygen-free MgB2 and understanding the real effect of oxygen in MgB2.
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