Modulation Dependence and Mechanism of Microwave Absorption in Granular YBa₂Cu₃O_x

Abstract

Modulation dependence of microwave absorption (MA) was measured on a ceramic, YBa2Cu3O x , at 77 K under a magnetic field to investigate the mechanism of the modulated MA signal in granular superconductors. The three factors of current transition region (CT), number of vortices (VN) and surface slope of the flux (SS) are proposed to interpret the signal. CT is based on the existence of the transition region of the shielding currents in the specimen where MA is smaller. VN is based on the absorption by vortices, and SS on the surface slope of flux distribution. The increasing behaviour of the signal with increasing modulation field (H m) depends fundamentally on VN. The signal behaviour is dominated by CT in the small H m regime and by SS in the large H m regime. CT- and SS-factors are derived from the shielding current mechanism.

Size-dependent properties of YBa2Cu3O6 xnanopowder

“…Recently the difference has been measured also in MgB 2 [24]. Since modulated non-resonant absorption in zero field is caused by shielding currents in the particles and the interparticle weak links [25,26], our result in figure 4 suggests that the absorption by the weak links is much stronger than absorption by the particles. Consistent with the magnetization results the strong rise in the signal below 60 K in the O (and L) sample can be assumed to be due to weak links.…”

“…Consistent with the magnetization results the strong rise in the signal below 60 K in the O (and L) sample can be assumed to be due to weak links. Above 60 K all the absorption is due to the shielding of the particles and is much smaller than below 60 K. Presumably due to vortex jumps in the weak links [25] the noise in the O (and L) curve below 60 K is also much larger than in the S (and M) curve, where no weak links are present. The resonant microwave absorption was measured using field modulation at 100 kHz and with a modulation amplitude of 0.2 mT.…”

Weak links in YBCO nanopowder

“…When cooling from all the curves in Fig. 2(a) first have a linear part (SC, above point I), which can be explained to be due to the shielding current absorption mechanisms [6]. Next the weak links are formed with the vortices in the liquid phase and nearly constant absorption and decreasing at low temperatures when the thermal motion of the vortex liquid decreases (between points I and II).…”