Screening of transverse ac fields and influence of supercurrent on NMR Knight shift in high-T_c crystals

In the nuclear magnetic resonance (NMR) configuration with perpendicular constant (dc) field and radiofrequency (rf) magnetic field, a strong screening supercurrent is generated by the rf field. In order to estimate this current which may influence Knight shift data, we performed a model experiment in the NMR configuration on a YBa₂Cu₄O₈ single crystal. The differential magneto-optical imaging technique is used to determine the ac screening current distribution in the sample. We find that at T > 20 K, NMR experiments can be straightforwardly interpreted. At T < 20 K, the rf field is completely excluded. In this situation, NMR only probes the crystal edges; moreover, the Knight shift may be affected by the supercurrent flow.