STRAIGHTENED VOLTAGE EFFECT IN HIGH-T-C SUPERCONDUCTORS

A new effect called ''ac-current straightening'' has been observed in ceramic (Bi,Pb)-2223 slabs carrying ac current I-dc+I-ac cos(omega t). The current-voltage (I-V) characteristics of the ceramic were measure d at 77 K at frequencies ranging from 50 to 20 000 Hz. A spectrum anal yzer showed a series of high harmonics in the voltage signal as well a s a constant voltage drop. The full set of experimental data has been explained theoretically using the Bean-Kim critical state model with a magnetic field dependent critical current j(c)(H) = j(c)(0)/(1+H/H-0) . A low transport ac current gives a voltage linearly proportional to the frequency and quadratically proportional to the ac-current amplitu de I-ac. It consists of odd harmonics only. If a bias de current is sw itched on, then even harmonics and a dc-voltage drop appear. Their amp litudes are proportional to the small parameter I-ac/cH(0) and depend on the I-dc/I-ac ratio.