Observation of current strings in Bi2Sr2CaCu2O8+x single crystals

A flux step between regions of magnetic flux of opposite polarities in the remanent state of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystals is observed by means of a high resolution magneto-optical technique. This step indicates a current string inside the crystal platelet which is similar to the string appearing in the solution of the critical state problem for a cylindrical type-II superconducting wire with transport current if one takes into account the finite lower critical field ${\mathit{H}}_{\mathit{c}1}$. The conditions for the existence of the current string and its properties are discussed.