In-plane current-voltage characteristics and oscillatory Josephson-vortex flow resistance in La-freeBi2+xSr2−xCuO6+δsingle crystals in high magnetic fields

Abstract

We have investigated the in-plane $I\text{\ensuremath{-}}V$ characteristics and the Josephson-vortex flow resistance in high-quality La-free ${\text{Bi}}_{2+x}{\text{Sr}}_{2\ensuremath{-}x}{\text{CuO}}_{6+\ensuremath{\delta}}$ (Bi-2201) single crystals in parallel and tilted magnetic fields at temperatures down to 40 mK. For parallel magnetic fields below the resistive upper critical field ${H}_{c2}^{\ensuremath{\ast}}$, the $I\text{\ensuremath{-}}V$ characteristics obey a power law with a smooth change with increasing magnetic field of the exponent from above 5 down to 1. In contrast to the double-layer cuprate Bi-2212, the observed smooth change suggests that there is no change in the mechanism of dissipation (no Kosterlitz-Thouless transition) over the range of temperatures investigated. At small angles between the applied field and the $ab$ plane, prominent current steps in the $I\text{\ensuremath{-}}V$ characteristics and periodic oscillations of Josephson-vortex flow resistance are observed. While the current steps are periodic in the voltage at constant fields, the voltage position of the steps, together with the flux-flow voltage, increases nonlinearly with magnetic field. The $ab$-flow resistance oscillates as a function of field with a constant period over a wide range of magnetic fields and temperatures. The current steps in the $I\text{\ensuremath{-}}V$ characteristics and the flow resistance oscillations can be linked to the motion of Josephson vortices across layers.