Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays

We study the dependence of the transport properties of square Josephson Jun ction arrays with the direction of the applied de current, both experimenta lly and numerically. We present computational simulations of current-voltag e curves at finite temperatures for a single vortex in an array of LXL junc tions (Ha(2)/Phi(0)=f = 1/L-2), and experimental measurements in 100x1000 a rrays under a low magnetic field corresponding to f approximate to 0.02. We find that the transverse voltage vanishes only in the directions of maximu m symmetry of the square lattice: the [10] and [01] direction (parallel bia s) and the [11] direction (diagonal bias). For orientations different from the symmetry directions, we find a finite transverse voltage that depends s trongly on the angle phi of the current. We find that vortex motion is pinn ed in the [10] direction (phi=0), meaning that the voltage response is inse nsitive to small changes in the orientation of the current near phi=0. We c all this phenomenon orientational pinning. This leads to a finite transvers e critical current for a bias at phi=0 and to a transverse voltage for a bi as at phi not equal 0. On the other hand, for diagonal bias in the [11] dir ection the behavior is highly unstable against small variations of phi, lea ding to a rapid change from zero transverse voltage to a large transverse v oltage within a few degrees. This last behavior is in good agreement with o ur measurements in arrays with a quasidiagonal current drive.