VORTEX MOTION AND VORTEX-FRICTION COEFFICIENT IN TRIANGULAR JOSEPHSON-JUNCTION ARRAYS

We study the dynamical response of triangular Josephson-junction array s, modeled as a network of resistively and capacitively shunted juncti ons. A flux-flow regime is found to extend between a lower vortex-depi nning current and a higher critical current, in agreement with previou s calculations for square arrays. The upper current corresponds either to row-switching events accompanied by steplike jumps in the array re sistance, or to a depinning of the entire array. In the flux-flow regi me, the dynamical response to the bias current is roughly Ohmic, and t he time-dependent voltage can be well understood in terms of vortex de grees of freedom. The vortex friction coefficient eta depends strongly on the McCumber-Stewart parameter beta, and at large beta is approxim ately independent of the shunt resistance R. To account for this, we g eneralize a model of Geigenmuller et al. to treat energy loss from mov ing vortices to the phase analog of optical spin waves in a triangular lattice. The value of eta at all values of beta agrees quite well wit h this model in the low-density limit. The vortex depinning current is estimated as 0.042I(c), independent of the direction of applied curre nt, in agreement with static calculations by Lobb et al. A simple argu ment suggests that quantum effects in vortex motion may become importa nt when the flux-flow resistivity is of order h/(2e)2 per unit frustra tion.