Multidimensional washboard ratchet potentials for frustrated two-dimensional Josephson junctions arrays on square lattices

Abstract

In this work, we derive an analytical procedure for obtaining a multidimensional washboard ratchet potential U f for two-dimensional Josephson junctions array (TDJJA) with an applied magnetic field. The magnetic field is given in units of the quantum flux per plaquette or frustration of the form f = M N . The derivation is done under the assumption that the checkerboard pattern ground state or unit cell of a two-dimensional Josephson junctions array (TDJJA) is preserved under current bias. The RCSJ with a white noise term models the dynamics for each junction phase in the array. The multidimensional potential is the unique expression of the collective effects that emerge from the array in contrast to the single junction. First step in the procedure is to write the equation for the phases for the unit cell by considering the constraints imposed for the gauge invariant phases due to frustration. Secondly and key idea of the procedure, is to perform a variable transformation from the original systems of stochastic equations to a system of variables where the condition for the equality of mixed second partial is forced via the Poincaré's theorem for differentials forms. This leads to a nonlinear matrix equation (equation ( 9) in the text), where the new coordinates variables x f are evaluated and where the potential exist. The transform matrix also permits the correct transformation of the original white noise terms of each junction to the intensities in the x f variables. The commensurate symmetries of the ground state pinned vortex lattice, leads to discrete symmetries to the part of the washboard potential that does not contain a tilt due to the external bias current(equation( 10) in the text). In this work we apply the procedure for the important cases f = 1 2 , 1 3 . For f = 1 2 , we show that previously efforts for finding the potential are restricted, leading to a reduced dimension of the potential. Our potential reduces to their expression, if one forces their assumption which represents an unstable situation. New physics emerge when currents are applied in the x and y directions, in particular, we confirm analytically previous numerical work for f = 1 2 , concerning the border of stationary states, a landmark of the potential. For f = 1 3 , we give a generalization of previous work, in which we include both the currents in the x and y directions as well the noise terms. We find the potentials realize tilted ratchets analogous to a combustion motor.