Patchwork Conditions for Holographic Nonlinear Responses: A Computational Method of Electric Conductivity and Friction Coefficient

We propose a new method to compute nonlinear transport coefficients in holography, such as nonlinear DC conductivity and nonlinear friction coefficient. The conventional method can be applied only to the models whose action in the gravity dual has the “square-root structure,” i.e., the Dirac-Born-Infeld action of the probe D-branes or the Nambu-Goto action of the probe strings. Our method is applicable to a wider range of holographic models whose action does not have such a square-root structure. We propose a condition to obtain regular physical configurations in the gravity dual in the form of two simultaneous equations, which we call the patchwork condition. Our method also enables us to estimate the effective temperature of the nonequilibrium steady states in a wider range of holographic models. We show that a general model exhibits different effective temperatures for different fluctuation modes.