Amplitude-dependent quantum hydrodynamics from a \(\coth\)-Madelung ansatz

Abstract

We investigate a nonlinear extension of the Madelung transformation based on a hyperbolic phase--amplitude coupling of the form \[ Ψ= R e^{\imathθ\coth R}, \] where \(R\) is a real amplitude field and \(θ\) is an auxiliary phase coordinate governed by Schrödinger's equation. In contrast to the conventional polar decomposition, this construction imposes a singular hyperbolic relation between amplitude and phase, thereby endowing the Bohmian or hydrodynamic description with an intrinsically geometric structure. We show that the associated velocity field acquires a density-gradient contribution, producing generalized continuity equations and modified quantum force terms. When interpreted as a complex macroscopic order parameter, this generalized phase structure leads to modified superconducting electrodynamics; in particular, the London equations acquire additional contributions that render the Meissner response sensitive to spatial density gradients. The proposed framework is motivated by broader developments involving complex group velocities, dissipative wave propagation, and amplitude-sensitive transport in quantum systems.