We construct a Schwinger-Keldysh effective field theory for relativistic hydrodynamics for charged matter in a thermal background using a superspace formalism. Superspace allows us to efficiently impose the symmetries of the problem and to obtain a simple expression for the effective action. We show that the theory we obtain is compatible with the Kubo-Martin-Schwinger condition, which in turn implies that Green's functions obey the fluctuation-dissipation theorem. Our approach complements and extends existing formulations found in the literature.

We construct a holographic dual of the Schwinger-Keldysh effective action for the dissipative low-energy dynamics of relativistic charged matter at strong coupling in a fixed thermal background. To do so, we use a mixed signature bulk spacetime whereby an eternal asymptotically anti-de Sitter black hole is glued to its Euclidean counterpart along an initial time slice in a way to match the desired double-time contour of the dual field theory. Our results are consistent with existing literature and can be regarded as a fully-ab initio derivation of a Schwinger-Keldysh effective action. In addition, we provide a simple infrared effective action for the near horizon region that drives all the dissipation and can be viewed as an alternative to the membrane paradigm approximation.

We classify all possible allowed constitutive relations of relativistic fluids in a statistical mechanical limit using the Schwinger-Keldysh effective action for hydrodynamics. We find that microscopic unitarity enforces genuinely new constraints on the allowed transport coefficients that are invisible in the classical hydrodynamic description; they are not implied by the second law or the Onsager relations. We term these conditions Schwinger-Keldysh positivity and provide explicit examples of the various allowed terms.

We present a minimal holographic model for s-wave superconductivity with unbalanced Fermi mixtures, in 2 + 1 dimensions at strong coupling. The breaking of a U(1) A "charge" symmetry is driven by a non-trivial profile for a charged scalar field in a charged asymptotically AdS 4 black hole. The chemical potential imbalance is implemented by turning on the temporal component of a U(1) B "spin" field under which the scalar field is uncharged. We study the phase diagram of the model and comment on the eventual (non) occurrence of LOFF-like inhomogeneous superconducting phases. Moreover, we study "charge" and "spin" transport, implementing a holographic realization (and a generalization thereof to superconducting setups) of Mott's two-current model which provides the theoretical basis of modern spintronics. Finally we comment on possible string or M-theory embeddings of our model and its higher dimensional generalizations, within consistent Kaluza-Klein truncations and brane-anti brane setups.