Reciprocal and nonreciprocal paraconductivity in bilayer multiphase superconductors

Thin-film multiphase superconductors are attracting much attention, and it is important to propose how to detect them in experiments. In this work, we study the reciprocal and nonreciprocal paraconductivity of a bilayer model with staggered Rashba-type spin-orbit coupling with and without the potential gradient and Zeeman field. This model shows the Bardeen-Cooper-Schrieffer phase, the pair-density-wave phase, and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, and we focus on how their properties are encoded to the charge transport. We show that the reciprocal paraconductivity has a peak associated with the phase transitions between different superconducting states due to the degeneracy of the transition temperatures as well as the paramagnetic depairing effect. We also show that the FFLO superconducting state shows a sizable nonreciprocal paraconductivity once the degeneracy of Cooper pairs is lifted by applying the potential gradient. Observation of the peaked reciprocal and nonreciprocal paraconductivity can be used as a probe of multiphase superconductivity.