The pair-density modulation in FeTe_0.55Se_0.45 thin flakes: A Landau theory perspective

A recent experiment on exfoliated flakes of FeTe_0.55Se_0.45 has revealed the pair-density modulation(PDM), in which the superconducting gap is modulated between different sublattices while preserving the lattice translation symmetry. Upon increasing the temperature, a two-step phase transition is observed: the PDM state evolves first into a superconducting phase with a spatially uniform gap and then into the normal state accompanied by nematic fluctuations. Motivated by this observation, we develop a phenomenological description of the PDM state based on Landau theory. We propose that the PDM state originates from the hybridization of two superconducting order parameters—one even-parity and the other odd-parity under a mirror reflection. A symmetry-allowed linear coupling between the superconducting and nematic orders naturally explains the observed sequence of transitions. Our results provide a framework for understanding the interplay between parity-mixed superconductivity and nematicity, offering guidance for constructing a corresponding microscopic theory.