A Landau Theory for Pair Density Modulation in Fe(Te,Se) flakes

Abstract

Motivated by recent scanning tunneling microscopy (STM) experiments reporting a pair-density modulation (PDM) in FeTe$_{0.55}$Se$_{0.45}$, we develop a Landau theory to elucidate the physical origin of this phenomenon. We analyze the PDM in terms of the screw symmetry of the single layer, interpreting it as a hybridized state of two order parameters of opposite glide and screw parity. Discussing the absence of PDM in the bulk where both glide and screw symmetry are present, we argue that the absence of glide symmetry on the surface allows nematic order to selectively stabilize the PDM in thin flakes. Finally, we discuss the symmetry constraints on the microscopic pairing mechanism, pointing out the opposite glide and screw parities of the order parameters favor a site, rather than a bond-based paring mechanism. This suggests that pairing in iron-based superconductors may be local to the iron atoms, possibly driven by Hunds coupling.

Figures (4)

• Figure 1: Schematic illustration of the two-step phase transition. The ground state evolves from PDM state into a uniform superconducting order and then into a normal state. The two critical temperatures are $T_{c1}\approx 9$K and $T_{c2}\approx 11$K kongCooperpairDensityModulation2025 .
• Figure 1: Three possible scenarios for the parity-mixing region in $g-T$ space. (a)$u_+, u_- >u_\pm,$ , (b)$u_+<u_\pm<u_-$ and (c) $u_+>u_\pm >u_-$
• Figure 2: a. Structure of single-layer Fe(Te,Se) showing the different distance of the upper and lower chalcogenide atoms from the iron plane. b. atomic displacements associated with nematic order. These displacements change sign under a $\pi$ rotation.
• Figure 3: Phase diagram of the Landau theory showing a. when $u_{\pm}\geq U_c = \sqrt{u_+u_-}$, where the co-existence region is absent and and b. where $u_{\pm}<U_c = \sqrt{u_+u_-}$ and a hybridized phase develops. c. Representative 3D phase diagram assuming a nematic coupling to the order parameters, exponentially dependent on flake thickeness $d$, calculated using $u_\pm=8,u_+=u_-=2,\lambda_0 = 7,\alpha =1, \xi = 1,T_0=1$.