Anisotropic Gap Structure and Sign Reversal Symmetry in Monolayer Fe(Se,Te)

Abstract

The iron-based superconductors are an ideal platform to reveal the enigma of the unconventional superconductivity and potential topological superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO3(001), which is proposed to be topological nontrivial, shows interface-enhanced high-temperature superconductivity in the two-dimensional limit. However, the experimental studies on the superconducting pairing mechanism of monolayer Fe(Se,Te) films are still limited. Here, by measuring the quasiparticle interference in monolayer Fe(Se,Te)/SrTiO3(001), we report the observation of the anisotropic structure of the large superconducting gap and the sign change of the superconducting gap on different electron pockets. The results are well consistent with the “bonding-antibonding” s ±-wave pairing symmetry driven by spin fluctuations in conjunction with spin-orbit coupling. Our work is of basic significance not only for a unified superconducting formalism in the iron-based superconductors, but also for understanding of topological superconductivity in high-temperature superconductors.

Two-Dimensional and Interface Superconductivity in Crystalline Systems

“…47 Additionally, the anisotropic structure and the sign change of the superconducting gap are visualized by the quasiparticle interference (QPI) pattern. 48 The zero-energy bound states, indicating the existence of the longsought Majorana zero modes, are discovered at both ends of a one-dimensional atomic line defect on the 1-UC Fe(Te,Se). 49 Remarkably, the pair density wave (PDW), an extraordinary state where Cooper pairs carry nonzero momentum, is observed at the domain wall on the surface of the 1-UC Fe(Te,Se), 50 characterized by the spatially oscillated superconducting order parameter.…”

Lateral Quantum Confinement Effect on High-TC Superconducting FeSe Monolayer

“…The modulus | g false( bold-italicq , normalE false) | (scattering intensity) exhibits three ring-like structures in Figure c, denoted by bold-italicq bold1 , bold-italicq bold2 and bold-italicq bold3 rings ( bold-italicq bold1 represents bold-italicq vectors on the ring). bold-italicq bold1 ring exhibits the highest scattering intensity, , therefore is used to extract the band dispersion. As bold-italicq is the change in the momenta bold-italick on the Fermi pockets after scattering, the band dispersion can be approximately given by E ( bold-italick ) = E ( bold-italicq bold1 /2) .…”

“…The STM topography of monolayer FeSe grown on the STO terraces is shown in Figure a, and the inset provides the top Se atom arrangement. In Figure b, a symmetrized d I /d V tunneling spectrum is obtained from STS measurements in the central uniform area of monolayer FeSe (see Supporting Information for spectra symmetrization), exhibiting prominent coherence peaks and U-shaped SC gaps at 4.2 K. The two-band Dynes model with anisotropic SC gap functions fits the d I /d V spectrum to give two gap values Δ 1 = 10.3 meV and Δ 2 = 18.1 meV …”

“…dI/dV spectrum to give two gap values Δ 1 = 10.3 meV and Δ 2 = 18.1 meV. 13 Quasiparticle interference (QPI) in the differential conductance mapping r g E ( , ) provides a powerful tool to analyze electronic states. 14−16 First, r g E ( , ) mappings (measured on monolayer FeSe) are Fourier transformed (FT) into q g( , E).…”

“…The modulus (scattering intensity) exhibits three ring-like structures in Figure c, denoted by , and rings ( represents vectors on the ring). ring exhibits the highest scattering intensity, , therefore is used to extract the band dispersion. As is the change in the momenta on the Fermi pockets after scattering, the band dispersion can be approximately given by E ( ) = E ( /2) .…”