Preformed Cooper Pairs in a Triclinic Iron Pnictide Superconductor

TL;DR

This study uncovers preformed Cooper pairs in a triclinic iron pnictide superconductor, revealing a pseudogap-like phase with spin resonance precursor phenomena related to phase fluctuations, distinct from conventional superconductivity.

Contribution

It provides evidence of pre-pairing behavior and pseudogap phenomena in a triclinic iron pnictide, expanding understanding of high-temperature superconductivity mechanisms.

Findings

Spin resonance peak observed below $T_c$ and decreases above it.

An anomaly around $T^{*} \\approx 45$ K indicating precursor pairing.

Suppression of density of states and Nernst signal below $T^{*}$.

Abstract

Electron pairing along with phase coherence generates superconductivity below the critical temperature ($T_c$). In underdoped high-$T_c$ cuprates, these two quantum phenomena may occur at separate temperatures, which was lately confirmed in the quasi-two-dimensional (quasi-2D) iron chalcogenide superconductors. Here, we report a systematic investigation on the pre-pairing behavior in a triclinic iron pnictide superconductor (Ca$_{0.85}$La$_{0.15}$)$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ with $T_c \approx $ 30 K, where the superconductivity is quasi-2D manifested by the Berezinskii-Kosterlitz-Thouless behaviors. Inelastic neutron scattering experiments unambiguously reveal a spin resonance peak around $E_R =$ 13 meV in the superconducting state, but its intensity continuously decreases when warming up across $T_c$, accompanied with an anomaly around $T^{*}\approx$ 45 K in spin correlations, and a suppression by an in-plane magnetic field persisting to the same temperature. Below $T^{*}$, a significant Nernst signal and a reduction of density of states at the Fermi level are also observed. These results suggest that the precursor of spin resonance is highly related to the preformed Cooper pairs driven by phase fluctuations, much like the pseudogap case in cuprates.