$Δ_T$ Noise, Quantum Shot Noise, and Thermoelectric Clues to the Pairing Puzzle in Iron Pnictides

Abstract

Quantum noise has long served as a powerful probe of quantum transport in mesoscopic junctions. Recently, temperature-driven noise, or $Δ_T$ noise, has attracted growing interest due to its presence even in the absence of average charge current. In this work, we investigate a normal metal-insulator-iron-pnictide junction and demonstrate how thermovoltage, Seebeck coefficient, zero temperature quantum shot noise, finite temperature quantum noise, and $Δ_T$ noise can discriminate between $S_{++}$ and $S_{+-}$ pairing symmetries, which are relevant to iron-based superconductors. We introduce $Δ_T$ noise as a novel probe for distinguishing between the two pairing symmetries. In contrast to conductance, which exhibits a single peak for both $S_{++}$ and $S_{+-}$ states with only a difference in magnitude, the $Δ_T$ noise reveals qualitatively distinct features: a twin-peak structure for the $S_{++}$ pairing symmetry and a single-peak profile for the $S_{+-}$ state. A similar symmetry-dependent contrast is observed in both zero temperature quantum shot noise and finite temperature quantum noise, where the $S_{++}$ state consistently exhibits a twin-peak structure, while the $S_{+-}$ state shows a single-peak response. Furthermore, both the thermovoltage and the Seebeck coefficient display sign reversals for the two pairing symmetries, with opposite trends in the $S_{++}$ and $S_{+-}$ cases. Our results demonstrate that noise-based measurements, together with Seebeck coefficient and thermovoltage, form a mutually reinforcing set of probes that enables reliable identification of superconducting gap symmetry in Iron Pnictide superconductors.

Figures (20)

• Figure 1: Schematic of the N-I-IP junction: Normal metal is at temperature $T_1$, Iron-pnictide superconductor is at temperature $T_2$, with voltage $V$ applied to normal metal while Iron-pnictide superconductor is grounded.
• Figure 2: $G$ (in units of $\tfrac{2e^2}{h}$) as a function of $eV$ at $T_1=10.5K$,$T_2=9.5K$, Z=1 for different interband coupling strengths($\alpha$). Panels (a), (b), (c) and (d) correspond to $\alpha = 0$, $1$, $2$ and $3$, respectively, showing the conductance spectra for the $S_{++}$ pairing symmetry (red) and the $S_{+-}$ pairing symmetry (blue).
• Figure 3: $G$ (in units of $\tfrac{2e^2}{h}$) as a function of $Z$ at $T_1=10.5K$,$T_2=9.5K$ for different interband coupling strengths($\alpha$). Panels (a), (b), (c) and (d) correspond to $\alpha = 0$, $1$, $2$ and $3$, respectively, showing the conductance spectra for the $S_{++}$ pairing symmetry (red) and the $S_{+-}$ pairing symmetry (blue).
• Figure 4: $G$ (in units of $\tfrac{2e^2}{h}$) as a function of $\alpha$ at $T_1=10.5K$,$T_2=9.5K$ for different interband coupling strengths($Z$). Panels (a), (b), and (c) correspond to $Z = 0$, $1$, $2$, and $3$, respectively, showing the conductance spectra for the $S_{++}$ pairing symmetry (red) and the $S_{+-}$ pairing symmetry (blue).
• Figure 5: Finite temperature $Q_{\mathrm{}}$ (in units of $\frac{4e^{2}}{h}k_BT$) at $eV=0.99\Delta_1$ as a function of the barrier strength $Z$ for different interband coupling strengths($\alpha$). Panels (a), (b), (c), and (d) correspond to $\alpha = 0,\,1,\,2,\,3$, respectively, showing the behaviour of the $S_{++}$ state (red) and the $S_{+-}$ state (blue), with $\Delta_2=1.5\Delta_1$