[Paper Review] Gap Anisotropy in Iron-Based Superconductors: A Point-Contact Andreev Reflection Study of BaFe$_{2-x}$Ni$_{x}$As$_2$ Single Crystals

Cong Ren, Zhaosheng Wang+8 more|arXiv (Cornell University)|Jun 15, 2011

Iron-based superconductors research6 citations

TL;DR

This study uses c-axis point-contact Andreev reflection (PCAR) spectroscopy on BaFe₂₋ₓNiₓAs₂ single crystals to probe doping-dependent superconducting gap anisotropy. It reveals a transition from a nodeless, two-gap state in underdoped samples to a highly anisotropic, nodal-like gap in overdoped samples, with quantitative fitting supporting orbital-dependent pairing mediated by antiferromagnetic spin fluctuations.

ABSTRACT

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At optimal doping ($x=0.1$) the PCAR spectrum feature the structures of two superconducting gap and electron-boson coupling mode. In the $s\pm$ scenario, quantitative analysis using a generalized Blonder-Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent, suggest a nodeless state in strong-coupling limit with gap minima on the Fermi surfaces. Upon crossing above the optimal doping ($x > 0.1$), the PCAR spectrum show an in-gap sharp narrow peak at low bias, in contrast to the case of underdoped samples ($x < 0.1$), signaling the onset of deepened gap minima or nodes in the superconducting gap. This result provides evidence of the modulation of the gap amplitude with doping concentration, consistent with the calculations for the orbital dependent pair interaction mediated by the antiferromagnetic spin fluctuations.

Motivation & Objective

• To investigate the doping evolution of superconducting gap anisotropy in electron-doped BaFe₂₋ₓNiₓAs₂ single crystals using high-resolution c-axis PCAR spectroscopy.
• To determine whether the superconducting gap structure evolves from nodeless to nodal-like with increasing doping, particularly across optimal doping.
• To quantitatively analyze the gap anisotropy using a generalized two-gap BTK model and assess consistency with the s± pairing scenario and spin-fluctuation-mediated pairing.
• To resolve discrepancies in prior experimental results on iron-based superconductors by using high-quality single crystals and reproducible c-axis contacts.
• To correlate the observed spectral evolution with theoretical predictions of orbital-dependent pair interactions mediated by antiferromagnetic spin fluctuations.

Proposed method

• Fabricated highly transparent c-axis point contacts using thick silver paste and gold wires on cleaved (001) surfaces of BaFe₂₋ₓNiₓAs₂ single crystals.
• Performed PCAR measurements at temperatures from 2 K to above Tc, acquiring normalized and raw G(V) conductance curves for multiple doping levels (x = 0.075, 0.085, 0.1, 0.12, 0.15).
• Applied a generalized two-gap Blonder-Tinkham-Klapwijk (BTK) formalism with one isotropic and one angle-dependent gap to fit the Andreev conductance spectra, particularly at optimal doping (x = 0.1).
• Used the fitting parameters to extract the maximum gap Δmax, the minimum gap Δmin, and the anisotropy ratio r = Δmin/Δmax, assessing the gap structure in strong-coupling limit.
• Analyzed the temperature dependence of conductance to identify spectral features such as in-gap peaks and kink structures, linking them to electron-boson coupling and nodal behavior.
• Correlated spectral evolution with bulk properties (Tc from resistivity) and prior measurements (e.g., neutron scattering for spin resonance energy Ωb).

Experimental results

Research questions

• RQ1 How does the superconducting gap structure evolve with electron doping in BaFe₂₋ₓNiₓAs₂ across the underdoped, optimally doped, and overdoped regimes?
• RQ2 Does the observed conductance spectrum in c-axis PCAR measurements indicate a nodeless or nodal superconducting gap in the overdoped region?
• RQ3 To what extent does the two-gap BTK model with anisotropic pairing reproduce the experimental Andreev conductance spectra, particularly at optimal doping?
• RQ4 Is the observed in-gap conductance peak in overdoped samples a signature of nodal quasiparticles or a finite density of states at low energy?
• RQ5 How does the measured gap anisotropy ratio r = Δmin/Δmax relate to theoretical predictions of orbital-dependent pairing mediated by antiferromagnetic spin fluctuations?

Key findings

• At optimal doping (x = 0.1), the PCAR spectrum exhibits two distinct superconducting gaps and a sharp in-gap peak at ~20 mV, indicating electron-boson coupling with a resonance energy Ωb ≈ 11–13 meV.
• Quantitative fitting using the generalized BTK model reveals a nodeless superconducting state with anisotropic gap minima on the Fermi surface, yielding an anisotropy ratio r = 0.3 in the strong-coupling limit.
• In underdoped samples (x = 0.075, 0.085), the conductance spectra show a conductance plateau near zero bias, characteristic of a fully gapped, nodeless state.
• In overdoped samples (x = 0.12, 0.15), the spectra evolve to exhibit a sharp in-gap peak at low bias, signaling the onset of a highly anisotropic or nodal gap structure.
• The systematic evolution from a full gap to an in-gap peak with increasing doping is consistent with the T-dependent penetration depth data showing a transition from n ≥ 2 to linear-T dependence in overdoped samples.
• The observed gap anisotropy and doping evolution are quantitatively consistent with theoretical models of orbital-dependent pairing mediated by antiferromagnetic spin fluctuations.