Decoupling of critical temperature and superconducting gaps in irradiated films of a Fe-based superconductor

Abstract

We report on direct measurements of the energy gaps (carried out by means of point-contact Andreev reflection spectroscopy, PCARS) and of the critical temperature in thin, optimally doped, epitaxial films of BaFe 2 (As 1−x P x ) 2 irradiated with 250-MeV Au ions. The low-temperature PCARS spectra (taken with the current flowing along the c axis) can be fitted by a modified Blonder-Tinkham-Klapwijk (BTK) model with two nodeless gaps; this is not in contrast with the possible presence of node lines suggested by various experiments in literature. Up to a fluence Φ = 7.3•10 11 cm −2 , we observe a monotonic suppression of the critical temperature and of the gap amplitudes ∆ 1 and ∆ 2 . Interestingly, while T c decreases by about 3%, the gaps decrease much more (by about 37% and 25% respectively), suggesting a decoupling between high-temperature and low-temperature superconducting properties. An explanation for this finding is proposed within an effective two-band Eliashberg model, in which such decoupling is inherently associated to defects created by irradiation.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

“…The number and symmetry of the SC energy gaps were determined via directional PCARS [27][28][29][30][31] in crystals with x = 0 and x = 0.05. Fig.…”

“…Prior to fitting, the experimental dI/dV curves need to be normalized, i.e. divided by the normal-state differential conductance curve [27,30,31]. In principle, the dI/dV spectrum recorded at a given T < Tc should be divided by the normal-state conductance at the same temperature, obtained by suppressing superconductivity with, e.g., a magnetic field.…”

“…The superconducting gap structure was investigated via pointcontact Andreev reflection spectroscopy (PCARS), which entails the measurement of the differential conductance (dI/dV ) of a pointlike contact between the superconductor (S) and a normal metal (N). Here, the contacts were fabricated via the so-called soft technique [27,[29][30][31], in which a thin Au wire is stretched over the crystal, until it touches the surface in a single point, is (optionally) mechanically stabilized by drop-casting a small (≈ 50 µm) droplet of Ag glue, and acts as a parallel of nanoscopic N/S contacts. Thanks to the platelet-like shape of the crystals, it was possible to control the direction of (main) current injection, by making the point contacts either on the topmost surface (thus injecting the current along the c axis) or on the side (thus injecting the current along the ab planes).…”

Ambipolar suppression of superconductivity by ionic gating in optimally doped BaFe2(As,P)2 ultrathin films

“…This may occur due to the introduction of extra scattering centers via the accumulation of ions at the film surface, an issue which is well-documented across a wide range of different materials [47][48][49][50][64][65][66][67][68]. However, in this case we expect this contribution to be negligible, since the width of the resistive transition of BaFe 2 (As,P) 2 is known to be very robust against the introduction of extrinsic disorder, both in the case of single-crystals [69] and epitaxial thin films [25,70]. More importantly, the scaling of ∆R/R ′ with ∆n 2D in the normal state indicates that disorder is more efficiently introduced by hole doping with respect to electron doping: therefore, if the broadening of the resistive transition was dominated by gate-induced disorder, one should observe a larger broadening upon hole doping and a smaller broadening upon electron doping.…”

Anisotropic s-Wave Gap in the Vicinity of a Quantum Critical Point in Superconducting BaFe2(As1–x P x )2 Single Crystals: A Study of Point-Contact Spectroscopy

“…[27] Point contact Andreev-reflection (PCAR) spectroscopy measurement on BaFe2(As1−𝑥P𝑥)2 thin film with a 𝑇c ≃ 30 K have provided evidence for the possible presence of nodeless gap. [28] What is more, addition of disorder/impurity can lift the nodes in nodal s-wave SCs, [29] leading to a nodal-nodeless gap structure transition in the vicinity of a quantum critical point in BaFe2(As1−𝑥P𝑥)2. [30,31] These studies with different conclusions reflect the complexity of the system, and systematic investigations on high-quality samples using different techniques are highly desired.…”