Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

Abstract

Underdoped PrFeAs(O,F), one of the lesser known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, magnetometry, nuclear magnetic resonance (NMR) measurements, and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electronboson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while 75 As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above T c and expected to mediate the superconducting pairing.

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.…”

“…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).…”

“…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.…”

Direct Observation of the Spin Exciton in Andreev Spectroscopy of Iron-Based Superconductors

“…Additionally, this is the reason to interpret the dips at mV as relating to the small gap rather than to the foot (otherwise, its temperature behavior would be similar to [ 12 ]). The observed temperature dependencies of the gap cannot be simulated in any conventional single-band model, however it is typical for the variety of the 1111 family of the FeBS studied before and could be described in the framework of the two-band model [ 17 , 18 ] or the three-band model [ 19 ].…”

Direct observation of the spin exciton in Andreev spectroscopy of iron-based superconductors

“…Additionally, this is the reason to interpret the dips at ±3.2 mV as relating to the small gap rather than to the foot (otherwise, its temperature behaviour would be similar to ∆ L (T ) [12]). The observed temperature dependencies of the gap cannot be simulated in any conventional single-band model, however it is typical for the variety of the 1111 family of the FeBS studied before and could be described in the framework of the two-band model [17,18] or the three-band model [19].…”