Scanning tunneling spectroscopy of ${\text{SmFeAsO}}_{0.85}$: Possible evidence for $d$-wave order-parameter symmetry

We report a scanning tunneling spectroscopy investigation of polycrystalline ${\text{SmFeAsO}}_{0.85}$ having a superconducting transition at 52 K. On large regions of the sample surface the tunneling spectra exhibited $\mathrm{V}$-shaped gap structures with no coherence peaks, indicating degraded surface properties. In some regions, however, the coherence peaks were clearly observed and the $\mathrm{V}$-shaped gaps could be fitted to the theory of tunneling into a $d$-wave superconductor, yielding gap values between 8 and 8.5 meV corresponding to the ratio $2\Delta /K_B{T}_C\sim 3.55–3.8$, which is slightly above the BCS weak-coupling prediction. In other regions the spectra exhibited zero-bias conductance peaks consistent with a $d$-wave order-parameter symmetry.

Figure 1

(Color online) (a) STM topographic image of a polycrystalline ${\text{SmFeAsO}}_{0.85}$ sample showing crystallites. (b) Zero-field-cooled (ZFC) and field-cooled (FC) temperature-dependent magnetization curves measured on the sample at 5 Oe.

Figure 2

(Color online) Tunneling spectra at 4.2 K taken (a) in a region where coherence peaks were observed and (b) in a region where only sharp gap edges were found. The red dashed curve in (a) is a fit to the theory of tunneling into a $d$-wave superconductor along the $c$ axis using the parameters denoted in the figure.

Figure 3

(Color online) (a) A tunneling spectrum exhibiting a zero-bias conductance peak with gaplike features. (b) A tunneling spectrum showing a shifted zero-bias conductance peak. The red dashed curves are theoretical fits calculated as described in the text, and the vertical dashed lines are guides to the eyes.