Point-contact Andreev-reflection spectroscopy in ReFeAsO_{1-x}F_x (Re = La, Sm): Possible evidence for two nodeless gaps
arXiv:0902.3441 · doi:10.1016/j.physc.2009.03.039
Abstract
A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with Tc ~ 27 K and SmFeAsO0.8F0.2 (Sm-1111) ones with Tc ~ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias, an experimental situation quite similar to that observed by the same technique in pure and doped MgB2. The single-band Blonder-Tinkham-Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO1-xFx (Re = La, Sm): a small gap, Delta1, smaller than the BCS value (2Delta1/kBTc ~ 2.2 - 3.2) and a much larger gap Delta2 which gives a ratio 2Delta2/kBTc ~ 6.5 - 9. In Sm-1111 both gaps close at the same temperature, very similar to the bulk Tc, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to Tc. The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive up to T* ~ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.
10 pages, 7 color figures, Special Issue of Physica C on Superconducting Pnictides
References in corpus (19)
- Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems
- Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsOF}
- LaFeAsOF: A low carrier density superconductor near itinerant magnetism
- Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2
- Very High Field Two-Band Superconductivity in LaFeAsO_0.89F_0.11
- The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
- As NMR studies of superconducting LaOFFeAs
- The role of critical current on point contact Andreev Reflection spectrum between a normal metal and a superconductor
- Probing superconducting energy gap from infrared spectroscopy on a BaKFeAs single crystal with T=37 K
- 19-F NMR Investigation of Iron-pnictide Superconductor LaFeAsO(0.89)F(0.11)
- Scanning tunneling spectroscopy of SmFeAsO0.85: Possible evidence for d-wave order parameter symmetry
- Nodal Superconductivity with Multiple Gaps in SmFeAsOF
- Possible two-gap superconductivity in NdFeAs(O,F) probed by point-contact Andreev-reflection spectroscopy
- Anisotropy of superconducting single crystal SmFeAsO_(0.8)F_(0.2) studied by torque magnetometry
- Evidence for gap anisotropy in CaC6 from directional point-contact spectroscopy
- Point contact Andreev reflection spectroscopy of NdFeAsO_0.85
- Multiple Gaps and Superfluid Density from Interband Pairing in Iron Oxypnictides
- Possible unconventional superconductivity in iron-based layered compound LaFePO: Study of heat capacity
- Linear Temperature Dependence of the Lower Critical Field in F-Doped LaOFeAs Superconductors