Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe1.84Co0.16As2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe2As2 pnictide
1 Sincrotrone Trieste, S.C.p.A., Area
Science Park, 34012 Basovizza, Trieste, Italy
2 Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
3 CNR-IOM and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
4 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
5 Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, 2031 East Paul Dirac Drive, Tallahassee, FL 32310, USA
6 CNR-SPIN and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
Present address: Center for Life NanoScience@LaSapienza, Istituto
Italiano di Tecnologia, Viale Regina Elena 295, Roma, Italy.
b e-mail: firstname.lastname@example.org
Received in final form: 22 March 2013
Published online: 17 June 2013
Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe1.84Co0.16As2 superconducting film with superconducting transition temperature Tc = 17.5 K. A detailed analysis of the combined reflectance and transmittance measurements showed that the optical properties of the superconducting system can be described in terms of a two-band, two-gap model. The zero temperature value of the large gap ΔB, which seems to follow a BCS-like behavior, results to be ΔB(0) = 17 cm-1. For the small gap, for which ΔA(0) = 8 cm-1, the temperature dependence cannot be clearly established. These gap values and those reported in the literature for the BaFe2−xCoxAs2 system by using infrared spectroscopy, when put together as a function of Tc, show a tendency to cluster along two main curves, providing a unified perspective of the measured optical gaps. Below a temperature around 20 K, the gap-sizes as a function of Tc seem to have a BCS-like linear behavior, but with different slopes. Above this temperature, both gaps show different supra-linear behaviors.
Key words: Solid State and Materials
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag, 2013