Eur. Phys. J. B (2013) 86: 274
https://doi.org/10.1140/epjb/e2013-30964-y

Regular Article

Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe_1.84Co_0.16As₂ 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

^a Present address: Center for Life NanoScience@LaSapienza, Istituto Italiano di Tecnologia, Viale Regina Elena 295, Roma, Italy.
^b e-mail: paolo.dore@roma1.infn.it

Received: 22 October 2012
Received in final form: 22 March 2013
Published online: 17 June 2013

Abstract

Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe_1.84Co_0.16As₂ superconducting film with superconducting transition temperature T_c = 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 Δ_B, which seems to follow a BCS-like behavior, results to be Δ_B(0) = 17 cm^-1. For the small gap, for which Δ_A(0) = 8 cm^-1, the temperature dependence cannot be clearly established. These gap values and those reported in the literature for the BaFe_2−xCo_xAs₂ system by using infrared spectroscopy, when put together as a function of T_c, 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 T_c seem to have a BCS-like linear behavior, but with different slopes. Above this temperature, both gaps show different supra-linear behaviors.