Energy gap evolution across the superconductivity dome in single crystals of (Ba1- x K x )Fe2As2

Kyuil Cho; Marcin Kończykowski; Serafim Teknowijoyo; Makariy A Tanatar; Yong Liu; Thomas A Lograsso; Warren E Straszheim; Vivek Mishra; Saurabh Maiti; Peter J Hirschfeld; Ruslan Prozorov

doi:10.1126/sciadv.1600807

Energy gap evolution across the superconductivity dome in single crystals of (Ba_{1- x} K _x )Fe₂As₂

Sci Adv. 2016 Sep 30;2(9):e1600807. doi: 10.1126/sciadv.1600807. eCollection 2016 Sep.

Affiliations

¹ Ames Laboratory, Ames, IA 50011, USA.; Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.
² Laboratoire des Solides Irradiés, École Polytechnique, CNRS, CEA, Université Paris-Saclay, 91128 Palaiseau Cedex, France.
³ Ames Laboratory, Ames, IA 50011, USA.
⁴ Ames Laboratory, Ames, IA 50011, USA.; Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA.
⁵ Joint Institute for Computational Sciences, University of Tennessee, Knoxville, TN 37996, USA.; Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
⁶ Department of Physics, University of Florida, Gainesville, FL 32611, USA.

Abstract

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba_1-x K _x )Fe₂As₂ has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x > 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, λ(T), across the full phase diagram for different concentrations of point-like defects introduced by 2.5-MeV electron irradiation. Fitting the low-temperature variation with the power law, Δλ ~ Tⁿ , we find that the exponent n is the highest and the T_c suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around x ≃ 0.8, indicating the onset of nodal behavior. Our analysis using a self-consistent t-matrix approach suggests the ubiquitous and robust nature of s_± pairing in IBSs and argues against a previously suggested transition to a d-wave state near x = 1 in this system.

Keywords: Superconductor; pairing symmetry; pnictides.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Anisotropy
Hot Temperature
Iron Compounds / chemistry*
Magnetics / methods
Phonons
Physical Phenomena*
Superconductivity*

Substances

Iron Compounds