Specific heat investigation for line nodes in heavily overdoped Ba_1-xK_xFe₂As₂

Journal Article · 09 June 2015 · Physical Review. B, Condensed Matter and Materials Physics

DOI: https://doi.org/10.1103/PhysRevB.91.214506 · OSTI ID:1227372

Kim, J. S. ^[1]; Stewart, G. R. ^[1]; Liu, Yong ^[2]; Lograsso, Thomas A. ^[3]

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1. Univ. of Florida, Gainesville, FL (United States)
2. Ames Lab., Ames, IA (United States)
3. Ames Lab., Ames, IA (United States); Iowa State Univ., Ames, IA (United States)

Previous research has found that the pairing symmetry in the iron-based superconductor Ba_1-xK_xFe₂As₂ changes from nodeless s-wave near optimally doped, x≈0.4-0.55 and T_c>30 K, to nodal (either d-wave or s-wave) at the pure endpoint, x=1 and T_c<4 K. Intense theoretical interest has been focused on this possibility of changing pairing symmetry, where in the transition region both order parameters would be present and time reversal symmetry would be broken. Here we report specific heat measurements in zero and applied magnetic fields down to 0.4 K of three individual single crystals, free of low temperature magnetic anomalies, of heavily overdoped Ba_1-xK_xFe₂As₂, x= 0.91, 0.88, and 0.81. The values for T_c^mid are 5.6, 7.2 and 13 K and for H_c2≈ 4.5, 6, and 20 T respectively. Furthermore, the data can be analyzed in a two gap scenario, Δ₂/Δ₁ ≈ 4, with the magnetic field dependence of γ (=C/T as T→0) showing an anisotropic ‘S-shaped’ behavior vs H, with the suppression of the lower gap by 1 T and γ ≈ H^1/2 overall. Although such a non-linear γ vs H is consistent with deep minima or nodes in the gap structure, it is not clear evidence for one, or both, of the gaps being nodal in these overdoped samples. Thus, following the established theoretical analysis of the specific heat of d-wave cuprate superconductors containing line nodes, we present the specific heat normalized by H^1/2 plotted vs T/H^1/2 of these heavily overdoped Ba_1-xK_xFe₂As₂ samples which – thanks to the absence of magnetic impurities in our sample - convincingly shows the expected scaling for line node behavior for the larger gap for all three compositions. There is however no clear observation of the nodal behavior C ∝ αT² in zero field at low temperatures, with α ≤ 2 mJ/molK³ being consistent with the data. Together with the scaling, this leaves open the possibility of extreme anisotropy in a nodeless larger gap, Δ₂, such that the scaling works for fields above 0.25 – 0.5 T (0.2 – 0.4 K in temperature units), where this an estimate for the size of the deep minima in the Δ₂ ~ 20-25 K gap. Furthermore, the location of the change from nodeless→nodal gaps between optimally doped and heavily overdoped Ba_1-xK_xFe₂As₂ based on the present work may be closer to the KFe₂As₂ endpoint than x=0.91.

• Research Organization: Ames Laboratory (AMES), Ames, IA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• Grant/Contract Number: AC02-07CH11358
• OSTI ID: 1227372
• Alternate ID(s): OSTI ID: 1184356
• Report Number(s): IS-J--8695
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Journal Name: Physical Review. B, Condensed Matter and Materials Physics Journal Issue: 21 Vol. 91; ISSN 1098-0121; ISSN PRBMDO
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English