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Title: Specific heat investigation for line nodes in heavily overdoped Ba 1-xK xFe 2As 2

Previous research has found that the pairing symmetry in the iron-based superconductor Ba 1-xK xFe 2As 2 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 xFe 2As 2, 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, Δ 21 ≈ 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 nodesmore » 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 xFe 2As 2 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 ∝ αT 2 in zero field at low temperatures, with α ≤ 2 mJ/molK 3 being consistent with the data. Together with the scaling, this leaves open the possibility of extreme anisotropy in a nodeless larger gap, Δ 2, 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 Δ 2 ~ 20-25 K gap. Furthermore, the location of the change from nodeless→nodal gaps between optimally doped and heavily overdoped Ba 1-xK xFe 2As 2 based on the present work may be closer to the KFe 2As 2 endpoint than x=0.91.« less
Authors:
 [1] ;  [1] ;  [2] ;  [3]
  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)
Publication Date:
Report Number(s):
IS-J-8695
Journal ID: ISSN 1098-0121; PRBMDO
Grant/Contract Number:
DMR1352604; AC02-07CH11358; FG02-86ER45268
Type:
Accepted Manuscript
Journal Name:
Physical Review. B, Condensed Matter and Materials Physics
Additional Journal Information:
Journal Volume: 91; Journal Issue: 21; Journal ID: ISSN 1098-0121
Publisher:
American Physical Society (APS)
Research Org:
Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE
OSTI Identifier:
1227372
Alternate Identifier(s):
OSTI ID: 1184356