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

Abstract

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:
Research Org.:
Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1227372
Report Number(s):
IS-J-8695
Journal ID: ISSN 1098-0121; PRBMDO
Grant/Contract Number:
DMR1352604; AC02-07CH11358; FG02-86ER45268
Resource Type:
Journal Article: 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)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Kim, J. S., Stewart, G. R., Liu, Yong, and Lograsso, Thomas A. Specific heat investigation for line nodes in heavily overdoped Ba1-xKxFe2As2. United States: N. p., 2015. Web. doi:10.1103/PhysRevB.91.214506.
Kim, J. S., Stewart, G. R., Liu, Yong, & Lograsso, Thomas A. Specific heat investigation for line nodes in heavily overdoped Ba1-xKxFe2As2. United States. doi:10.1103/PhysRevB.91.214506.
Kim, J. S., Stewart, G. R., Liu, Yong, and Lograsso, Thomas A. 2015. "Specific heat investigation for line nodes in heavily overdoped Ba1-xKxFe2As2". United States. doi:10.1103/PhysRevB.91.214506. https://www.osti.gov/servlets/purl/1227372.
@article{osti_1227372,
title = {Specific heat investigation for line nodes in heavily overdoped Ba1-xKxFe2As2},
author = {Kim, J. S. and Stewart, G. R. and Liu, Yong and Lograsso, Thomas A.},
abstractNote = {Previous research has found that the pairing symmetry in the iron-based superconductor Ba1-xKxFe2As2 changes from nodeless s-wave near optimally doped, x≈0.4-0.55 and Tc>30 K, to nodal (either d-wave or s-wave) at the pure endpoint, x=1 and Tc<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 Ba1-xKxFe2As2, x= 0.91, 0.88, and 0.81. The values for Tcmid are 5.6, 7.2 and 13 K and for Hc2≈ 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 ≈ H1/2 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 H1/2 plotted vs T/H1/2 of these heavily overdoped Ba1-xKxFe2As2 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 Ba1-xKxFe2As2 based on the present work may be closer to the KFe2As2 endpoint than x=0.91.},
doi = {10.1103/PhysRevB.91.214506},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 21,
volume = 91,
place = {United States},
year = 2015,
month = 6
}

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  • 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>30K, to nodal (either d wave or s wave) at the pure end point, x=1 and T c<4K. 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-temperaturemore » 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. 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 nonlinear 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 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≤2mJ/molK 3 being consistent with the data. This, together with the scaling, 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 is an estimate for the size of the deep minima in the Δ 2~20-25K gap. Therefore, the location of the change from nodeless to 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 end point than x=0.91.« less
    Cited by 2
  • Low-temperature specific heat, C, in magnetic fields up to H c2 is reported for underdoped Ba(Fe₀.₉₅₅Co₀.₀₄₅)₂As₂ (T c = 8 K) and for three overdoped samples Ba(Fe₁₋ xCo x)₂As₂ (x = 0.103, 0.13, and 0.15; T c = 17.2, 16.5, and 11.7 K, respectively). Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable-composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials, respectively. The present work shows that the measured behavior of the specific heat γ (∝C/T as T → 0, i.e., a measure ofmore » the electronic density of states at the Fermi energy) as a function of field approximately obeys γ ∝ H 0.5±0.1, similar to the Volovik effect for nodal superconductors, for both the underdoped and the most overdoped Co samples. However, for the two overdoped compositions x = 0.103 and 0.13, the low-field (H ≤ 10 T) data show a Volovik-like behavior of γ ∝ H 0.3–0.4, followed by an inflection point, followed at higher fields by γ ∝ H¹. We argue that, within the two-band theory of superconductivity, an inflection point may occur if the interband coupling is dominant.« less
    Cited by 12
  • No abstract prepared.
  • The dependence of the low-temperature specific heat of (La{sub 1.85}Sr{sub 0.15})CuO{sub 4} on magnetic field (H) is reported. Low concentrations of paramagnetic centers allow a different approach to analysis of the data that minimizes the problem of identifying the effects of the line nodes in the energy gap that are expected for d-wave pairing. As a consequence, these effects can be recognized even in the raw data. The data show evidence of the T{sup 2} term expected for H=0, and a well defined H{sup 1/2}T term for H{ne}0. They conform to a scaling relation recently predicted for d-wave pairing. (c)more » 2000 The American Physical Society.« less
  • The authors have measured the magnetic field dependence of the electronic specific heat {gamma}{sup el} (= C{sup el}/T) of polycrystalline Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} from 8 K to above T{sub c}, for samples with T{sub c} = 24 K to 72 K using a high resolution, differential technique. They find that in applied fields of up to 13 T the position of the specific heat jump does not shift significantly for any T{sub c}. This is in strong disagreement with H{sub c2}(T) inferred from magnetoresistance experiments on the same compound, where the resistive transition is shifted down considerably by similarmore » fields. However, there is a dramatic collapse of the specific heat anomaly height in field - signifying a loss in entropy around T{sub c} - compensated by a corresponding increase in {gamma}{sup el} at lower temperatures. The superconducting anomaly is almost completely suppressed in 13 T for the lowest T{sub c} sample, compared to a reduction of 65 % for the sample with T{sub c} = 72 K. The authors find simple vertical scaling of {gamma}{sup el}(T) {minus} {gamma}{sub N} suggesting that the field forces a proportion of the superconductor into the normal state, while the remainder in the superconducting state is unaffected.« less