Specific heat investigation for line nodes in heavily overdoped Ba1−xKxFe2As2
Specific heat investigation for line nodes in heavily overdoped Ba1−xKxFe2As2
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DOI：
10.1103/physrevb.91.214506
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发表时间：
2015-05
影响因子：
3.7
通讯作者：
J. Kim;G. Stewart;Yong Liu;T. Lograsso
中科院分区：
文献类型：
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作者：
J. Kim;G. Stewart;Yong Liu;T. Lograsso
先前的研究发现，铁基超导体Ba_（1-x）K_xFe_2As_2的配对对称性从最佳掺杂附近的无节点s波（x = 0.4-0.55，Tc>30 K）转变为纯端点处的节点（d波或s波）（x=1，Tc<4 K）。强烈的理论兴趣一直集中在这种可能性改变配对对称性，在过渡区的两个顺序参数将存在和时间反演对称性将被打破。在这里，我们报告比热测量在零和施加的磁场下降到0.4 K的三个单独的单晶，无低温磁异常，严重过掺杂的Ba 1-xKxFe 2As 2，x= 0.91，0.88，和0.81。Tcmid的值分别为5.6、7.2和13 K，Hc 2的值分别为4.5、6和20 T。此外，数据可以在两个间隙的情况下进行分析，Δ2/Δ1 <$4，其中γ（=C/T，T→0）的磁场依赖性显示出相对于H的各向异性“S形”行为，其中较低间隙被1 T抑制，并且γ <$H 1/2总体上。尽管这种非线性γ对H与差距结构中的深最小值或节点一致，但在这些过掺杂样品中，间隙中的一个或两个是节点不是清楚的证据。因此，在对含有线节点的d波铜氧化物超导体的比热进行了建立的理论分析之后，我们提出了这些重度过掺杂的Ba 1-xKxFe 2As 2样品的由H1/2绘制的归一化比热与T/H1/2的关系，由于我们的样品中没有磁性杂质，因此令人信服地显示了所有三种成分的较大间隙的线节点行为的预期缩放。然而，在低温下零场中没有清楚地观察到节点行为C <$αT2，α ≤ 2 mJ/molK 3与数据一致。与标度一起，这使得在无节点的较大间隙Δ2中存在极端各向异性的可能性，使得标度适用于0.25 - 0.5 T（温度单位为0.2 - 0.4 K）以上的场，其中这是对Δ2 ~ 20-25 K间隙中深极小值的大小的估计。此外，基于本工作的最佳掺杂和重过掺杂Ba 1-xKxFe 2As 2之间从无节点到节点间隙的变化的位置可能比x=0.91更接近KFe 2As 2端点。«少
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 gapsmore » 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.« less