arXiv 1007.35972010-07-21cond-mat.supr-con

角分辨Specific Heat揭示的FeSe0.45Te 0.55序参量的各向异性结构

Anisotropic Structure of the Order Parameter in FeSe0.45Te0.55 Revealed by Angle Resolved Specific Heat

B. Zeng, G. Mu, H. Q. Luo, T. Xiang, H. Yang, L. Shan, C. Ren, I. I. Mazin, P. C. Dai, H. -H. Wen

备注：: 32 pages, 7 figures
发表信息：: Nature Communications 1 (2010) 112
DOI：: 10.1038/ncomms1115

摘要：

铁基超导体中超导能隙的对称性和结构是理解这些新材料的核心问题。到目前为止，实验数据和理论模型一直存在很大争议。一些实验倾向于两个或多个恒定或接近恒定的间隙，其他实验表明强各向异性，还有一些实验表明间隙为零（“节点”）。理论模型也各不相同，这表明节点的存在或不存在定量地取决于模型参数。一个已经得到广泛传播的观点是，与所有其他已知的超导体（包括铜酸盐）不同，在同一家族中的不同化合物中，间隙结构可能不同。解决这个问题的一个独特的方法，是体积和角度分辨的极少数方法之一，要求测量旋转磁场中的电子比热，作为相对于晶轴的场取向的函数。在本通讯中，我们提出了第一个这样的测量铁基高温超导体（FeBSC）。我们观察到比热随面内磁场方向的四倍振荡，这使我们能够确定费米表面上的带隙最小值（或节点）的位置。我们的结果与扩展的s波模型的预期一致，该模型在电子袋上具有显著的能隙各向异性，并且能隙最小值沿ΓM（或Fe-Fe键）方向。

英文摘要：

The symmetry and structure of the superconducting gap in the Fe-based superconductors are the central issue for understanding these novel materials. So far the experimental data and theoretical models have been highly controversial. Some experiments favor two or more constant or nearly-constant gaps, others indicate strong anisotropy and yet others suggest gap zeros ("nodes"). Theoretical models also vary, suggesting that the absence or presence of the nodes depends quantitatively on the model parameters. An opinion that has gained substantial currency is that the gap structure, unlike all other known superconductors, including cuprates, may be different in different compounds within the same family. A unique method for addressing this issue, one of the very few methods that are bulk and angle-resolved, calls for measuring the electronic specific heat in a rotating magnetic field, as a function of field orientation with respect to the crystallographic axes. In this Communication we present the first such measurement for an Fe-based high-Tc superconductor (FeBSC). We observed a fourfold oscillation of the specific heat as a function of the in-plane magnetic field direction, which allowed us to identify the locations of the gap minima (or nodes) on the Fermi surface. Our results are consistent with the expectations of an extended s-wave model with a significant gap anisotropy on the electron pockets and the gap minima along the ΓM (or Fe-Fe bond) direction.

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