Orbital fluctuation theory in iron-based superconductors: s++-wave superconductivity, structure transition, and impurity-induced nematic order
Orbital fluctuation theory in iron-based superconductors: s++-wave superconductivity, structure transition, and impurity-induced nematic order
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DOI：
10.1016/j.ssc.2012.01.012
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发表时间：
2012-04-01
影响因子：
2.1
通讯作者：
Onari, Seiichiro
中科院分区：
文献类型：
--
作者：
Kontani, Hiroshi;Inoue, Yoshio;Onari, Seiichiro
铁基超导体的主要特征是（i）伴随着剪切模量显着软化的正交相变，（ii）接近正交相的高温超导性，以及（iii）四方相的向列相转变。在本文中，我们基于轨道涨落理论，考虑了 e-ph 和库仑相互作用，对它们提出了统一的解释。研究发现，较小的电声子耦合常数（lambda类似于0.2）足以产生较大的轨道（=电荷四极O-xz/yz）涨落，从而导致无符号反转的s波超导（s（++）波态）。推导的轨道涨落还由于两个动量相反的轨道形成束缚态而导致结构转变的不稳定，这被称为“双轨道过程”。此外，当轨道涨落较强时，可以获得具有C-2对称性的杂质引起的非局域轨道序。这种“杂质引起的向列态”解释了解孪生样品中电阻率的面内各向异性。我们强调，只有当 O-xz 和 O-yz 电荷四极杆的轨道涨落最发散时，(i)-(iii) 才可重现。这一事实保证了本模型哈密顿量和计算的可靠性。 (C) 2012 Elsevier Ltd. 保留所有权利。
The main features in iron-based superconductors would be (i) the orthorhombic transition accompanied by remarkable softening of shear modulus, (ii) high-T-c superconductivity close to the orthorhombic phase, and (iii) nematic transition in the tetragonal phase. In this paper, we present a unified explanation for them, based on the orbital fluctuation theory, considering both the e-ph and the Coulomb interaction. It is found that a small e-phonon coupling constant (lambda similar to 0.2) is enough to produce large orbital (= charge quadrupole O-xz/yz) fluctuations, which causes the s-wave superconductivity without sign reversal (s(++)-wave state). The derived orbital fluctuations also cause the instability toward the structure transition due to the bound state formation of two orbitons with opposite momenta, which is called the "two-orbiton process". Moreover, impurity-induced non-local orbital order with C-2-symmetry is obtained when the orbital fluctuations are strong. This "impurity-induced nematic state" explains the in-plane anisotropy of resistivity in detwinned samples. We stress that (i)-(iii) are reproducible only when orbital fluctuations with respect to O-xz and O-yz charge quadrupoles are the most divergent. This fact ensures the reliability of the present model Hamiltonian and calculation. (C) 2012 Elsevier Ltd. All rights reserved.