Origin of orthorhombic transition, magnetic transition, and shear-modulus softening in iron pnictide superconductors: Analysis based on the orbital fluctuations theory
Origin of orthorhombic transition, magnetic transition, and shear-modulus softening in iron pnictide superconductors: Analysis based on the orbital fluctuations theory
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DOI：
10.1103/physrevb.84.024528
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发表时间：
2011-07-25
影响因子：
3.7
通讯作者：
Onari, Seiichiro
中科院分区：
文献类型：
--
作者：
Kontani, Hiroshi;Saito, Tetsuro;Onari, Seiichiro
铁磷属元素化物超导体的主要特征是：（i）伴随剪切模量显著软化的正交相转变，（ii）接近正交相的高T_c超导性，（iii）由正交性诱导的条形磁有序。为了对这些特征给出统一的解释，我们基于轨道涨落理论分析了含铁离子光学声子的多轨道Hubbard-Holstein模型。在随机相位近似（RPA）中，一个小的电子-声子耦合常数（λ近似为0.2）就足以产生大的轨道（电荷四极）波动。最发散的磁化率是O-xz-反铁四极（AFQ）磁化率，它导致s波超导性，而没有符号反转（s（++）波状态）。同时，O-x ~ 2-（y ~ 2）-铁四极（FQ）磁化率的发散发展是由AFQ涨落的“双轨道过程”引起的，这在RPA中是不存在的。由此导出的FQ涨落导致C-66剪切模量软化，其长程有序不仅触发了正交结构的转变，而且诱发了条型反铁磁态的不稳定性.换句话说，由两个轨道组成的复合玻色子的凝聚引起了FQ级和结构跃迁。因此，理论上预测的多轨道临界性对铁磷属元素化物超导体的上述特性提供了统一的解释。
The main features in iron pnictide superconductors are summarized as (i) the orthorhombic transition accompanied by a remarkable softening of the shear modulus, (ii) high-T-c superconductivity close to the orthorhombic phase, and (iii) stripe-type magnetic order induced by orthorhombicity. To present a unified explanation for these features, we analyze the multi-orbital Hubbard-Holstein model with Fe-ion optical phonons based on the orbital fluctuation theory. In the random-phase approximation (RPA), a small electron-phonon coupling constant (lambda similar to 0.2) is enough to produce large orbital (charge quadrupole) fluctuations. The most divergent susceptibility is the O-xz-antiferroquadrupole (AFQ) susceptibility, which causes s-wave superconductivity without sign reversal (s(++)-wave state). At the same time, divergent development of O-x2-(y2)-ferroquadrupole (FQ) susceptibility is brought about by the "two-orbiton process" with respect to the AFQ fluctuations, which is absent in the RPA. The derived FQ fluctuations cause the softening of the C-66 shear modulus, and its long-range order not only triggers the orthorhombic structure transition, but also induces the instability of the stripe-type antiferromagnetic state. In other words, the condensation of composite bosons made of two orbitons gives rise to the FQ order and structure transition. Therefore, the theoretically predicted multi-orbital criticality presents a unified explanation for the above-mentioned features of iron pnictide superconductors.