期刊
PHYSICAL REVIEW B
卷 84, 期 2, 页码 -出版社
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.84.024528
关键词
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资金
- MEXT of Japan
- JST, TRIP
- Grants-in-Aid for Scientific Research [22740222, 20102008] Funding Source: KAKEN
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.
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