Lee C H, Kihou K, Park J T, Horigane K, Fujita K, Waßer F, Qureshi N, Sidis Y, Akimitsu J, Braden M
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.
Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, D-85748 Garching, Germany.
Sci Rep. 2016 Mar 23;6:23424. doi: 10.1038/srep23424.
The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δs), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δs in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2Δs to higher energies, peaking at values of 3Δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.
铁基超导体中库珀对形成的机制仍然是一个有争议的话题。主要问题是自旋涨落还是轨道涨落对配对机制起作用。为了解决这个问题，通过研究超导相中出现的磁中子散射信号的显著增强可以获得一个关键线索。对于自旋涨落模型，这种增强被称为自旋共振，在该模型中，它们的能量被限制在超导能隙值的两倍（2Δs）以下，而在其他模型如轨道涨落模型中，能量可能更大。在此，我们报告了通过非弹性中子散射研究的0.5 < x < 0.84时Ba1-xKxFe2As2中低能磁激发光谱的掺杂依赖性。我们发现，自旋共振的行为在最佳掺杂区到过掺杂区发生了显著变化。在最佳掺杂区，在2Δs以下清晰地观察到强共振峰，而过掺杂区则没有。相反，谱权重从低于2Δs的能量转移到更高能量，对于x = 0.84，在3Δs处达到峰值。这些结果表明，在过掺杂区，磁性对库珀对形成的影响减小。
