High-energy spin fluctuation in low-Tc iron-based superconductor LaFePO0.9
Motoyuki Ishikado1),2),3), Shin-ichi Shamoto1), Katsuaki Kodama4), Ryoichi Kajimoto5),
Mitsutaka Nakamura5), Tao Hong6), and Hannu Mutka7)1) : R. G. for Nanoscale Structure and Function of Advanced Materials, ASRC, JAEA
2) : Comprehensive Research Organization for Science and Society (CROSS)3) : National Institute of Advanced Industrial Science and Technology (AIST)
4) : Material Science Research Center, JAEA, 5) : J-PARC Center, JAEA
6) : Quantum Condensed Matter Division, Oak Ridge National Laboratory, USA,7) : Institut Laue-Langevin, France

Spin fluctuations are widely believed to play an importantrole in the superconducting mechanisms of unconventional hightemperature superconductors. Spin fluctuations have beenobserved in iron-based superconductors as well [1]. In the seriesof iron-based superconductors, the superconductivity appears in
the vicinity of an antiferromagnetic (AF) phase and a structuralphase transition from tetragonal to orthorhombic phases
accompanied by orbital ordering. Therefore, the spin and multiorbital dynamics are believed to play an important role in thesuperconducting mechanisms. The spin dynamics have beenstudied intensively by inelastic neutron scattering (INS),because it is one of the best powerful tools for the study. In thesuperconducting states, magnetic resonance modes have beencommonly observed in the INS spectra of iron-based
superconductors so far. According to the doping-dependence,
the magnetic resonance energies are proportional to thesuperconducting gap energies. At the same time, the low-energy
spin dynamics are well explained by the Fermi surface nestingmodel. On the other hand, the magnetic resonance mode nor thespin fluctuation itself has not been observed in the low-Tc ironbased superconductor LaFePO1-y[2]. This superconductor
LaFePO1-y was discovered as the first superconductor among theiron-based pnictogen compounds [1]. For this system, the lowenergy spin dynamics have also been studied by nuclear
magnetic resonance, suggesting no AF spin fluctuations.
Nevertheless, electron-doped LaFePO1-y shows line-node
symmetry revealed by temperature dependence of magnetic
penetration depth and thermal conductivity measurements [3].
The line-node symmetry corresponds to a sign-reversing orderparameter. It is typically originated by the spin fluctuation.
Therefore, no AF spin fluctuation in LaFePO1-y system has beena long standing mystery. Consequently, it was quite important toobtain dynamical information on spin fluctuations of LaFePO0.9
in a wide energy range in order to examine the existence of thespin fluctuation on the superconductivity with line-node gap
symmetry.
Inelastic neutron scattering measurements have been
performed on powder samples of LaFePO0.9 with Tc= 5 K and
optimally doped LaFeAsO0.918F0.082 with Tc= 29 K as a reference.
In conclusion, the spin fluctuation has been discovered on thislow-Tc LaFePO0.9 at 30-50 meV with similar intensity to the
optimally doped LaFeAsO0.918F0.082 at the normal state,suggesting the universality of the correlation between line-node
symmetry and spin fluctuations [4]. However, the observed highenergy was very unusual for low-Tc superconductors. This
research is expected to trigger a research of searching highenergy spin fluctuation in low-Tc superconductors.
This work was supported by JST, Transformative Research-Project on Iron Pnictides (TRIP), and Grant-in-Aid for Specially
Promoted Research, Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan (No. 17001001). INSexperiments at J-PARC MLF were carried out under project
numbers 2009A0087 and 2014A0114. The experiment at ILL
was conducted under the experiment number DIR-97 as asupport program (Director’s Discretion Time (DIR)) to theearthquake damage of Japanese neutron facilities. A portion ofthis research used resources at the High Flux Isotope Reactor, aDOE Office of Science User Facility operated by the Oak Ridge
National Laboratory, and was partly supported by the US-Japan
Collaborative Program on Neutron Scattering.
References

Fig.1. Momentum-integrated dynamical spin susceptibility
χ”(E) for LaFePO0.9 (red filled circles) andLaFeAsO0.918F0.082 (blue filled circles). All the points
were measured at the normal state of T = 30 K [4]. The
solid and broken lines are guides for the eye.
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[2] A. E. Taylor et al., J. Phys.: Cond. Mat. 25, 425701 (2013).
[3] T. M. McQueen et al., Phys. Rev. B 78, 024521 (2008).
[4] M. Ishikado, S. Shamoto, K. Kodama, R. Kajimoto, M.
Nakamura, T. Hong and H. Mutka, Sci. Rep. 8, 16343 (2018).