Observing spin fluctuations by neutrons; recent development ofneutron inelastic spectroscopy
Taku J Sato
Neutron Science Laboratory, Institute for Solid State Physics, University ofTokyo, 106-1 Shirakata, Tokai, Ibaraki 319-1106, Japan
In this talk, I will briefly overview the present status of the neutron inelastic
spectroscopy in Japan. As the construction of the new spallation neutron source
J-PARCC/MLF has completed, a number of powerful inelastic neutron spectrometers
are online. In addition, after roughly 20 years of operation, the research reactor JRR-3
is now fully equipped with matured inelastic spectrometers; recent development ofneutron optics enables much efficient experiments at the reactor spectrometers. The
combination of the inelastic spectrometers at the stead-state and pulsed neutron sources
will provide novel opportunities for condensed matter sciences in this country.
The neutron inelastic scattering is an unparalleled technique for detecting spin
fluctuations in quantum magnets, since it can observe the Q and  dependence of thegeneralized magnetic susceptibility. In this talk I would try to exemplify theuniqueness of the neutron magnetic inelastic scattering technique, and to foresee its
future direction, by using our recent research outcomes.
The first one is the role of the spin fluctuations in the Fe-based superconductors. TheFe-based superconductors were discovered in 2008, and now has the second highest
superconducting transition temperature, next to the cuprates, and thus is actively
investigated by many researchers all over the world. A close relation between theantiferromagnetic fluctuations and the high-Tc superconductivity was inferred from thebeginning. To experimentally confirm the relation, we have performed neutron
inelastic scattering experiments on the BaFe2As2 parent (antiferromagnetic) compound,and its superconducting phases realized by electron doping (Ba(Fe,Co)2As2) [1]. We
have confirmed that the spin fluctuations are significantly suppressed in the overdoped
region, where the superconductivity is also suppressed, evidencing the close relation
between the antiferromagnetic spin fluctuations and the superconductivity.
Secondly, I will talk on the formation of the valence-bond-solid (VBS) state in thequantum kagome antiferromagnet [2]. Ground state of the s=1/2 quantum kagome
antiferromagnet has been studied intensively for decades. Nevertheless, it is still
controversial if there is a gapless resonating-valence-bond (RVB) state, or its solidified
kind VBS, is realized or not. Using the single-crystalline sample of the deformed
kagome compound Rb2Cu3SnF12, we have unambiguously confirmed that the pinwheel
VBS state is stabilized. This confirmation can be only obtained by inelastic neutron
spectroscopy, since the ground state has no detectable spins. This way, the neutron
inelastic spectroscopy is the key tool to study the quantum coherent state of theantiferromagnetically coupled systems.
[1] K. Matan, S. Ibuka, R. Morinaga, S. Chi, J. W. Lynn, A. D. Christianson, M. D.
Lumsden, and T. J. Sato, Doping dependence of spin dynamics in electron-doped
Ba(Fe1-xCox)2As2, Phys. Rev. B 82 (2010) 054515-1-5.
[2] K. Matan, T. Ono, Y. Fukumoto, T. J. Sato, J. Yamaura, M. Yano, K. Morita, and H.
Tanaka, Pinwheel valence-bond solid and triplet excitations in the two-dimensional
deformed kagome lattice, Nature Phys. 6 (2010) 865-869.