No Evidence for "Small-Moment Antiferromagnetism" under Ambient Pressure in URu_2Si_2 : Single-Crystal ^<29>Si NMR Study(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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The still unidentified hidden order (HO) phase below T_0 = 17.5K in the heavy-electron superconductor URu_2Si_2 has been investigated by ^^<29>Si NMR on a single crystal under external magnetic field H_<ext> ‖ c-axis and ambient pressure P. For a previously proposed model of the small-moment antiferromagnetism (SMAF), in which tiny-moment antiferromagnetism (AF) under ambient P is ascribed to spatially homogeneous AF with an ordered moment μ_<SMAF> = 0.02-0.04 μ_B, splitting of the line would be expected on the basis of a scaling from the corresponding splitting in the P-induced AF. However, we observed at T = 4.5 K no splitting of the line. This clearly indicates that the SMAF model cannot describe the ambient-P properties. Below T_0 down to 〜14 K, the ^^<29>Si NMR line shows additional broadening over the paramagnetic width. The additional contribution, however, disappears below 〜14K, which indicates that the HO itself produces almost zero internal field at the ^^<29>Si site along the c-axis at low temperatures in the HO phase.
社団法人日本物理学会の論文
2007-03-15