Tokunaga Y, Aoki D, Homma Y, Kambe S, Sakai H, Ikeda S, Fujimoto T, Walstedt R E, Yasuoka H, Yamamoto E, Nakamura A, Shiokawa Y
ASRC, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan.
Phys Rev Lett. 2006 Dec 22;97(25):257601. doi: 10.1103/PhysRevLett.97.257601. Epub 2006 Dec 19.
We report a microscopic investigation of multipolar order parameters in the ordered state of NpO2 conducted via 17O NMR on a single crystal. From the angular dependence of hyperfine fields at 17O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below T0 = 26 K. We have also observed oscillatory spin-echo decay, which is well understood in terms of small electric field gradients created by antiferro-quadrupolar ordering. This reveals that the quadrupolar order parameter is directly observable by means of NMR. The present NMR studies provide definitive support for a proposed longitudinal triple-q type octupolar-quadrupolar ordering model for NpO2.
我们报告了一项通过对单晶进行¹⁷O核磁共振（NMR）对NpO₂有序态中的多极序参量进行的微观研究。从¹⁷O核处超精细场的角度依赖性，我们获得了明确的证据，表明在T₀ = 26 K以下出现了场诱导的反铁磁八极矩以及场诱导的反铁磁偶极矩。我们还观察到了振荡自旋回波衰减，这可以根据反铁磁四极有序产生的小电场梯度很好地理解。这表明四极序参量可通过NMR直接观测到。目前的NMR研究为所提出的NpO₂纵向三q型八极 - 四极有序模型提供了确凿的支持。
