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Evidence of a Rattling Transition in the Caged Compounds LaRu2Zn20 and LaIr2Zn20: 139La NMR Studies
Hideki Tou,Kenji Asaki,Hisashi Kotegawa,Takahiro Onimaru,Keisuke T. Matsumoto,Yukihiro F. Inoue,Toshiro Takabatake 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
We report a study of 139La(I=7/2) nuclear magnetic resonance (NMR) to investigate the structuralphase transition in the two caged compounds LaIr2Zn20 and LaRu2Zn20 from microscopic viewpoints. The former compound shows a second-order structural phase transition around Ts = 200 Kwhereas the latter one shows a first-order structural phase transition at Ts = 150 K. For both compounds,quite narrow La-NMR lines of less that 10 kHz without any electric field gradient (EFG) atthe La site were observed at temperatures above Ts, indicating that the local symmetry at the Lasite holds a cubic symmetry. On the other hand, an EFG at La site appears at temperatures belowTs, evidencing symmetry lowering occurs at the La site at temperatures below Ts. At temperaturesaround Ts, both the nuclear spin-lattice relaxation rate (1/T1) and the nuclear spin-spin relaxationrate (1/T2) show unusual enhancements associated with the slowing down of the EFG fluctuations. However, the temperature dependences of 1/T1T and 1/T2 are different from the typical slowingdownphenomena of the EFG fluctuations due to the classical motional narrowing. The lowering ofthe local symmetry at the La site is ascribed to a freezing of the rattling motion.
Pressure Evolution of the Metamagnetic Transition in UCoAl As Measured Using 59Co NMR
Hisashi Kotegawa,Hiroki Nohara,Hideki Tou,Tatsuma D. Matsuda,Etsuji Yamamoto,Yoshinori Haga,Zachary Fisk,Yoshichika Onuki,Dai Aoki,Jacques Flouquet 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
We have performed NMR measurements under pressure in UCoAl with a quantum critical endpointof the metamagnetic transition from the paramagnetic phase to the ferromagnetic (FM) phase. 59Co-NMR sensitively detects the evolution of the internal field caused by applying the externalfield. The metamagnetic field Hm increases with increasing pressure consistently with other experimentalmethods, accompanied by the suppression of the magnetization in the field-induced FMphase and the magnetization jump at Hm. The loss of the NMR signal on approaching the QCEPindicates the development of the spin fluctuations.
Cu-NMR Studies of the Heavy-Fermion Compound CeCu6 under High Magnetic Fields
Keisuke Kuroda,Kyohei Morita,Hisashi Kotegawa,Hitoshi Sugawara,Hideki Tou 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.12
We report a Cu NMR Knight shift K in a prototypical heavy-fermion system CeCu6 in thetemperature range of 1.5 K - 100 K at applied magnetic fields up to 15 T. With increasing magneticfield, the 63Cu NMR Knight shift for Hk[001] is suppressed drastically. The suppression of K withincreasing field is consistent with the field dependence of the susceptibility. In order to explainthis behavior, we adopted the Kondo resonance level model, where the thermodynamics at lowtemperatures are proportional to the density of states at the energy µH. The resonant model wasfound to correctly predict the suppression of the Knight shift with increasing magnetic field.