Dynamical properties of guest ions in the type-I clathrate compoundsX8Ga16Ge30(X

Abstract

The resistivity and Hall effect in CeNiSn are measured at temperatures down to 35 mK and in magnetic fields up to 20 T with the current applied along the b axis. The resistivity at zero field exhibits a quadratic temperature dependence below ϳ0.16 K with a huge coefficient of the T 2 term (54 ⍀ cm/K 2 ). The resistivity as a function of field shows an anomalous maximum and dip, the positions of which vary with field directions. Shubnikov-de Haas ͑SdH͒ oscillations with a frequency F of ϳ100 T are observed for a wide range of field directions in the ac and bc planes, and the quasiparticle mass is determined to be ϳ(10-20)m e . The carrier density is estimated to be ϳ10 Ϫ3 electron/Ce. In a narrow range of field directions in the ac plane, where the magnetoresistance-dip anomaly manifests itself clearer than in other field directions, a higherfrequency (Fϭ300-400 T) SdH oscillation is found at high fields above the anomaly. This observation is discussed in terms of possible field-induced changes in the electronic structure.Single-crystalline ingots of CeNiSn were grown by the Czochralski method and were purified by a solid-state elec-PHYSICAL REVIEW B 66, 075127 ͑2002͒

Optical conductivity of rattling phonons in type-I clathrateBa8Ga16Ge30

“…On the contrary, recent more detailed structural analyses [6] indicate that the Ba 2+ (2) ions are slightly displaced from the center depending on temperature and also carrier-type. This structural indication seems to be consistent with Raman scattering measurements of low lying additional modes [7].…”

“…3. With decreasing temperature down to 6.6 K, the low-lying ν 1 and ν 2 modes soften by 4.2%, being consistent with the Raman-active modes at 1.05 THz (35 cm −1 ) with E g symmetry [7], and 1.8% respectively. On the other hand, other higher-frequency modes remain almost constant or harden slightly.…”

“…On the contrary, recent more detailed structural analyses [6] indicate that the Ba 2+ (2) ions are slightly displaced from the center depending on temperature and also carrier-type. This structural indication seems to be consistent with Raman scattering measurements of low lying additional modes [7].Single crystals of p-type BGG are grown by a self-flux method [8]. To obtain transmitting signals as strong as possible through the conductive sample, single crystal disks of 2 -5 mm in diameter and 0.5 -1.0 mm in thickness are polished down to about 20 µm in thickness using diamond abrasive films.…”

“…For the assignment of the phonon modes, we have made first-principle calculations [7] based on the density functional approximation for infrared-active optical modes of T 1u symmetry. Table I lists the phonon frequencies la- beled as ν i (i = 1 − 6) together with fitting parameters at T = 6.6 K. The low-lying peaks observed at 1.15 and 1.80 THz can be assigned as the modes labeled ν 1 and ν 2 of the Ba 2+ (2) in the oversized cage vibrating within the plane perpendicular to the fourfold axis and along the fourfold axis, respectively.…”

NMR relaxation and rattling phonons in the type-I Ba8Ga16Sn30clathrate

“…Understanding the guest atom vibration modes, often called "rattling", has been considered to be one of the most important ways to reveal the essence of these phenomena. Many methods including Raman scattering 10,11 , inelastic neutron scattering 12 , optical conductivity 13,14 , NMR relaxation 15 and theoretical calculations 16,17 have been reported to analyze the rattling atoms in clathrates using different models.…”