Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically Robust Heavy Fermion System SmOs₄Sb₁₂

Abstract

Elastic properties of the filled skutterudite compound SmOs 4 Sb 12 have been investigated by ultrasonic measurements. The elastic constant C 11 (ω) shows two ultrasonic dispersions at ∼15 and ∼53 K for frequencies ω between 33 and 316 MHz, which follow a Debye-type formula with Arrhenius-type temperaturedependent relaxation times and remain unchanged even with applied magnetic fields up to 10 T. The corresponding activation energies were estimated to be E 2 = 105 K and E 1 = 409 K, respectively. The latter, E 1 , is the highest value reported thus far in the Sb-based filled skutterudites. The presence of magnetically robust ultrasonic dispersions in SmOs 4 Sb 12 implies a possibility that an emergence of a magnetically insensitive heavy fermion state in this system is associated with a novel local charge degree of freedom, which causes the ultrasonic dispersion.KEYWORDS: filled skutterudite, heavy fermion, rattling, elastic constant, ultrasonic dispersionIntermetallic compounds with cagelike structures have been studied both experimentally and theoretically for several decades.1, 2) Recently, this area of research has received intense interest, since various exotic physical properties, e.g., heavy-electron states of heavier rare-earth (Pr, Nd, and Sm) and La-based systems, multipole ordering, unconventional superconductivity, high thermoelectric figures of merit, and anharmonic vibrations of the guest ions, are observed in these types of compounds.3-5) It has been suggested that these exotic phenomena originate from the characteristic cage structure in these systems. A well-known intermetallic system, with a cagelike structure, that exhibits a variety of correlated electron phenomena, are the filled skutterudite.3-6) For instance, an unconventional heavy fermion (HF) state is observed for SmOs 4 Sb 12 , 7) where the electronic specific heat coefficient γ = 820 mJ mol −1 K −2 is insensitive to an applied magnetic field H ∥ [100] up to 8 T.8) This result is in contrast to the expected suppression of γ with magnetic field for the conventional (magnetic) Kondo effect, where the heavy fermion ground state is derived from spin degrees of freedom, as is often observed for Ce-and Yb-based compounds. In contrast, the exotic HF state in SmOs 4 Sb 12 might be explained by a novel many-body effect, such as the multichannel Kondo effect, which is derived from multipole degrees of freedom and/or local charge fluctuations originating from offcenter degrees of freedom of guest ions. 9,10) By ultrasonic measurements, a frequency-dependent upturn of the elastic constant with an ultrasonic attenuation peak, called an ultrasonic dispersion (UD), has been observed in the cage compounds, such as R 3 Pd 20 Ge 6 (R = La, Ce, Pr) and ROs 4 Sb 12 (R = La, Pr, Nd).11-13) Moreover, an unusual decrease in elastic constant has also been observed at very low temperatures in the La-based compounds. Owing to the copresence of UD and low-temperature elastic softening in non-4 f La-based compounds, the origins of these phenomena are expected to ...

Study of localized character of4felectrons and ultrasonic dispersions inSmOs4Sb12by high-pressure high-frequency ultrasonic measurements

“…As seen in Ref. 26, UD in ROs 4 Sb 12 systems appears at higher temperatures as the atomic number increases. In other words, as the ionic mass increases and the radius of the R ion decreases, the temperature at which the UD appears shifts toward higher temperatures, whereas the size of the atomic cage does not change in the ROs 4 Sb 12 systems.…”

“…At ambient pressure, C 11 reveals shoulder-like anomalies near 15 and 55 K, which are ultrasonic dispersions (UDs), and an abrupt decrease near T C due to magnetic ordering, as described in Ref. 26. The elastic anomalies of the UDs reveal a clear dependence on pressure.…”

“…These results also suggest the existence of a Kondo-like screened state and its suppression by strong magnetic fields. Furthermore, ultrasonic dispersions, i.e., frequency-dependent elastic anomalies accompanied by a peak in ultrasonic attenuation, appear near 20 and 55 K in SmOs 4 Sb 12 [26]. These anomalies are considered to be resonances between thermally activated rattling and ultrasonic strain.…”

Quadrupole Ordering and Rattling Motion of Clathrate Compound Pr3Pd20Ge6

“…16. 4,[32][33][34] The activation energies E and the attempt times 0 are both listed in Table II. For these systems, activation energies E in the range of 100-200 K and attempt times 0 of 5 Â 10 À11 -5 Â 10 À12 s are common features for the thermally activated rattling motions of the guest rare-earth ions in cages.…”

Simultaneous Measurements of Elastic Constant and Resistivity near the Tc of Non-BCS-Type Superconductor PrOs4Sb12

“…Thus, the softening of C 11 in the 30 MHz data may be caused by an accidentally mixed C 44 mode. Ultrasonic measurements on a small crystal grown by flux methods often encounter such problems [20]. Figure 2 (a) shows the temperature dependence of the elastic constant C 11 in 0, 1, and 3 T for H 001 .…”