Isikawa Yosikazu

Universal scaling in the dynamical conductivity of heavy fermion Ce and Yb compounds

Dynamical conductivity spectra [σ(ω)] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the infrared region of σ(ω) for all the compounds, and has been analyzed in terms of a simple model based on conduction ( c )– f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c – f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c – f hybridized band can generally and quantitatively describe the low-energy charge excitations in a wide range of HF compounds.

High Field Magnetoresistance and de Haas-van Alphen Effect in LaNi

The transverse magnetoresistance and de Haas-van Alphen (dHvA) effect in LaNi have been studied in the fields up to 150 kOe. It was found from the magnetoresistance that the Fermi surface of LaNi contains two open surfaces elongated along the b - and c -axes. Many dHvA branches with the frequencies ranged from 7×10 5 Oe to 1.5×10 8 Oe were found in the fundamental planes. The cyclotron effective masses range from 0.4 m 0 . to 3.3 m 0 .

Isotropic Γ6 Ground State in Caged Cubic Compound NdRu2Zn20

The magnetic susceptibility χ, magnetization \(M\), and specific heat \(C\) were measured for the caged cubic compound NdRu2Zn20 at temperatures down to 0.5 K in magnetic fields \(H\) along the three principal crystallographic axes, [100], [110], and [111]. A ferromagnetic phase transition was observed at the Curie temperature \(T_{\text{C}}=1.9\) K in the temperature dependences of both χ(T) and \(C(T)\). Below \(T_{\text{C}}\), the magnetization curves \(M(H)\) show very weak magnetic anisotropy, and the \(C(T)\) curves in the fields along the three principal axes show no magnetic anisotropy both below and above \(T_{\text{C}}\). The easy direction of magnetization at 0.5 K changes from the [111] direction at 0 T to the [110] direction at 7 T. We analyzed these characteristic behaviors theoretically by taking into account the crystalline-electric-field energy, the magnetic exchange interaction, and the Zeeman energy. It was found that these features originate from the isotropic Γ6 ground state mixing wi...The magnetic susceptibilityχ, the magnetizationM, and the specific heat C were measured for the caged cubic compound NdRu 2Zn20 at temperatures down to 0.5 K in the magnetic field H along the three crystallographic principal axes, [100], [110], and [111]. The ferromagnetic phase tran sition was observed at the Curie temperature TC = 1.9 K in both the temperature dependence of χ(T ) andC(T ). BelowTC, the magnetization curves M(H) show very weak magnetic anisotropy, and the C(T ) curves in the fields along the three principal axes show no ma gnetic anisotropy both below and aboveTC. The easy direction of magnetization at 0.5 K changes from th e [111] direction in 0 T to the [110] diretion in 7 T. We analyzed these characteristic behaviors theoretica lly taking into account the crystalline-electric-field ene rgy, the magnetic exchange interaction, and the Zeeman energy. It wa s found that these features are originated from the isotropi c Γ6 ground state mixing with the magnetic Γ (1) 8 excited state. The temperature and the magnetic field depend nces of M(T,H) andC(T,H) are quantitatively well analyzed by this theoretical calc ulation.

Optical Conductivity of CeNiSn, CeRhSb, and CeRhAs

The optical reflectivity spectra of the Kondo semiconductors CeNiSn, CeRhSb, and CeRhAs have been measured in a wide photon energy range to obtain the optical conductivity spectra σ(ω). For CeNiSn and CeRhSb, two pronounced peaks in the mid-infrared (IR) region and small pseudogaps in the far-IR region were observed at low temperatures ( T ). The two peaks are attributed to optical transitions related with the two bands arising from the spin–orbit splitting of Ce 4 f . For CeRhAs, σ(ω) exhibits a gap which develops gradually with cooling. The observed (pseudo)gap widths and T dependent variations are largest for CeRhAs and smallest for CeNiSn, well corresponding to their Kondo temperatures. For CeNiSn and CeRhSb, the T dependence of σ(ω) in the far-IR suggests the formation of a narrow Drude peak caused by a c – f mixing. The results may indicate that the pseudogap is opened in a heavy quasiparticle band.

Thermoelectric Power Anomaly of PrTi2Al20 and PrV2Al20 with Non-Kramers Γ3 Ground State

We investigated thermoelectric properties for PrTi2Al20 and PrV2Al20 below room temperature down to 0.13 K. A clear positive peak has been observed at 75 K in the thermoelectric power \(S\) for PrTi2Al20. Also for PrV2Al20, the similar \(S\) peak has been detected at 40 K. These peaks arise from the Kondo effect in the magnetic excited levels of the \(4f\)-electronic state under a cubic crystalline electric field (CEF). In addition, for PrTi2Al20, \(S\) takes a minimum with a negative sign at 10 K owing to the CEF effect, and shows an additional tiny negative hump below 2 K as a signature of quadrupole ordering. On the other hand, \(S\) for PrV2Al20 manifests a large negative peak at 3 K with the magnitude of 3 μV/K. This gives a large enhancement in absolute \(S\) divided by temperature \(T\), \(|S|/T\), approximately below 10 K on cooling. \(|S|/T\) reaches 4 μV/K2 at 0.2 K, which is larger than that in the zero-\(T\) limit for a heavy-electron compound CeRu2Si2. This observation strongly indicates that...

Study of 57Fe Mössbauer Effect on DyFe2Zn20 and YFe2Zn20

The 57Fe Mossbauer effect of DyFe2Zn20 and YFe2Zn20 was studied at temperatures between 8 and 301 K, and at room temperature in the presence of an external magnetic field. The spectra of the two compounds appeared almost the same at temperatures above 77 K. Each spectrum exhibited a double peak, which is attributed to the electric field gradient (EFG) present at the Fe nuclei. However, the spectrum of DyFe2Zn20 at 8 K exhibited a complicated shape, resulting from a magnetic hyperfine field and the EFG at the Fe nuclei. From the detailed analyses of the spectra, we concluded that Fe magnetic moments interact antiferromagnetically with Dy magnetic moments and that the Fe magnetic moments in DyFe2Zn20 at 8 K are approximately 0.2 µB/Fe.

Magnetic and Specific Heat Properties of NdxFe4Sb12

Nd x Fe 4 Sb 12 single crystals with x = 0.774–0.901 were synthesized by different heat treatment methods in the Sb self-flux method. The magnetization and the specific heat for the single crystalline samples of Nd x Fe 4 Sb 12 were measured. The ferromagnetism of Nd x Fe 4 Sb 12 was found to strongly correlate with x ; with increasing x from 0.774 to 0.901 the ferromagnetic transition temperature, T C , increases from 8.6 to 15 K. From this variation, T C is expected to reach 20 K at x = 1.0.

Electronic Structures of CeNi 2Al 5 and CeNiAl 4 Studied by Photoelectron Spectroscopy

We have studied the electronic structures of ternary intermetallic compounds CeNi 2 Al 5 and CeNiAl 4 on the basis of Ce 3 d core level spectra by X-ray photoelectron spectroscopy and high resolution Ce 4 d →4 f resonant photoemission spectra. In the 4 f photoemission spectra, fine structures near the Fermi edge manifest that hybridization strength is considerably larger in CeNiAl 4 than in CeNi 2 Al 5 . The electronic structures of these compounds are discussed in connection with those of Ce–Ni and Ce–Al intermetallic compounds.

Possible Heavy Fermion State of the Caged Cubic Compound NdV2Al20

The magnetic susceptibility M/H, the magnetization M, and the specific heat C were measured for the caged cubic compound NdV2Al20 single crystal at temperatures down to 0.5 K in magnetic fields H along the three principal crystallographic axes [001], [101], and [111]. The electrical resistivity ρ was measured in the temperature range of 1.9–300 K. A ferromagnetic phase transition was observed at a Curie temperature TC of 1.81 K. The easy direction of magnetization was the [001] direction at low fields, and changed to the [101] direction above 2 T. On cooling below 2 K at zero field, the specific heat divided by temperature, C/T, resulted in a large electronic specific heat coefficient γ of about 2 J/(K2 mol). A large C/T of more than 1 J/(K2 mol) remained at 9 T below 2 K. The temperature dependence of ρ showed the −ln T dependence at around 10 K. These behaviors possibly originate from a heavy fermion state.

Magneto-Acoustic Effect of Heavy Fermion Compound CeInCu2

We measured the temperature dependences of elastic constants of the heavy fermion compound CeInCu 2 and the reference compound LaInCu 2 . The transverse C 44 and (C 11 -C 12 )/2 modes in CeInCu 2 exhibit monotonous temperature dependences. The bulk modulus C B , however, shows an anomalous softening of 6% below 130 K, which is in contrast to the normal behavior in LaInCu 2 . The anomalies of C B in CeInCu 2 indicate the coupling between the volume strain and the heavy fermion state with a large electronic Gruneisen constant Ω g ≃50 at 5 K.