A. Ochiai

New observations concerning the low-temperature properties of the carrier-poor heavy-fermion system Yb4As3

Abstract We report low-temperature specific-heat and thermal-expansion results on the carrier-poor system Yb 4 As 3 as well as Yb 4 (As 1− y X y ) 3 , X: P, Sb. We infer that Yb 4 As 3 behaves as a two-fluid system with almost decoupled spin and charge excitations. Quasi-one-dimensional spin excitations within S = ½ Heisenberg chains of Yb 3− ions formed in the charge-ordered state dominate the low-temperature thermodynamic properties. The effect of a magnetic field is to open a (pseudo-)gap in the low-energy spin-excitation spectrum. The Gruneisen parameter shows rather anomalous temperature and field dependences.

Low-temperature magnetization study of the heavy-electron systems

Abstract The ground-state magnetic properties of Ce0.75La0.25B6 and Sm3Te4 have been studied by means of static magnetization measurements at very low temperatures T ⩾ 50 mK. In Ce0.75La0.25B6, the H − T phase diagram reveals three different ordered phases: in addition to the two phases (phase II and III) known for CeB6, a new antiferromagnetic state appears before the antiferro-quadrupolar order sets in. In the valence fluctuation compound Sm3Te4, a spin-glass ordering is evidenced at Tg ∼ 1.3 K. It is likely that the two valence states (Sm2+, Sm3) are randomly frozen at low temperature to form effectively a randomly diluted magnet.

Strongly correlated electron system without free carrier Yb4(As1-xPx)3

Abstract Mixed valence compound Yb4As3 which shows the charge ordering of Yb3+ and Yb2+ around 300 K has been considered to be a heavy fermion system with extremely low carrier concentration. So the main interest in this system is to know whether the heavy fermion behavior surely comes from such low carrier concentration. To make this situation clear, single crystals of Yb4(As1-xPx)3 were grown and their transport properties, magnetic susceptibility and specific heat were measured. With increasing concentration of phosphor the system becomes semiconductive, however, the specific heat and magnetic susceptibility are almost the same as those of pure Yb4As3. This result shows that the large γ-value of Yb4As3 is not due to the mass enhancement as seen in usual heavy fermion systems but arises from another magnetic correlation.

Charge order and one-dimensional properties of Yb4As3

Abstract A polarized neutron diffraction experiment gives direct evidence for a charge order in the extremely low-carrier system Yb4As3. Inelastic neutron scattering experiments on Yb4As3 revealed the existence of low-energy spin excitations that are characteristic of a one-dimensional antiferromagnetic coupling at low temperatures. The results indicate that the one-dimensional arrangement of Yb3+ ions caused by the charge order plays a main role for the heavy-electron behavior in Yb4As3.

Systematic study in mixed valence system Yb4(As1−xSbx)3

Abstract In Yb pnictides with anti-Th 3 p 4 structure, the valence of Yb decreases from about 2.25 in Yb 4 As 3 to about 2 in Yb 4 Bi 3 with increasing weight of pnictogen. In Yb 4 As 3 , charge ordering around 300 K is observed and in the charge ordered phase heavy fermion behavior is seen in spite of extremely low carrier concentration. On the other hand, Yb 4 Sb 3 is a valence fluctuating system. So, to make systematic studies from Yb 4 As 3 to Yb 4 Sb 3 we grew single crystals of Yb 4 (As 1− x Sb x ) 3 . With increasing Sb content, the absolute value of the resistivity decreases due to the increase of the number of carriers and the charge ordering temperature decreases reflecting the increase of the screening effect, and at 29% charge ordering seems to disappear. The γ-value, however, is still large even in Sb 29% sample. The large γ-value of Yb 4 (As 1− x Sb x ) 3 is considered to be independent ofthe carrier concentration.

X-ray diffraction studies of lattice properties in CeX (X = P and As) and Yb4As3

Abstract X-ray diffraction experiments were performed in order to study the lattice properties of the low-carrier-density strongly correlated electron systems CeX ( X = P and As) and Yb 4 As 3 . The lattice constants of CeP and CeAs take minima at 120 and 90 K, respectively. These results are explained by the model in which each crystal-field state is supposed to have its own effective ionic radius. The cell angle of Yb 4 As 3 shows the first-order-like increase from 90.0° to 90.7° accompanied with the charge ordering at T C = 288K. It gradually increases to 90.8° with decreasing temperature and is almost independent of temperature below 200 K. The temperature dependence of the cell angle is discussed in connection with the process of the charge ordering.

Anomalous transport and electronic properties of Yb4As3 under high pressure

Abstract Yb4As3 shows anomalous transport phenomena below the phase-transition temperature associated with an order-disorder transition due to the atomic arrangement between Yb3+ and Yb2+. Although the LIII spectrum shows no change in valence above and below the temperature, the large increase of the trivalent component has been clearly observed with increasing pressure. Comparing the pressure- and temperature-dependent resistivity between Yb4As3 and Yb4Sb3, it is suggested that the anomalous transport properties of Yb4As3 are due to a characteristic electronic structure modulated with the phase transition.

Low energy excitation of Yb4As3

Abstract Temperature dependence of electronic structure near the Fermi level of Yb 4 As 3 was investigated by the reflectivity measurement. An absorption band with strong temperature dependence was observed around 0.4 eV. The band shifts to the low energy side and becomes broad as the temperature increases. The electronic state which makes the absorption crosses the Fermi level at the temperature of 200 K. The anomalous conducting property is considered to relate to the electronic band.

Magnetic Compton study of a USe single crystal

Magnetic Compton scattering experiments on a USe single crystal have been performed at several temperatures around the Curie temperature of the specimen. The spin and the orbital moments of USe are deduced by combining the magnetic Compton scattering experiment with the magnetization measurement. Furthermore, the spin moment of USe is decomposed into a 5f electron component and a diffused component. We have obtained the spin moment of the 5f electron , the orbital moment of the 5f electron and the diffused moment . From comparison with the previous results of UTe, it is concluded that the increase of the total magnetization from USe to UTe can mainly be ascribed to the decrease in magnitude of spin moment of the 5f electron with a slight increase of the orbital moment of the 5f electron. The results are also discussed from the points of view on the hybridization between U 5f and U 6d electrons and/or U 5f electrons and valence electrons of chalcogenide.

Elastic anomalies of charge ordering in Yb4As3

Elastic constants of the mixed valence compound Yb4As3 with extremely low carrier 0.001 per Yb atom have been measured by the ultrasonic method. We found that the transverse C44 mode exhibits a pronounced softening around the structural phase transition point at Tc = 292 K due to the charge ordering of Yb3+ ions along <111 direction. We interpret the softening of C44 with the coupling of the strain eyz, ezx and exy to charge fluctuation mode processing the electric quadrupole moment in Yb3+ linear chain.