T. Suzuki

Ferromagnetic Mixed-Valence and Kondo-Lattice State in TmTe at High Pressure

Neutron diffraction experiments on TmTe at pressures up to 7 GPa are reported. The semiconductor-to-metal transition occurring at 2 GPa in this compound is accompanied by the appearance of strong ferromagnetic exchange interactions producing a Curie temperature as high as 14 K. This behavior is characteristic for the incipient mixed-valence regime just above the transition pressure and traced back to the release of a small concentration of free charge carriers in the material. The steep decrease of both the Curie temperature and ordered magnetic moment occurring at higher pressures emphasizes the role of Kondo fluctuations, and raises the interesting possibility that a quantum critical transition to a non-magnetic ground state of mixed-valence Tm might take place around 6 GPa.

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.

Low-energy excitations of the semimetallic one-dimensional S=1/2 antiferromagnet Yb4As3 ☆

We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

Neutron scattering study of crystal-field excitations in TmTe

Abstract Inelastic neutron scattering measurements have been carried out on TmTe to settle the controversial issue of the crystal-field level scheme in this compound. By comparing data obtained with different energy resolutions, it is shown that the total splitting cannot exceed 2 meV. The energies and intensities of the excitations are consistent with a Γ 8 quarter ground-state, but their width is appreciably larger than the resolution. This broadening may be due to spin fluctuations and/or a weak quadrupolar splitting of the quartet state.

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.

Magnetic excitations in the charge ordered state of Yb4As3 and Yb4(As0.6P0.4)3

Abstract Detailed inelastic neutron scattering studies on the low-carrier heavy-electron system Yb4As3 and the semi-conducting mixed compound Yb4(As0.6P0.4)3 reveal that the charge ordering phenomenon, which leads to one-dimensional properties, is common to both compounds. No definite magnetic response other than the one-dimensional spin excitations was observed from the pure material Yb4As3, which exhibits a heavy-electron anomaly in its transport properties, for energy transfers above about 0.3xa0meV.

X-Ray Diffraction Study on Thermal Properties of Crystal Lattices in CeP and CeAs

X-ray diffraction studies at low temperatures have been performed in order to investigate thermal properties of crystal lattices in the low-carrier-density systems cerium monopnictides. CeP and CeAs show large crystal-lattice contractions with increasing temperatures up to about 120 K and 90 K, respectively. Due to the strong p-f mixing, the excited crystal field Γ 8 -like state is expected to produce a shorter interatomic bond length between cerium ions and neighboring pnictogens than that realized by the Γ 7 ground state. The experimental results of the lattice contractions at low temperatures are explained by the thermal average of these crystal field states.

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.

Polarized-neutron study of the magnetic-polaron state in CeP

Abstract The Ce magnetic form factor in different magnetic phases of CeP has been studied by unpolarized and polarized neutron diffraction. In the type-I antiferromagnetic phase ( H=0, T=2 xa0K), the form factor agrees with the calculation for a Γ7 crystal-field ground state. On the other hand, the results for H=3xa0T and T=1.7xa0K (in so-called “phase I”) indicate that the Ce ions with large moments occurring in the structure are in a strongly polarized state very close to the crystal-field state Γ8. This result supports the view that a Γ8-like wave-function is stabilized at ordered Ce sites by the formation of a magnetic-polaron state.