Yasushi Okayama

Normal and Anomalous Hall Effect in CeSb and CeBi

Anomalous behavior of Hall constant and magnetoresistance in CeSb and CeBi under high magnetic field and high pressure is studied critically. At first, anomalous field dependence in the reference systems LaSb and LaBi is investigated based on the three band model in which two kinds of electrons are treated independently and it is concluded that the anomalous behavior is due to a small deviation from the stoicheometry, which is enhanced by a large mobility. In CeSb without pressure, no anomalous Hall effect is observed even in the ferromagnetic state. On the other hand, the normal Hall constant in CeSb and CeBi shows strong electron scattering in the whole temperature region confirming the model that the electrons also substantially contribute to the Γ 8 Kondo state. To study the anomalous Hall effect under high pressure and high magnetic field, general equations for anomalous Hall effect under high magnetic field are derived, and the anomalous Hall effect is identified to exit in the region in which a kin...

Pressure effects on crystal electric field excitations in ultra-low-carrier Kondo-lattice systems CeP and CeAs

Pressure effects on crystal electric field (CEF) excitations in ultra-low carrier systems CeP and CeAs have been studied up to 2.8 GPa. There are two critical pressures P c and P B : 0.25 GPa and 1.1 GPa for CeP, while 1.3 GPa and 2.5 GPa for CeAs, respectively. CEF excitations are well-defined for P < P B , and there is no anomaly at the onset pressure of the ferromagnetic order, P c . In contrast, striking anomalies have been observed around P B . For CeP, the CEF excitation decreases to 12 meV and a new CEF excitation appears at 9 meV near P B , then it gradually replaces the first one above P B . For CeAs, a CEF excitation exhibits strong softening as well as broadening near P B . Finally, the CEF spectra exhibit strong broadening around 40 K in the low tempe rature phase near and above P B .

Localization effects of kondo semimetals CeNiSn and CeRhSb

Abstract We have found that the semiconductor-like increase in resistivity previously reported on CeNiSn and CeRhSb below about 8 K is suppressed with decreasing impurity concentration in single-crystalline samples. Nevertheless, the absolute values of the Hall coefficient strongly increase as temperature decreases. Therefore, we interpret the metallic behaviour found in the a -axis resistivity of a purified crystal of CeNiSn as a consequence of the significant increase of relaxation time of carriers. Even for the purified crystal, the residual γ value is 40 mJ/K 2 mol at 0.03 K. These results suggest that the energy gap is closed along the a -axis and that residual carriers are strongly localized in the presence of impurities.

Novel magnetic structures of the low-carrier system CeP under high pressure

Abstract The results of neutron scattering experiments on the low-carrier system CeP under high pressure up to 1.7 GPa are presented. The magnetic structures derived from the scattering patterns at various pressures at low temperatures are similar to those of CeP under magnetic field, forming the periodic stacking of the double Γ8 Ce (0 0 1) planes with sandwiching the Γ7 Ce (0 0 1) planes. The difference is a wide and systematic change of the period of the stacking of Γ8 Ce (0 0 1) planes with changing pressure. These results support strongly the magnetic polaron model for the unusual properties of CeP.

Pressure Effect on the Charge-Density Waves and the Superconductivity in NbSe3

The pressure effect on the CDW formation and the superconductivity in NbSe 3 is investigated up to 18 kbar by resistive and diamagnetic measurements. Both the T 1 -CDW and the T 2 -CDW are suppressed under pressure, as reported earlier. The superconductivity appears at pressures above P c =7.5 kbar where the T 2 -CDW is suppressed totally. It was confirmed that the superconducting transition temperature T c exhibits the maximum value just above P c while it decreases gradually at higher pressures. It was found that the initial slope of ln T 2 and the slope of ln T c above 9.5 kbar are almost the same. The results are discussed in terms of both collapse of the nesting of the Fermi surfaces and reduction of a dimensionless electron-phonon coupling constant under pressure.

Neutron Diffraction Study of the Magnetic Ordering of Low Carrier Kondo Lattice System CeAs under High Pressure

Neutron diffraction measurements were performed on CeAs under high pressure. Above the critical pressure, P c =1.1 GPa, two magnetically ordered phases appear. The ferromagnetic ordering is established in the upper temperature phase. The type-I ordering is established in the lower temperature phase in addition to the ferromagnetic ordering, which brings about the canted spin structure. The temperature dependence of the Bragg intensity of the type-I ordering has a long tail above the Neel temperature. The ferromagnetic moment increases with increasing pressure, while the antiferromagnetic moment decreases particularly above 2.0 GPa. The total magnetic moment, which is the expected Γ 7 value below P c , decreases abruptly at P c .

Effect of pressure on the electrical resistivity and Hall effect in single crystals of CeRhSb

Abstract The Hall effect and electrical resistivity have been measured under pressure up to 7.3 kbar using purified single crystals of CeRhSb. Electrical resistivity and especially the Hall coefficient show a large increase under pressure at low temperatures. These results could be due to an increase of the gap energy under pressure (confirmed by an Arrhenius plot of the resistivity and Hall effect data). These pressure effects are anisotropic and the minimum is for Ia, Hb. It is notable that the b direction coincides with the minimum lattice constant of the orthorhombic structure.

Anomalous Hall Effect in Ce Monopnictides

The anomalous Hall Effect in CeAs and CeP is studied critically based on the recent detailed experimental study under pressure of up to 2.6 GPa, as well as a vast amount of data on other rare-earth monopnictides and monochalcogenides. It is shown that the anomalous Hall effect is observed only in the intermediate region where the magnetic polaron liquid state becomes unstable and the transition to the valence fluctuation state begins to occur. Strong field dependence of the anomalous Hall effect is explained very well by a simple skew scattering model.

Pressure-induced electronic phase transition in CeP

High pressure effects on the temperature dependent resistivity of single crystalline CeP were studied at temperature from 4.2 to 300K under pressure up to 8GPa. The resistivity anomalies associated with the magnetic dense Kondo state are found to change drastically with pressure. A resistivity jump in the residual resistivity is observed at a pressure between 5 and 6GPa. This abrupt change in resistivity clearly indicates a pressure-induced electronic phase transition between a localized to an itinerant state of 4f electron. A pressure-temperature phase diagram of CeP is proposed.

Unusual electronic state of the low-carrier system CeSb under high pressure studied by simultaneous measurement of electrical resistivity and lattice parameter

We have studied the origin of the anomalous peak structure of resistivity of the low-carrier system CeSb appeared around the antiferromagnetic transition temperature T N under high pressures by means of simultaneous measurement of lattice parameter by X-ray diffraction and resistivity together with neutron diffraction technique. It was found that there is a strong relationship between the unusual decrease of lattice parameter and the unusual increase of resistance below T N under high pressure. Both of these phenomena can be explained by the appearance of Γ 8 -like magnetic state of Ce 3+ ion below T N .