M. Kasaya

Mixed valence properties of YbB12

Abstract The magnetic susceptibility, electrical resistivity, Hall effect, NMR, XPS and specific heat measurements indicate that (1) YbB12 is a mixed valence compound with the valence of + 2.9, (2) as the temperature decreases χ goes through a pronounced peak at 75 K and then decreases rapidly to a constant value after subtraction of impurity effect, (3) the transport properties are very similar to those of SmB6, (4) 1/T1 obtained from 11B NMR shows a temperature dependence of activation type, (5) the magnetic specific heat is of Schottky type. These results suggest that YbB12 is a new mixed valence compound accompanied with the formation of an energy gap. Some of the experimental results are analysed by the model used in SmB6.

Experimental study of lattice dynamics in LaB6 and YbB6

Abstract Lattice dynamics of La 11 B 6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B 6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB 6 , the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB 6 , no softening of C 12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds.

A new and typical valence fluctuating system, YbB12

Abstract Magnetic susceptibility, electrical resistivity, Hall effect and L III absorption measurements indicate that YbB 12 exhibits unique behavior uncharacteristic of hitherto studied ytterbium compounds. We suggest that YbB 12 is a valence fluctuating system accompanied with the formation of an energy gap.

Magnetic and transport properties of pure and carbon‐doped divalent RE hexaboride single crystals

Single crystals of pure and carbon‐substituted divalent hexaborides EuB6 and YbB6 have been prepared by the Al flux method. Lattice parameters, density measurements, and microprobe analysis show that the single crystals are very close to stoichiometry. EuB6 is ferromagnetic (TC=12.5 K and ϑp=15 K) whileYbB6 is diamagnetic. Resistivity and Hall effect measurements carried out over a wide temperature range show that both pure hexaborides are semiconductors with a small intrinsic gap. The carbon‐substituted hexaborides RE B6−xCx (RE=Eu, Yb) have a metallic behavior due to the donation of electrons to the conduction band by carbon atoms. Metallic conduction, at low temperature, in the ’’pure’’ hexaborides is attributed to the formation of an impurity band containing more than 1018 electrons per cm3.

Field-induced metallic state in YbB12 under high magnetic field

High field magnetization and magnetoresistance of YbB 12 are investigated up to 550 kOe at low temperatures with special interest in the semiconductive band gap energy under the field. Very large negative magnetoresistance is found and the system is perfectly metallic around 500 kOe with anomalous increase in the magnetization. The experimental results are explained by introducing the model that the band gap produced by the mixing of the f- and conduction electrons is destroyed by applying the field and the released f-electron gives an anomalous magnetization. The g J -value is estimated as 1.9 for the f-level and this means that \(\varGamma_{8}\) may be most favourable for the responsible f-electron at the Fermi energy.

Mechanisms for anomalous properties in SmB6

Abstract The present situation in SmB 6 is reviewed. Properties of the gap in the 4f pseudo-band and its mechanism are studied on the new experimental results for transport, ultrasonic and NMR. They are interpreted on the model of the amorphous Wigner crystallization with an opening gap consistently.

Semiconducting properties of the isomorphous compounds, Ce3Au3Sb4 and Ce3Pt3Sb4

Abstract Lattice parameters, electrical resistivity, Hall constant, magnetic susceptibility and specific heat measurements have been made on R 3 Au 3 Sb 4 ( R = La, Ce, Pr) and R 3 Pt 3 Sb 4 (R = Ce, Pr). It is found that La 3 Au 3 Sb 4 and Ce 3 Au 3 Sb 4 show semi-metallic or semiconducting behaviors, while Pr 3 Pt 3 Sb 4 shows metallic one. Furthermore, it is revealed that the value of the valence of Ce ions in Ce 3 Au 3 Sb 4 is well-defined 3+. These results indicate that R 3 3+ Au 3 Sb 4 and R 3 3+ Pt 3 Sb 4 are narrow-gap semiconductors and metals, respectively, in agreement with our band calculation on La 3 Au 3 Sb 4 . As for the Ce 3 Pt 3 Sb 4 , it also shows semiconducting property and the Ce ions is in a mixed valence state, similar to that in CeO 2 .

Systematics of valence fluctuating states in f-electron systems

Abstract Results of our theoretical and experimental studies on the anomalous Kondo ferromagnet CeRh3B2 based on the f0 (lz = 0) band and the d0 screening of the 4f hole are shown. Gap formation in the unique compounds YbB12 and Ce(Pd1−xCux)3 is described by comparing with SmB6. Gap creation on the Kondo lattice is thereby established. Finally, a dense Kondo state in samarium compounds is shown in our recent experiments compared with that in cerium compounds.

Quadrupolar ordering and dense kondo behaviour in SmSn3

Abstract From the specific heat measurement on SmSn 3 , it is found that the ground state of Sm 3+ ( J = 5 2 ) ions is Γ 8 quartet and that the magnetic phase transition at T N ≈ 10 K reported so far is not simple but of successive transitions due to probably quadrupolar ordering. We also observed that SmSn 3 exhibits dense Kondo behaviour. These results are compared with our experimental results on isomorphous compound SmIn 3 .

XPS study of rare earth dodecaborides: TmB12, YbB12 and LuB12

Abstract Single phase TmB12, YbB12 and LuB12 were successfully obtained by the arc-melting and floating zone methods. The XPS measurements of these compounds have indicated that (1) only YbB12 is a valence fluctuating compound with the valency of +2.9 at room temperature, (2) the correlation energy between Yb2+ and Yb3+ ions is about 6.0 eV, (3) Tm and Lu ions in the dodecaborides are in a trivalent state, and (4) the binding energy of the 4f level in TmB12 and LuB12 is 5.0 and 7.7 eV, respectively.