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We examined the chemical doping effect on the Kondo semiconductor CeFe2Al10 with a nonmagnetic ground state by means of the magnetic susceptibility, specific heat, electrical resistivity, and thermopower. The effect of Ru doping on the ground state is small. On the other hand, by a small amount of Rh doping, the magnetic susceptibility is strongly enhanced along the orthorhombic a axis, and a Curie-Weiss behavior is observed in a wide temperature range. The low-temperature specific heat is also strongly enhanced by the doping, and a metallic ground state is realized at low temperatures. These results suggest the collapse of the spin and charge gap due to the suppression of the c-f hybridization effect. From the results of a crystalline electric field analysis on the magnetic susceptibility of Ce(Fe1-xxRhx)2Al10, it was revealed that the Rh-doping effect on the c-f hybridization effect is anisotropic, especially for the a axis. Similar doping effects are seen in the Rh-doped CeRu2Al10, Ir-doped CeOs2Al10, and Si-doped CeRu2Al10. From these results, we conclude that the collapse of the spin and charge gap by such an excess electron doping is one of the universal features of the Kondo semiconductor Cer2Al10 (T = Fe, Ru, and Os). [ABSTRACT FROM AUTHOR]

Electrical resistivity (σ), thermopower, and specific heat measurements have been performed on the novel Kondo semiconductor Ce(Ru1-xRhx)2Al10 (x=0, 0.02, 0.03, and 0.05), which has been attracting a great deal of interest due to an unusual antiferromagnetic (AFM) order below T0, in order to clarify the Rh doping effect on the anisotropy of the electronic properties in the ordered state. In CeRu2Al10, σ shows an anisotropic increase below T0 independently of the electric current direction. We propose the existence of two different mechanisms to explain the anisotropic increase of σ. One is an isotropic charge gap which enhances σ below T0 isotropically, although its origin is not known at present. The other is an anisotropic suppression of σ which originates from the anisotropic c-f hybridization and is largest along the orthorhombic a axis. By the Rh doping, the anisotropic temperature dependence of σ below T0 is drastically changed. For I||b, the increase is almost completely suppressed and a metallic-like behavior is observed, whereas it is small and isotropic for I||a and c. From these results, we propose that as a result of the destruction of the spin-gap excitation by the Rh doping, a metallic-like electronic state is formed along the b axis and the small isotropic charge gap is opened in the ac plane. By taking into account the present results and the still high T0 even in x=0.05, we conclude that the AFM order in the Rh-doped CeRu2Al10 should be viewed as unusual as the AFM order in CeRu2Al10 although the localized character of the Ce-4f electron is apparently enhanced by the Rh doping. We have also examined the evolution of the AFM ordered state from x=0 to x=0.05, where the AFM ordered moment (mAF) is aligned along the c axis in x=0 and a axis in x=0.05. From the results of those experiments in magnetic field, we have revealed that the spin reorientation from mAF||c to mAF||a takes place quite abruptly just at xc~0.03, which could be related to the topology change of the Fermi surfaces. [ABSTRACT FROM AUTHOR]

We performed the detailed studies of the pressure effect on the anisotropic electrical resistivity (ρ) of CeRu2Al10 in order to clarify if the pressure effect is also anisotropic or not. The results clearly showed the large anisotropic pressure effect on ρ. By applying the pressure, the large peak of ρ along the c axis below the antiferromagnetic transition temperature (T0) is rapidly suppressed, and becomes small and similar to the large peak along the a axis where the peak is already small at the ambient pressure. On the other hand, that along the b axis at the ambient pressure is enhanced up to 2 GPa. Also, in the temperature dependence of ρ between T0 and ~50 K, the similar anisotropic pressure effect is observed. Along the a and c axes, the semiconducting behavior is suppressed, but that along the b axis is enhanced by pressure. From these results, we concluded that the two-dimensional electronic nature of CeRu2Al10 is reinforced by pressure and becomes close to that of CeOs2Al10. The pressure effect of the magnetization along the a axis and the thermal expansion of CeRu2Al10 along the three crystal axes were also investigated. These results and the pressure effect on ρ indicate that the anisotropic c-f hybridization effect is important to clarify the unusual long-range order in this system. [ABSTRACT FROM AUTHOR]

We studied the applied magnetic field direction (θ) dependence of the magnetic and transport properties of CeT2Al10 (T= Ru, Os) in the ab, bc, and ca planes focusing on the magnetic anisotropy in the ordered state. The magnetization of NdOs2Al10 was also studied as a reference of CeT2Al10. The results of NdOs2Al10 in the antiferromagnetic (AFM) state could be understood as a normal localized AFM compound. In CeT2Al10, the magnetic susceptibility in the ab, bc, and ca planes exhibits a smooth cosθ-like θ dependence both in the ordered state and the paramagnetic region. This suggests that the antiferromagnetic moment (mAFM) is not so strongly fixed along the c axis in the ordered phase, which indicates that the anisotropic AFM exchange interaction along the c axis is not strong and a coupling between the orbital moment of the Ce ion and the lattice is also small. The θ dependence of H* in the bc and ca planes strongly supports the spin-flop transition from mAFM∥c to mAFM∥b at H* for H∥c unexpected from the smallest magnitude of the magnetic susceptibility along the b axis, which was recently proposed by our 27Al NMR studies. From the θ dependence of H* in the ca plane, we propose that the phase II observed in a high-field magnetization curve a little below the critical field to the paramagnetic region for H∥a is the spin-flop phase B with mAFM∥b. H* in CeOs2Al10 is about twice larger than that in CeRu2Al10, and when the temperature approaches to the transition temperature (T0) from the low temperature, H* of CeOs2Al10 exhibits the unusual increase, but that of CeRu2Al10 is reduced. As the origin of the different behavior of H* in these two compounds, we propose that the relative weight of the singlet component against the AFM component in the ordered state close to T0 and also the c-f hybridization along the b axis are larger in CeOs2Al10 than in CeRu2Al10. We conclude that the anisotropic c-f hybridization is the origin of the unusual magnetic anisotropy of CeT2Al10 in the ordered state. [ABSTRACT FROM AUTHOR]