Spill fluctuations and antiferromagnetic phase transitions in heavy fermions of Kramers (Ce compound) and non-Kramers (U compound) doublets.

The low temperature properties of Ce(Ru1-xRhx)(2)Si-2 (0 less than or equal to x less than or equal to 1), U(Ru1-xRhx)(2)Si-2 (0 less than or equal to x less than or equal to 1) and UxLa1-xRu2Si2 (0 less than or equal to x le ss than or equal to 0.15) are reviewed with emphasis on experiments of susc eptibility, non-linear susceptibility, specific heat, resistivity, thermal expansion, neutron scattering and mu SR. New experimental data has led to n ew insights to the physical understanding of these heavy fermions. In Rh ri ch side, antiferromagnetic order associated with the localized character of the f-electrons sets in for both Ce(Ru1-xRhx)(2)Si-2 (0 less than or equal to x less than or equal to 1) and U(Ru1-xRhx)(2)Si-2 (0 less than or equal to x less than or equal to 1) compounds. For Ce(Ru1-xRhx)(2)Si-2 compound, x-dependence of the material is investigated in detail. In Ru rich side, t he ground state is a Fermi liquid for x < 0.03; an anisotropic spin density wave (SDW) appears for 0.03 < x < 0.4 with a maximum T-N = 5.5 K at x = 0. 15. The nature of the anisotropic SDW is characterized by the anisotropic a nomaly in the resistivity below T-N The anisotropy of the SDW is discussed from the view point of nesting of the Fermi surfaces of itinerant heavy fer mion quasi-particle bands. Another interesting feature is the non-Fermi liq uid behavior observed for x = 0.4 and 0.5. Appearance of quantum Griffiths' s like phase is indicated for x = 0.5 from the measurements of mu SR, low f ield ac-susceptibilty and nonlinear susceptibiltiy. The temperature depende nce of the susceptibility and the magnetoresistance with 0.1 T de-field are well explained in terms of the mean field theory based on spin fluctuation s. The phase transition of URu2Si2 at 17.5 K(= T-o) is considered to be due to quadrupolar ordering from the behaviors of a sharp anomaly in nonlinear susceptibility at T-o and very large consumption of entropy below T-o comp ared to a small staggered ordered moment, 0.02 mu(B) We recently found that the phase transition is not due to magnetic origin by neutron scattering e xperiment under pressure. The susceptibility and magnetization in dilute al loys of UxLa1-xRu2Si2 are well explained by assuming a non-Kramers doublet ground state of the J = 4 crystalline field level. From these experiments, the phase transition of URu2Si2 at T-o is understood by the model of quadru polar ordering of the non-Kramers doublet which coexists with antiferromagn tic short range order.