Department of Physics, Graduate School of Science, Nagoya University

Universal Relation in Heavy Fermion Systems:

Strongly correlated metals composed of 4f and 5f elements are called the heavy fermions systems, because the effective electron mass is drastically enlarged (m^*/m_e=100~1000) due to the strong Coulomb interaction of f-electrons. In these systems, the specific heat and the resistivity follows the Fermi liquid relations C = γT and ρ = AT². In addition, in may Ce-based compounds, empirical universal relation A/γ² ~ a₀ = 1×10^-5μΩcm(K mol/mJ)² holds, known as Kadowaki-Woods relation. However, strong violation of this “universal relation” has been reported recently in various heavy fermions, especially Yb-based compounds. This finding casted a serious and fundamental question on the ground state of heavy fermion systems.

In many heavy fermion systems, the Kondo temperature T_K can be larger than the crystalline electric field (CEF). In this case, the degeneracy N becomes as large as 6 (J=5/2 in Ce) and 8 (J=7/2 in Yb). Based on the microscopic Fermi liquid theory, we derived the “ground Kadowaki-Woods relation A/γ² ~ 2a₀/N(N+1)” [26]. Original Kadowaki-Woods relation is satisdied when N=2 (Kramers douoblet case). Figure shows the KW ratio of various heavy-fermion compounds with N=2~8. Since various compounds with N>2 has been discovered now, the ground Kadowaki-Woods relation is the universal relation of heavy fermion systems.

[1] H. Kontani, J. Phys. Soc. Jpn. 73, 515 (2004). (Editor’s choice)
[2] N. Tsujii, H. Kontani and K. Yoshimura, Phys. Rev. Lett. 94, 057201 (2005).