YbRh2Si2:: Quantum tricritical Behavior in itinerant electron systems

We propose that the proximity of the first-order transition manifested by the quantum tricritical point (QTCP) explains non-Fermi-liquid properties of YbRh2Si2. Here, at the QTCP, a continuous phase transition changes into first order at zero temperature. The non-Fermi-liquid behaviors of YbRh2Si2 are veiled in several prominent mysteries; diverging ferromagnetic susceptibility at the antiferromagnetic transition and enhancement of magnetization as well as specific heat. These puzzles are solved by an unconventional criticality derived from our spin fluctuation theory for the QTCP; especially, diverging ferromagnetic susceptibility is quantitatively reproduced.