  We propose that 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
puzzling in two aspects; diverging ferromagnetic susceptibility at the
antiferromagnetic transition and unconventional power-law dependence in
thermodynamic quantities. These puzzles are solved by an unconventional
criticality derived from our spin fluctuation theory for the QTCP.
