  Quantum tricriticality, a unique form of high-order criticality, is expected
to exhibit fascinating features including unconventional critical exponents and
universal scaling laws. However, a quantum tricritical point (QTCP) is much
harder to access, and the corresponding phenomena at tricriticality have rarely
been investigated. In this study, we explore a tricritical quantum Rabi model,
which incorporates a nontrivial parameter for adjusting the coupling ratio
between a cavity and a three-level atom. The QTCP emerges at the intersection
of a first- and second-order superradiant phase transitions according to Landau
theory. By using finite-frequency scaling analyses for quantum fluctuations and
the mean photon number, universal critical exponents differentiate the QTCP
from the second-order critical point. We find that the phase transition at the
tricritical point goes beyond the conventional second-order phase transition.
Our work explores an interesting direction in the generalization of the
well-known Rabi model for the study of higher-order critical points due to its
high control and tunability.
