Renormalized φ6 model for quantum phase transitions in systems of itinerant fermions

We study the impact of quantum and thermal fluctuations on properties of quantum phase transitions occurring in systems of itinerant fermions with main focus on the order of these transitions. Our approach is based on a set of flow equations derived within the functional renormalization-group framework, in which the order parameter is retained as the only degree of freedom and where the effective potential is parametrized with a phi(6) form allowing for both first-order and second-order scenarios. We find a tendency to turn the first-order transitions within the bare model into second-order transitions upon accounting for the order-parameter fluctuations. We compute the first-order and second-order phase boundary lines T-c as a function of a nonthermal control parameter a(2) in the vicinity of a quantum phase transition. We analyze crossovers of the shift exponent psi governing the shape of the T-c line when the system is tuned close to a quantum tricritical scenario, where a second-order phase-transition line terminates at a quantum tricritical point.