Universal behaviour at discontinuous quantum phase transitions

Abstract

Discontinuous quantum phase transitions besides their general interest are clearly relevantto the study of heavy fermions and magnetic transition metal compounds. Recentresults show that in many systems belonging to these classes of materials themagnetic transition changes from second order to first order as they approach thequantum critical point (QCP). We investigate here some mechanisms that may beresponsible for this change, specifically the coupling of the order parameter tosoft modes and the competition between different types of order near the QCP.For weak first order quantum phase transitions general results are obtained. Inparticular, we describe the thermodynamic behaviour at this transition when it isapproached from finite temperatures. This is the discontinuous equivalent of thenon-Fermi-liquid trajectory close to a conventional QCP in a heavy-fermion material.