Temperature-induced phase transitions in the correlated quantum Hall state of bilayer graphene*

The quantum Hall systems can be used to study many-body physics owing to its multiple internal electronic degrees of freedom and tunability. While quantum phase transitions have been studied intensively, research on the temperature-induced phase transitions of this system is limited. We focused on the easy-plane antiferromagnetic quantum Hall state with U(1) symmetry, which is established at the zero-energy of bilayer graphene. Using Corbino samples, we measured the pure bulk conductivity of a quantum Hall antiferromagnetic state in bilayer graphene over a wide range of temperatures and observed nonmonotonic temperature dependence which is characterized by two different energy scales. Based on previous studies and theoretical considerations, we found that they are associated with the breaking of the long-range order, i.e., the Kosterlitz–Thouless transition, and short-range antiferromagnetic order. This is the first study for the temperature-induced phase transition in quantum Hall antiferromagnet and indicates a similarity between quantum Hall magnetic systems and other strongly correlated systems such as Mott insulators.