Off-diagonal Geometric Phase of Two-Qubit System

This paper focuses on the off-diagonal geometric phase of the thermal state of a two-qubit system under the external magnetic field. The properties of geometric phases of the state in critical and non-critical regions are discussed respectively. The sudden change of structure of degeneracy at the critical point do not affect the geometric phase of the model. Increasing temperature tends to suppress the off-diagonal geometric phase. The relationship between the geometric phase and external magnetic field is also discussed.