Coherence Factors and Quantum Interferences in Excitonic Condensation of Ta₂NiSe₅

In order to elucidate whether Ta₂NiSe₅ is in an excitonic condensation state or not, we study macroscopic quantum interferences in ultrasonic attenuation rate and nuclear magnetic resonance relaxation rate. Using the three-chain model describing the excitonic condensation of Ta₂NiSe₅, we demonstrate analytically that the ultrasonic attenuation rate shows a characteristic peak just below the transition temperature of the excitonic condensation, while the nuclear magnetic resonance relaxation rate shows a rapid drop. In particular, we find that the constructive interference originates from the hybridization between the conduction and valence bands induced by an external field.