Coherence Factors and Quantum Interferences in Excitonic Condensation of Ta₂NiSe₅

Abstract

Abstract. 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. IntroductionIt is known that, in narrow-gap semiconductors or semimetals, pairs of electrons and holes (excitons) are spontaneously formed and go into a condensed state with macroscopic phase coherence. This state is referred to as an excitonic condensation [1], and it entails the flattering of the band edges in the case of semiconductors, and the opening of the band gap in the case of semimetals. It was predicted half a century ago, but actual materials of this phase are not known. However, thanks to the recent development of angle-resolved photoemission spectroscopy experiments that enable us to observe band dispersions directly, the realizations of excitonic condensation in some materials have been suggested [2,3,4,5,6]. Especially, Ta 2 NiSe 5 , which is known as a direct-gap semiconductor [7,8], is regarded as one of the most promising candidates of excitonic condensation [2,3,9].A measurement of transformation of the band dispersion is, however, merely an indirect evidence for excitonic condensation. In this respect, there have been some proposals that the ultrasonic attenuation rate shows a coherence peak in a simple electron-gas model [10,11] although the connection to an actual material lacks. In order to elucidate whether Ta 2 NiSe 5 is actually in the excitonic phase or not, we have recently proposed that measurements of the ultrasonic attenuation rate and nuclear magnetic resonance (NMR) relaxation rate can lead to a direct evidence for excitonic condensation in Ta 2 NiSe 5 [12]. In our proposal, we predicted that the ultrasonic attenuation rate shows a characteristic peak just below the transition temperature, while the NMR relaxation rate shows a rapid drop just below it. These behaviors originate from the quantum interferences due to the superposition of the wave functions of electrons and holes in the excitonic condensation state.