Photo-excited dynamics in the excitonic insulator Ta2NiSe5

Daniel Werdehausen,Hidenori Takagi,Gelon Albrecht,Tomohiro Takayama,Yangfan Lu,Stefan Kaiser

doi:10.1088/1361-648x/aacd76

Abstract

The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.

Highlights

The excitonic insulator state can be formed in both a semiconductor that exhibits a small band gap or a semimetal which is distinguished by a small band overlap [2,3,4,5,6,7,8]
In their comprehensive work on high temperature superconductors Demsar, Kabanov et al have shown that the dynamics observed in an 800nm pump probe experiment in these materials depend critically on the properties of the energy gap around the Fermi level [1,13]. Their models allow to determine the gap’s properties from temperature dependent measurements of the photoinduced reflectivity changes, and were found to provide a good agreement with the experimental results [1,13,14,16,17,18]. We show that these models us to describe dynamics observed in Ta2NiSe5: The results of our analysis support the assumption of a BEC-like phase transition into the excitonic insulator ground state, and show that there is a an additional temperature depended contribution to the gap
Our measurements revealed that the excitonic insulator state remains intact after photoexcitation, even at excitation densities close to the damage threshold

Summary

Introduction

The excitonic insulator state can be formed in both a semiconductor that exhibits a small band gap or a semimetal which is distinguished by a small band overlap [2,3,4,5,6,7,8]. In both regimes the excitonic insulator ground state is distinguished by a characteristic band flattening (see figure 1a) and the size of the gap around the Fermi level is determined by the binding energy of the excitons [8] The properties of this gap can, for example, be probed using ARPES or infrared measurements of the optical conductivity [9,10,11,12]. Recent time dependent measurements under high excitation fluence found a low frequency phonon showing amplitude mode like behavior revealing a strong fingerprint towards the existence of a coherent excitonic insulator ground state [22]

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