Different Time Characteristics of Conduction and Valence Bands in a Photo-Excited Excitonic Insulator

Half a century ago, theoretical studies proposed the concept of a new type of insulator called an “excitonic insulator”. This material is originally a semimetal or a semiconductor with a narrow gap; however, the electrons and holes produced in the ground state form excitons owing to Coulomb attractive interactions, which condense to form an insulator. Excitonic insulators have not been discovered for a long time. Recently, however, their candidates have been discovered in transition metal chalcogenides and cobalt oxides and have attracted significant attention. The central material is Ta₂NiSe₅, in which two Ta chains and one Ni chain are connected by Se ions to form a quasi-one-dimensional electron system along the a-axis, as shown in Fig. 1. The conduction band consists of Ta 5d orbitals, and the valence band consists of a mixture of Ni 3d and Se 4p orbitals. With decreasing temperature, it undergoes a semiconductor-to-insulator transition at T_c = 328 K, at which point the Ta ions are displaced, changing the crystal system from orthorhombic to monoclinic.

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Fig. 1. Crystal structure of Ta₂NiSe₅, in which the Ni chain runs along the a-axis. The figures were obtained from Fig. 1(b) of Ref. 8.

An excitonic insulator is expected to form flat dispersions at the bottom of the conduction band and the top of the valence band. A previous angle-resolved photoelectron spectroscopy (ARPES) measurement on Ta₂NiSe₅ indeed revealed that the top of the valence band became flat and decreased in energy below T_c [1]. This suggested that an excitonic insulator phase was formed and possibly stabilized by the structural changes in Ta₂NiSe₅. Moreover, lattice fluctuations related to the structural changes have been observed even above T_c [2].

Subsequently, the change in the electronic structure when Ta₂NiSe₅ is irradiated with a femtosecond laser pulse with 1.55 eV, which is sufficiently higher than the band gap with approximately 0.2 eV, was investigated using time-resolved ARPES (Tr-ARPES) [3–5]. The results showed that when the excitation density I_ex is low, the excitonic insulator phase is not significantly destabilized [3]. By contrast, when I_ex is high, the excitonic-insulator phase is destabilized, and a semimetallic state is generated [4,5]. The detailed I_ex dependence of the optical responses was investigated by transient reflection spectroscopy using the 1.55-eV excitation [6,7]. The results revealed that at low I_ex, the exciton condensed state does not melt; however, when I_ex exceeds 1 mJ/cm², a Drude-like increase in reflectivity was observed in the midinfrared region [7]. Through such a strong excitation, coherent oscillations at 2 THz were observed in the time evolutions of PE signals [5] and reflectivity changes [7]. These results suggested that screening by many carriers melted the exciton condensed state and generated a transient metallic state, which released the displacements of Ta ions. However, the role of structural changes in the formation of the excitonic insulator phase remains controversial. In addition, the origin of the I_ex-dependent changes in the exciton condensed state after photoexcitation is not fully understood. An effective approach to elucidate these issues is to study the change in the band structure after photoexcitation while varying the magnitudes of I_ex.

Based on this background, Takahashi et al. have focused on Ta₂Ni_0.9Co_0.1Se₅, in which part of the Ni in Ta₂NiSe₅ was replaced by Co, and performed Tr-ARPES measurements upon 1.55-eV excitation at various I_ex magnitudes [8]. The gap in the excitonic insulator phase is smaller in this material than in Ta₂NiSe₅, suggesting that a photoinduced metallic state is more likely to occur. Detailed measurements have revealed that the photoinduced changes in the dispersions of the conduction and valence bands exhibited different time characteristics.

Figure 2(I) shows the PE spectra before and after the photoexcitation under strong excitation at I_ex = 3 mJ/cm². A finite intensity just above the Fermi level E_F appears 70 fs after the photoexcitation, indicating that the conduction band drops rapidly near E_F. By contrast, the top of the valence band increases only slightly at 70 fs but increases to E_F at 170 fs, indicating the formation of a metallic state. To investigate the I_ex dependence of the transient changes in the conduction and valence bands near E_F, Takahashi et al. performed a deconvolution analysis of the energy distribution curves (EDCs) at k = 0. The results are presented in Fig. 2(II), where the blue lines represent the deconvoluted EDCs. At a relatively low I_ex below 1 mJ/cm², the bottom of the conduction band approaches E_F at 50–70 fs, whereas the top of the valence band does not change significantly even at 160–170 fs. Takahashi et al. deduced that the delay and suppression of the increase at the top of the valence band observed at high and low excitation densities, respectively, were due to lattice fluctuations. They speculated that in the photoinduced phase, lattice fluctuations similar to those observed above T_c without photoexcitation [2] had an important influence on the transient changes in the valence band.

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Fig. 2. (I) Tr-ARPES spectra for a fluence of 3.00 mJ cm⁻² at delay times of (a) −680 fs, (b) 70 fs, and (c) 170 fs (100 K). The figures were obtained from Figs. 5(a)–5(c) of Ref. 8. (II) Deconvoluted EDCs at various excitation fluences and delay times, which were obtained by integrating the data in the k_x range between 0.05 and −0.05 Å⁻¹ (blue lines). The red and green lines show the original EDCs and EDCs convoluted from the blue lines, respectively. The figure was obtained from Fig. 6 of Ref. 8.

The results presented in this study are interesting because they indicate the possibility that variations in the band structure and electron itinerancy due to structural changes play essential roles in the excitonic insulator phases of Ta₂NiSe₅ and related materials, in addition to the excitonic effect. This study is expected to serve as a basis for further clarification of the role of structural changes in the formation of exciton condensed states.

References

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Author Biographies