JPSJ News Comments 20, 08 (2023) [2 Pages]

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

Yu Takahashi, Takeshi Suzuki, Masaki Hattori, Mario Okawa, Hidenori Takagi, Naoyuki Katayama, Hiroshi Sawa, Minoru Nohara, Yigui Zhong, Kecheng Liu, Teruto Kanai, Jiro Itatani, Shik Shin, Kozo Okazaki, Takashi Mizokawa
J. Phys. Soc. Jpn. 92,  064706 (2023).

+ Affiliations
Department of Advanced Materials Science, the University of Tokyo

Photoinduced electronic state changes in the excitonic insulator Ta2Ni0.9Co0.1Se5 were investigated using transient photoemission spectroscopy. The results revealed that the conduction and valence bands exhibit different temporal changes during the photoinduced melting of the exciton condensed state.

©2023 The Physical Society of Japan

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 Tc = 328 K, at which point the Ta ions are displaced, changing the crystal system from orthorhombic to monoclinic.

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 Tc [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 Tc [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) [35]. The results showed that when the excitation density Iex is low, the excitonic insulator phase is not significantly destabilized [3]. By contrast, when Iex is high, the excitonic-insulator phase is destabilized, and a semimetallic state is generated [4,5]. The detailed Iex 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 Iex, the exciton condensed state does not melt; however, when Iex 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 Iex-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 Iex.

Based on this background, Takahashi et al. have focused on Ta2Ni0.9Co0.1Se5, in which part of the Ni in Ta2NiSe5 was replaced by Co, and performed Tr-ARPES measurements upon 1.55-eV excitation at various Iex 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 Iex = 3 mJ/cm2. A finite intensity just above the Fermi level EF appears 70 fs after the photoexcitation, indicating that the conduction band drops rapidly near EF. By contrast, the top of the valence band increases only slightly at 70 fs but increases to EF at 170 fs, indicating the formation of a metallic state. To investigate the Iex dependence of the transient changes in the conduction and valence bands near EF, 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 Iex below 1 mJ/cm2, the bottom of the conduction band approaches EF 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 Tc without photoexcitation [2] had an important influence on the transient changes in the valence band.

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 kx 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.


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Author Biographies

About the Author: Hiroshi Okamoto

Hiroshi Okamoto graduated with a Ph.D. in Applied Physics from the University of Tokyo in 1988. Subsequently, he joined Institute for Molecular Science as a research associate. He was appointed as a lecturer at Research Institute for Scientific Measurements, Tohoku University in 1992 and promoted to associate professor at the same institute in 1995. He was appointed as an associate professor at Graduate School of Engineering, the University of Tokyo in 1998 and moved to Graduate School of Frontier Sciences of the same university in 1999. Since 2005, he has been a professor at Graduate School of Frontier Sciences at the University of Tokyo. He is currently studying photoinduced phase transitions and nonlinear optical responses in correlated electron systems, ferroelectrics, and multiferroics using ultrafast laser, nonlinear, and terahertz spectroscopies.