Excitonic insulator due to condensation of excitons is theoretically predicted to be formed close to the border between semiconductor with small energy gap and semimetal with small band overlap [1]. In low carrier density system, Coulomb interaction between electron and hole is weakly screened. If the excitonic binding energy E_b exceeds the energy gap E_g, the system undergoes an excitonic phase transition. The possibility of excitonic gap formation have been discussed in 1T-TiSe₂ and TmSe_1-xTe_x so far, however, this identification remains controversial [2,3]. Recently, Ta₂NiSe₅ is a prime candidate for excitonic insulator. Ta₂NiSe₅ is a direct gap semiconductor with energy gap E_g ~ 0.14 eV and exhibits a semiconductor to insulator transition at T_C ~ 328 K [4]. In particular, a flattening of the valence band at Γ point is observed across the T_C. Furthermore, electronic phase diagram by tuning the energy gap is similar to theoretical prediction [5,6]. High-pressure experiment on Ta₂NiSe₅ revealed that semiconducting behavior and the excitonic transition at T_C is suppressed with increasing pressure [6,7], and the first order semiconductor to semimetal transition occurs at P_c1 ~ 3.0 GPa. Above P_c1, a new anomaly at T* possibly associated with the excitonic transition is also suppressed with increasing pressure and reaches to zero at P_c2 ~ 8.0 GPa. More interestingly, pressure-induced superconductivity emerges in the vicinity of P_c2. It is suggested that applying pressure tends to increase the dimensionality of the system, destabilizing the excitonic phase. However, there has been no comprehensive experimental study of the pressure-induced change of the dimensionality. Here we report the resistive anisotropy in Ta₂NiSe₅ under pressure.

  Reflecting a quasi-one dimensional nature of the crystallographic structure of Ta₂NiSe₅, the anisotropic conductivity is observed at ambient pressure [6]. In the pressure dependence of resistivity at room temperature, we found the discontinuous change at P_c1 for all axes, which is consistent with that in x-ray diffraction measurement under pressure [8]. The interplane anisotropy above P_c1 becomes almost half of low-pressure phase, indicating the larger Inter-plane coupling. In this presentation, we will discuss the influence of anisotropy change on the formation of the excitonic and superconducting phase.

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[2] H. Cercellier et al., Phys. Rev. Lett. 99, 146403 (2007).

[3] J. Neuenschwander and P. Wachter Phys. Rev B 41, 12693 (1990)

[4] F. J. DiSalvo et al., J. Less-Common Met. 116, 51 (1986).

[5] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009).

[6] Y. F. Lu et al., Nat. Commun. 8, 14408 (2017).

[7] K. Matsubayashi et al., submitted.

[8] A. Nakano et al., IUCrJ 5, 158 (2018).