Recently, a nonlinear electrical conductivity has been reported in single crystals of 4$d$ and 5$d$ transition metal (TM) oxides. In the layered perovskite oxides Ca₂RuO₄ and Sr₂IrO₄, conductivity is increased when current is applied and nonlinear electrical conductivity is observed [1–3]. In the layered structure, the TM–O–TM bond angle, which is important for conductivity, is changed by the application of current. It has been reported that the crystal structure of these materials under the application of current is in a non-equilibrium state, which is different from the thermal equilibrium state. Spin–orbit coupling is thought to play an important role in the nonlinear electrical conductivity of these 4$d$ and 5$d$ transition metal oxides. Therefore, such spin–orbit coupled materials are expected to have applications in devices based on a new mechanism.

We have reported that Ca₅Ir₃O₁₂ exhibits nonlinear electrical conductivity along the $c''-''$axis [4,5]. Ca₅Ir₃O₁₂ has a hexagonal crystal structure without centrosymmetry, in which one-dimensional chains of edge-sharing IrO₆ octahedra along the $c''-''$axis form triangular lattices in the $c''-''$plane (Fig. 1). Recently, to investigate the effect of current application on the crystal structure, Raman scattering experiments on Ca₅Ir₃O₁₂ were performed under the application of DC current [6]. Changes in the Raman spectrum caused by the application of DC current, which are different from the effect of self-heating due to DC current, were also observed. These results indicate that the oxygen bonds in the edge-sharing IrO₆ chain along the $c''-''$axis are strengthened by applying a DC current.

In this study, we report current-induced lattice distortion in iridium oxide Ca₅Ir₃O₁₂ with a hexagonal structure that exhibits nonlinear conductivity along the $c''-''$axis [7]. Using the synchrotron radiation X-ray diffraction experiments on the single crystal, the change in the lattice constant due to thermal expansion at temperatures above room temperature and the change in the lattice constant due to the application of current along the $c''-''$axis were investigated. We found that the current application along the $c''-''$axis caused anisotropic lattice distortion that expanded within the $c''-''$plane direction. The observed distortion due to the electric current is not caused by the piezoelectric effect that would be expected given the symmetry of the crystal structure of this material; the evaluated piezoelectric constant is a very large value compared to typical piezoelectric materials. These results show that the change in the lattice constant ratio $a''/''c$ due to the application of current is greater than the change in thermal expansion caused by self-heating. In spin–orbit coupled oxides, this result reveals the possibility of significant current-induced distortion based on a mechanism different from the conventional piezoelectric effect.

References

• [1] G. Cao, J. Phys.: Condens. Matter 32, 423001 (2020).
• [2] R. Okazaki, Y. Nishina, Y. Yasui, F. Nakamura, T. Suzuki, and I. Terasaki, J. Phys. Soc. Jpn. 82, 103702 (2013).
• [3] G. Cao, J. Terzic, H. D. Zhao, H. Zheng, L. E. De Long, and P. S. Riseborough, Phys. Rev. Lett. 120, 017201 (2018).
• [4] K. Matsuhira, K. Nakamura, Y. Yasukuni, Y. Yoshimoto, D. Hirai, and Z. Hiroi, J. Phys. Soc. Jpn. 87, 013703 (2018).
• [5] H. Hanate, K. Nakamura, and K. Matsuhira, J. Magn. Magn. Mater. 498, 166203 (2020).
• [6] M. Hayashida, T. Hasegawa, K. Kadohiro, H. Hanate, S. Kawano, and K. Matsuhira, J. Phys. Soc. Jpn. 93, 104704 (2024).
• [7] K. Kadohiro, H. Hanate, M. Hayashida, T. Hasegawa, H. Nakao, Y. Okamoto, K. Miyazaki, S. Tsutsui, and K. Matsuhira, J. Phys. Soc. Jpn. 94, 023601 (2025).

Authors

• K. Matsuhira^a and Y. Okamoto
• ^aKyushu Institute of Technology