Eiji Hitsuda is a master's student in the Master's Program in Engineering Sciences at the University of Tsukuba, conducting his research at the Frontier Superconducting Materials Group, National Institute for Materials Science (NIMS). His research focuses on physical property measurements under high pressure using diamond anvil cells.

Layered Ruddlesden–Popper nickelates REₙ₊₁NiₙO₃ₙ₊₁ have emerged as promising candidates for next-generation high-temperature superconductors. The n = 2 compound La₃Ni₂O₇ exhibits superconductivity with Tc > 80 K under high pressure [1], and superconductivity with Tc > 30 K under high pressure in the n = 3 compound La₄Ni₃O₁₀ was first discovered by our group [2]. Both transitions are accompanied by a pressure-induced structural change from a non-superconducting orthorhombic phase to a superconducting tetragonal phase, with suppression of a competing density-wave phase near this transition considered key to the emergence of superconductivity. Furthermore, ambient-pressure superconductivity (Tc > 40 K) has been achieved in La₃Ni₂O₇ thin films via strain-induced chemical pressure, using pulsed laser deposition on SrLaAlO₄ substrates [3].

An electric double-layer transistor (EDLT) is an effective means of further enhancing Tc through carrier density control, and thin-film samples are well suited for EDLT fabrication. Superconductivity in La₃Ni₂O₇ thin films arises via chemical pressure in ultrathin films (≤5 nm) or via applied physical pressure in thicker films (>5 nm).

In this study, we fabricated thin films in which the n = 2 and n = 3 phases coexist by pulsed laser deposition. We also developed a measurement system (EDLT-DAC) combining a diamond anvil cell with boron-doped diamond electrodes [4] and an EDLT, which enables simultaneous application of high pressure and electric-field-induced carrier density control while performing in situ four-terminal resistance measurements [5]. In future work, we plan to control the carrier density using an electric field and investigate the material properties.

1. H. Sun, et al., Nature, 621, 493-498 (2023) DOI: 10.1038/s41586-023-06408-7
2. H. Sakakibara, et al., Phys. Rev., 132, 106002, (2024) DOI: 10.1103/PhysRevLett.132.106002
3. E. Ko, et al., Nature, 638, 935-940 (2025) DOI: 10.1038/s41586-024-08525-3
4. R. Matsumoto, et al., Rev. Sci. Instrum., 89, 076103 (2016) DOI: 10.1063/1.4959154
5. S. Adachi, et al., Appl. Phys. Lett. 116, 223506 (2020) DOI: 10.1063/5.0004973