Debye temperature, electron-phonon coupling constant, and three-dome shape of crystalline strain as a function of pressure in highly compressed La3Ni2O7−δ

Besides ongoing studies of phase structural transitions, pairing mechanism, and physical properties of recently discovered highly compressed high-temperature superconductor La3Ni2O7−δ, here we explored a possibility for the electron-phonon pairing mechanism as an origin of the superconducting state and determined the microcrystalline strain, ε, in high-pressure Fmmm-phase, and low-pressure Amam-phase of this nickelate. To do this, we analysed temperature dependent resistance and extracted pressure dependent Debye temperature, ΘD (P), in La3Ni2O7−δ with an approximate value of ΘD(25 GPa) = 550 K. From this we established that the La3Ni2O7−δ is strong-coupled superconductor with the electron-phonon coupling constant λe-ph(P = 22.4 GPa) =1.75. This value is close to λe-ph =1.70 of ambient pressure superconductors Nb3R (R = Sn, Al). To address ongoing discussion that the lattice strain can be the origin for the emergence of high-temperature superconductivity in the La3Ni2O7−δ, we determined the microcrystalline strain, 0.011≤ ε(P), in the high-pressure Fmmm-phase, and ε(P) < 0.011 of low-pressure Amam-phase. Our analysis showed that ε(P) has three-dome shape in the pressure range of 1.6 GPa ≤ P ≤ 41.2 GPa. One of these two ε(P) deeps at P =15 GPa coincides with the pressure at which the Amam into the Fmmm phase transition occurs. Based on our analysis, we proposed probable condition to observe the zero-resistance state in La3Ni2O7−δ.

1. Ministry of Science and Higher Education of the Russian Federation - theme “Pressure” No. 122021000032-5
2. Ministry of Science and Higher Education of the Russian Federation - theme “Spin” No. 122021000036-3