Debye temperature, electron-phonon coupling constant, and three-dome shape of crystalline strain as a function of pressure in highly compressed La${}_3$Ni${}_2$O${}_{7-\delta }$

Abstract

Besides ongoing studies of phase structural transitions, pairing mechanism, and physical properties of recently discovered highly compressed high-temperature superconductor La${}_3$Ni${}_2$O${}_{7-\delta }$, here we explored a possibility for the electron-phonon pairing mechanism as an origin of the superconducting state and determined the microcrystalline strain, $\epsilon (P)$, in high pressure $Fmmm$-phase, and low-pressure $Ammm$-phase of this nickelate. To do this, we analyzed temperature dependent resistance and extracted pressure dependent Debye temperature, ${\Theta }_D(P)$, in La${}_3$Ni${}_2$O${}_{7-\delta }$ with an approximate value of ${\Theta }_D(P=25GPa)=550$ $K$. From this we established that the La${}_3$Ni${}_2$O${}_{7-\delta }$ is strong-coupled superconductor with the electron-phonon coupling constant ${\lambda }_{e-ph}(P=22.4GPa)=1.75$. This value is close to ${\lambda }_{e-ph}=1.70$ of ambient pressure superconductors $N{b}_{3}R(R=Sn,Al)$. To address ongoing discussion that the lattice strain can be the origin for the emergence of high-temperature superconductivity in the La${}_3$Ni${}_2$O${}_{7-\delta }$, we determined the microcrystalline strain, $0.011<\epsilon (P)$, in the high-pressure $Fmmm$-phase, and $\epsilon (P)<0.011$ of low-pressure $Fmmm$-phase. Our analysis showed that $\epsilon (P)$ has three-dome shape in the pressure range of $1.6GPa<P<41.2GPa$. One of these two $\epsilon (P)$ deeps at $P\approx 15GPa$ coincides with the pressure at which the $Ammm$-phase into the $Fmmm$-phase phase transition occurs. Based on our analysis, we proposed probable condition to observe the zero-resistance state in La${}_3$Ni${}_2$O${}_{7-\delta }$.

11 pages, 5 figures, 62 references and Supplementary Information