Evolution of the superconductivity in pressurized La3-xSmxNi2O7

TL;DR

This study systematically explores Sm substitution in La3-xSmxNi2O7 under pressure to assess its impact on superconductivity and structure. By combining sol-gel synthesis with ambient and high-pressure transport and synchrotron diffraction, the authors show that Sm doping contracts lattice constants and enhances in-plane distortion, driving insulating behavior at ambient pressure for high x, while pressure induces superconductivity with $T_c$ onsets up to ~89 K for x=0.9–1.5. A pressure-induced orthorhombic Amam to tetragonal I4/mmm transition accompanies the superconducting state, and the maximum $T_c$ appears to saturate beyond moderate Sm doping, highlighting interlayer couplings as the primary lever for $T_c$ tuning. These findings provide insight into how isovalent substitution and lattice engineering influence nickelate superconductivity and point toward strategies to push $T_c$ higher via interlayer interaction optimization.

Abstract

Motivated by the discovery of superconductivity in bilayer La$_3$Ni$_2$O$_7$ at 80 K and the increased superconducting transition temperature, $T_\text{c}$, up to 92 K in single crystals of La$_2$SmNi$_2$O$_7$ under pressure, we systematically study the effect of Sm doping on the superconductivity and structure of La$_{3-x}$Sm$_x$Ni$_2$O$_7$ (0 $\leq$ x $\leq$ 1.5) under pressure. Experimental investigations in polycrystalline samples reveal that Sm doping monotonically decreases the lattice constants $c$ and $a$, thereby enhancing crystal structure distortion and leading to an evolution of the metallic ground state in La$_3$Ni$_2$O$_7$ to an insulating state in La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$. The maximum onset $T_\text{c}$ in compounds $x=0.9$ and 1.5 is 89 K, while the pressure that drives the emergence of superconductivity is higher for higher doping levels. The results suggest that the enhancement of $T_\text{c}$ in La$_{3-x}$Sm$_x$Ni$_2$O$_7$ is mainly affected by the compressed $c$ lattice before saturation, and the structure transition is critical for the emergence of superconductivity. Our experimental results provide insight into the influence of elemental substitution on nickelate superconductors, offering a means to increase the transition temperature further.

Figures (4)

• Figure 1: (a) Crystal structure of the La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$ polycrystalline sample at ambient pressure. (b) Rietveld refinement results of the powder X-ray diffraction pattern, indexed to the orthorhombic $Amam$ space group. (c) The variation of lattice parameters $a$, $b$, $c$ in La$_{3-x}$Sm$_x$Ni$_2$O$_7$ with Sm doping concentration (0 $\leq$$x$$\leq$ 1.5). (d) Temperature dependence of electrical resistivity for La$_{3-x}$Sm$_x$Ni$_2$O$_7$ ($x$ = 0, $x$ = 0.9, and $x$ = 1.5) at ambient pressure. (e) Temperature-dependent magnetic susceptibility of La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$ measured under 0.4 T magnetic field in zero-field-cooled (ZFC) and field-cooled (FC).
• Figure 2: (a) The curve of resistance versus temperature for La$_{2.1}$Sm$_{0.9}$Ni$_2$O$_7$ in the pressure range of 23.4 to 39.0 GPa in run 1 and (b) 20.0 to 25.7 GPa in run 2, where the dashed line indicates the superconducting onset transition temperature, $T_c^{\text{onset}}$. (c) Temperature-dependent resistance of La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$ in run 1 under pressures ranging from 31.6 to 43.8 GPa, (d) run 2 from 20.4 to 37.1 GPa, and (e) run 3 from 37.1 to 43.1 GPa. The inset shows a photograph of the electrodes for resistance measurements in the high-pressure DAC, with a scale bar of 100 $\mu$m. (f) Magnetic field-dependent resistance in run 3 at 37.1 GPa. The inset presents the upper critical field fitted using $T_c^{\text{onset}}$ and 90% of $T_c^{\text{onset}}$, where open circles represent experimental data and the solid lines are the results of fitting with the empirical Ginzburg-Landau formula.
• Figure 3: (a) High-pressure synchrotron XRD panoramic patterns of the La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$ powder sample at room temperature, ranging from 2.7 to 58.4 GPa (wavelength $\lambda$=0.4834 Å). (b) Detailed evolution of (020)/(200) peaks within the range of 9.5$^\circ$$\leq$ 2$\theta$$\leq$ 11.5$^\circ$, showing their pressure-induced merging into a single (110) peak above 30.5 GPa. (c) Variations of lattice parameters $a, b$, and $c$ under pressure. (d) Rietveld refinements at 31.9 GPa and 2.7 GPa using the $I4/mmm$ and $Amam$ space groups, respectively.
• Figure 4: The temperature-pressure phase diagram of La$_{2.1}$Sm$_{0.9}$Ni$_2$O$_7$ and La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$, where the green and orange squares represent the superconducting onset transition temperature $T_c^{\text{onset}}$ on the resistance-temperature curves for samples La$_{2.1}$Sm$_{0.9}$Ni$_2$O$_7$ run1 and run2, respectively. The pink pentagrams, purple diamonds, and cyan spheres denote the $T_c^{\text{onset}}$ on the resistance-temperature curves for samples La$_{1.5}$Sm$_{1.5}$Ni$_2$O$_7$ run 1, run 2, and run 3, respectively.