Pressure-Induced Superconductivity In Polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$

We synthesized polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ ($\delta \approx 0.07$) samples by using the sol-gel method without postannealing under high oxygen pressure, and then measured temperature-dependent resistivity under various hydrostatic pressures up to 18 GPa by using the cubic anvil and two-stage multianvil apparatus. We find that the density-wave-like anomaly in resistivity is progressively suppressed with increasing pressure and the resistivity drop corresponding to the onset of superconductivity emerges at pressure as low as $\sim 6\mathrm{GPa}$. Zero resistivity is achieved at 9 GPa below $T_{\mathrm{c}}^{\mathrm{zero}}\approx 6.6\mathrm{K}$, which increases quickly with pressure to 41 K at 18 GPa. However, the diamagnetic response was not detected in the ac magnetic susceptibility measurements up to 15 GPa, indicating a filamentary nature of the observed superconductivity in the studied pressure range. The constructed $T\text{−}P$ phase diagram reveals an intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. The observation of zero-resistance state in the polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ samples under high pressures not only corroborates the recent report of superconductivity in the pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ crystals but also facilitates further studies on this emerging family of nickelate high-$T_{\mathrm{c}}$ superconductors.

Popular Summary

The groundbreaking discovery of high-temperature superconductivity in cuprates motivated many efforts in the past decades to unveil its mechanism and to find more superconducting families with high transition temperatures. Thanks to sharing structural and electronic similarities with cuprates, certain nickelates offer a tantalizing avenue for those endeavors. Very recently, researchers reported the signature of high-temperature superconductivity in crystals of one nickelate, ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, at pressures above 14 GPa. Here, we report that it can also be achieved at lower pressures.

For our experiments, we synthesize high-quality ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline samples and then conduct a comprehensive study on their electronic transport properties under various pressures. We observed superconductivity over a wide range of pressures, from 9 to 18 GPa, with a transition temperature as high as 78 K. In addition, we construct a temperature-pressure phase diagram of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ samples and find that it reveals a close relationship between the superconductivity, charge-density wave order, and strange metal behavior in this system.

The observation of a zero-resistance state in the polycrystalline ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ samples under high pressures not only corroborates the recent report of superconductivity in the pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ crystals but also facilitates further studies on this emerging family of nickelate high-temperature superconductors.

Figure 1

(a) Rietveld refinements on the room temperature XRD pattern of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ polycrystalline sample. The obtained lattice parameters are shown. The bottom marks and line correspond to the calculated Bragg diffraction positions and the difference between observed and calculated data, respectively. (b) Thermogravimetric curves for ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ in 10% ${\mathrm{H}}_2/\mathrm{Ar}$. (c) Rietveld refinements on the NPD pattern of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ polycrystalline sample. (d) The SEM EDS elemental mapping of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7-\delta }$ polycrystalline sample.

Figure 2

(a),(b) Temperature dependence of resistivity $\rho (T)$ of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline sample no. 1 and no. 2 under various hydrostatic pressures up to 14.5 GPa measured in PCC and CAC at the Institute of Physics (IOP) CAS. Here, the $T_{\mathrm{c}}^{\text{onset}}$ is determined as the interception between two straight lines below and above the superconducting transitions. (c),(d) Temperature dependence of resistivity $\rho (T)$ of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline sample no. 3 under various hydrostatic pressures up to 18 GPa measured in MA at the Institute for Solid State Physics (ISSP), University of Tokyo.

Figure 3

(a) The low-temperature resistivity $\rho (T)$ at 14.5 GPa of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline sample no. 2 under various magnetic fields up to 8.5 T. (b) Temperature dependence of the upper critical field ${\mu }_0{H}_{\mathrm{c}2}(T)$ for ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline sample at 14.5 GPa. The solid line is the fitting curve by using the formula $H_{\mathrm{c}2}=H_{\mathrm{c}2}(0)(1-t^2)/(1+t^2)$, where $t=T/T_{\mathrm{c}}$.

Figure 4

The $T\text{−}P$ phase diagram of the ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{6.93}$ polycrystalline sample. The solid and open circles represent the DW-like transition $T_{\mathrm{DW}}$ measured at various pressures using PCC, CAC, and MA. The solid and open squares and pentagons represent the onset and zero-resistance superconducting transition temperatures determined from the present measurements in CAC and MA. The solid and open hexagons represent the critical temperature $T_{\mathrm{sm}}$ for the strange-metal-like behavior, above which the $\rho (T)$ curve deviates from linearity.