Superconductivity in monolayer-trilayer phase of La$_3$Ni$_2$O$_7$ under high pressure

Abstract

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La$_3$Ni$_2$O$_7$. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic $Cmmm$ to the tetragonal $P4/mmm$ space group at 13~GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO$_6$ block as the essential structural motif for achieving high-$T_\text{c}$ superconductivity in the RP nickelates.

Figures (7)

• Figure 1: $|$Structure, resistance, and magnetization of hybrid La$_3$Ni$_2$O$_7$ at ambient pressure.a, Crystal structure of the monolayer-trilayer (1313) hybrid phase of La$_3$Ni$_2$O$_7$, with an enlarged view illustrating the alternating stacking of monolayer and trilayer NiO$_6$ octahedra. The Ni--O--Ni bond angle along the $c$-axis is 180$^\circ$. b,c, Photograph of a single crystal and its corresponding Laue diffraction pattern. d, Rietveld refinement of the powder XRD pattern using the 1313 structural model. A simulated pattern for the bilayer (2222) phase of La$_3$Ni$_2$O$_7$ is shown for comparison. e, HAADF-STEM image of La$_3$Ni$_2$O$_7$, with a zoomed-in view of the dotted area clearly showing the alternating monolayer (ML) and trilayer (TL) sequences. f, Temperature-dependent resistance of a 1313 phase single crystal at ambient pressure, showing two distinct anomalies at 243 K and 185 K. g, Temperature-dependent magnetic susceptibility ($M/H$) measured at 1000 Oe for magnetic fields applied in-plane and out-of-plane. The inset shows data from 100 to 300 K, revealing an anomaly at 193 K.
• Figure 2: $|$Electrical transport properties of hybrid La$_3$Ni$_2$O$_7$ under high pressure.a,b, Temperature-dependent in-plane resistance for sample S1 at various pressures. Two anomalies are visible at 240 K and 181 K at 1.0 GPa. A sharp superconducting transition emerges at $T_\text{c}^\text{onset} = 3.6$ K and 19.0 GPa. The inset shows a photo of the sample configured for high-pressure transport measurements. c, Detailed view of the low-temperature resistance of S1 at 24.3, 26.5, 30.6, and 35.0 GPa, demonstrating the development of a zero-resistance superconducting state with increasing pressure. d, Temperature dependence of the upper critical field $\mu_0H_{\text{c2}}(T)$ at 24.3, 30.6, and 35.0 GPa, with solid lines representing fits using the Ginzburg-Landau model. e-g, Evolution of the superconducting transition with applied magnetic field at selected pressures.
• Figure 3: $|$High-pressure structural evolution of hybrid La$_3$Ni$_2$O$_7$.a, Rietveld refinements of synchrotron XRD patterns for the 1313 phase of La$_3$Ni$_2$O$_7$ at 4.6 GPa and 17.5 GPa, measured at room temperature. The refinements correspond to the orthorhombic $Cmmm$ and tetragonal $P4/mmm$ space groups, respectively. b, Pressure evolution of selected reflection peaks between 8$^\circ$ and 10$^\circ$, showing the merging of the (0 2 0) and (2 0 0) peaks, which signals the transition to the tetragonal phase. Neon was used as the pressure-transmitting medium. c, Raman spectra of the 1313 phase at room temperature under various pressures. Helium was used as the pressure-transmitting medium. d, Pressure dependence of the lattice constants $a$, $b$, and $c$. e, Color map of the Raman intensity as a function of pressure and wavenumber. The positions of peaks P2, P3, and P2$^\prime$, determined by Lorentzian fits, are overlaid. The intensity of peak P2 is normalized to unity at each pressure for comparison. The dashed line at 13 GPa marks the merging of two peaks, indicating a structural transition. f, Schematic of the trilayer structure unit in the 1313 phase before and after the structural transition.
• Figure 4: $|$Phase diagram of hybrid La$_3$Ni$_2$O$_7$.Upper panel: Pressure dependence of the resistance at 20 K for samples S1 and S2. Lower panel: Temperature-pressure phase diagram for the 1313 phase of La$_3$Ni$_2$O$_7$. The superconducting onset temperature $T_\text{c}^\text{onset}$ and zero-resistance temperature $T_\text{c}^\text{zero}$ for S1, along with $T_\text{c}^\text{onset}$ for S2, are indicated. The pressure-induced structural transition from the orthorhombic $Cmmm$ phase to the tetragonal $P4/mmm$ phase at 13 GPa is marked by a vertical dashed line. The background color represents the normalized resistance $R/R_\text{50 K}$ for S1, highlighting the superconducting region and the pronounced resistance change at the structural transition.
• Figure : Extended Data Fig.1$|$High-pressure electrical transport properties of S2.a, Temperature dependence of resistance at pressures from 0.9 to 30.7 GPa. b, Enlarged display of the resistance at selected pressures below 6 K. c, Temperature dependence of the resistance under various magnetic fields below 6 K. d, Upper critical field $\mu_0 H_{\text{c2}}(T)$ at 30.7 GPa with a fit based on the Ginzburg-Landau model.