Pressure-enhanced spin-density-wave transition in double-layer nickelate La₃Ni₂O₇−δ

摘要

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for La₃Ni₂O₇−δ under pressure. This finding immediately stimulated intense interest in the possible mechanism of high-T_c superconductivity in double-layer nickelates. Notably, the pressure-dependent phase diagram inferred from transport measurements indicates that the superconductivity under high pressure emerges from the suppression of density-wave-like order at ambient pressure, which is similar to high-temperature superconductors. Therefore, clarifying the exact nature of the density-wave-like transition is important for determining the superconducting mechanism in double-layer nickelates. Here, nuclear magnetic resonance (NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like transition in a single crystal of La₃Ni₂O₇−δ. At high temperatures, two sets of sharp 139La NMR peaks are clearly distinguishable from a broad background signals, which are ascribed to La(1) sites from two bilayer Ruddlesden-Popper phases with different oxygen vacancy δ. As the temperature decreases, the temperature-dependent 139La NMR spectra and nuclear spin-lattice relaxation rate (1/T₁) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature (TSDW), which is approximately 150 K. The anisotropic splitting in the NMR spectra suggests the formation of a possible double spin stripe with magnetic moments aligned along the c-axis. Furthermore, we studied the pressure-dependent SDW transition up to ∼ 2.7 GPa. Surprisingly, the TSDW inferred from NMR measurements of both La(1) sites increases with increasing pressure, which is opposite to the results from previous transport measurements under pressure and suggests an intriguing phase diagram between superconductivity and SDW. In contrast, the present 139La NMR is insensitive to the possible charge-density-wave (CDW) order in the Ni-O planes. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

材料

• La₃Ni₂O₇-δ

方法

• Nuclear magnetic resonance (NMR) spectroscopy of 139La nuclei
• Nuclear spin-lattice relaxation rate (1/T₁) measurements

关键词

• spin density wave ordering
• double spin stripe
• magnetic moments along c axis
• pressure enhanced tsdw

亮点

• Anisotropic splitting in NMR spectra suggests formation of a possible double spin stripe with magnetic moments aligned along the c-axis.
• NMR is insensitive to possible charge-density-wave order in the Ni-O planes.

结论

• Spin-density-wave ordering occurs below TSDW approximately 150 K.
• TSDW increases with pressure, opposite to transport results, suggesting an intriguing phase diagram.

主要论断

• Spin-density-wave ordering occurs at ≈150 K in La₃Ni₂O₇−δ single crystals.
  • 证据: 139La NMR line splitting below 150 K (Fig.2b),1/T₁T peak at ≈150 K (Fig.3a),Field-independent splitting confirms internal magnetic field
• The SDW transition temperature increases with pressure, in contrast to the resistivity anomaly which is suppressed.
  • 证据: 1/T₁T peak shifts to higher temperature with pressure (Fig.4a,b),Transport anomaly disappears above 2 GPa (Fig.4c)
• The magnetic structure is consistent with a double spin stripe model with wavevector q=(π/2,π/2) and moments along c-axis.
  • 证据: Anisotropic splitting pattern (splitting for B//ab, broadening for B//c),Simulations reproduce NMR spectra (Fig.2i-l),Estimated Ni moments ≈0.08 μB (La(1)a) and ≈0.018 μB (La(1)b)

研究流程

• Sample growth — Single crystal with two distinct La(1) sites due to different oxygen content.
  • 材料: La₃Ni₂O₇ precursors
  • 方法: High oxygen pressure floating zone technique
  • 观察: Single crystal with size ≈2mm×1mm, dominant bilayer RP phase; Oxygen content inhomogeneity leads to two orthorhombic phases (Fmmm and Amam)
• 139La NMR measurements — SDW transition at ≈150 K detected via NMR line shape changes and 1/T₁ anomalies.
  • 材料: La₃Ni₂O₇ single crystal
  • 方法: NMR spectroscopy; Spin echo method; Fast Fourier transform sum
  • 观察: Two sets of sharp NMR lines for La(1) sites (La(1)a and La(1)b); Below 150 K, lines broaden (B//c) or split (B//ab); Field-dependent splitting confirms magnetic origin; Anisotropic splitting suggests spin stripe order with moments along c
• Spin-lattice relaxation rate measurements — Spin fluctuations and magnetic order coexist; oxygen vacancies induce spin freezing at low T.
  • 材料: Same sample
  • 方法: Saturation method; Fitting recovery with stretched exponential
  • 观察: 1/T₁T shows peak at ≈150 K for both La(1) sites; La(1)a exhibits significant spin fluctuations above TSDW; Stretched exponent decreases below TSDW indicating inhomogeneous dynamics; At low T (<50 K), additional peak due to spin freezing
• Pressure-dependent study — TSDW and transport density wave show opposite pressure dependence, indicating different origins (SDW vs CDW).
  • 材料: Sample in BeCu piston-cylinder cell
  • 方法: 1/T₁T measurements under pressure
  • 观察: TSDW increases slightly with pressure (e.g., from 152 K to 155 K); Transport anomaly (density wave) decreases with pressure and disappears above 2 GPa