Unconventional Pressure Evolution of Spin-Density-Wave State in La₃Ni₂O₇

Summary

We performed systematic electronic Raman scattering studies on single crystals of the bilayer nickelate La₃Ni₂O₇ under hydrostatic pressures up to 16.51 GPa, tracing the evolution of the spin-density-wave (SDW) state with pressure. The results show that both the SDW gap energy and the SDW transition temperature generally increase with increasing pressure, while the dimensionless coupling ratio 2Δ_SDW/(k_B T_SDW) remains approximately 7.5, indicating robust strong-coupling character of the SDW state. Concurrently, the SDW Raman peak progressively broadens with pressure, suggesting that the long-range SDW ordering coherence gradually weakens. This work reveals an unusual pressure evolution in which the SDW energy scale is enhanced while the SDW state becomes increasingly incoherent, thereby providing spectroscopic constraints on magnetic correlations relevant to high-temperature superconductivity in bilayer nickelates.

Materials

• La₃Ni₂O₇

Methods

• polarization-resolved electronic Raman scattering
• hydrostatic pressure
• diamond anvil cell
• ruby fluorescence pressure calibration
• Lorentzian fitting
• memory-function formalism

Keywords

• spin density wave
• strong coupling
• superconductivity
• interlayer exchange coupling
• pressure induced broadening
• magnetic correlations

Highlights

• This work provides the first Raman spectroscopic characterization of the pressure evolution of the SDW state in bilayer nickelates.
• Unlike cuprates and iron-based superconductors, both the SDW gap and T_SDW in La₃Ni₂O₇ increase monotonically with pressure with no downturn over the measured range.
• The pressure-independent coupling ratio remains around 7.5, well above the weak-coupling mean-field value of 3.52, showing that the strong-coupling nature is preserved under pressure.

Conclusions

• Both the SDW gap energy and the SDW transition temperature increase monotonically with pressure, while the dimensionless coupling ratio 2Delta_SDW/(k_B T_SDW) remains near 7.5, indicating robust strong-coupling character of the SDW state.
• The SDW-related Raman peak progressively broadens with pressure, suggesting reduced coherence of the long-range SDW order.
• These results reveal an unconventional pressure evolution in which the SDW energy scale is enhanced while the SDW state becomes progressively less coherent, providing spectroscopic constraints on magnetic correlations relevant to superconductivity in bilayer nickelates.

Main claims

• Electronic Raman scattering shows an SDW-related feature in La₃Ni₂O₇ that evolves systematically with hydrostatic pressure up to 16.51 GPa.
  • Evidence: full_text: The SDW-related feature remains discernible over nearly the entire pressure range. With increasing pressure, its characteristic energy shifts progressively upward.
• The SDW gap energy and T_SDW both increase with pressure, while the dimensionless coupling ratio 2Delta_SDW/(k_BT_SDW) remains about 7.5.
  • Evidence: full_text: The SDW gap [Fig. 3(a)] increases monotonically with pressure, from approximately 41.3 meV at 1.23 GPa to about 49.6 meV near 13 GPa. Notably, T_SDW and Delta evolve in parallel with increasing pressure. Consequently, the dimensionless coupling ratio remains constant around 7.5 over the entire pressure range.
• The progressive broadening of the SDW-related Raman peak indicates reduced long-range SDW coherence under pressure.
  • Evidence: full_text: As shown in Fig. 3(c), Gamma increases progressively with pressure. The increase in Gamma therefore suggests enhanced damping and/or a broader distribution of these transitions, indicating that the SDW-related electronic response becomes progressively less well defined under pressure.
• The enhanced SDW energy scale is naturally explained by pressure-induced strengthening of Ni-O orbital overlap and magnetic exchange interactions.
  • Evidence: full_text: Hydrostatic compression shortens the Ni-O bond length and modifies the Ni-O-Ni bond angle, thereby enhancing the overlap between the Ni and O orbitals and increasing the effective hopping integral. The resulting enhancement of magnetic exchange interactions provides a natural explanation for the observed increases in both T_SDW and Delta_SDW.

Workflow

• sample_preparation — High-qualityLa₃Ni₂O₇ single crystals suitable for Raman experiments were prepared.
  • Materials: La₃Ni₂O₇ single crystals
  • Methods: vertical optical floating-zone growth under 15 bar oxygen pressure using a 5 kW xenon arc lamp; selection of shiny and flat crystals for Raman measurements
• measurement — Pressure- and temperature-dependent Raman spectra were collected for the SDW state.
  • Materials: La₃Ni₂O₇ single crystals; argon pressure-transmitting medium; ruby fluorescence pressure calibrant; membrane-driven diamond anvil cell
  • Methods: polarization-resolved electronic Raman scattering; pressure generation using a membrane-driven diamond anvil cell; cooling to 20 K and subsequent warming to 300K; excitation with a 532 nm laser at about 1.3 mW focused to about 10 um spot
  • Observations: SDW-related Raman feature centered near 700 cm-1 at 20 K and 1.23 GPa; SDW-related feature remains discernible over nearly the entire measured pressure range
• analysis — Quantitative spectral analysis reveals pressure-dependent SDW energy scale and linewidth broadening.
  • Methods: modeling the low-energy electronic continuum using the memory-function formalism; fitting the SDW-related Raman peak with a Lorentzian function; extracting SDW gap energy, transition temperature, and linewidth; constructing the phase diagram from present and previously reported data
  • Observations: SDW gap increases from about 41.3 meV at 1.23 GPa to about 49.6 meV near 13 GPa; T_SDW increases monotonically with pressure; 2Delta_SDW/(k_BT_SDW) remains around 7.5; SDW Raman peak linewidth increases progressively with pressure
• interpretation — Pressure enhances the SDW energy scale while reducing long-range SDW coherence, preserving the strong-coupling character of the SDW state.
  • Methods: comparison with strong-coupling bilayer models; interpretation of linewidth broadening as reduced long-range SDW coherence and possible inhomogeneity