[Paper Review] Pressure-enhanced spin-density-wave transition in double-layer nickelate $La_{3}Ni_{2}O_{7-δ}$

Dan Zhao, Yanbing Zhou+8 more|arXiv (Cornell University)|Feb 6, 2024

Magnetic and transport properties of perovskites and related materials12 citations

TL;DR

The paper uses 139La NMR to show a spin-density-wave order in La3Ni2O7-δ around 150 K, and finds that the SDW transition temperature increases with pressure up to ~2.7 GPa, suggesting a nontrivial relation between SDW and potential superconductivity.

ABSTRACT

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for $La_{3}Ni_{2}O_{7-δ}$ 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. Here, nuclear magnetic resonance (NMR) spectroscopy of $^{139}La$ nuclei was performed to study the density-wave-like transition in a single crystal of $La_{3}Ni_{2}O_{7-δ}$. At high temperatures, two sets of sharp $^{139}La$ 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 $^{139}La$ NMR spectra and nuclear spin-lattice relaxation rate $(1/T_{1}$) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature ($T_{SDW}$), which is ~ 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 $T_{SDW}$ 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. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

Motivation & Objective

• Motivate understanding of density-wave order in double-layer nickelates as a potential precursor to superconductivity under pressure.
• Identify the SDW transition temperature and its pressure dependence using 139La NMR.
• Disentangle contributions from multiple La sites due to oxygen vacancy variations in the crystal.
• Explore the spin orientation and possible double spin stripe structure from NMR anisotropy.
• Assess how pressure modifies magnetic correlations and its relation to superconductivity in this system.

Proposed method

• Perform 139La NMR spectroscopy on a single crystal of La3Ni2O7-δ across temperatures to detect SDW onset.
• Measure nuclear spin-lattice relaxation rate (1/T1) for La(1) sites to identify SDW ordering.
• Analyze high-temperature spectra to assign peaks to La(1) sites from two bilayer Ruddlesden-Popper phases with different δ.
• Apply pressure up to ~2.7 GPa and monitor changes in T_SDW inferred from NMR signals for both La(1) sites.
• Interpret anisotropic NMR peak splitting to infer possible double spin stripe order with moments along the c-axis.

Experimental results

Research questions

• RQ1 Does La3Ni2O7-δ host SDW order at ambient pressure as detected by 139La NMR?
• RQ2 How does the SDW transition temperature T_SDW respond to applied pressure up to ~2.7 GPa?
• RQ3 What is the nature of the magnetic order (e.g., spin orientation, stripe structure) suggested by NMR anisotropy?
• RQ4 How do multiple La sites and oxygen vacancies influence the NMR signatures and SDW detection?
• RQ5 What implications do the pressure dependence of SDW have for the relation between magnetism and potential superconductivity in double-layer nickelates.

Key findings

• SDW order is detected below T_SDW ~ 150 K from temperature-dependent NMR spectra and 1/T1 for both La(1) sites.
• Two sets of sharp La NMR peaks at high temperature originate from La(1) sites in two bilayer Ruddlesden-Popper phases with different oxygen vacancy δ.
• Anisotropic splitting of NMR spectra suggests a possible double spin stripe with magnetic moments along the c-axis.
• Under pressure up to ~2.7 GPa, T_SDW increases with pressure, contrary to some transport studies under pressure.
• Results highlight a nontrivial relationship between SDW magnetism and high-pressure superconductivity candidates in La3Ni2O7-δ.