Spin Fluctuations in the Rare-Earth Doped Bilayer Nickelates

TL;DR

The paper investigates spin fluctuations in rare-earth doped bilayer nickelates to shed light on possible pairing mechanisms for high-temperature superconductivity. Using inelastic neutron scattering on Nd- and Pr-doped La3Ni2O7-δ, it reveals that the previously observed flat 45 meV spin mode splits into two components and an additional ~60 meV excitation, with Nd-doped samples showing stronger fluctuations. SpinW linear spin-wave theory applied to stripe-type antiferromagnetic orders shows that enhanced interlayer exchange (J_perp) of about 69–73 meV can account for the splitting, while intralayer exchanges remain comparatively small. These findings emphasize the crucial role of interlayer coupling in promoting superconductivity in nickelates, suggesting interlayer s± pairing as a potential route to higher Tc; a simple estimate indicates Tc could approach ~104 K if J_perp is maintained at large values under pressure.

Abstract

Spin fluctuations have been generally believed as the pairing glue of high-$T_c$ superconductivity. Recent inelastic neutron scattering (INS) studies have revealed a weak flat spin-fluctuation signal around 45 meV in the bilayer nickelate La$_3$Ni$_2$O$_{7-δ}$, suggesting strong interlayer and weak intralayer magnetic couplings ($SJ_{\perp}\approx$ 60 meV, $SJ_{\parallel}\leq$ 3.5 meV) in contrast to cuprate and pnictide superconductors. Here, we report further INS studies on the Pr and Nd doped La$_3$Ni$_2$O$_{7-δ}$ powder samples at ambient pressure. Besides the crystalline electric field excitations at low energies, we have found that the 45 meV flat mode splits into two modes in doped compounds, along with another weak mode at about 60 meV, where the spin fluctuations in La$_2$NdNi$_2$O$_{7-δ}$ are stronger than La$_3$Ni$_2$O$_{7-δ}$ and La$_2$PrNi$_2$O$_{7-δ}$. Based on an effective Heisenberg model by only considering the nearest-neighbor exchange couplings on the stripe-type antiferromagnetic orders, we conclude that the interlayer coupling $SJ_{\perp}$ is enhanced to about 69 meV and 73 meV for Pr and Nd doped samples, respectively. Our results highlight the crucial role of interlayer coupling in the rare-earth doped bilayer nickelates, which towards to promote high $T_c$ via interlayer $s\pm$ pairing.

Figures (9)

• Figure 1: (a)-(h) SpinW calculations based on the double spin stripe (DSS) and the single spin-charge stripe (SCS) AF orders using different combinations of exchange couplings. The nearest-neighbor intralayer exchange couplings $J_1$,$J_1^{\prime}$ in DSS model (or $J_2$,$J_2^{\prime}$ in SCS model) and interlayer exchange coupling $J_{\perp}$ are indicated in (a) and (e). The green, red and white balls represent the spin up, spin down and nonmagnetic Ni atoms, respectivelyxchen2024. (i) Magnetic susceptibility $\chi$ of La$_3$Ni$_2$O$_{7-\delta}$. The anomaly at $T_N=147$ K probably responses to the AF transition. (j) $\chi$ and its inverse $1/\chi$ of La$_2$NdNi$_2$O$_{7-\delta}$ and La$_2$PrNi$_2$O$_{7-\delta}$. The dashed straight lines are Curie-Weiss fittings below 50 K. (k) Heat capacity $C_p$ versus $T$ for three compounds. (l) Magnetic entropy of Pr and Nd doped samples from the integration of $C_m/T$. Here $C_m$ is obtained by subtracting the $C_p$ of La$_3$Ni$_2$O$_{7-\delta}$.
• Figure 2: INS spectra of La$_2$NdNi$_2$O$_{7-\delta}$ collected at MERLIN. (a)-(d) Data measured at 5 K with incident energy $E_i=$ 15, 24, 50 and 79 meV. (e)-(h) Data measured at 110 K with similar $E_i$s. (i)-(l) $E$-cuts at $T=$ 5 K, 110 K, and their differences with Bose correctionssupplementary. The blue arrows mark the CEF excitations, and the green arrows mark the possible spin excitations. (m)-(p) $Q$-cuts at $T=$ 5 K at the typical energy windows of excitations. The data is vertically shifted for clarity. The elastic cut with $E=$[-0.5, 0.5] meV is also presented in (m), where all nuclear peaks are marked by vertical bars.
• Figure 3: INS spectra of La$_2$NdNi$_2$O$_{7-\delta}$ collected at PANTHER with $E_i=$ 19 and 76 meV. (a) and (b) Spectra after subtracting 170 K from 1.5 K data. (c) and (d) Spectra after subtracting La$_3$Ni$_2$O$_{7-\delta}$ from La$_2$NdNi$_2$O$_{7-\delta}$ data at $T=1.5$ K. (e)-(h) $E$-cuts at different $Q$ regions corresponding to (a)-(d). Bose corrections are applied to (e) and (f)supplementary. The blue arrows mark the CEF excitations at 5.5 and 22 meV, and the green arrows mark the spin excitations at 43, 48 and 60 meV.
• Figure 4: INS spectra of La$_2$PrNi$_2$O$_{7-\delta}$ collected at PANTHER with $E_i=$ 12.5, 19 and 76 meV. (a) Spectrum of subtracting 50 K from 1.5 K data ($E_i=$ 12.5 meV). (b)(c) Spectra after subtracting 170 K from 1.5 K data ($E_i=$ 19 and 76 meV). (d) Spectra for $E_i=$ 76 meV after subtracting La$_3$Ni$_2$O$_{7-\delta}$ from La$_2$PrNi$_2$O$_{7-\delta}$ data at $T=1.5$ K. (e)-(h) $E$-cuts at different regions corresponding to (a)-(d). Bose corrections are applied to panel (e), (f) and (g)supplementary The blue arrows mark the CEF excitations at 2, 3.5 and 6 meV, and the green arrows mark the spin excitations at 44 meV, 47 meV and 60 meV. (i)-(l) $Q$-cuts at $T=$ 1.5 K by focusing energy windows. The elastic cut with $E=$[-0.5, 0.5] is also presented in panel (i), where all nuclear peaks are marked by vertical bars.
• Figure S1: Powder X-ray diffraction and refinement results of (a) La$_3$Ni$_2$O$_{7-\delta}$ (b) La$_2$NdNi$_2$O$_{7-\delta}$ (c) La$_2$PrNi$_2$O$_{7-\delta}$ samples.