Electronic layer decoupling driven by density-wave order in La$_4$Ni$_3$O$_{10}$

Abstract

We probe the density-wave transition of the trilayer nickelate La$_4$Ni$_3$O$_{10}$ with polarization-resolved infrared spectroscopy. The low-energy electrodynamics is strongly anisotropic, with metallic in-plane and insulating out-of-plane character. In the ordered phase, the anisotropy grows more than an order of magnitude as the out-of-plane conductivity is sharply suppressed. We interpret this enhancement as an effective electronic decoupling of the Ni-O layers, driven by a spin-density-wave-induced redistribution of Ni-$d_{z^2}$ occupation within the trilayers. This electronic response is accompanied by clear shifts and splittings of the out-of-plane phonons, compatible with a density-wave instability of electronic origin.

Figures (4)

• Figure 1: (a) Structure of the trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$, consisting of three NiO$_3$ octahedral layers, where the inner and outer layers are colored differently to reflect their distinct local electronic environment. (b) Reflectivity and (c) real part of the optical conductivity $\sigma_1$ at base (blue) and room temperature (red). Out-of-plane ($E\parallel c^*$) and in-plane ($E\parallel ab$) optical responses are shown in solid lines and broken lines, respectively. The DW transition temperature is approximately 140 K.
• Figure 2: (a)-(d) Temperature-dependent real and imaginary optical conductivity of La$_4$Ni$_3$O$_{10}$ in- ($E\parallel ab$) and out-of-plane ($E\parallel c^*$), denoted as solid lines. Drude$+\varepsilon_\infty$ model fits are shown as dashed lines. (e) Temperature dependence of the extrapolated DC resistivity obtained from the Drude fits. Left- and right-pointing triangles with error bars represent the out-of-plane ($\rho_{c^*}$) and in-plane ($\rho_{ab}$) resistivities, respectively. Broken lines denote DC resistivity obtained from transport measurements. $\rho_{ab}$ is adapted from Ref. zhang2020high (see Supplementary Material Sec. S6 for further details). (f) Inset: Temperature-dependent resistivity anisotropy ratio $\rho_{c^*}/\rho_{ab}$.
• Figure 3: (a) Real-space depiction of the DW modulation. Below $T_{\mathrm{DW}}$, an antiphase SDW develops on the outer layers, leaving the inner layer as a magnetic node, while the intertwined CDW is in-phase across the trilayer. The SDW is indicated by the purple wave, and the CDW by the blue/white variation of the lattice color. (b) Proposed qualitative schematic of DW-driven redistribution of Ni $d_{z^2}$ orbital occupation within a trilayer. Above $T_{\mathrm{DW}}$ the trilayer is nonmagnetic and the $d_{z^2}$ occupation is comparable in inner and outer Ni-O layers despite their inequivalent local environments. Below $T_{\mathrm{DW}}$, the SDW residing on the outer layers (with a magnetic node on the inner layer) is accompanied by an enhanced $d_{z^2}$ weight on the outer layers and a reduced weight on the inner layer, thereby suppressing interlayer transport. The inverted colors on the outer layer $d_{z^2}$ orbitals illustrate the phase of the SDW modulation.
• Figure 4: (a) Temperature evolution of the out-of-plane vibrational response. Spectra are vertically offset by 150 $\Omega^{-1}$cm$^{-1}$ for clarity. Symbols mark phonon frequencies and dotted lines are guides to the eye. Drude-Lorentz fits to the 4 K spectrum with individual contributions are shown as grey lines and shaded areas, respectively. (b) Temperature-dependent phonon frequencies for selected modes; triangles denote split modes in the DW phase (blue shading). (c)-(f) Calculated atomic displacements for infrared-active modes matching the 160 K data in panel (b); only large-amplitude displacements are shown for clarity. Owing to the monoclinic angle $\beta$, the crystallographic $c$ axis is tilted by $\sim 11^{\circ}$ relative to the experimental out-of-plane $c^*$ direction. (c)-(d) and (e)-(f) are views of the same mode from two different directions.