Electronic Structure of Epitaxial Films of the Bilayer Strontium Ruthenate: Sr$_{3}$Ru$_2$O$_{7}$

TL;DR

The study addresses how epitaxial strain tunes the low-energy electronic structure of the bilayer ruthenate Sr3Ru2O7 by combining in-situ ARPES on MBE-grown films under tensile (STO) and compressive (LSAT) strain with DFT calculations using lattice parameters from reciprocal-space mapping. The results show strain-driven Fermi-surface topology changes and symmetry reductions: tensile strain preserves tetragonal-like structure while compressive strain induces orthorhombicity and RuO6 octahedral rotations, with DFT reproducing the main features when realistic in-plane geometries are used. Notably, flat bands appear within ~$15$ meV below $E_F$ along $Γ$–$X$ in the orthorhombic phase and near $Γ$ in the tetragonal phase, signaling potential van Hove singularities that could promote magnetic instabilities in the films. The work demonstrates strong strain sensitivity of Sr3Ru2O7’s electronic structure, validating strain engineering as a route to access correlated phases in bilayer ruthenates and guiding future high-resolution ARPES and transport studies.

Abstract

We report the first combined study of the low-energy electronic band structure of epitaxial Sr$_3$Ru$_2$O$_7$ films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The complete Fermi-surface topography of the near-Fermi-level bands is determined from in-situ ARPES measurements. To investigate the effects of substrate-induced strain on the band structure, Sr$_3$Ru$_2$O$_7$ thin films are epitaxially grown on SrTiO$_3$ (STO) and (LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$TaAlO$_{6}$)$_{0.7}$ (LSAT) substrates using molecular beam epitaxy. The combination of the measured Fermi-surfaces along with the theoretical interpretation, clearly show dramatic changes in the Fermi surface topologies that result from the underlying strain states of the films on the two substrates. We find that the Sr$_3$Ru$_2$O$_7$ films prepared on STO are tensile strained with tetragonal symmetry, whereas those grown on LSAT are compressively strained with orthorhombic symmetry. Within $\sim15~\text{meV}$ below the Fermi level, we observe two flat bands along $Γ$-$X$ in the orthorhombic phase and around $Γ$ in the tetragonal phase. These features could be favorable for van Hove singularities near the Fermi level, and highlight the emergence of magnetic instabilities in epitaxial Sr$_3$Ru$_2$O$_7$ films.

Figures (4)

• Figure 1: (a) crystal structure of bilayered ruthenate Sr$_3$Ru$_2$O$_7$ in the tetragonal I4/mmm phase. The blue contour encompasses the unit cell. (b) Schematic of the photoexcitation process in ARPES experiments viewed from the sample surface. The emission angle, $\theta$, the sample rotation angle, $\varphi$, and the photoelectron wave vector, $\vec{K}$, are indicated. (c) and (d) RHEED images of Sr$_3$Ru$_2$O$_7$ (SRO) grown on STO and LSAT along (110) and (100) directions, respectively. (e) Close up view of XRD $\theta$-2$\theta$ scans of representative samples of thickness 18 nm (on LSAT) and 24 nm (on STO), around the (002) and (0010) reflections of the substrates and SRO films, respectively. The vertical dashed line indicates the position of the (0010) reflection for a fully relaxed SRO film. (f) and (g) Reciprocal space maps around (103) peaks of the substrates and (1015) peaks of the SRO films grown on STO and LSAT, respectively.
• Figure 2: (a) Symmetrized ARPES Fermi-surface map of Sr$_3$Ru$_2$O$_7$ grown on LSAT. The white square marks the reconstructed Brillouin zone arising from octahedral rotations. (b) Calculated Fermi surface of Sr$_3$Ru$_2$O$_7$ for LSAT-induced strain, which drives the system into an orthorhombic structure. (c) ARPES Fermi-surface map of Sr$_3$Ru$_2$O$_7$ grown on STO. (d) Calculated Fermi surface of Sr$_3$Ru$_2$O$_7$ under STO-induced strain resulting in a tetragonal phase.
• Figure 3: (a) DFT-calculated band structure of Sr$_3$Ru$_2$O$_7$ grown on LSAT, including the folded bands for an orthorhombic phase. (b) and (c) ARPES-measured band dispersions of Sr$_3$Ru$_2$O$_7$ on LSAT along the $\Gamma$–X direction at two different manipulator angles, sampling distinct regions of the Brillouin zone. The MDC at E$_F$ are overlaid (red curves). (d) DFT-calculated band structure of Sr$_3$Ru$_2$O$_7$ grown on STO. (e) and (f) ARPES-measured band structure of Sr$_3$Ru$_2$O$_7$ on STO along two momentum regions. The insets in (b), (c), (e), and (f) indicates the momentum-cut locations in the Brillouin zone. To visualize the edges of the narrow features around E$_F$ in (c) and (e), we performed their first derivatives as shown in the supplementary information.
• Figure S1: Figure S1:First derivatives of the original ARPES maps of the Sr$_{3}$Ru$_2$O$_{7}$ thin films plotted in Fig 3(c) and 3(e) of the main text. (a) and (b) are the data of the film deposited on STO [Fig 3(e) of the main text]. (a) represent the derivative with respect to the energy dimension, and (b) is that along the momentum axis. The edges of the nearly narrow feature located about 15 meV below E$_F$ and centered at $\Gamma$ are visible in (a). This feature seems to have been completely quenched when the derivative is applied along the momentum direction, attesting to its nearly flat character [1]. (c) and (d) are the data for the film prepared on LSAT [Fig 3(c) of the main text]. Similarly, the edges of a flat band around 0.6--0.9 Å$^{-1}$ are visible when the derivative is performed with respect to the energy axis in (c). Only steep bands are visible in (d), when the derivative is performed along the momentum axis.